U.S. patent application number 09/821119 was filed with the patent office on 2001-12-27 for rolling bearing.
This patent application is currently assigned to NSK LTD.. Invention is credited to Ohori, Manabu, Ueda, Kouji.
Application Number | 20010055432 09/821119 |
Document ID | / |
Family ID | 27310909 |
Filed Date | 2001-12-27 |
United States Patent
Application |
20010055432 |
Kind Code |
A1 |
Ueda, Kouji ; et
al. |
December 27, 2001 |
Rolling bearing
Abstract
The present invention provides a rolling bearing having an
excellent corrosion resistance and toughness which can fairly
operate at a high rotary speed. At least the inner race is formed
by a titanium alloy, and the rolling elements are formed by
ceramics. Alternatively, at least one of the inner race and the
outer race is formed by a .beta. type titanium alloy. The percent
cold working of the .beta. type titanium alloy is predetermined to
not less than 20% or a range of from 5 to 20%. The cold working is
followed by shot peening. Further, the surface hardness Hv is
predetermined to not less than 600. The volumetric ratio of
residual .beta. phase in the .beta. type titanium alloy is
predetermined to a range of from 30 to 80%.
Inventors: |
Ueda, Kouji; (Kanagawa,
JP) ; Ohori, Manabu; (Kanagawa, JP) |
Correspondence
Address: |
CROWELL & MORING LLP
Intellectual Property Group
P. O. Box 14300
Washington
DC
20044-4300
US
|
Assignee: |
NSK LTD.
|
Family ID: |
27310909 |
Appl. No.: |
09/821119 |
Filed: |
March 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09821119 |
Mar 30, 2001 |
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09108391 |
Jul 1, 1998 |
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6250812 |
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Current U.S.
Class: |
384/492 ;
29/898.066; 29/90.7 |
Current CPC
Class: |
F16C 33/30 20130101;
Y10T 29/479 20150115; Y10S 384/912 20130101; Y10T 29/49689
20150115 |
Class at
Publication: |
384/492 ;
29/90.7; 29/898.066 |
International
Class: |
F16C 033/32; F16C
033/64 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 1, 1997 |
JP |
P. HEI. 9-188898 |
Dec 2, 1997 |
JP |
P. HEI. 9-345718 |
Apr 3, 1998 |
JP |
P. HEI. 10-107102 |
Claims
What is claimed is:
1. A rolling bearing comprising races composed of an outer race and
an inner race and rolling elements which are provided between the
outer race and the inner race such that the rolling elements rotate
freely, wherein at least said inner race is made of a titanium
alloy and said rolling elements are made of a corrosion-resistant
material.
2. The rolling bearing of claim 1, wherein said titanium alloy is
selected from the group consisting of .beta. type titanium alloy
and (.alpha.+.beta.) type titanium alloy and said
corrosion-resistant material is selected from the group consisting
of ceramics and martensite stainless steel.
3. The rolling bearing of claim 1, wherein the surface hardness
(Hv) of the finished raceway track on at least one race selected
from the group consisting of said outer race and said inner race is
not less than 600.
4. The rolling bearing of claim 1, wherein the surface of said
finished raceway track on at least one race selected from the group
consisting of said outer race and said inner race comprises a
mixture of .alpha. phase texture and .beta. phase texture, the
proportion of said .beta. phase in said mixture being from 30 to 80
vol %.
5. A method for producing a rolling bearing, which comprises
preparing at least one race selected from the group consisting of
an outer race and an inner race according to a method which
comprises steps of: (a) selecting at least one from the group
consisting of .beta. type titanium alloy and (.alpha.+.beta.) type
titanium alloy as a race material; (b) heating and keeping said
race material at the temperature falling within the range of .beta.
phase temperature of not lower than .beta. phase transition point
to effect solution treatment such that the phase of the texture of
said race material is converted to .beta. phase; (c) rapidly
cooling said race material so that the texture of said race
material normally stays in .beta. single phase; (d) subjecting said
race material to plastic working so that it is shaped as desired
and given work strain, which enables formation of nuclei of .alpha.
phase which is harder than .beta. phase and the .alpha. phase to be
finely deposited in .beta. phase; (e) subjecting said race material
to aging at a predetermined temperature lower than .beta. phase
transition point, whereby nuclei of .alpha. phase are formed and
grown and the .alpha. phase is finely deposited in .beta. phase;
and then (f) machining said race material to a race.
6. The method of claim 5, wherein the percent plastic working at
the step (d) is not less than 20%.
7. The method of claim 5, wherein said percent plastic working is
from 5 to 30% and the surface of said raceway track is subjected to
shot peening before aging.
8. The method of claim 7, wherein shot peening is effected after
aging.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a rolling bearing, and,
more particularly, a rolling bearing which is used which is used
under a special environment, for example, under an environment
requiring corrosion resistance to water content, sea water and
chemicals, e.g., in a food machine, a semiconductor producing
apparatus and a chemical fiber producing machine, or in a tool
machine which operates at a high rotary speed.
BACKGROUND OF THE INVENTION
[0002] As a bearing which must be corrosion-resistant there has
been heretofore used relatively often a sliding bearing made of a
material having an excellent corrosion resistance. In recent years,
rolling bearings have been used more and more from the standpoint
of torque reduction that prevents dynamic loss or eliminates the
necessity of maintenance and improvement of product quality.
[0003] As the material for such rolling bearings there is mostly
used a low-alloy steel such as two kinds of high carbon chromium
bearing steels (SUJ2) and case hardening steel (SCR420). However,
rolling bearings are used in various working conditions. Thus, if
such a rolling bearing made of a low-alloy steel is used under
environmental conditions which can be contaminated by water content
or sea water, the contamination by even a slight amount of water
content or sea water corrodes the bearing portion thereof corrodes
with rust that disables the rolling bearing from working. Thus,
martensite stainless steel having an excellent corrosion resistance
and a high chromium content (e.g., SUS440C) is used under such
environmental conditions.
[0004] However, a rolling bearing comprising races and rolling
elements both of which are made of martensite-based stainless steel
(hereinafter simply referred to as "stainless steel") can exhibit
an insufficient corrosion resistance in some working atmospheres.
In this case, corrosion occurs with chromium-deficient layer in the
vicinity of coarse eutectic carbide as a starting point to reduce
precision such as surface smoothness, possibly making it impossible
to secure the desired bearing life. In particular, a rolling
bearing adapted for use in semiconductor producing apparatus, etc.
is subject to attack by a corrosive gas or chemical that can
corrode stainless steel. Thus, it is required that such a rolling
bearing comprise a material having a better corrosion resistance
than stainless steel.
[0005] From this standpoint of view, as a bearing material
constituting a rolling bearing adapted for use in corrosive working
atmospheres there has heretofore been used a ceramic material such
as silicon nitride (Si.sub.3N.sub.4) (hereinafter referred to as
"first conventional technique").
[0006] In the machine tool industry, on the other hand, the recent
trend is for more machines to operate at higher rotary speed. To
this end, it is required for the rolling bearing for supporting the
rotary portion of machine tools to have higher precision and
withstand severer working conditions. When a machine tool operates
at a raised rotary speed, the so-called bearing clearance is
reduced, causing further rolling friction that adds to heat
generation. As a result, the temperature of the bearing rises.
[0007] The rise in the heat generation due to rolling friction is
considered to be attributed to the rise in the centrifugal force
applied to the rolling elements. In order to lessen the centrifugal
force and hence lower the temperature of the rolling elements, a
rolling bearing comprising rolling elements made of ceramic
material, which exhibit a small density (specific gravity), rather
than low-alloy steel has heretofore been put into practical use.
However, with the recent trend for more machine tools to operate at
even higher rotary speed, mere reduction of the weight of the
rolling elements cannot prevent the rise in the bearing
temperature.
[0008] By the way, the heat generated in the outer race during high
speed rotation normally is radiated to the exterior through the
housing. Since the heat generated in the inner race can be
difficultly radiated from the rotary axis, the temperature of the
inner race is higher than that of the outer race. Thus, if the
outer race and the inner race are formed by the same material, and
the temperature of the inner race is raised by heat generation, the
inner race undergoes a great thermal expansion that reduces the
bearing clearance from the initial value. The resulting preload is
excessive, accelerating the heat generation. This phenomenon occurs
in a vicious circle. Eventually, the bearing undergoes seizing that
can lead to the destruction of the bearing.
[0009] From this standpoint of view, a rolling bearing has been
proposed comprising an inner race formed by a material having a
smaller linear expansion coefficient than the outer race material
(see JP-B-7-30788 (The term "JP-B" as used herein means an
"examined Japanese patent publication")) (hereinafter referred to
as "second conventional technique"). In accordance with the
foregoing second conventional technique, the inner race is formed
by a material having a smaller linear expansion coefficient than
the outer race material. For example, the outer race may be formed
by a high carbon chromium bearing steel (SUJ2) while the inner race
may be formed by a stainless steel (SUS440C) or ceramic material.
In this arrangement, even if the temperature of the inner race is
higher than that of the outer race, the expansion of the inner race
caused by the temperature difference between the inner race and the
outer race can be inhibited. As a result, the variation of preload
accompanying the change in the bearing clearance is reduced, making
it possible to prevent the bearing from seizing.
[0010] A titanium alloy has a lighter weight and a higher strength
than a steel material and a very excellent corrosion resistance
among metallic materials and thus is expected to be a bearing
material for use in special corrosive atmospheres such as those
contaminated by water content, sea water, chemical, etc.
[0011] In a rolling bearing, however, a very great face pressure is
applied to the portion at which the races and the rolling elements
come in contact with each other. Thus, it is required for a rolling
bearing to exhibit a high surface hardness. However, a titanium
alloy which has been merely subjected to ordinary heat treatment
such as solution treatment and aging cannot be provided with a
desired surface hardness.
[0012] From this standpoint of view, a technique for enhancing the
surface hardness of a titanium alloy by a predetermined surface
treatment has been proposed (JP-B-61-2747) (hereinafter referred to
as "third conventional technique").
[0013] In the foregoing third conventional technique, a titanium
alloy is subjected to gaseous nitriding or carburizing so that
penetrating elements such as C, N and O are diffused in the form of
solid solution therein, thereby securing the surface hardness
required for the races.
[0014] In the foregoing first conventional technique, a ceramic
material is used as bearing material. Thus, the bearing exhibits an
extremely good corrosion resistance as compared with stainless
steel. However, the first conventional technique is disadvantageous
in that a ceramic material is inferior to stainless steel in
strength or toughness and thus cannot be used without any trouble
in atmospheres subject to great load. In particular, the use of
ceramic material as the race material is undesirable from the
standpoint of reliability of bearing.
[0015] Further, a ceramic material is remarkably inferior to
metallic material in formability and grindability. Thus, if all the
essential parts of a bearing are formed by a ceramic material, it
disadvantageously adds to the production cost.
[0016] Moreover, a ceramic material has an extremely smaller linear
expansion coefficient than a metallic material. Thus, the foregoing
conventional technique has some disadvantages. For example, if the
outer race is formed by the foregoing high carbon chromium steel
(SUJ2) and the inner race is formed by a ceramic material, the
difference in thermal expansion between the metallic rotary axis
and the inner race made of ceramic material becomes too great when
the temperature rises to relax the thermal expansion of the rotary
axis, possibly cracking the inner race made of ceramic material and
hence causing the destruction of the bearing.
[0017] On the other hand, if the outer race is formed by a high
carbon chromium bearing steel (SUJ2) and the inner race is formed
by a stainless steel (SUS440C), the change in the bearing clearance
caused by the temperature rise can be minimized because the linear
expansion coefficient of stainless steel is as small as 80% of that
of high carbon chromium bearing steel. Further, since a stainless
steel is a metallic material, the inner race made of stainless
steel is considered to be insusceptible to cracking due to the
difference in thermal expansion between the rotary axis and the
inner race unlike the inner race made of ceramic material.
[0018] However, since the stainless steel used as inner race
material has a higher density (higher specific gravity) than the
ceramic material, the rise in the centrifugal force applied to the
inner race cannot be neglected. In other words, since centrifugal
force increases in proportion to mass and speed, the inner race
expands due to the centrifugal force produced by rotation as the
rotary speed increases. As a result, the bearing clearance is
reduced, accelerating the heat generation.
[0019] The foregoing third conventional technique is
disadvantageous in that the resulting surface hardness and depth of
hardening differ greatly with the kind of penetrating elements to
be incorporated in the form of solid solution by surface treatment.
Further, some titanium alloys used have too low a strength in the
core to fulfill a sufficient function as bearing.
[0020] In accordance with the third conventional technique, the
surface hardness of the titanium alloy can be enhanced by diffusing
penetrating elements in the titanium alloy in the form of solid
solution. However, these penetrating elements can embrittle the
titanium alloy, making it impossible to obtain a desired bearing
life.
SUMMARY OF THE INVENTION
[0021] It is therefore an object of the present invention to
provide a rolling bearing excellent in corrosion resistance,
toughness and high rotary speed operation.
[0022] The foregoing and other objects of the present invention
will become more apparent from the following detailed description
and examples.
[0023] The objects are achieved by the following embodiments
mainly.
[0024] (1) A rolling bearing comprising races composed of an outer
race and an inner race and rolling elements which are provided
between the outer race and the inner race such that the rolling
elements rotate freely, wherein at least the inner race is made of
a titanium alloy and the rolling elements are made of a
corrosion-resistant material.
[0025] (2) The rolling bearing of item (1), wherein the titanium
alloy is selected from the group consisting of .beta. type titanium
alloy and (.alpha.+.beta.) type titanium alloy and the
corrosion-resistant material is selected from the group consisting
of ceramics and martensite stainless steel.
[0026] (3) The rolling bearing of item (1), wherein the surface
hardness (Hv) of the finished raceway track on at least one race
selected from the group consisting of the outer race and the inner
race is not less than 600.
[0027] (4) The rolling bearing of item (1), wherein the surface of
the finished raceway track on the at least one race comprises a
mixture of .alpha. phase texture and .beta. phase texture, the
proportion of the .beta. phase in the mixture being from 30 to 80
vol %. (5) A method for producing a rolling bearing, which
comprises preparing at least one race selected from the group
consisting of an outer race and an inner race according to a method
which comprises steps of:
[0028] (a) selecting at least one from the group consisting of
.beta. type titanium alloy and (.alpha.+.beta.) type titanium alloy
as a race material;
[0029] (b) heating and keeping said race material at the
temperature falling within the range of .beta. phase temperature of
not lower than .beta. phase transition point (.beta.-phase transus)
to effect solution treatment such that the phase of the texture of
said race material is converted to .beta. phase;
[0030] (c) rapidly cooling said race material so that the texture
of said race material normally stays in .beta. single phase;
[0031] (d) subjecting said race material to plastic working (cold
working) so that it is shaped as desired and given work strain,
which enables formation of nuclei of .beta. phase which is harder
than .beta. phase and the .alpha. phase to be finely deposited in
.beta. phase;
[0032] (e) subjecting said race material to aging at a
predetermined temperature lower than .beta. phase transition point,
whereby nuclei of .alpha. phase are formed and grown and the
.alpha. phase is finely deposited in .beta. phase; and then
[0033] (f) machining said race material to a race.
[0034] (6) The method of item (5), wherein the percent plastic
working at the step (d) is not less than 20%.
[0035] (7) The method of item (5), wherein the percent plastic
working is from 5 to 30% and the surface of the raceway track is
subjected to shot peening before aging.
[0036] (8) The method of item (7), wherein shot peening is effected
after aging.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] By way of example and to make the description more clear,
reference is made to the accompanying drawings in which:
[0038] FIG. 1 is a chart illustrating a second embodiment of the
method for the production of the bearing material according to the
embodiment of the present invention;
[0039] FIG. 2 is a chart illustrating a third embodiment of the
method for the production of the bearing material according to the
embodiment of the present invention;
[0040] FIG. 3 is a chart illustrating a modification of the third
embodiment of the method for the production of the bearing material
according to the embodiment of the present invention;
[0041] FIG. 4 is a diagram illustrating the inner structure of a
submerged thrust bearing life testing machine for use in the
submerged life test;
[0042] FIG. 5 is a sectional view illustrating a high speed rotary
testing machine for use in the high speed rotary test;
[0043] FIG. 6 is a characteristic curve illustrating the
relationship between percent cold working .eta. and hardness Hv
after aging in the fourth group of examples;
[0044] FIG. 7 is a characteristic curve illustrating the
relationship between percent cold working .eta. and hardness Hv
after aging in the fifth group of examples;
[0045] FIG. 8 is a characteristic curve illustrating the
relationship between aging time and residual .beta. phase content
and hardness Hv after aging in the sixth group of examples; and
[0046] FIG. 9 is a characteristic curve illustrating the
relationship between residual .beta. phase and submerged life
L.sub.10 in the sixth group of examples, wherein the reference
numeral 3 indicates an inner race, the reference numeral 4
indicates an outer race, the reference numeral 5 indicates rolling
elements, the reference numeral 12 indicates a outer race, the
reference numeral 13 indicates an inner race, and the reference
numeral 14 indicates rolling elements.
DETAILED DESCRIPTION OF THE INVENTION
[0047] The present invention will be further described
hereinafter.
[0048] The inventors made extensive studies of rolling bearing
having an excellent corrosion resistance. As a result, it was found
that the use of a titanium alloy having a higher toughness than
ceramics as a race material makes it possible to drastically
improve corrosion resistance as compared with the use of stainless
steel.
[0049] It was also found that an inner race formed by such a
titanium alloy, which has a lighter weight and a smaller linear
expansion coefficient than stainless steel, shows a smaller
temperature rise during high speed operation than that formed by
stainless steel, making it possible to avoid the reduction of
clearance and hence inhibit the rise in heat generation.
[0050] The present invention has been worked out on the basis of
these knowledges. As the first feature, the rolling bearing
according to the present invention comprises an outer race and an
inner race and rolling elements rotatably provided between the
outer race and the inner race, wherein at least the inner race is
made of a titanium alloy and the rolling elements are made of a
corrosion-resistant material.
[0051] The inventors obtained a knowledge that among titanium
alloys having an excellent corrosion resistance a .beta. type
titanium alloy exhibits a high strength and an excellent
cold-workability in the form of solid solution and then made
extensive studies. As a result, it was found that the use of a
.beta. type titanium alloy cold-worked at a percent cold working
(percent plastic working) of not less than 20% as a bearing
material makes it possible to provide a race having a Rockwell
hardness HRC (hereinafter simply referred as "HRC") of not less
than 57 through a short aging.
[0052] Thus, as the second feature, in the rolling bearing
according to the present invention, at least one of the inner race
and outer race is formed by a .beta. type titanium alloy
cold-worked at a percent cold working of not less than 20%.
[0053] When a titanium alloy which has been cold-worked by not less
than 20% is subjected to aging, a rolling bearing having a desired
surface hardness as defined above can be obtained. However, the
resulting .beta. type titanium alloy tends to have a hardened
texture as a whole. In particular, if the percent cold working is
predetermined high, the .beta. phase titanium alloy hardens more
than necessary even in its core and to thereby exhibit a reduced
toughness. Accordingly, from the point of view of obtaining good
toughness, it appears to be preferred that the titanium alloy be
not subjected to cold working or, if any, be subjected to cold
working at a low percent working to obtain a good toughness.
[0054] If a steel material such as stainless steel is used as a
race material, it is subjected to heat treatment such as hardening
and tempering and then to shot peening to have an enhanced surface
hardness. In other words, when subjected to shot peening, the
stainless steel material undergoes transformation of residual
austenite to martensite, producing stress that gives a huge strain
energy to the surface layer of the race. The work-hardening makes
it possible to enhance the surface hardness of the race.
[0055] However, the inventors' studies made it obvious that if a
titanium alloy which has been subjected to heat treatment is
subjected to shot peening alone, the amount and depth of work
strain thus provided are restricted, making it difficult to obtain
a desired surface hardness required for rolling bearing.
[0056] Paying their attention to the rise in surface hardness by
shot peening, the inventors made further extensive studies. As a
result, it was found that a titanium alloy which has been
cold-worked at a percent working of from 5 to 20% can be subjected
to shot peening to obtain a rolling bearing having a good toughness
as well as a high surface hardness.
[0057] As the other feature, in the rolling bearing according to
the present invention, at least one of the inner race and outer
race is formed by a .beta. type titanium alloy obtained by cold
working at a percent working of from 5 to 20%, followed by shot
peening.
[0058] In the foregoing aspect of the present invention, if a
titanium alloy which has been cold-worked is subjected to shot
peening followed by aging, a rolling bearing having a Vickers
surface hardness Hv (hereinafter simply referred to as "Hv") of not
less than 600 (corresponding to HRC of about 57) can be obtained.
In order to improve fatigue resistance, the titanium alloy which
has been aged is preferably again subjected to shot peening.
[0059] During their study of the life of a race made of a titanium
alloy, the inventors found that the bearing shows a shorter life
when the lubricant is contaminated by foreign matters than when the
lubricant is free of foreign matters similarly to the case where
the race is made of a steel material such as stainless steel.
[0060] For the conventional rolling bearings made of steel
material, a technique for improving the life of bearing by
optimizing the carbon content and residual austenite, optionally
carbon nitride content, in the surface layer of the bearing is
proposed in JP-B-7-88851. In the known technique, by optimizing and
restricting the content of carbon, residual austenite and carbon
nitride in the material to a specific range, the concentration of
stress on the edge portion of impression produced by foreign
matters can be relaxed, inhibiting the generation of cracks. As a
result, the life of the bearing can be improved.
[0061] The optimum relationship between the amount of residual
austenite and the surface hardness is found by adjusting and
optimizing the average grain diameter of carbide or carbon nitride
and a technique for prolonging the life of bearing based on the
relationship is proposed in JP-B-8-26446.
[0062] In other words, the foregoing known techniques
(JP-B-7-88851, JP-B-8-26446) contemplate optimizing the amount of
soft austenite to improve the life of bearing when the lubricant is
contaminated by foreign matters. Accordingly, it is considered that
even a .beta. phase titanium alloy can provide a bearing which can
operate over a prolonged life even when the lubricant is
contaminated by foreign matters if the volumetric proportion of
residual .beta. phase being a soft phase is optimized.
[0063] The inventors made extensive studies from such a standpoint
of view. As a result, it was found that if the volumetric
proportion of residual .beta. phase in soft phase in the texture of
.beta. phase titanium alloy is optimized, a bearing can be obtained
which can operate over a desired life even when the lubricant is
contaminated by foreign matters.
[0064] As the fourth feature, in the rolling bearing according to
the present invention, at least one of the inner race and outer
race is formed by a .beta. phase titanium alloy obtained by cold
working at a percent working of not less than 20%, and the
volumetric proportion of residual .beta. phase in the .beta. type
titanium alloy is from 30 to 80%.
[0065] In the foregoing aspect of the present invention, if the
percent cold working is predetermined to 5 to 20% on condition that
a titanium alloy which has been cold-worked is subjected to shot
peening, a rolling bearing which satisfies both the two
requirements for toughness and surface hardness can be
obtained.
[0066] First Embodiment
[0067] In the rolling bearing according to the first embodiment of
the present invention, at least the inner race is formed by a
titanium alloy, and the rolling elements are formed by a
corrosion-resistant material such as ceramics.
[0068] The reason why the rolling bearing and the rolling elements
are formed by these materials will be described hereinafter.
[0069] (1) Races
[0070] The terminology "a race" as used hereinafter inclusively
means an inner race and an outer race.
[0071] A race formed by a titanium alloy exhibits a drastically
improved corrosion resistance as compared with that formed by
stainless steel.
[0072] The corrosion resistance of titanium is attributed to the
formation of a stable passive film on the surface thereof similarly
to stainless steel. The passive film of titanium is known to be
TiO.sub.2(or Ti.sub.2O.sub.3) (see Goro Ito, "Fushoku kagaku to
boshoku gijutsu (Corrosion science and corrosion prevention
technique)", revised edition, page 282, Corona Co., Ltd., 1979).
Thus, the excellent corrosion resistance of titanium is attributed
to properties inherent to the passive film of titanium.
[0073] In other words, TiO.sub.2, which is the passive film of
titanium, exhibits a high oxygen overvoltage. As the potential
applied to titanium rises, the anodization proceeds. The resulting
passive film exhibits an excellent corrosion resistance even in a
high temperature high concentration oxidizing atmosphere such as
high temperature high concentration nitric acid. Unlike stainless
steel, titanium does not undergo corrosion due to
overpassivation.
[0074] On the other hand, TiO.sub.2 corrodes in a nonoxidizing
atmosphere such as hydrochloric acid and sulfuric acid easily but
less easily than stainless steel. Further, TiO.sub.2 requires a low
passivation potential for forming a passive film. Therefore, a
titanium alloy can be easily passivated merely by dipping it in a
corrosive solution comprising an extremely small amount of an
oxidizing agent incorporated therein. Accordingly, a titanium alloy
can be corrosion-resistant even in a nonoxidizing atmosphere such
as hydrochloric acid and sulfuric acid.
[0075] Further, the passive film is tough and does not break even
when attacked by chloride ion. Thus, the passive film is little
liable to erosion, void corrosion, stress corrosion cracking, etc.,
which are remarkable in stainless steel. Accordingly, the passive
film exhibits an extremely excellent corrosion resistance against
sea water. As a result, a rolling bearing formed by a titanium
alloy cannot be disabled even when sea water enters thereinto.
[0076] Moreover, a titanium alloy also exhibits an excellent
corrosion resistance against many organic acids and is not liable
to deterioration of cold-workability or deterioration by impure
elements.
[0077] Thus, a titanium alloy exhibits an extremely excellent
corrosion resistance as compared with stainless steel.
[0078] The comparison of titanium alloy with ceramics material such
as Si.sub.3N.sub.4 in corrosion resistance shows that a titanium
alloy undergoes so-called overall corrosion against some alkaline
solutions such as NaOH and KOH solutions and thus cannot be used in
such an alkaline atmosphere but exhibits corrosion resistance equal
to ceramics in special atmospheres other than the alkaline
atmosphere.
[0079] A ceramics material exhibits a low toughness and thus is not
suitable for use under working conditions subject to great impact
load while a titanium alloy exhibits a toughness about three times
that of Si.sub.3N.sub.4. In other words, a titanium alloy exhibits
a toughness equal to stainless steel. Thus, if a titanium alloy is
used as a race., it is extremely unlikely that the bearing can
break as compared with the case where a ceramics material is
used.
[0080] Further, ceramics cannot be subjected to plastic working as
metallic materials. Therefore, in order to produce a race from
ceramics, ceramics must be subjected to a continuous complicated
production method which comprises compressing a powdered ceramics
into a ring, sintering the material, subjecting the material to HIP
(hot isostatic pressing) so that it is densed, and then grinding
the material. Thus, ceramics materials exhibit a poor productivity
as compared with metallic materials. Further, a large-sized race
can hardly be produced from a ceramics material. Moreover, ceramics
materials exhibit a remarkably deteriorated grindability as
compared with metallic materials, thereby increasing the production
cost.
[0081] On the other hand, a titanium alloy exhibits a deteriorated
workability as compared with a steel material such as stainless
steel but a sufficient plastic deformability. A titanium alloy
exhibits an excellent grindability as compared with ceramics.
Further, working facilities for steel material can be used for
titanium alloy. Therefore, existing facilities can be used,
eliminating the necessity of equipment investment. The production
cost can be reduced.
[0082] A titanium alloy is a nonmagnetic material. Thus, even if a
titanium alloy is used in a magnetic atmosphere such as
semiconductor producing apparatus and superconduction-related
apparatus, disturbance in the magnetic field can be avoided.
Further, the rise or variation in the rotary torque of the bearing
due to magnetic field can be inhibited.
[0083] The inhibition of the rise or variation in the rotary torque
is more remarkable when the rolling elements are formed by a
nonmagnetic ceramics.
[0084] On the other hand, in order to avoid the rise in the bearing
temperature even in a tool machine which operates at a high rotary
speed, it is effective to form the races, particularly inner race,
by a titanium alloy.
[0085] As previously mentioned, the rise in the bearing temperature
developed when the bearing rotates at a high speed is attributed to
the reduction of bearing clearance accompanying the high speed
rotation. The reduction of bearing clearance is attributed riot
only to the thermal expansion due to the difference in temperature
between the inner race and the outer race but also to the expansion
of the inner race due to the centrifugal force caused by the
rotation of the rotary axis.
[0086] Accordingly, in order to inhibit the rise in the bearing
temperature accompanying the high speed rotation, it is necessary
that a material having a small linear expansion coefficient be
selected to inhibit the thermal expansion. In order to reduce the
centrifugal force, it is necessary that a material having a small
density be selected.
[0087] The comparison of Ti-6Al-4V alloy as a titanium alloy with
SUS440C as a stainless steel shows that the linear expansion
coefficient of Ti-6Al-4V alloy is as small as 80% of that of
SUS440C. Therefore, if Ti-6Al-4V alloy is used as an inner race
material, the reduction in the bearing clearance accompanying the
difference in temperature between the inner race and the outer race
can be drastically reduced as compared with the use of stainless
steel.
[0088] Further, the density of Ti-6Al-4V alloy is as small as about
60% of that of SUS440C. As a result, Ti-6Al-4V alloy gives a lower
centrifugal force than stainless steel. Thus, the inner race formed
by Ti-6Al-4V alloy expands less than that formed by stainless
steel.
[0089] As mentioned above, by using a titanium alloy as an inner
race, the reduction in the bearing clearance during high speed
rotation can be avoided, thereby inhibiting the rise in friction.
As a result, the rise in the bearing temperature can be
inhibited.
[0090] When the race and the rolling elements come in contact with
each other under a predetermined load, the contact portion
undergoes elastic deformation to form a contact ellipse the size of
which depends on the Young's modulus of the race and the rolling
elements.
[0091] Ceramics exhibit a greater Young's modulus than metallic
materials and thus undergo little elastic deformation. Accordingly,
the race receives a higher face pressure when the rolling elements
are formed by ceramics than when the rolling elements are formed by
a metallic material. On the other hand, a titanium alloy exhibits a
Young's modulus as small as about half that of stainless steel.
Accordingly, the contact ellipse is larger when the race is formed
by a titanium alloy than when the race is formed by a stainless
steel. Thus, the contact portion receives a lower face pressure
when the race is formed by a titanium alloy than when the race is
formed by a stainless steel. Therefore, the use of a titanium alloy
as a race makes it possible to relax the rise in the contact face
pressure which can occur when rolling elements made of ceramics is
used and improve the rolling fatigue life of the bearing.
[0092] As the titanium alloy to be used for race there may be used
(.alpha.+.beta.) type titanium alloy such as Ti-6Al-4V, Ti-3Al-2.5V
and Ti-6Al-2Sn-4Zr-6Mo or .beta. type titanium alloy such as
Ti-15Mo-5Zr, Ti-15Mo-5Zr-3Al, Ti-15V-3Sn-3Al-3Cr, Ti-10V-2Fe-3Al,
Ti-3Al-8V-6Cr-4Zr and Ti--22V-3Al, which can be subjected to heat
treatment to have a high strength and a high toughness.
[0093] Preferred among the titanium alloys listed above are .beta.
type titanium alloys, which exhibit an excellent cold-workability,
taking into account workability. Particularly preferred among these
.beta. type titanium alloys are Ti-15Mo titanium alloys such as
Ti-15Mo-5Zr and Ti-l5Mo-5Zr-3Al, which are particularly excellent
in corrosion resistance. (.alpha.+.beta.) type titanium alloys have
a great content of alloying elements having a smaller density than
Ti. Thus, (.alpha.+.beta.) type titanium alloys, which have a small
mass, are preferably used in terms of reduction of centrifugal
force.
[0094] In order to secure the bearing strength, the titanium alloy
needs to be subjected to heat treatment so that it is reinforced as
a (.alpha.+.beta.) two-phase texture.
[0095] Pure titanium and .alpha. type titanium alloy such as
Ti-0.3Mo-0.8Ni have an .alpha. single phase microstructure and
hence a lower strength than the foregoing (.alpha.+.beta.) type
titanium alloys or .beta. type titanium alloys and thus cannot used
as race materials.
[0096] It is said that the surface hardness HRC of the race needs
to be not less than 57 to provide an endurable bearing. However, if
the foregoing titanium alloy is used as a race, even if the
material has been hardened by aging after solution treatment, the
resulting surface hardness is as small as about 40 to 45, making it
impossible to provide a surface hardness required for bearing.
Further, the resulting bearing exhibits a poor seizing resistance
and thus is liable to adhesive abrasion.
[0097] The foregoing titanium alloy is preferably subjected to heat
treatment such as atmospheric oxidation, gaseous nitriding,
boriding, wet plating, TiC or TiN coating by CVD method or PVD
method and ion injection to obtain a desired surface hardness HRC.
Taking into account the convenience of treatment, atmospheric
oxidation or gaseous nitriding is desirable.
[0098] In the present embodiment, at least the inner race is formed
by a titanium alloy. In a preferred embodiment, both the inner race
and the outer race are formed by a titanium alloy to provide a
better corrosion resistance in a working atmosphere such as food
machine, semiconductor producing apparatus and chemical fiber
producing machine which is liable to be contaminated by a corrosive
material such as water content, sea water and chemicals. In a
machine tool or other machines which operate at a high rotary
speed, it is important to inhibit the rise in the inner race
temperature. Therefore, the inner race needs to be formed by a
titanium alloy, but the outer race is preferably formed by a steel
material such as SUJ2 and stainless steel, which exhibits a greater
linear expansion coefficient than the titanium alloy constituting
the inner race.
[0099] (2) Rolling Elements
[0100] The reason why the rolling elements are formed by ceramics
in the present embodiment will be described hereinafter.
[0101] Ceramics are insulating materials. Rolling elements formed
by ceramics is not liable to so-called galvanic corrosion even when
it comes in contact with a race formed by a titanium alloy and thus
is extremely excellent in corrosion resistance as compared with
that formed by a metallic material.
[0102] Ceramics are also nonmagnetic materials. Thus, rolling
elements formed by ceramics causes no variation of rotary torque of
bearing even when used in a magnetic field. Accordingly, ceramics
are suitable for use in a special working atmosphere subject to
magnetic field such as semiconductor producing apparatus and
superconduction-related apparatus.
[0103] Further, ceramics have a smaller density than stainless
steel. The comparison of Si.sub.3N.sub.4 as ceramics with SUS440C
as stainless steel shows that the density of Si.sub.3N.sub.4 is
about 40% of that of SUS440C. Accordingly, the use of ceramics,
which have a smaller density than stainless steel, makes it
possible to provide rolling elements having a lighter weight. When
the rolling bearing operates at a high rotary speed, such rolling
elements give a reduced centrifugal force that applies a reduced
load to the outer race, making it possible to inhibit the
deterioration of durability.
[0104] In other words, when a rolling bearing operates at a high
rotary speed, the high speed rotation is accompanied by the rise in
centrifugal force that causes the rolling elements to apply
nonneglible load to the outer race. Thus, the contact load of the
rolling elements on the outer race is raised, reducing the life of
bearing or raising the amount of heat generated by friction.
Further, since the centrifugal force of the rolling elements are
proportional to the mass of the rolling elements as well known, the
greater the mass of the rolling elements are, the greater is the
foregoing contact load.
[0105] Thus, in the present embodiment, the use of ceramics as
rolling element material provides rolling elements having a reduced
weight that inhibits the generation of heat by friction and hence
the reduction of the life of bearing.
[0106] In a rolling bearing having a contact angle such as angular
contact ball bearing, the rolling elements are acted upon by
gyroscopic moment. When the gyroscopic moment becomes greater than
the frictional force at the portion where the rolling elements come
in contact with the race, a violent revolutionary slip called
skidding occurs to cause further friction. The reduction of the
weight of the rolling elements also makes it possible to reduce the
gyroscopic moment.
[0107] Further, rolling elements formed by the same titanium alloy
having an excellent corrosion resistance as used for the race
exhibit a strong adhesion and thus is liable to seizing or galling.
On the contrary, rolling elements formed by ceramics, which differ
from the material of the race, exhibit improved seizing resistance
and galling resistance. In particular, a titanium alloy is an
active metal and thus exhibits a deteriorated seizing resistance.
Accordingly, the use of ceramics as rolling element material makes
it possible to improve the seizing resistance of the titanium alloy
used as race.
[0108] As the ceramics to be used as rolling element material there
may be used SiAlON, zirconia (ZrO.sub.2), silicon carbide (SiC),
alumina (Al.sub.2O.sub.3) or the like besides Si.sub.3N.sub.4.
Si.sub.3N.sub.4 exhibits a small density, a low linear expansion
coefficient, a high thermal impact resistance and excellent
flexural strength and fracture toughness and thus can be used as
rolling elements for use under high speed rotary conditions.
[0109] The present invention is not limited to the present
embodiment. With respect to the bearing for use in a corrosive
working atmosphere, the rolling elements are preferably formed by a
stainless steel depending on the application.
[0110] In this case, the rolling elements are formed by a stainless
steel, which differ from the material of the race, i.e., titanium
alloy as in the case where the rolling elements are formed by
ceramics. When the bearing rotates, the different kinds of metals
come in contact with each other.
[0111] In general, when different kinds of metals come in contact
with each other in a solution, galvanic corrosion occurs to
accelerate the corrosion of the metal which is electronegatively
greater than the other. Accordingly, when rolling elements made of
stainless steel, which is electronegatively greater than titanium
alloy, come in contact with a race made of titanium alloy, the
rolling elements corrode remarkably, possibly causing a drastic
reduction of the bearing life.
[0112] Stainless steel is electronegatively greater than titanium
alloy in the order of corrosion tendency in sea water. However, the
two metals have an extremely small potential difference (see "Titan
Kako Gijutsu (Titanium Processing Technique)", compiled by Japan
Titanium Society, page 208 (published by Nikkan Kogyo Shinbunsha,
1992). Thus, little or no galvanic corrosion occurs even when a
titanium alloy and a stainless steel come in contact with each
other in sea water.
[0113] Accordingly, as the rolling element material there may be
used a general-purpose stainless steel in some cases. In other
words, in some cases, the use of stainless steel as rolling element
material rather than expensive ceramics makes it possible to
maintain sufficient corrosion resistance and hence reduce the
production cost. Further, the use of stainless steel as rolling
element material also makes it possible to reduce the contact face
pressure as compared with ceramics material which is little liable
to elastic deformation.
[0114] Second Embodiment
[0115] In the rolling bearing according to the second embodiment of
the present invention, at least one of the inner race and outer
race is formed by a .beta. type titanium alloy and the percent cold
working of the race is predetermined to not less than 20%.
[0116] Among the titanium alloys having an excellent corrosion
resistance, a .beta. phase titanium alloy exhibits a high strength
and an excellent cold-workability in the form of solid solution. In
other words, a .beta. phase titanium alloy which has been subjected
to solution treatment at a predetermined temperature can be rapidly
cooled to obtain a soft .beta. single phase having a body-centered
cubic lattice (bcc) structure at room temperature. Among materials
belonging to .beta. type titanium alloy, there is a reinforcible
material having a percent cold working q of not less than 90% as
represented by the following equation (1). The use of such a
material makes it possible to omit the grinding step.
.eta.={(l.sub.0-l)/l.sub.0}.times.100 (1)
[0117] wherein l.sub.0 represents the height of the material before
cold working; and l represents the height of the material after
cold working.
[0118] In other words, a titanium alloy exhibits an excellent
corrosion resistance but a small thermal conductivity and thus
generates heat at the area where it comes in contact with the
grinding tool during grinding that gives a great stress to the
cutting edge. Thus, a titanium alloy is disadvantageous in that it
exhibits a deteriorated grindability. In the second embodiment of
the present invention, .beta. type titanium alloy, which exhibits
an excellent cold-workability, is used. The .beta. type titanium
alloy is subjected to solution treatment to give a soft .beta.
single phase which is then subjected to cold working. This cold
working causes the production of a large amount of lattice defects
that cause dislocation. Thus, hard .alpha. phase is uniformly and
finely deposited in .beta. crystalline grains. In this manner, both
the surface hardness HRC and the strength of the material can be
enhanced, making it possible to enhance the durability of the
rolling bearing itself.
[0119] In other words, it is a common practice that the bearing
material which has been subjected to solution treatment is
subjected to aging for hardening. However, if the bearing material
which has been subjected to solution treatment is not subjected to
cold working before aging, .alpha. phase is deposited
preferentially at the grain boundary in layer during aging but less
in .beta. crystalline grains, providing an extremely nonuniform
aged texture.
[0120] On the contrary, if the bearing material which has been
subjected to solution treatment is subjected to cold working before
aging, the cold working (plastic working) causes a large amount of
dislocation to be introduced into .beta. crystalline grains, and
the dislocation becomes a nucleus production ground for deposition
of .alpha. phase. Thus, hard .alpha. phase is uniformly and finely
deposited in soft .beta. crystalline grains, increasing the surface
hardness of the material.
[0121] In other words, a .beta. phase titanium alloy obtained by
aging a titanium alloy which has been subjected to solution
treatment free from cold working has a surface hardness HRC of
about from 40 to 48. On the contrary, a titanium alloy obtained by
subjecting a solution-treated titanium alloy to cold working
followed by aging can be provided with a surface hardness HRC of
not less than 57 and hence a raised strength that improves the life
of the rolling bearing.
[0122] The solution treatment temperature, percent cold working q
and aging time T will be described hereinafter.
[0123] (1) Solution Treatment Temperature
[0124] If solution treatment is effected at a temperature of not
higher than the critical temperature at which .beta. transition,
i.e., .beta. phase is transformed to (.alpha.+.beta.) phase,
initial .alpha. phase is deposited, causing a remarkable
deterioration of workability. Accordingly, the solution treatment
temperature needs to be not lower than .beta. transition. On the
contrary, if solution treatment is effected at an excessively high
temperature, the resulting .beta. crystalline grains are remarkably
coarse, causing a strength drop. Thus, in the present embodiment,
the solution treatment temperature is predetermined to a range of
from 13 transition to (.beta.+150.degree. C.).
[0125] (2) Percent Cold Working .eta.
[0126] A titanium alloy obtained by subjecting a solution-treated
titanium alloy to cold working before aging exhibits enhanced
surface hardness HRC and strength. As described later, such a
titanium alloy which has been subjected to cold working can be aged
in a reduced time. However, the density of dislocation introduced
by cold working varies, affecting the surface hardness HRC or
strength. In other words, if the percent cold working .eta. is
predetermined to not more than 20%, the resulting dislocation is
nonuniform, causing .alpha. phase to be deposited preferentially at
the grain boundary. Further, when .alpha. phase is deposited in
layer at the grain boundary, break can easily occur at the
interface of .beta. crystalline grain with .alpha. phase, causing a
strength drop.
[0127] On the contrary, if the percent cold working q is not less
than 20%, dislocation is uniformly introduced into crystalline
grains. Thus, .alpha. phase is uniformly and finely deposited in
.beta. crystalline grains with the foregoing dislocation as a
nucleus production ground during aging, enhancing the surface
hardness HRC and strength.
[0128] It is considered that the degree of reinforcing by cold
working follows n-order hardening rule represented by the equation
(2):
.sigma.=AE.sup.n (2)
[0129] wherein .sigma. represents true stress; E represents true
strain; A represents reinforcement coefficient; and n represents
work-hardening index. A .beta. type titanium alloy exhibits a
smaller work-hardening index than steel material and thus is akin
to completely plastic material. Thus, the percent cold working
.eta. can be raised without any problem. In particular, when the
percent cold working .eta. is within the range of not less than
30%, a bearing material having a stabilized hardness can be
obtained. Accordingly, cold working may be effected at a percent
cold working .eta. of not less than 20% to obtain a predetermined
height.
[0130] From these standpoints of view, the percent cold working q
is predetermined to not less than 20%, preferably not less than
30%, in the present embodiment.
[0131] (3) Aging Time T
[0132] As mentioned above, the dislocation introduced during cold
working becomes a nuclear production ground which accelerates the
deposition of .alpha. phase in .beta. crystalline grains. As a
result, the time required until averaging is reached can be
reduced, making it possible to drastically reduce the aging time T.
However, if aging is effected over an excessively prolonged period
of time, averaging occurs, causing hard .alpha. phase to grow
coarsely. Thus, the material softens, causing a drop of surface
hardness HRC and hence a reduction of the bearing life. Further, if
the aging time T is predetermined excessively long, an
intermetallic compound is deposited as a final stable phase,
remarkably embrittling the bearing material. As a result, the
surface hardness and submerged life of the bearing can be reduced.
From these standpoints of view, the aging time T is preferably
predetermined to 5 to 10 hours in the present embodiment.
[0133] FIG. 1 is a chart illustrating the method for the production
of the bearing material according to the embodiment of the present
invention.
[0134] In other words, a .beta. type titanium alloy is subjected to
solution treatment at a temperature (.beta. transition to (.beta.
transition +150.degree. C.), e.g., 800.degree. C. to 1,000.degree.
C., in an Ar atmosphere or in vacuum, and then rapidly cooled to
give a soft .beta. single phase having bcc structure. The titanium
alloy thus treated is then subjected to cold working at a percent
working .eta. of not less than 20% to form races. The titanium
alloy is then formed into a race. Referring to the method for
forming race, the titanium alloy is subjected to near net shaping
(semi-finished shaping) to minimize the number of steps required
for grinding. Accordingly, the titanium alloy is preferably
subjected to cold working by cold rolling forging. The titanium
alloy thus cold-worked is then subjected to aging at a temperature
of from 400.degree. C. to 550.degree. C. for 5 to 10 hours. In this
manner, a race material having .alpha. phase deposited uniformly
and finely in .beta. crystalline grains can be produced. The race
material thus obtained can be then subjected to a predetermined
finishing such as grinding to finally obtain a race made of .beta.
type titanium alloy.
[0135] As mentioned above, a .beta. phase titanium alloy exhibits
an excellent cold-workability. Thus, the kind of .beta. type
titanium alloy to be used in the present invention is not
specifically limited. However, even an alloy belonging to .beta.
type titanium alloy is liable to instabilization of residual .beta.
phase depending on its alloy composition. If subjected to cold
working, such a .beta. phase titanium alloy can form a work-induced
martensite. However, the foregoing work-induced martensite can
crack if the percent cold working .eta. is great. Accordingly,
among .beta. type titanium alloys, a .beta. type titanium alloy
which hardly forms such a work-induced martensite is preferably
used. In particular, a Ti-Mo-based .beta. type titanium alloy such
as Ti-15Mo-5Zr and Ti-15Mo-5Zr-3Al is preferably used for positions
requiring corrosion resistance.
[0136] Third Embodiment
[0137] In the rolling bearing according to the third embodiment of
the present invention, at least one of the inner race and the outer
race is formed by a .beta. type titanium alloy, the percent cold
working is predetermined to a range of from 5 to 20%, and the cold
working is followed by shot peening.
[0138] In the third embodiment of the present invention, as shown
in FIG. 2, a titanium alloy is subjected to solution treatment, and
then rapidly cooled in the same manner as in the second embodiment
of the present invention. The titanium alloy thus treated is then
subjected to cold working such as cold rolling forging. The
titanium alloy is then subjected to shot peening. The titanium
alloy is then finally subjected to aging to produce a rolling
bearing having a surface hardness Hv of not less than 600.
[0139] The reason why a .beta. phase titanium alloy which has been
subjected to shot peening has a hardened surface layer will be
described hereinafter.
[0140] In other words, the shot peening of .beta. single phase
texture obtained by rapidly cooling the solution-treated titanium
alloy causes the surface layer to undergo plastic deformation that
causes the introduction of a large amount of dislocation. When the
titanium alloy thus treated is then aged, hard .alpha. phase is
deposited in the plastically-deformed surface layer with a high
density dislocation as nucleating site. Thus, the shot peening
causes the surface layer to have more nucleating sites at which
.alpha. phase is deposited than the core which undergoes not
plastic deformation. As a result, .alpha. phase is finely and
uniformly deposited in the surface layer similarly to cold working,
drastically hardening the surface layer alone.
[0141] However, as mentioned above, if a .beta. type titanium alloy
material which has been subjected to solution treatment is then
directly subjected to shot peening, the work strain thus provided
and its depth are limited, limiting the rise in the surface
hardness.
[0142] Thus, in the third embodiment of the present invention, a
titanium alloy is subjected to cold working at a percent working of
from 5 to 20% before shot peening to obtain a rolling bearing
having a good toughness as well as a surface hardness Hv of not
less than 600.
[0143] The reason why the percent cold working is predetermined to
a range of from 5 to 20% will be described hereinafter.
[0144] In other words, if a titanium alloy is subjected to cold
working, there is a fear that the metallic texture is hardened to
the core to thereby impair its toughness. Therefore, in order to
obtain a good toughness, it is preferred that a titanium alloy be
not subjected to cold working or be subjected to cold working at a
low percent working. However, if the percent cold working falls
below 5%, a titanium alloy exhibits a surface hardness Hv as small
as not more than 600 even when subjected to shot peening and thus
cannot provide a surface hardness required for rolling bearing. On
the contrary, if the percent cold working exceeds 20%, a titanium
alloy exhibits a remarkably reduced toughness. Accordingly, in the
present embodiment:, the percent cold working is predetermined to a
range of from 5 to 20%.
[0145] If a titanium alloy is subjected to cold working at a
percent working of from 5, to 20%, followed by shot peening, it is
provided with a work strain in the surface layer as much as
obtained when it is subjected to cold working at a high percent
working. When the titanium alloy is then subjected to aging, its
core undergoes aged hardening to an extent such that the toughness
thereof is not impaired, and the micro-deposition of hard .alpha.
phase in the surface layer proceeds to cause hardening.
[0146] Thus, in accordance with the third embodiment of the present
invention, a rolling bearing suitable for use in working
atmospheres requiring toughness can be obtained.
[0147] FIG. 3 is a chart illustrating a modification of the third
embodiment of the present invention. In this modification, a
titanium alloy which has been subjected to aging is again subjected
to shot peening.
[0148] Shot peening originally exerts an effect of applying
residual compression stress to the surface layer to enhance its
fatigue strength.
[0149] Shot peening after cold working can enhance the surface
hardness of a titanium alloy. However, since work strain which has
been given by shot peening can be released during a prolonged
heating and storage at the aging step, the residual compression
stress is reduced after the termination of aging, possibly making
it impossible to enhance the fatigue strength of the titanium
alloy.
[0150] Thus, in this modification, a titanium alloy which has been
subjected to aging is again subjected to shot peening as shown in
FIG. 3 so that the surface layer thereof is provided with a high
residual compression stress to enhance the fatigue strength
thereof.
[0151] In other words, if a .beta. type titanium alloy is used as a
race material, even when residual .beta. phase is subjected to
plastic deformation, the residual .beta. phase which has been aged
has .beta. phase-stabilizing elements in a high concentration to
show a high degree of stabilization of .beta. phase. Thus, unlike
steel material such as stainless steel, the .beta. type titanium
alloy does not undergo work-induced martensite transformation.
However, since the residual .beta. phase exhibits a very great
plastic transformability, it can have a large amount of work strain
accumulated therein as compared with steel materials when subjected
to shot peening. As a result, the .beta. type titanium alloy can be
provided with a high residual compression stress, making it
possible to enhance the fatigue strength thereof.
[0152] In accordance with this modification, a rolling bearing
suitable for use in working atmospheres particularly requiring
excellent fatigue life and fatigue strength can be obtained.
[0153] Fourth Embodiment
[0154] In the rolling bearing according to the fourth embodiment of
the present invention, at least one of the inner race and the outer
race is formed by a .beta. type titanium alloy, the percent cold
working is predetermined to not less than 20%, and the content of
residual .beta. phase in the .beta. type titanium alloy is
predetermined -to a range of from 30 to 80 vol %.
[0155] When the rolling bearing operates with a lubricant
contaminated by foreign matters, impressions are formed by the
foreign matters on the surface layer of the race, possibly reducing
the bearing life. Thus, when a steel material such as stainless
steel is used, the following countermeasure is taken. As previously
mentioned, the edge of the impressions are allowed to undergo
plastic deformation when they repeatedly come in contact with the
rolling elements which pass thereby during the period between the
formation of the impressions and the generation of cracks in the
edge of the impressions so that the concentration of stress on the
edge of the impressions is relaxed, making it possible to prolong
the life of bearing when the lubricant is contaminated by foreign
matters.
[0156] In other words, residual austenite contained in steel
materials is a soft texture liable to plastic deformation. When a
high stress is concentrated on the edge of impressions formed by
foreign matters which have entered in the lubricant on the surface
layer of a race made of steel material, the edge of the impressions
can easily undergo plastic deformation as well as stress-induced
transformation so that it is transformed to a hard martensite
texture. As a result, the edge of the impressions shows a hardness
rise. When the drop of concentration of stress and the hardness
rise are balanced, the edge of the impressions no longer undergoes
plastic deformation. To be short, when a race made of a steel
material operates with a lubricant contaminated by foreign matters,
the residual austenite texture exerts an effect of enhancing
fatigue strength due to stress relaxation and martensite
transformation to improve the bearing life.
[0157] In the case of .beta. type titanium alloy, residual .beta.
phase exerts the same effect as exerted by residual austenite in
steel materials. In other words, a .beta. type titanium alloy is
subjected to solution treatment at a .beta. phase temperature of
not lower than .beta. transition, and then rapidly cooled to give a
residual .beta. single phase which normally stays soft.
Subsequently, the titanium alloy is subjected to aging to cause
hard .alpha. phase to be uniformly and finely deposited in the
surface layer, thereby forming an (.alpha.+.beta.) texture and
enhancing the surface hardness.
[0158] In other words, the .beta. type titanium alloy forms a
two-phase texture having a hard .alpha. phase deposited in a soft
.beta. phase texture. Thus, when the rolling bearing operates with
a lubricant contaminated by foreign matters, the edge of
impressions formed on the soft residual .beta. phase is allowed to
undergo plastic deformation when it repeatedly comes in contact
with the rolling elements passing thereby during the period between
the formation of the impressions and the generation of cracks in
the edge of the impressions, making it possible to relax the
concentration of stress on the edge of the impressions.
[0159] Further, unlike steel materials, the .beta. type titanium
alloy forms an (.alpha.+.beta.) two-phase texture when subjected to
aging. Thus, .beta. stabilizing elements are concentrated in .beta.
phase to raise the stability of .beta. phase, preventing martensite
transformation during working and hence causing no enhancement of
the hardness of the periphery of the impressions.
[0160] In other words, since the residual .beta. phase in the
.beta. type titanium alloy exhibits an extremely high
transformability, it can repeatedly form impressions therein. As a
result, the impressions can easily undergo plastic deformation to
relax stress concentration thereon even when they come in contact
with the rolling elements passing thereby. Further, the .beta.
phase titanium alloy exhibits a smaller work-hardening index n (see
the equation (2) in the second embodiment) than steel material. By
making the best use of the characteristics, the .beta. type
titanium alloy undergoes no extreme hardening even when repeatedly
subjected to plastic deformation that causes the introduction of a
large amount of strain and thus is little liable to cracking,
making it possible to improve the life of the bearing which
operates with a lubricant contaminated by foreign matters.
[0161] In the fourth embodiment of the present invention, too, if a
titanium alloy is merely subjected to solution treatment and aging,
it cannot be provided with a surface hardness Hv required for
bearing. Thus, the titanium alloy which has been subjected to
solution treatment followed by rapid cooling needs to be subjected
to cold working similarly to the second and third embodiments.
[0162] The residual .beta. phase, percent cold working .eta., and
aging temperature will be described hereinafter.
[0163] (1) Residual .beta. Phase
[0164] As mentioned above, the presence of residual .beta. phase is
effective for the prevention of reduction of the life of bearing
even when the lubricant is contaminated by foreign matters. If the
content: of residual .beta. phase falls below 30 vol %, the
proportion of residual .beta. phase in the bearing material is too
small to provide a stably prolonged bearing life when the lubricant
is contaminated by foreign matters. On the contrary, since the
residual .beta. phase is soft, if the content of residual .beta.
phase is too great, the resulting rolling bearing exhibits an
insufficient hardness and thus cannot operate over a desired life.
To be short, if the content of residual .beta. phase exceeds 80 vol
%, the amount of .alpha. phase deposited in the surface layer of
the .beta. phase titanium alloy is too small to provide a
sufficient surface hardness at the initial stage of aging. As a
result, even after aging, a desired surface hardness cannot be
obtained, making it impossible to provide a desired bearing life.
Accordingly, the volumetric proportion of residual .beta. phase
needs to be from 30 to 80 vol %.
[0165] The volumetric proportion of residual .beta. phase can be
obtained by removing the surface layer of an alloy material by a
depth of about 50 .mu.m by means of chemical polishing (e.g., with
an aqueous solution of hydrofluoric acid and hydrogen peroxide),
and then quantitatively analyzing the surface exposed by means of
X-ray diffraction.
[0166] (2) Percent Cold Working .eta.
[0167] As mentioned in the second embodiment of the present
invention, if a titanium alloy which has been subjected to solution
treatment is subjected to cold working such as cold rolling
forging, it can exhibit an enhanced surface hardness HRC or
strength when subjected to aging. In other words, cold working
causes dislocation to be uniformly introduced into the crystalline
grains. Thus, .alpha. phase is uniformly and finely deposited in
.beta. crystalline grains with the dislocation as a nucleus
production ground, making it possible to enhance surface hardness
HRC and strength. Thus, it is normally necessary that the percent
cold working .eta. be not less than 20%, preferably not less than
30%, similarly to the second embodiment of the present
invention.
[0168] If an emphasis is placed on toughness, the percent cold
working .eta. is preferably predetermined to a range of from 5 to
20% on condition that the cold working is followed by shot peening
as mentioned in the third embodiment.
[0169] (3) Aging Temperature
[0170] A titanium alloy which has been subjected to cold working
needs to be subjected to aging for hardening. If the aging
temperature falls below 400.degree. C., .omega. phase is
preferentially deposited. This .omega. phase remarkably hardens the
surface layer but exerts an embrittling effect. Thus, the
deposition of this .omega. phase needs to be avoided as much as
possible. On the contrary, if the aging temperature exceeds
550.degree. C., hard .alpha. phase can be deposited in the surface
layer in a short period of time. However, grain boundary reaction
type deposition becomes dominant, causing .alpha. phase to be
preferentially deposited in layer at the residual .beta. phase
grain boundary. As a result, coarse acicular .alpha. phase is
deposited in .beta. grains, constituting a hindrance to surface
hardening. In order to enhance surface hardness, it is preferred
that the aging temperature be lowered. However, the aging time is
prolonged. Accordingly, the aging temperature is preferably
predetermined to a range of from 450.degree. C. to 500.degree.
C.
[0171] The present invention will be further described in the
following examples, but the present invention should not be
construed as being limited thereto.
[0172] First Group of Examples
[0173] The inventors prepared disc-shaped specimens as races made
of various titanium alloys and various steel materials.
[0174] Table 1 shows the name of. the material of various
specimens, the surface hardening method, the solution treatment
conditions (or hardening conditions), and the aging conditions (or
tempering conditions).
1TABLE 1 Solution Treatment Conditions Aging Conditions Surface (or
Hardening (or Tempering Race No. Name of Material Hardening Method
Conditions) Conditions) A Ti--6Al--4V 850.degree. C./10 hr.
950.degree. C. water cooling 540.degree. C./4 hr. gaseous nitriding
B Ti--6Al--2Sn--4Zr--6Mo 850.degree. C./10 hr. 910.degree. C. oil
cooling 590.degree. C./4 hr. gaseous nitriding C Ti--15Mo--5Zr
850.degree. C./10 hr. 730.degree. C. water cooling 500.degree.
C./16.7 hr. gaseous nitriding D Ti--15Mo--5Zr--3Al 850.degree.
C./10 hr. 735.degree. C. water cooling 450.degree. C./16.7 hr.
gaseous nitriding E Ti--15V--3Cr--3Sn--3Al 850.degree. C./10 hr.
800.degree. C. water cooling 450.degree. C./6 hr. gaseous nitriding
F Ti--10V--2Fe--3Al 850.degree. C./10 hr. 760.degree. C. water
cooling 400.degree. C./8 hr. gaseous nitriding G Ti--0.3Mo--0.8Ni
850.degree. C./10 hr. 700.degree. C. annealing " gaseous nitriding
H Ti--5Ta 850.degree. C./10 hr. 700.degree. C. annealing " gaseous
nitriding I Pure titanium (JIS3) 850.degree. C./10 hr. 700.degree.
C. annealing " gaseous nitriding J SUS630H Immersion 1,050.degree.
C. oil cooling 500.degree. C./1 hr. hardening K SUS440C Immersion
1,050.degree. C. oil cooling 180.degree. C./2 hr. hardening L
SCR420 930.degree. C./4 hr. 850.degree. C. oil cooling 180.degree.
C./2 hr. carburizing M SUJ2 Immersion 850.degree. C. oil cooling
180.degree. C./2 hr. hardening
[0175] The races A and B were made of (.alpha.+.beta.) type
titanium alloys, the races C to F were made of .beta. phase
titanium alloys, the races G and H were made of .alpha. type
titanium alloys, the race I was made of pure titanium (JIS3), and
the races J to M were made of predetermined steel materials.
[0176] The races A to H. which had been made of titanium alloys,
and the race I, which had been made of pure titanium, were
subjected to gaseous nitriding at a temperature of 850.degree. C.
as surface treatment, and then cooled with nitrogen. The races A to
F were subjected to solution treatment at a temperature of from 730
to 950.degree. C. while being subjected to water cooling or oil
cooling, and then subjected to aging at a temperature of from 450
to 590.degree. C. for 4 to 10 hours to undergo hardening. On the
other hand, the races G to I were subjected to gaseous nitriding,
and then subjected to annealing at a temperature of 700.degree.
C.
[0177] The races J, K and M were subjected to immersion hardening
at a temperature of from 850 to 1,050.degree. C., and then
subjected to tempering at a temperature of from 180 to 500.degree.
C. for 1 to 2 hours.
[0178] The race L was subjected to carburizing at a temperature of
930.degree. C. for 4 hours, subjected to hardening at a temperature
of 850.degree. C., and then subjected to tempering at a temperature
of 180.degree. C. for 2 hours.
[0179] Table 2 shows the surface hardness HRC of the races, the
results of salt spray corrosion test on these races, and the
results of submerged life test on rolling bearings having rolling
elements made of Si.sub.3N.sub.4.
2TABLE 2 Material of Results of Submerged Rolling Surface Salt
Spray Life L.sub.10 Example No. Race No. Elements Hardness (HRC)
Corrosion Test (.times. 10.sup.6 Cycle) Example 1 A Si.sub.3N.sub.4
58.1 Good 25.3 Example 2 B " 58.3 Good 29.4 Example 3 C " 60.2 Good
33.4 Example 4 D " 60.0 Good 31.5 Example 5 E " 59.8 Good 28.3
Example 6 F " 58.1 Good 24.8 Comparative G " -46.2 Good 3.8 Example
101 Comparative H " 46.5 Good 2.8 Example 102 Comparative I " 38.7
Good 3.6 Example 103 Comparative J Si.sub.3N.sub.4 43.0 Fair 2.9
Example 104 Comparative K " 59.7 Poor 2.5 Example 105 Comparative L
" 62.1 Poor 1.4 Example 106 Comparative M " 62.0 Poor 1.3 Example
107
[0180] The .alpha. type titanium alloy used in Comparative Examples
101 and 102 and pure titanium used in Comparative Example 103 don't
undergo hardening when subjected to heat treatment. Thus, all these
races exhibit a surface hardness as low as not more than 47, making
it impossible to provide a surface hardness sufficient for
bearing.
[0181] On the contrary, the (.alpha.+ ) type titanium alloy used in
Examples 1 and 2 and the .beta. phase titanium alloy used in
Examples 3 to 6 exhibit a surface hardness HRC of not less than 57
when subjected to heat treatment, making it possible to provide a
surface hardness sufficient for bearing. Thus, a race which
exhibits an excellent seizing resistance and thus is not liable to
adhesive abrasion can be obtained.
[0182] For the salt spray corrosion test, a 5% aqueous solution of
NaCl was used. The 5% aqueous solution of NaCl was sprayed onto the
various races A to M at a temperature of 35.degree. C. for 150
hours. After spraying, corrosion products were removed from these
races A to M. The change in the weight of these races A to M was
then determined. From these measurements, the corrosion rate per
year was calculated, and the saline resistance was then evaluated.
Referring to criterion for evaluation, when the corrosion rate is
not more than 0.13 mm/year, the corrosion resistance is rated as
"good". When the corrosion rate is from 0.13 to 1.3 mm/year, the
corrosion resistance is rated as "fair (slightly poor)". When the
corrosion rate is not less than 1.3 mm/year, the corrosion
resistance is rated as "poor (unacceptable)".
[0183] Table 2 shows that all Comparative Examples 104 to 107,
which comprise races made of steel material, corrode remarkably
with rust and thus exhibit an insufficient corrosion resistance
while Examples 1 to 6 and Comparative Examples 101 to 103, which
comprise races made of titanium alloy, give good test results. In
other words, concerning the saline resistance, the races made of
steel material didn't give satisfactory results while the races
made of titanium alloy, that is, not only .beta. type titanium
alloy or (.alpha.+.beta.) type titanium alloy but also .alpha. type
titanium alloy or pure titanium, gave satisfactory results.
[0184] The submerged life test will be described hereinafter.
[0185] FIG. 4 is a schematic diagram illustrating the structure of
a submerged thrust bearing life testing machine for use in the
submerged life test. The various races (A to M) and the rolling
elements made of Si.sub.3N.sub.4 were assembled into a thrust ball
bearing 1. For the submerged life test, the thrust ball bearing 1
was immersed in the water in a testing tank 2. A rotary axis 7 was
then allowed to rotate while the bearing was under a predetermined
test load applied from the lower side. In FIG. 4, the reference
numeral 3 indicates an inner race, the reference numeral 4
indicates an outer race, the reference numeral 5 indicates a ball,
and the reference numeral 6 indicates a cage. As the water which
fills the testing tank 2 there was used tap water. The tap water
was supplied from the lower side of the testing tank 2, and then
overflown from the upper side of the testing tank 2.
[0186] The submerged life test conditions will be described
hereinafter.
[0187] Test Conditions
[0188] Bearing tested: Thrust ball bearing (Designation
3 Bearing tested: Thrust ball bearing (Designation No. 51305)
Rotary speed of rotary axis: 1,000 rpm Test load: 150 kgf Material
of rolling elements: Si.sub.3N.sub.4 Material of cage:
Fluororesin
[0189] The inner race and outer race in each bearing to be used in
the submerged life test were prepared from the same material, which
is indicated in Table 3.
[0190] The submerged life L.sub.10 indicates the time at which 10%
of the specimens show a vibration level of 5 times the initial
value as detected by an acceleration pick up sensor. The submerged
life is quantitatively evaluated by the number of rotations
cumulated until this point is reached.
[0191] Table 2 shows that Comparative Examples 101 to 107 exhibit
an extremely short submerged life L.sub.10. This is probably
because Comparative Examples 101 and 102 and Comparative Example
103 use .alpha. type titanium alloys and pure titanium,
respectively, and thus exhibit a reduced strength and a reduced
surface hardness HRC and hence undergo early flaking due to surface
fatigue. In Comparative Examples 104 to 107, the races were made of
alloy steel and thus undergo remarkable corrosion abrasion and
exhibit an extremely short bearing life.
[0192] On the contrary, in Examples 1 to 6, the races were made of
a .beta. phase titanium alloy or (.alpha.+.beta.) type titanium
alloy. Combined with rolling elements made of Si.sub.3N.sub.4,
these races exhibit a remarkably prolonged submerged life
L.sub.10.
[0193] Second Group of Examples
[0194] The inventors prepared rolling elements made of SUS440C and
SUJ2. Combined with these rolling elements, the races A to M set
forth in Table 1 were subjected to salt spray corrosion test and
submerged life test in the same manner as mentioned above.
[0195] Table 3 shows the combination of races and rolling elements
and the results of the various tests on these combinations.
4TABLE 3 Material of Results of Submerged Rolling Surface Salt
Spray Life L.sub.10 Example No. Race No. Elements Hardness (HRC)
Corrosion Test (.times. 10.sup.6 Cycle) Example 11 A SUS440C 58.1
Good 12.3 Example 12 B " 58.3 Good 13.0 Example 13 C " 60.2 Good
15.9 Example 14 D " 60.0 Good 16.5 Example 15 E " 59.8 Good 15.6
Example 16 F " 58.1 Good 14.2 Comparative A SUJ2 58.1 Good 1.2
Example 111 Comparative B " 58.3 Good 1.0 Example 112 Comparative C
" 60.2 Good 0.9 Example 113 Comparative D " 60.0 Good 1.2 Example
114 Comparative E SUJ2 59.8 Good 1.5 Example 115 Comparative F "
58.1 Good 1.3 Example 116 Comparative G SUS440C 46.2 Good 3.8
Example 117 Comparative H " 46.5 Good 2.8 Example 118 Comparative I
" 38.7 Good 3.6 Example 119 Comparative K " 59.7 Poor 2.5 Example
120
[0196] As can be seen in Comparative Examples 111 to 116, if SUJ2
(high carbon chromium bearing steel) is used as rolling element
material, even when the race is made of .beta. phase titanium alloy
or (.beta.+.beta.) type titanium alloy, the resulting rolling
bearing exhibits a reduced submerged life L.sub.10. This is because
titanium alloy and SUJ2 greatly differ electronegatively from each
other to cause galvanic corrosion that attacks and drastically
wears the rolling elements made of SUJ2, which is electronegatively
greater than titanium alloy.
[0197] In Comparative Examples 117 to 119, races made of .alpha.
type titanium alloy or pure titanium and rolling elements made of
SUS440C were combined However, the .alpha. type titanium alloy or
pure titanium used in the races exhibits a deteriorated strength
and surface hardness. The resulting surface fatigue causes early
flaking that reduces the submerged life L.sub.10. In Comparative
Example 120, a race made of SUS440C and rolling elements made of
SUS440C were combined. However, this combination accelerates the
corrosion, deteriorating both the submerged bearing life and saline
resistance.
[0198] On the contrary, Examples 11 to 16 concern a combination of
race made of .beta. type titanium alloy or (.alpha.+.beta.) type
titanium alloy and rolling elements made of SUS440C. These
combinations exhibit a reduced submerged bearing life as compared
with the case where the race is made of Si.sub.3N.sub.4 (see Table
2). However, since there is little difference in electronegativity
between titanium alloy and SUS440C, the progress of galvanic
corrosion is inhibited, making it possible to secure some submerged
bearing life.
[0199] As can be seen in the foregoing first and second groups of
examples, the combination of (.alpha.+.beta.) type or .beta. phase
titanium alloy as race material and Si.sub.3N.sub.4 as rolling
element material is most suitable for corrosion resistance. It is
also made obvious that even rolling elements made of SUS440C can
provide a sufficient bearing life in water or sea water.
[0200] Third Group of Examples
[0201] The inventors prepared combined angular ball bearings from
various titanium alloys and steel materials. The change in the
bearing clearance and the expansion of the inner race during high
speed rotation were then calculated. The rise in the temperature of
the outer race was measured.
[0202] Table 4 shows various bearing materials used in Examples 21
and 22 and Comparative Examples 131 to 136, the solution treatment
conditions (hardening conditions) and the aging conditions (or
tempering conditions).
5 TABLE 4 Inner Race Solution Inner Race Material Treatment Aging
Rolling Conditions Conditions Outer Inner Ele- (or Hardening (or
Tempering Race Race ments Conditions) Conditions) Example SUJ2
Ti-6Al- Si.sub.3- 900-950.degree. C. 500-540.degree. C./ 21 4V
N.sub.4 water cooling 4 hr. Example SUJ2 Ti-22V- Si.sub.3-
750-800.degree. C. 450-500.degree. C./ 22 4Al N.sub.4 water cooling
4 hr. Compara- SUJ2 SUS- Si.sub.3- 1050.degree. C. 180 .degree. C./
tive 440C N.sub.4 oil cooling 2 hr. Example 131 Compara- SUJ2 SUJ2
Si.sub.3- 840.degree. C. 180.degree. C./ tive N.sub.4 oil cooling 2
hr. Example 132 Compara- SUJ2 Ti-6Al- SUJ2 900-950.degree. C.
500-540.degree. C./ tive 4V water cooling 4 hr. Example 133
Compara- SUJ2 Ti-22V- SUJ2 750-800.degree. C. 450-500.degree. C./
tive 4Al water cooling 4 hr. Example 134 Compara- Ti-6Al- Ti-6Al-
Si.sub.3- 900-950.degree. C. 500-540.degree. C./ tive 4V 4V N.sub.4
water cooling 4 hr. Example 135 Compara- Ti-22V- Ti-22V- Si.sub.3-
750-800.degree. C. 450-500.degree. C./ tive 4Al 4Al N.sub.4 water
cooling 4 hr. Example 136
[0203] The inner races of Example 22 and Comparative Examples 134
and 136 were made of .beta. phase titanium alloy, and the inner
races of Example 21 and Comparative Examples 133 and 135 were made
of (.alpha.+.beta.) type titanium alloy. These materials were each
subjected to solution treatment and aging under conditions set
forth in Table 4.
[0204] The inner races of Comparative Examples 131 and 132 were
made of alloy steel. The alloy steel was subjected to hardening at
a predetermined temperature, and then subjected to tempering at a
predetermined temperature.
[0205] The inner races made of titanium alloy were coated with TiN
on the raceway track to secure sufficient abrasion resistance and
seizing resistance.
[0206] The rolling bearings of Examples 21 and 22 and Comparative
Examples 131 to 136 were measured for change in the bearing
clearance and expansion of the inner race during high speed
rotation using a high speed rotary testing machine shown in FIG. 5.
The rise in the temperature of the outer race was then determined.
In FIG. 5, the reference numeral 12 indicates an outer race, the
reference numeral 13 indicates an inner race, and the reference
numeral 14 indicates rolling elements.
[0207] In other words, the outer race 12 was incorporated in a
housing 15, and the inner race 13 was put on a rotary axis 16 so
that a back-to-back type combined angular ball bearing 11 was
mounted in the high speed rotary testing machine. The rotary axis
16 was then rotated. The temperature of the outer race 12 was then
measured by means of a thermocouple 17 inserted in the housing
15.
[0208] The test conditions will be described hereinafter.
6 High Speed Test Bearing tested: Back-to-back type angular ball
bearing (Designation No. 7013C) Preload during mounting: 10 kgf
Lubrication: Grease Grease used: Isoflex NBU15 (produced by NOK
Kluber Co., Ltd.) Rotary speed of rotary axis: 12,000 rpm Table 5
shows the results of high speed rotary test.
[0209] Table 5 shows the results of high speed rotary test.
7TABLE 5 Linear Density Tempera- Expansion of ture Coefficient
Inner Density of Difference Rise in of Inner Race Rolling Between
Change in Inner Race Mate- Element Inner Race Bearing Expansion
Tace Material rial Material and Outer Clearance of Inner Tempera-
Example No. (/.degree. C.) (g/cm.sup.3) (g/cm.sup.3) Race (.degree.
C.) (.mu.m) Race (.mu.m) ture (.degree. C.) Example 21 0.0000088
4.43 3.2 7 2.1 2.9 8.7 Example 22 0.0000085 4.69 3.2 7 2.8 3.0 8.5
Comparative 0.0000101 7.70 3.2 7 -0.7 4.5 10.8 Example 131
Comparative 0.0000125 7.83 3.2 7 -8.1 5.1 12.4 Example 132
Comparative 0.0000088 4.43 7.83 7 2.1 2.9 11.4 Example 133
Comparative 0.0000085 4.69 7.83 7 2.8 3.0 11.2 Example 134
Comparative 0.0000088 4.43 3.2 7 -5.7 4.9 11.6 Example 135
Comparative 0.0000085 4.69 3.2 7 -5.5 5.1 11.5 Example 136
[0210] For the evaluation of the bearing clearance, the change
developed when the temperature difference between the inner race
and the outer race reaches 7.degree. C. was determined.
[0211] In Comparative Example 131, the outer race was made of SUJ2,
the inner race was made of SUS440C, and the rolling elements were
made of Si.sub.3N.sub.4. Since SUS440C exhibits a greater linear
expansion coefficient than titanium alloy, the bearing clearance is
reduced with the temperature difference between the inner race and
the outer race being 7.degree. C. Further, since SUS440C has a
great density, it exhibits a great expansion due to centrifugal
force, causing a great rise in the temperature of the outer race.
In Comparative Example 132, both the inner race and the outer race
were made of SUJ2, and the rolling elements were made of
Si.sub.3N.sub.4. Since both the inner race and the outer race were
made of SUJ2, the bearing clearance showed a remarkable drop, and
the expansion of the inner race and the rise in the temperature of
the outer race were raised.
[0212] In Comparative Examples 133 and 134, the outer race was made
of SUJ2, the inner race was made of titanium alloy, and the rolling
elements were made of Si.sub.3N.sub.4. Since the inner race was
made of titanium alloy, the bearing clearance showed a rise rather
than drop. The expansion of the inner race was small. However, the
outer race showed a great temperature rise. This is probably
because the rolling elements are made of SUJ2, which has a greater
density than ceramics, and thus is given a great centrifugal force,
resulting in the rise in the friction between the track on the race
and the rolling surface of the rolling elements.
[0213] In Comparative Examples 135 and 136, both the inner race and
the outer race were made of titanium alloy, and the rolling
elements were made of Si.sub.3N.sub.4. Since the inner race and the
outer race was made of the same material, the bearing clearance
shows a drop and the expansion of the inner race is raised if
evaluated with the temperature difference between the inner race
and the outer race being 7.degree. C. As a result, the rise in the
temperature of the outer race is raised. Accordingly, taking into
account the high speed rotary operation, the inner race and the
outer race should not be made of the same material. However, since
titanium alloy exhibits a smaller linear expansion coefficient than
SUJ2, the reduction of the bearing clearance can be less than
Comparative Example 132 in which both the inner race and the outer
race are made of SUJ2. Accordingly, the rise in the temperature of
the outer race can be inhibited more than in Comparative Example
132.
[0214] On the contrary, in Examples 21 and 22, the inner race was
made of titanium alloy, and the rolling elements were made of
Si.sub.3N.sub.4. Even if there occurs a temperature difference of
7.degree. C. between the inner race and the outer race, the bearing
clearance does not show a drop but increases. The expansion of the
inner race due to centrifugal force is far less than in Comparative
Examples 131 to 136. Thus, the rise in the temperature of the outer
race during high speed rotation can be reduced to not more than
10.degree. C. Accordingly, the rolling bearings according to these
examples are suitable for high speed rotation.
[0215] As can be seen in the present group of examples, a
combination of an inner race made of titanium alloy, an outer race
made of steel material such as SUJ2 and rolling elements made of
Si.sub.3N.sub.4 is optimum for bearing for use in machines which
operate at a high rotary speed such as machine tool.
[0216] Fourth Group of Examples
[0217] The inventors prepared a disc-shaped specimen from
Ti-15V-3Cr-3Sn-3Al as .beta. type titanium alloy. The specimen was
subjected to solution treatment at a temperature of 850.degree. C.
in an Ar atmosphere, water-cooled, and then subjected to cold
rolling (cold working) at various percent cold working .eta.. The
specimen was subjected to aging a a temperature of 450.degree. C.
for 5 to 8 hours, and then measured for surface hardness Hv by
means of a Vickers hardness testing machine.
[0218] FIG. 6 is a characteristic curve illustrating the
relationship between percent cold working .eta. and Vickers
hardness Hv after aging.
[0219] There is a relationship represented by the following
equation (3) between surface hardness Hv (Vickers hardness) and
surface hardness HRC (Rockwell C hardness).
Hv=10HRC+30 (3)
[0220] Accordingly, in order to obtain a surface hardness of not
less than 57 as calculated in terms of HRC, it is necessary that
the surface hardness Hv be not less than 600 according to the
equation (3).
[0221] However, as evident from FIG. 6, if the percent cold working
.eta. is less than 20%, the surface hardness Hv after aging is not
more than 600, making it impossible to obtain a sufficient
hardness. On the contrary, if the percent cold working .eta. is not
less than 20%, the surface hardness Hv after aging is not less than
600, making it possible to obtain a bearing material having a
sufficient hardness. Further, if the percent cold working q is not
less than 30%, a bearing material having a stabilized hardness of
not less than 600 can be obtained.
[0222] The same disc-shaped specimen as used above (.beta.-titanium
alloy, Ti-15V-3Cr-3Sn-3Al) was subjected to solution treatment,
water cooling, cold working, etc. in the same manner as mentioned
above. The specimen was then subjected to aging under isothermal
conditions (450.degree. C.) for 5 to 50 hours. The specimen was
then measured for surface hardness Hv. The specimen was also
subjected to submerged life test in the same manner as in the first
group of examples.
[0223] Table 6 shows the results of measurement of hardness Hv and
submerged life L.sub.10 vs. percent cold working .eta..
8TABLE 6 Percent Cold Aging Time Submerged Life Example No. Working
.eta. (%) (hr) Hardness (Hv) L.sub.10 (.times. 10.sup.6 cycle)
Example 41 25 5 618 16.8 Example 42 30 5 623 17.1 Example 43 50 5
629 17.1 Example 44 80 5 631 18.9 Example 45 25 7 620 17.3 Example
46 30 7 622 18.5 Example 47 50 7 631 19.7 Example 48 80 7 638 20.1
Comparative Example 141 25 50 583 4.2 Comparative Example 142 30 50
585 4.4 Comparative Example 143 50 50 590 4.5 Comparative Example
144 80 50 597 4.8 Comparative Example 145 0 5 424 1.2 Comparative
Example 146 0 7 455 1.6 Comparative Example 147 0 10 451 1.4
Comparative Example 148 0 50 448 1.4 Comparative Example 149 15 5
568 4.3 Comparative Example 150 15 7 572 5.0 Comparative Example
151 15 10 572 5.1
[0224] As can be seen in Table 6, in Comparative Examples 141 to
144, cold working was effected at a percent working .eta. of from
25 to 80%. In other words, cold working was effected at a percent
working .eta. of not less than 20%. However, since aging was
effected for a period of time as long as 50 hours, the bearing
materials were softened and thus exhibited a reduced surface
hardness Hv and submerged life L.sub.10. This is probably because
the aging time T is too long, giving overaging that causes hard
.alpha. phase to grow coarsely or .alpha. phase to be deposited at
grain boundary and hence causing a hardness drop. In Comparative
Examples 145 to 151, cold working was effected at a percent working
.eta. of not more than 20%, making it impossible to obtain
satisfactory results for use in special corrosive atmospheres
concerning hardness Hv and submerged life L.sub.10. This is
probably because if the percent cold working .eta. is low,
dislocation is nonuniformly introduced, making it difficult for
.alpha. phase to be uniformly and finely deposited in .beta.
crystalline grins. Thus, the resulting degree of reinforcement is
small. Further, .alpha. phase is preferentially deposited at grain
boundary to reduce the grain boundary strength, causing early
flaking.
[0225] On the contrary, in Examples 41 to 48, the percent cold
working .eta. is not less than 20%, and the aging time T is as
short as 5 to 7 hours, making it possible to obtain a hardness Hv
of not less than 600 and hence a sufficient submerged life
L.sub.10.
[0226] The inventors measured the relationship between percent cold
working .eta. and aging time T (hr) required until the highest
hardness is reached. Table 7 shows the measurements.
9TABLE 7 Aging Time (hr) Percent Cold Required Until Working .eta.
Highest Hardness Example No. (%) is Reached Example 51 25 5 Example
52 30 5 Example 53 50 4 Example 54 70 4 Comparative Example 161 0 7
Comparative Example 162 15 6
[0227] As can be seen in Table 7, in Comparative Example 161, no
cold working is effected, requiring 7 hours of aging time T until
the highest hardness is reached. In Comparative Example 162, the
percent cold working .eta. is as low as 15%, requiring 6 hours of
aging time T. On the contrary, in Examples 51 to 54, the percent
cold working .eta. is not less than 20%, requiring aging time T as
short as 4 to 5 hours. Thus, a great effect of accelerating the
deposition of .alpha. phase in .beta. crystalline grains can be
exerted.
[0228] Fifth Group of Examples
[0229] The inventors examined a bearing material which had been
subjected to shot peening after cold working and a bearing material
which had not been subjected to shot peening after cold working for
the relationship between percent cold working .eta. and surface
hardness after aging.
[0230] In some detail, Ti-b 15Mo-5Zr as .beta. phase titanium alloy
was subjected to solution treatment at a temperature of 750.degree.
C. in an Ar atmosphere, water-cooled to form a residual .beta.
single phase texture, subjected to cold rolling (cold working) at a
predetermined percent working .eta., subjected to shot peening
using a straight-hydraulic air blast machine, and then subjected to
aging at a temperature of 475.degree. C. for 5 hours to prepare a
specimen. Separately, a specimen was prepared in the same manner as
mentioned above except that the titanium alloy was not subjected to
shot peening after cold working. The cold working was effected at a
percent working .eta. of 0%, 5%, 10%, 15%, 20%, 30%, and 50%,
respectively.
[0231] The shot peening conditions will be described below.
[0232] Shot Peening Conditions
[0233] Shot: Shot intensity 6A
[0234] Shooting material: Cast steel
[0235] Grain diameter: 400 .mu.m
[0236] Surface hardness Hv: 420
[0237] These specimens were each measured for surface hardness Hv
by means of a Vickers hardness tester.
[0238] FIG. 7 shows a characteristic curve illustrating the
relationship between percent cold working .eta. and surface
hardness Hv after aging in the present examples, wherein
.circle-solid. indicates the case where shot peening is effected
after cold working, and .smallcircle. indicates the case where only
cold working is effected.
[0239] As can be seen in FIG. 7, cold working, if not followed by
shot peening, must be effected at a percent working .eta. of not
less than 20% to obtain a bearing material having a surface
hardness Hv of not less than 600. On the contrary, cold working,-if
followed by shot peening, may be effected even at a percent working
.eta. as low as 5% to obtain a bearing material having a surface
hardness Hv of not less than 600. Further, if the percent cold
working .eta. is low, the bearing material can be prevented from
hardening to the core, making it possible to obtain a good
toughness.
[0240] The inventors prepared specimens from Ti-15Mo-5Zr as .beta.
type titanium alloy. These specimens were subjected to solution
treatment, water cooling, cold rolling, shot peening and aging.
These specimens were then measured for surface hardness Hv and
residual compression stress. These specimens were also subjected to
submerged life test. For comparison, specimens which had not been
subjected to shot peening or cold working were prepared and
subjected to the same tests as mentioned above.
[0241] For the measurement of residual compression stress, an X-ray
residual stress meter was used. The measurement conditions will be
described hereinafter.
[0242] Conditions for the Measurement of Residual Compression
Stress
[0243] Target: Cu--Ka
[0244] Filter: Ni
[0245] Tube voltage: 40 kV
[0246] Tube current: 300 mA
[0247] For the submerged life test, the same testing machine (see
FIG. 4) as used in the first group of examples was used. The test
was effected in the same manner as in the first group of examples.
However, when the specimens were subjected to shot peening, the
race showed a raised surface roughness. In order to eliminate the
effect of this surface roughness, the surface of these specimens
was polished before the submerged life test.
[0248] Table 8 shows the measurements of various specimens which
had been subjected to cold working at different percent working
.eta..
10 TABLE 8 Residual Submerged Production Conditions Surface
Compression Life L.sub.10 % Cold Shot Aging Shot Hardness Stress
(.times. 10.sup.6 Example No. Working Peening Conditions Peening
(Hv) (kg/mm.sup.2) cycle) Example 61 5 Yes 475.degree. C./5 hr No
603 0 17.1 Example 62 10 Yes " No 625 -2 17.3 Example 63 15 Yes "
No 632 -1 17.7 Example 64 25 Yes " No 640 0 18.6 Example 65 30 Yes
" No 644 0 19.5 Example 66 5 Yes " Yes 631 -34 19.7 Example 67 10
Yes " Yes 639 -31 20.1 Example 68 15 Yes " Yes 644 -37 20.6 Example
69 25 Yes " Yes 649 -37 21.3 Example 70 30 Yes " Yes 651 -35 21.5
Comparative 0 No 475.degree. C./5 hr No 458 0 1.6 Example 171
Comparative 0 Yes " No 521 -2 2.2 Example 172 Comparative 0 Yes "
Yes 521 -30 4.3 Example 173 Comparative 5 No " No 508 0 4.6 Example
174 Comparative 5 No " Yes 532 -29 4.8 Example 175
[0249] As can be seen in Table 8, in Comparative Example 171, the
bearing material is subjected to neither cold working nor shot
peening but aging after solution treatment. Thus, the resulting
specimen exhibits a low surface hardness Hv and a reduced submerged
life L.sub.10.
[0250] In Comparative Example 172, shot peening is effected,
causing .alpha. phase to be uniformly and finely deposited in the
surface layer. Thus, the rise in the surface hardness Hv can be
recognized as compared with Comparative Example 171. However, since
solution treatment is not followed by cold working but by shot
peening, the resulting specimen exhibits a surface hardness Hv of
not more than 600, making it impossible to provide a surface
hardness Hv required for bearing. In Comparative Example 173, shot
peening is effected after aging as well in addition to the
conditions used in Comparative Example 172, providing a residual
compression stress. However, since no cold working is effected as
in Comparative Example 172, a surface hardness Hv required for
bearing cannot be obtained.
[0251] In Comparative Example 174, cold working is effected at a
percent working .eta. as low as 5%. However, since no shot peening
is effected, a surface hardness Hv required for bearing cannot be
obtained. In Comparative Example 175, shot peening is effected
after aging as well, providing a residual compression stress.
However, since a bearing material which has been subjected to
solution treatment followed by cold working at a low percent
working is not subjected to shot peening before aging as in
Comparative Example 174, a surface hardness Hv required for bearing
cannot be obtained.
[0252] On the contrary, in Examples 61 to 70, cold working is
effected at a percent working .eta. of from 5 to 30% before shot
peening. Thus, the resulting specimen exhibits a surface hardness
Hv of not less than 600 and shows a drastic enhancement of
submerged life L.sub.10 as compared with Comparative Examples 171
to 175.
[0253] In particular, Examples 66 to 70 involve another shot
peening after aging. Thus, the resulting specimens exhibit a
further rise in surface hardness Hv if the percent cold working
.eta. remains the same. Further, the bearing material can be
provided with a residual compression stress. As a result, the
submerged life L.sub.10 can be enhanced.
[0254] In Examples 64, 65, 69 and 70, the percent cold working
.eta. is predetermined to not less than 20%. It is thus likely that
the bearing material can be hardened to the core to exhibit a
reduced toughness. However, a surface hardness Hv of not less than
600 can be obtained, and the submerged life L.sub.10 cannot be
reduced. Accordingly, if the rolling bearing is used in positions
requiring toughness, it is preferred that a bearing material which
has been subjected to cold working at a percent working .eta. of
from 5 to 20% followed by shot peening be used. If the rolling
bearing is used in positions where emphasis is placed on surface
hardness rather than toughness, it is preferred that the bearing
material be subjected to cold working at a percent working q of not
less than 20% and then directly to aging without shot peening as in
the fourth group of examples. If it is desired to enhance fatigue
strength in particular, it is preferred that the bearing material
which has been thus aged be subjected to shot peening to have a
residual compression stress applied thereto.
[0255] Sixth Group of Examples
[0256] The inventors examined the relationship between aging time T
and residual .beta. phase content and surface hardness Hv and the
relationship between residual .beta. phase content and bearing life
when the lubricant is contaminated by foreign matters.
[0257] In some detail, Ti-15V-3Cr-3Sn-3Al as .beta. type titanium
alloy was subjected to solution treatment at a temperature of
800.degree. C. in an Ar atmosphere, water-cooled to form a residual
.beta. single phase, subjected to cold rolling at a percent working
.eta. of 50%, and then subjected to aging at a temperature of
450.degree. C. for various periods of time to prepare various
specimens composed of (.alpha.+.beta.) texture. These specimens
were then determined for residual .beta. phase content and measured
for surface hardness Hv.
[0258] Firstly, the specimen was subjected to chemical polishing
with an aqueous solution comprising 60% hydrogen peroxide and 10%
hydrofluoric acid so that a processed layer formed on the surface
thereof was removed to a depth of about 50 .mu.m. Subsequently,
using an X-ray diffractometer, the volumetric ratio (vol %) of
residual .beta. phase was calculated with Co-K.alpha. line as a
target. As the X-ray diffractometer there was used Type RAD-III
X-ray diffractometer Geiger Flex (produced by Rigaku Corp.).
[0259] For the measurement of surface hardness Hv, a Vickers
hardness testing machine was used as in the fourth and fifth groups
of examples.
[0260] FIG. 8 is a characteristic curve illustrating the
relationship between aging time T and residual .beta. phase content
and surface hardness Hv.
[0261] As can be seen in FIG. 8, concerning the relationship
between aging time T and surface hardness Hv, as the aging time T
increases, the deposition of .alpha. phase proceeds to reduce the
residual .beta. phase content. In particular, when the aging time T
exceeds 1 hour, the volumetric ratio of residual .beta. phase shows
a sudden drop.
[0262] On the other hand, concerning the relationship between aging
time T and surface hardness Hv, when the aging time T exceeds 1
hour, and the deposition of .alpha. phase becomes remarkable, the
rise in surface hardness Hv becomes remarkable. However, when the
aging time T exceeds 10 hours, the content of .alpha. phase shows a
continuous rise, and the volumetric ratio of residual .beta. phase
continues to drop. Thus, the surface hardness Hv shows a continuous
drop. Accordingly, if aging is effected for 10 hours or longer,
averaging occurs.
[0263] The inventors conducted submerged life test on rolling
bearings containing residual .beta. phase which had been aged for
different periods of time shown in FIG. 8 using a submerged thrust
bearing life testing machine shown in FIG. 4.
[0264] The conditions for submerged life test will be described
hereinafter.
[0265] Test Conditions
[0266] Bearing tested: Thrust ball bearing (Designation
11 Bearing tested: Thrust ball bearing (Designation No. 51305)
Rotary speed of rotary axis: 1,000 rpm Test load: 150 kgf Material
of rolling elements: Si.sub.3N.sub.4 Material of cage: Fluororesin
Foreign matters: Fe.sub.3C powder (300 ppm in water) Grain diameter
of foreign matters: 74-147 .mu.m Surface hardness HRC of 52 foreign
matters:
[0267] The inner race and outer race in each bearing to be used in
the submerged life test were prepared from the same material, which
is indicated in Table 9.
[0268] The submerged life L.sub.10 indicates the time at which 10%
of the specimens undergo cracking or flaking which can be observed
under microscope or visually. The submerged life is quantitatively
evaluated by the number of rotations cumulated until this point is
reached.
[0269] FIG. 9 is a characteristic curve illustrating the
relationship between the residual .beta. phase content and the
submerged life L.sub.10 of the specimens which have been aged for
different periods of time as shown in FIG. 8.
[0270] As can be seen in FIG. 9, if the volumetric ratio of
residual .beta. phase falls below 30 vol %, the submerged life
L.sub.10 is extremely low, although the content of hard .alpha.
phase is greater than that of residual .beta. phase. This is
because the specimen are overaged. Thus, .alpha. phase grows
coarsely or is deposited at .beta. phase grain boundary to cause
rapid softening. Therefore, the resulting bearing exhibits an
insufficient hardness. Further, since there is a small residual
.beta. phase content, the impressions possibly formed by foreign
matters exert a small effect of relaxing stress. On the other hand,
when the bearing material which has been subjected to solution
treatment is rapidly cooled, a residual .beta. single phase is
formed. Therefore, if the volumetric ratio of residual .beta. phase
exceeds 80 vol %, this state corresponds to that obtained at the
initial stage of aging. Thus, the specimen is not sufficiently
hardened. Accordingly, a sufficient surface hardness Hv cannot be
obtained. The submerged life L.sub.10 is extremely reduced.
[0271] On the contrary, if the volumetric ratio of residual .beta.
phase falls within the range of from 30 to 80 vol %, the residual
.beta. phase relaxes stress on the impressions formed by foreign
matters even when the lubricant is contaminated by foreign matters.
Further, .alpha. phase is deposited to an ideal extent, making it
possible to provide a surface hardness Hv of not less than 600 and
a stabilized prolonged submerged life L.sub.10.
[0272] The inventors prepared various specimens from Ti-15Mo-5Zr as
.beta. type titanium alloy and Ti-6Al-4V as (.alpha.+.beta.) type
titanium alloy. These titanium alloys were subjected to heat
treatment (solution treatment and aging) under different conditions
or cold working at different percent working .eta.. These specimens
were measured for volumetric ratio (vol %) of residual .beta.
phase, surface hardness Hv and submerged life L.sub.10 under the
same conditions as mentioned above (lubricant contaminated by
foreign matters).
[0273] Table 9 shows the production conditions of these .beta. type
titanium alloys and the measurements of the various specimens.
12TABLE 9 Resi- Sub- dual merged .beta. life Solution Aging Aging
Surface phase L.sub.10 Example Treatment % Cold Temp. Time Hardness
(vol- (.times. 10.sup.6 No. Alloy Temp. (.degree. C.) Working
(.degree. C.) (hr) (Hv) %) cycle) Example 71 Ti-15Mo-5Zr
750.degree. C. 50 475 3 615 75 9.3 (.beta. type water titanium)
cooling Example 72 " " 50 475 5 625 59 10.1 Example 73 " " 50 475 7
630 51 10.4 Example 74 " " 50 475 10 621 45 9.8 Example 75 " " 30
475 3 608 78 8.9 Example 76 " " 30 475 5 611 70 9.2 Example 77 " "
30 475 7 615 64 9.3 Example 78 " " 30 475 10 609 58 9.5 Comparative
Ti-15Mo-5Zr 750.degree. C. 50 400 5 658 -- 1.3 Example 181 (.beta.
type water titanium) cooling Comparative " " 50 400 7 666 -- 1.4
Example 182 Comparative " " 50 550 5 573 50 3.5 Example 183
Comparative " " 50 550 7 561 45 2.8 Example 184 Comparative
Ti-6Al-4V 950.degree. C. 0 540 4 421 58 0.8 Example 185 ((.alpha. +
.beta.) type water titanium) cooling Comparative " 900.degree. C. 0
540 4 423 43 0.9 Example 186 water cooling
[0274] As can be seen in Table 9, Comparative Examples 181 and 182
provide a surface hardness Hv of not less than 600 but an extremely
short submerged life L.sub.10. In Comparative Examples 181 and 182,
the surface hardness itself is raised. However, since the aging
temperature is as low as 400.degree. C., .omega. phase is formed,
reducing the plastic deformability. Thus, the concentration of
stress on the edge of impressions formed by foreign matters is
raised, causing early flaking.
[0275] In Comparative Examples 181 and 182, the residual .beta.
phase content was not calculated. This is because the deposition of
.omega. phase makes it impossible to accurately determine the
residual .beta. phase content. However, since .omega. phase is
extremely brittle, it has an adverse effect on the texture even if
the volumetric ratio of residual .beta. phase falls within the
range of from 30 to 80 vol %. Accordingly, the condition for aging
so that .omega. phase is deposited even in a slight amount should
be avoided.
[0276] In Comparative Examples 183 and 184, the aging temperature
is predetermined too high as 550.degree. C. Thus, .alpha. phase
which would be deposited in layer at residual .beta. phase grain
boundary or inside .beta. phase grain boundary grows coarsely,
making it impossible to undergo sufficient aged hardening.
[0277] In Comparative Examples 185 and 186, (.alpha.+.beta.) type
titanium alloy is used as titanium alloy. When subjected to
solution treatment followed by rapid cooling, an (.alpha.+.beta.)
type titanium alloy forms a martensite texture of a
(.alpha.+.beta.) two-phase texture which cannot be subjected to
cold working. Accordingly, since this titanium alloy cannot be
subjected to cold working, it exhibits a reduced surface hardness
Hv and an extremely reduced submerged life L.sub.10 even when
subsequently aged.
[0278] On the contrary, in Examples 71 to 78, the aging temperature
is predetermined to 475.degree. C., which is the optimum aging
temperature for the present alloy, the aging time is predetermined
to a range of from 3 to 10 hours, and the residual .beta. phase
content is varied. All these examples exhibit a surface hardness Hv
of not less than 600 and provide a remarkable improvement of
submerged life L.sub.10 as compared with the comparative examples.
Thus, these examples can provide rolling bearings suitable for use
in the conditions where corrosion resistance is required and
foreign matters are incorporated.
[0279] As mentioned above, the rolling bearing according to the
present invention comprises an outer race and an inner race and
rolling elements which are provided between the outer race and the
inner race such that the rolling elements rotate freely,
characterized in that at least the inner race is made of a titanium
alloy and the rolling elements are made of ceramics. Thus, the
rolling bearing according to the present invention exhibits a
drastically improved corrosion resistance as compared with the case
where the race is made of a steel material such as stainless steel
and thus is suitable for use in corrosive atmospheres such as food
machine, semiconductor producing machine and chemical fiber
producing machine which must be resistant to corrosion with sea
water or chemical.
[0280] Further, the use of a titanium alloy having a small linear
expansion coefficient and a small density as an inner race material
makes it possible to inhibit the rise in the temperature of the
outer race during high speed rotation and hence provides a rolling
bearing suitable for use in machine tools which operate at a high
rotary speed.
[0281] Moreover, in accordance with the present invention, the rise
in the production cost can be inhibited as compared with the case
where the bearing is totally made of ceramics.
[0282] Further, by forming at least one of the inner race and outer
race by a .beta. type titanium alloy and predetermining the percent
cold working .eta. to not less than 20%, .alpha. phase is deposited
in .beta. crystalline grains to enhance hardness and bearing
strength, making it possible to improve the durability of the
bearing.
[0283] Moreover, by predetermining the percent cold working .eta.
of .beta. type titanium alloy to a range of from 5 to 20% and
subjecting the .beta. phase titanium alloy thus cold-worked to shot
peening, .alpha. phase is finely deposited, enabling drastic rise
in the hardness of the surface layer alone without impairing the
toughness. Further, by subjecting the .beta. type titanium alloy to
shot peening after aging as well, the .beta. type titanium alloy
can be provided with a residual compression stress, making it
possible to improve the bearing life in a special atmosphere.
[0284] Further, by predetermining the volumetric ratio of residual
.beta. phase in the foregoing .beta. type titanium alloy to a range
of from 30 to 80%, the concentration of stress on the edge of
impressions formed on the surface of the race can be relaxed even
when the lubricant is contaminated by foreign matters, making it
possible to provide a rolling bearing which exhibits an excellent
corrosion resistance and a prolonged life even when the lubricant
is contaminated by foreign matters.
[0285] While the invention has been described in detail and with
reference to specific embodiments thereof, it will be apparent to
one skilled in the art that various changes and modifications can
be made therein without departing from the spirit and scope
thereof.
* * * * *