U.S. patent number RE48,336 [Application Number 16/398,979] was granted by the patent office on 2020-12-01 for rolling apparatus.
This patent grant is currently assigned to NSK LTD.. The grantee listed for this patent is NSK LTD.. Invention is credited to Nobuaki Mitamura, Naoya Seno, Koji Ueda, Toru Ueda.
View All Diagrams
United States Patent |
RE48,336 |
Ueda , et al. |
December 1, 2020 |
Rolling apparatus
Abstract
A rolling apparatus including an external member having a
raceway surface on an inner peripheral surface thereof, an internal
member having a raceway surface on an outer peripheral surface
thereof, and a plurality of rolling elements which are rotatably
provided between the raceway surface of the external member and the
raceway surface of the internal member. A surface of at least one
of the internal member, the external member, and the rolling
elements is subjected to carbonitriding or nitriding; an area
percentage of a nitride containing Si and Mn is 1% or more and 20%
or less; surface hardness is HV750 or more. When depth from the
raceway surface or depth from a rolling surface of the rolling
element is defined as Z and diameter of the rolling element is
defined as d, hardness at Z=0.045 d is HV650 to 850, and hardness
at Z=0.18 d is HV400 to 800.
Inventors: |
Ueda; Koji (Fujisawa,
JP), Ueda; Toru (Fujisawa, JP), Seno;
Naoya (Fujisawa, JP), Mitamura; Nobuaki
(Fujisawa, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
NSK LTD. |
Tokyo |
N/A |
JP |
|
|
Assignee: |
NSK LTD. (Tokyo,
JP)
|
Family
ID: |
38723242 |
Appl.
No.: |
16/398,979 |
Filed: |
April 30, 2019 |
PCT
Filed: |
May 16, 2007 |
PCT No.: |
PCT/JP2007/060073 |
371(c)(1),(2),(4) Date: |
September 16, 2008 |
PCT
Pub. No.: |
WO2007/135929 |
PCT
Pub. Date: |
November 29, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
12293189 |
May 16, 2007 |
8088230 |
Jan 3, 2012 |
|
|
Foreign Application Priority Data
|
|
|
|
|
May 19, 2006 [JP] |
|
|
2006-140111 |
May 29, 2006 [JP] |
|
|
2006-148497 |
May 30, 2006 [JP] |
|
|
2006-150375 |
Apr 16, 2007 [JP] |
|
|
2007-107250 |
Apr 23, 2007 [JP] |
|
|
2007-112995 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C
38/04 (20130101); F16C 33/64 (20130101); C21D
1/10 (20130101); C23C 8/26 (20130101); F16C
33/34 (20130101); C21D 1/06 (20130101); C21D
1/10 (20130101); C21D 9/40 (20130101); F16C
33/34 (20130101); C23C 8/32 (20130101); F16C
33/64 (20130101); C22C 38/18 (20130101); C22C
38/04 (20130101); C22C 38/02 (20130101); F16C
33/62 (20130101); C23C 8/26 (20130101); C21D
9/40 (20130101); C23C 8/32 (20130101); C21D
1/06 (20130101); F16C 33/62 (20130101); C22C
38/18 (20130101); F16C 33/32 (20130101); C22C
38/02 (20130101); F16C 33/32 (20130101); F16C
2202/04 (20130101); F16C 19/06 (20130101); C21D
2211/008 (20130101); C21D 2211/001 (20130101); Y10S
148/906 (20130101); Y02P 10/25 (20151101); F16C
19/364 (20130101); F16C 29/0647 (20130101); F16C
19/06 (20130101); C21D 2211/001 (20130101); Y02P
10/25 (20151101); C21D 2211/008 (20130101); Y10S
148/906 (20130101); F16C 19/364 (20130101); F16C
29/0647 (20130101) |
Current International
Class: |
C22C
38/02 (20060101); C22C 38/04 (20060101); C22C
38/18 (20060101); F16C 33/64 (20060101); F16C
29/06 (20060101); C21D 9/40 (20060101); C23C
8/26 (20060101); C23C 8/32 (20060101); F16C
33/32 (20060101); F16C 33/34 (20060101); F16C
19/06 (20060101); F16C 19/36 (20060101); C21D
1/10 (20060101); C21D 1/06 (20060101); F16C
33/62 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
61-270331 |
|
Nov 1986 |
|
JP |
|
64-55423 |
|
Mar 1989 |
|
JP |
|
7-41934 |
|
Feb 1995 |
|
JP |
|
8-311603 |
|
Nov 1996 |
|
JP |
|
9-170624 |
|
Jun 1997 |
|
JP |
|
09-257041 |
|
Sep 1997 |
|
JP |
|
2003-193200 |
|
Jul 2003 |
|
JP |
|
2004-52997 |
|
Feb 2004 |
|
JP |
|
2005-282854 |
|
Oct 2005 |
|
JP |
|
2005-337361 |
|
Dec 2005 |
|
JP |
|
2006-105363 |
|
Apr 2006 |
|
JP |
|
Primary Examiner: Johnson; Jerry D
Attorney, Agent or Firm: Sughrue Mion, PLLC
Claims
The invention claimed is:
1. A rolling apparatus comprising: an external member having a
raceway surface on an inner peripheral surface thereof; an internal
member having a raceway surface on an outer peripheral surface
thereof; and a plurality of rolling elements which are rotatably
provided between the raceway surface of the external member and the
raceway surface of the internal member, wherein .Iadd.the rolling
elements are made of steel which contains 0.95 to 1.10 mass % of C,
0.4 to 0.7 mass % of Si, 0.9 to 1.15 mass % of Mn, 0.9 to 1.20 mass
% of Cr, remaining Fe and inevitable impurities, and wherein
.Iaddend.a surface of at least one of the internal member, the
external member, and the rolling elements is subjected to
carbonitriding or nitriding, an area percentage of a nitride
containing Si and Mn is 1% or more and .[.20%.]. .Iadd.3.65%
.Iaddend.or less, a hardness on a surface is HV750 or more, .Iadd.a
concentration of nitrogen on a surface layer of at least one of the
internal element, the external element, and the rolling elements is
0.2 mass % to 0.64 mass %, .Iaddend.and when depth from the raceway
surface or depth from a rolling surface of the rolling element is
defined as Z and diameter of the rolling element is defined as d,
hardness at Z=0.045d is .[.HV650.]. .Iadd.HV770 .Iaddend.to
.[.850.]. .Iadd.816.Iaddend., and hardness at Z=0.18d is
.[.HV400.]. .Iadd.HV700 .Iaddend.to .[.800.]. .Iadd.771, when an
amount of retained austenite on the raceway surfaces is defined as
.gamma.r.sub.AB and when an amount of retained austenite on the
rolling surface of the rolling element is defined as
.gamma.r.sub.C,
.gamma.r.sub.AB-15.ltoreq..gamma.r.sub.C.ltoreq..gamma.r.sub.AB+15
(0.ltoreq..gamma.r.sub.AB, .gamma.r.sub.C.ltoreq.50, and a unit is
vol. %) is satisfied.Iaddend..
2. The rolling apparatus according to claim 1, wherein .[.a
concentration of nitrogen on a surface layer of at least one of the
internal element, the external element, and the rolling elements is
0.2 mass % or more, and.]. a number of nitride of which size is
0.05 .mu.m to 1 .mu.m and which contains Si and Mn in an area of
375 .mu.m.sup.2 is 100 or more.
.[.3. The rolling apparatus according to claim 2, wherein at least
one of the carbonitrided or nitrided internal member, external
member, and rolling elements is made of steel which contains 0.3 to
1.2 mass % of C, 0.3 to 2.2 mass % of Si, 0.3 to 2.0 mass % of Mn,
0.5 to 2.0 mass % of Cr, 5 or less of Si/Mn, remaining Fe and
inevitable impurities..].
4. The rolling apparatus according to claim .[.3.].
.Iadd.1.Iaddend., wherein a carbonitrided or nitrided member is the
rolling elements.
.[.5. The rolling apparatus according to claim 4, wherein, when an
amount of retained austenite on the raceway surfaces is defined as
.gamma.r.sub.AB and when an amount of retained austenite on the
rolling surface of the rolling element is defined as
.gamma.r.sub.C,
.gamma.r.sub.AB-15.ltoreq..gamma.r.sub.C.ltoreq..gamma.r.sub.AB+15
(0.ltoreq..gamma.r.sub.AB, .gamma.r.sub.C.ltoreq.50, and a unit is
vol. %) is satisfied..].
Description
.Iadd.This application is a reissue application of U.S. Pat. No.
8,088,230, which was filed as U.S. application Ser. No. 12/293,189
on Sep. 16, 2008, issued on Jan. 3, 2012, which is a U.S. national
stage application under 35 U.S.C. 371 of International Application
No. PCT/JP2007/060073 filed on May 16, 2007 in the Japan Patent
Office, and which claims priority to JP 2007-112995 filed Apr. 23,
2007, JP 2007-107250, filed Apr. 16, 2007, JP 2006-150375, filed
May 30, 2006, JP 2006-148497, filed May 29, 2006, and JP
2006-140111, filed May 19, 2006, the disclosures of each of which
are hereby incorporated by reference in their
entirety..Iaddend.
TECHNICAL FIELD
The present invention relates to a rolling apparatus, such as a
rolling bearing, a ball screw, a linear guide, and the like.
BACKGROUND ART
In a rolling bearing, such as a ball bearing, a cylindrical roller
bearing, a conical roller bearing, a self-aligning roller bearing,
a needle bearing, and the like, used in an environment involving
harsh lubrication conditions as in; for instance, an automobile,
construction machinery, agricultural machinery, an iron and steel
fixture, and the like, there is a high probability of: foreign
substances being introduced into lubricating oil; indentation
arising in a raceway surface as a result of biting of the foreign
substances; and early exfoliation starting from the
indentation.
A method proposed for solving the problem is to make an attempt to
elongate flaking life by subjecting inner and outer rings and
rolling elements to carburizing or carbonitriding, to thus
precipitates a predetermined amount of retained austenite and relax
the concentration of stress due to the indentation developed in the
raceway surface (see Patent Document 1).
Another proposed method to make an attempt to extend the life is to
enhance the hardness of a raceway surface by high-concentration
carburizing, thereby enhancing the strength of a material (see
Patent Document 2).
These methods are to strengthen individual components by taking the
inner and outer rings and the rolling elements as individual
components. Accordingly, when extension of life of the raceway ring
is desired, an idea for subjecting a raceway ring to predetermined
life-extending treatment is usually conceived.
Bearing steel typified by JIS SUJ2 or SUJ3 has hitherto been used
for a rolling bearing, and the steel is usually used at hardness
HRC 60 or more after having undergone quenching and tempering
treatment.
However, in an environment where a foreign substance is
contaminated to lubricant or insufficient lubrication is achieved
as result of diversification of an environment where a rolling
bearing is used, it may be the case where the bearing steel will
fail to provide sufficient life or where seizure will arise. For
these reasons, the steel is subjected to carbonitriding called
marstressing, thereby making nitride into a solid solution and
increasing the amount of retained austenite on the raceway surface.
Thus, an attempt is made to relax the stress developed along an
edge of the indentation in the foreign substances contaminated
lubrication environment or enhance seizure resistance by an effect
of nitrogen.
However, the environment where the rolling bearing is used has
recently become harsh further, and there arises a case where a
sufficient effect is not yielded by merely subjecting SUJ2 to
carbonitriding. In order to solve the problem, a material provided
with a large amount of added Si is used, and carbide or
carbonitride containing Si and Mn is precipitated in an area
percentage of 1 to 30%, thereby enhancing abrasion resistance and
seizure resistance in an environment entailing occurrence of slide
contact or an environment where lubricating oil becomes depleted.
(see Patent Document 3).
Further, it has been well known that foreign substances, such as
metal chips, shavings, burrs, dust resulting from abrasion, and the
like, which are mixed in bearing lubricating oil inflict damage on
a raceway ring or rolling elements of a rolling bearing, to thus
significantly shorten the life of the rolling baring. Accordingly,
the present inventor proposes that a content of C in the rolling
surface layer of the bearing, the amount of retained austenite, and
a content of carbonitride should be set to appropriate values even
when the rolling bearing is used in the foreign substances
contaminated lubrication environment, to thus relax the stress
concentrated at an edge of indentation caused by the foreign
substances, prevent occurrence of cracking, and extend the life of
the rolling bearing (Patent Publication 1). Patent Document 1:
JP-A-64-55423 Patent Document 2: JP-A-7-41934 Patent Document 3:
JP-A-2003-193200
DISCLOSURE OF THE INVENTION
Problem that the Invention is to Solve
As mentioned above, enhancement of durability has been variously
considered in terms of a material or surface treatment. However, a
market environment surrounding bearings has recently become severe
in conjunction with miniaturization and speedup of machinery
achieved in these days. The number of cases where a problem cannot
be resolved by only the related-art life-extending technique is
increasing.
Early exfoliation arising in a foreign substances contaminated
lubrication environment, such as Patent Publication 4, is said to
start from indentation formed as a result of biting of foreign
substances between a rolling element and a raceway ring and to be
attributable to concentration of stress induced by formation of the
indentation. However, indentation-originated flaking is not caused
by only concentration of stress but is also attributable to the
influence of tangential force acting between the rolling elements
and the raceway ring. Factors affecting tangential force include
surface roughness and a surface geometry in addition to including
sliding velocity and contact pressure. As the surface roughness
becomes smaller and the surface geometry becomes superior, the
tangential force acting between the rolling elements and the
raceway ring becomes smaller, and the life of the bearing acquired
in the foreign substances contaminated lubrication environment
becomes longer.
However, an increase in the amount of retained austenite on a
rolling surface results in a decrease in indentation resistance as
well as a decrease in surface hardness and abrasion resistance.
Therefore, when the amount of retained austenite on the rolling
surface is large, indentation becomes more likely to be formed on
the rolling surface by the influence of foreign substances or
static excessive load. The rolling surface on which indentation is
formed induces deformation or an increase in surface roughness. As
the indentation becomes larger and the number of indentations
increases, an increase in deformation and surface roughness becomes
more noticeable. Specifically, in the foreign substances
contaminated lubrication environment, as the amount of retained
austenite on the rolling surface becomes greater, indentation
becomes more likely to be formed, and therefore tangential force
acting between the rolling elements and the raceway ring becomes
greater.
When the amount of retained austenite on the rolling surface is
large in the environment of foreign substances contaminated
lubrication, even when the tangential force has become greater, the
life of a member containing a large amount of austenite is not
reduced because there is an effect of relaxing concentration of
stress yielded by the influence of the retained austenite as
described in Patent Document 1. However, since the tangential force
of the same magnitude acts on two objects contacting each other,
the life of a partner member is reduced. For instance, when the
amount of retained austenite on the rolling surface of the raceway
ring is increased, the life of the bearing is extended because of
the effect of relaxing the concentration of stress. However, on the
other hand, the life of rolling elements which are the partner
member is reduced as a result of an increase in tangential
force.
Even when the rolling elements are subjected to flaking or even
when the raceway ring is subjected to flaking, the life of the
bearing is affected, and hence the life of the rolling elements and
the life of the raceway ring must be extended in order to extend
the life of the entire bearing. Specifically, a sufficient
life-extending effect is not yielded by only a technique of
increasing the retained austenite on the rolling surface. Depending
on conditions for use of the bearing, it may also be the case where
the technique of extending the life of the bearing by increasing
the amount of retained austenite cannot be adopted. For instance,
when the bearing is used at high temperatures, the retained
austenite deteriorates the dimensional stability of the bearing.
For this reason, a small amount of austenite is desirable.
As described in Patent Document 3, under the method for forming a
nitride containing Si and Mn, an appropriate composition of a
material and an appropriate concentration of nitrogen are not
specified, and there may be a case where sufficient performance
cannot be exhibited.
The present invention has been conceived in view of the drawbacks,
such as those mentioned above, and aims at providing a rolling
bearing apparatus which enhances flaking resistance, abrasion
resistance, and seizure resistance to a much greater extent while
preventing an increase in cost and which can extend its life even
in a foreign substances contaminated lubrication environment.
Means for Solving the Problem
Through assiduous studies, the present inventors have carried out
investigations in order to find out material factors which
sufficiently ensure indentation-originated flaking life of the
material (e.g., rolling elements); which enhances indentation
resistance and abrasion resistance of the material; and which also
extends the life of a partner member (e.g., a raceway ring) by
preventing deterioration of surface roughness and surface geometry
and reducing tangential force acting between two objects (the
rolling elements and the raceway ring). Consequently, the present
inventors have found that surface hardness, residual austenite, the
surface nitrogen concentration, and an area percentage of nitride
which is precipitated on a surface and which contains Si and Mn
(hereinafter described as a "Si.Mn-based nitride") are relevant
material factors for enhancing indentation resistance and abrasion
resistance, and has achieved the present invention.
Specifically, in order to achieve the object, the present invention
provides the following rolling apparatus.
(1) A rolling apparatus including:
an external member having a raceway surface on an inner peripheral
surface thereof;
an internal member having a raceway surface on an outer peripheral
surface thereof; and
a plurality of rolling elements which are rotatably provided
between the raceway surface of the external member and the raceway
surface of the internal member, wherein
a surface of at least one of the internal member, the external
member, and the rolling elements is subjected to carbonitriding or
nitriding,
an area percentage of a nitride containing Si and Mn is 1% or more
and 20% or less,
a hardness on a surface is HV750 or more, and
when depth from the raceway surface or depth from a rolling surface
of the rolling element is defined as Z and diameter of the rolling
element is defined as d, hardness at Z=0.045 d is HV650 to 850, and
hardness at Z=0.18 d is HV400 to 800.
(2) The rolling apparatus according to (1), wherein
a concentration of nitrogen on a surface layer of at least one of
the internal element, the external element, and the rolling
elements is 0.2 mass % or more, and a number of nitride of which
size is 0.05 .mu.m to 1 .mu.m and which contains Si and Mn in an
area of 375 .mu.m.sup.2 is 100 or more.
(3) The rolling apparatus according to (2), wherein
at least one of the carbonitrided or nitrided internal member,
external member, and rolling elements is made of steel which
contains
0.3 to 1.2 mass % of C,
0.3 to 2.2 mass % of Si,
0.3 to 2.0 mass % of Mn,
0.5 to 2.0 mass % of Cr,
5 or less of Si/Mn,
remaining Fe and
inevitable impurities.
(4) The rolling apparatus according to (3), wherein
a carbonitrided or nitrided member is the rolling elements.
(5) The rolling apparatus according to (4), wherein,
when a amount of retained austenite on the raceway surfaces is
defined as .gamma.r.sub.AB and
when a amount of retained austenite on the rolling surface of the
rolling element is defined as .gamma.r.sub.C,
.gamma.r.sub.AB-15.ltoreq..gamma.r.sub.C.ltoreq..gamma.r.sub.AB+15
(0.ltoreq..gamma.r.sub.AB, .gamma.r.sub.C.ltoreq.50, and a unit is
vol. %) is satisfied.
(6) The rolling apparatus according to (5), wherein,
at least one of the internal member and the external member is made
of steel which contains:
0.15 to 1.2 mass % of C,
0.1 to 1.5 mass % of Si,
0.2 to 1.5 mass % of Mn,
0.5 to 2.0 mass % of Cr,
remaining Fe and
inevitable impurities.
(7) The rolling apparatus according to (6), wherein
at least one of the internal member and the external member is made
of high-carbon chromium bearing steel.
(8) The rolling apparatus according to (7), wherein
on the raceway surface of a raceway ring is made of high-carbon
chromium bearing steel among the internal member and the external
member, a surface layer section hardened by heat treatment
including carburizing or carbonitriding is formed,
hardness of the surface layer section is HRC58 or more and HRC66 or
less, and
hardness of an internal core of the surface layer section is HRC56
or more and HRC64 or less.
(9) The rolling apparatus according to (6), wherein
a surface nitrogen concentration on the raceway surface of the
internal member and the external member is 0.05 mass % or less.
(10) The rolling apparatus according to (9), wherein
a Si content and a Mn content in the rolling elements is 1.0 mass %
or more.
Advantage of the Invention
A rolling apparatus of the present invention enhances indentation
resistance and abrasion resistance by defining the hardness of an
internal member, the hardness of an external member, and the
hardness of rolling elements which are achieved at a surface level
and a specific depth and an area percentage of an Si.Mn-based
nitride, thereby preventing an increase in tangential force between
the rolling elements and the raceway ring during use of a bearing
and enhancing flaking resistance strength.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a deep groove ball bearing
which is an example of a rolling apparatus;
FIG. 2 is across-sectional view of a conical roller bearing which
is another example of the rolling apparatus;
FIG. 3 is a perspective view of a linear guide which is still
another example of the rolling apparatus;
FIG. 4 is a cross-sectional view of a ball screw which is yet
another example of the rolling apparatus;
FIG. 5 is a graph showing a relationship between the distribution
of static shearing force and yield pressure;
FIG. 6 is a schematic view showing the structure of an indentation
resistance test;
FIG. 7 is a schematic view showing the structure of a two-cylinder
abrasion test;
FIG. 8 is a graph showing a relationship between surface hardness
and indentation resistance;
FIG. 9 is a graph showing a relationship between surface hardness
and indentation resistance;
FIG. 10 is a graph showing the influence of the concentration of
surface nitride exerted on indentation resistance and abrasion
resistance;
FIG. 11 is a graph showing a relationship between the surface
nitrogen concentration acquired through a Charpy impact test and
absorbed energy;
FIG. 12 is a graph showing the influence of an area percentage of a
Si.Mn-based nitride exerted on indentation resistance and abrasion
resistance;
FIG. 13 is an example electron microscope photograph of the surface
of a rolling element at the time of measurement of an area
percentage of the Si.Mn-based nitride;
FIG. 14 is a graph showing a relationship between the amount of
nitride and an area percentage of the Si.Mn-based nitride;
FIG. 15 is a graph showing a relationship between an area
percentage of the Si.Mn-based nitride and indentation-originated
flaking life;
FIG. 16 is a graph showing a relationship between the area
percentage of the Si.Mn-based nitride acquired through a Charpy
impact test and absorbed energy;
FIG. 17 is a graph showing a relationship between the number of
pieces of Si.Mn-based nitride measuring 0.05 to 1 .mu.m and the
life thereof;
FIG. 18 is a graph showing a relationship between the amount of
Si+Mn and the indentation depth;
FIG. 19 is an example analysis result of components of the
Si.Mn-based nitride;
FIG. 20 is a graph showing a relationship between the amount of
retained austenite on a rolling element and the life thereof;
and
FIG. 21 is a graph showing a relationship between a ratio of a
Si/Mn and the area percentage of the Si.Mn-based nitride.
TABLE-US-00001 Descriptions of Reference Numerals 1 INNER RING 1a
RACEWAY SURFACE 2 OUTER RING 2a RACEWAY SURFACE 3 ROLLING ELEMENT
3a ROLLING SURFACE 4 CAGE 10 LINEAR GUIDE 11 GUIDE RAIL 12 SLIDER
13 BALL 22 BALL SCREW 23 BALL SCREW NUT 24 BALL 25 CIRCULATOR TUBE
26 SPACER
BEST MODES FOR IMPLEMENTING THE INVENTION
The present invention will be described in detail hereunder.
A rolling bearing can be mentioned as an example of the rolling
apparatus of the present invention. No limitations are imposed on
the type and structure of a rolling bearing, and a deep groove ball
bearing shown in FIG. 1 can be exemplified. The deep groove ball
bearing has an inner ring 1 (an internal member) having a raceway
surface 1a on an outer peripheral surface thereof; an outer ring 2
(an external member) having a raceway surface 2a on an inner
peripheral surface thereof opposing the raceway surface 1a of the
inner ring 1; balls which are rotatably provided between the
raceway surfaces 1a and 2a and which correspond to a plurality of
rolling elements 3; a cage 4 for retaining the rolling elements 3
between the inner ring 1 and the outer ring 2; and seals 5 and 5
for covering an opening of clearance between the inner ring 1 and
the outer ring 2. Lubrication between the raceway surfaces 1a and
2a and rolling surfaces 3a of the rolling elements 3 is embodied by
lubricant 6 such as grease, lubricating oil, and the like. The cage
4 and the seal 5 may be omitted.
A conical roller bearing having bearing number L44649/610, such as
that shown in FIG. 2, can be exemplified as the rolling bearing,
wherein conical rollers serving as the rolling elements 3 are
retained between the inner ring 1 and the outer ring 2 by the cage
4. Moreover, although omitted from the drawings, an angular ball
bearing, a cylindrical roller bearing, a self-aligning roller
bearing, a needle roller bearing and the like, are also
available.
A linear guide, such as that shown in FIG. 3, can be also
exemplified as the rolling apparatus. The linear guide 10 has a
guide rail 11 (an internal member), a slider 12 (an external
member) which is provided on a guide rail 11 and which is attached
to the guide rail 11 so as to be movable in an axial direction, and
a plurality of balls 13 serving as rolling elements. A rail surface
14 is formed over an upper surface of the guide rail 11, and rail
raceway surfaces 15, 16 for enabling slidable movement of the
rolling elements 13 are formed in two rows; namely, one is an upper
row and the other is a lower row, along either side surface of the
guide rail 11. A bolt hole 17 vertically penetrating through the
rail surface 14 is formed at a plurality of positions along the
axial direction. As a result of bolts being screwed into the bolt
holes 17, the guide rail 11 is secured to a machining bed. In a
slider 12, slider raceway surfaces 19 for enabling slidable
movement of the rolling elements 13 are formed in a rolling-element
circulation channel 18 which retains the rolling elements 13 in a
circulating manner.
A ball screw, such as that shown in FIG. 4, can also be exemplified
as the rolling apparatus. A ball screw 21 has a screw shaft 22 (an
internal member) having a helical thread groove 22a on an outer
peripheral surface thereof; a ball screw nut 23 having a helical
thread groove 23a on an inner peripheral surface thereof
corresponding to the thread groove 22a of the screw shaft 22 and
which is screw-engaged with the screw shaft 22 by way of a
plurality of balls 24 serving as rolling elements which are fitted
into a helical ball rolling space formed from the thread groove 22a
of the screw shaft 22 and the thread groove 23a of the ball screw
nut 23 (an external member) rotatably and which are spaced apart
from each other with a spacer 26 sandwiched therebetween; and a
circulator tube 25. The circulator tube 25 is for circulating the
balls 24 rotatably in conjunction with the spacers 26 by rotation
of either the screw shaft 22 or the ball screw nut 23, and is
attached to the ball screw nut 23.
By such a configuration, the balls 24 moving rotatably, through the
ball rolling space travel through the ball rolling space along with
the spacers 26 and are guided upwardly at one end of the circulator
tube 25 after turning around the screw shaft 22 a plurality of
times; and pass through the circulator tube 25 and return to the
ball rolling space from the other end of the circulator tube,
thereby repeating a circulation. A cross-sectional profile of the
thread grooves 22a and 23a can be selected appropriately, as
required, and may also be realized as a Gothic arch; namely, an
essentially-V-shaped form made by combination of two circular arcs
whose curvature centers are different, or a circular-arc form.
The present invention is characterized in that the hardness of a
material forming the internal members (the inner ring, the guide
rail, and the screw shaft), the hardness of the external members
(the outer ring, the slider, and the ball screw nut), and the
hardness of the rolling elements (balls, conical rollers, and
balls) of the rolling apparatus are specified.
In these rolling apparatuses, a material factor which is most
effective for enhancing indentation resistance is hardness. The
type of indentation includes foreign substance indentation
generated as a result of biting of foreign substances and Brinell
indentation which is formed as a result of rolling elements biting
into a raceway ring when excessive load acts on the rolling
apparatus, thereby flattening the rolling elements. In the case of
the foreign substance indentation, formation of indentation can be
prevented by increasing the hardness of a neighborhood of the
surface. However, in the case of the Brinell indentation, the
hardness of a core of a material as well as the hardness of a
surface thereof is important. Indentation is formed by static
shearing force (shearing force in a direction at an angle of
45.degree. with respect to the direction of rolling) arising in a
material as a result of the raceway ring contacting the rolling
elements, thereby imposing load on the rolling elements. A
phenomenon of formation of indentation is caused by plastic
deformation of the material. Hence, when yield shearing stress of
the material is equal to or greater than the static shearing
stress, indentation is not formed.
The load acting on the rolling bearing is usually designed so as to
be come equal to or less than static rated load. Therefore, it is
important for a material to have material strength which prevents
formation of indentation even when static rated load acts on the
material. In the case of a ball bearing, static rated load is
defined as load which causes contact pressure of 4200 MPa for the
case of a ball bearing, as well as being defined as load which
causes contact pressure of 4000 MPa for the case of a rolling
bearing. Indentation does not arise, so long as the static shearing
force caused when the contact pressure has acted on the bearing is
equal to or smaller than the yield shearing stress of a material of
the bearing. In the meantime, the yield shearing stress of the
material is proportional to the hardness of the material, and a
relationship of .tau.y=1/6.times.HV exists between the yield
shearing stress and Vickers hardness.
Accordingly, as shown in FIG. 5, in order to prevent formation of
Brinell indentation, it is important to set hardness in such a way
that a distribution of yield shearing stress (a distribution of
hardness) exceeds a distribution of static shearing stress at the
time of application of static rated load. In the meantime, an
excessive increase in the hardness of the core leads to a decrease
in toughness, thereby raising a problem of cracking.
A correlation exists between the depth of action of the maximum
static shearing stress (the distribution of static shearing stress)
and the diameter of a rolling element, and hence hardness is
specified as follows. Specifically, provided that a depth from a
raceway surface or a depth from a rolling surface of a rolling
element is taken as "Z" and that the diameter of the rolling
element is "d,"
the surface hardness of at least one of the inner ring, the outer
ring, and the rolling element is set to HV750 or more; preferably
to HV800 or more; and more preferably to HV820 or more;
hardness at Z=0.045 d is set to HV650 to 850, and preferably the
hardness is set to HV770 to 816; and
hardness at Z=0.18 d is set to HV400 to 800, preferably to HV700 to
771, and more preferably to HV718 to 771.
As a result, formation of Brinell indentation, which would
otherwise be caused by contacting of the raceway ring with the
rolling elements, can be prevented, and longer life of the bearing
can be achieved by reducing tangential force acting between the
raceway ring and the rolling elements. Applying these requirements
to the rolling elements is more preferable.
In particular, the surface hardness Hv of the rolling surface of
the rolling element is preferably 750 or more, more preferably 800
or more, furthermore preferably 820 or more. The most important
fact or of a material for enhancing indentation resistance and
abrasion resistance is surface hardness. In order to study the
influence of surface hardness on indentation resistance and
abrasion resistance, an indentation resistance test shown in FIG. 6
and a two-cylinder abrasion test shown in FIG. 7 were carried
out.
The indentation resistance test was conducted by a method for
pressing a steel ball having a diameter of 2 mm against a sample at
5 GPa and subsequently measuring the depth of an indentation. In
the meantime, the two-cylinder abrasion test was carried out under
a method for rotating a drive side (a high-speed side) at 10
min.sup.-1 under conditions including a contact pressure of 0.8
GPa; rotating a driven side (a low-speed side) at 7 min.sup.-1 by
reducing the speed with a gear, to thus forcefully impart slippage
to both the drive side and the driven side. A mean value of amounts
of abrasion of the drive side acquired after elapse of 20 hours
since initiation of the test and a mean value of amounts of
abrasion of the driven side acquired after elapse of 20 hours since
initiation of the test were measured.
FIG. 8 is a graph showing a relationship between surface hardness
and an indentation resistance characteristic, and FIG. 9 is a graph
showing a relationship between surface hardness and an abrasion
resistance characteristic. It is clear from the graphs that the
indentation characteristic and the abrasion resistance
characteristic become superior as the surface hardness becomes
higher. In particular, when the surface hardness is equal to or
greater than Hv750, both the indentation resistance characteristic
and the abrasion resistance characteristic are extremely superior.
Moreover, it has also been known that fatigue strength is higher as
surface hardness becomes higher, and an increase in the hardness of
the rolling surface of the rolling elements enables enhancement of
indentation-originated flaking strength as well as enhancement of
the indentation resistance characteristic and the abrasion
resistance characteristic.
In the present invention, carbonitriding is performed in order to
enrich predetermined nitrogen on the surface layer of the raceway
ring or the surface layer of the rolling elements. As in the case
of carbon, nitrogen has also the property of forming a nitride or
carbonitride to enhance the indentation resistance characteristic
and the abrasion resistance characteristic, as well as having the
property of intensifying solid-solution of martensite and stable
securing of residual austenite.
FIG. 10 shows the influence of nitrogen exerted on the indentation
resistance characteristic and the abrasion resistance
characteristic determined by an indentation resistance
characteristic test and a two-cylinder abrasion test analogous to
those mentioned above. An electron probe micro analyzer (EPMA) is
used for measuring the amount of surface nitrogen. In order to
investigate only the effect of a nitrogen concentration, hardness
and the amount of residual austenite other than the surface
nitrogen concentration were held constantly. FIG. 10 shows that the
abrasion resistance characteristic and the indentation resistance
characteristic become superior with an increase in the surface
nitrogen concentration. A noticeable effect appears when the
surface nitrogen concentration exceeds 0.2 mass %. However, the
surface nitrogen concentration is set to more preferably 0.45 mass
% or more.
In the meantime, an excessively high concentration of nitrogen
involves a drawback of a decrease in toughness and static strength.
Since toughness and static strength are required performances for
the rolling elements of the rolling bearing, an excessively-high
concentration of nitrogen is not preferable. FIG. 11 shows results
of a Charpy impact test, and it is seen that a drastic drop arises
in toughness when the concentration of nitrogen exceeds 2.0 mass %.
Accordingly, the upper limit of the nitrogen concentration in the
present invention is set to 2.0 mass %.
As mentioned above, it has become clear that the indentation
resistance characteristic and the abrasion resistance
characteristic of the material are enhanced as the surface nitrogen
concentration increases. However, the present inventors also found
that, even in the case of the same nitrogen concentration, the
indentation resistance characteristic and the abrasion resistance
characteristic change according to the state of presence of
nitrogen in the material. Nitrogen is present in two cases; namely,
a case where nitrogen is present in the form of a solid solution
within a material and a case where nitrogen is precipitated as a
nitride. Although detailed numerals will be described later, a
nitrogen content included in a material in the form of a solid
solution becomes greater, even at the same concentration of
nitrogen, than a nitrogen content in an Si.Mn-based nitride
precipitated on the surface of the material when a material
containing large amounts of Si and Mn is carbonitrided.
FIG. 12 shows area-percentage influence of the Si.Mn-based nitride
exerted on the indentation resistance characteristic and the
abrasion resistance characteristic determined by the indentation
resistance characteristic test and the two-cylinder abrasion test
analogous to those mentioned above. In order to investigate only
the effect of the Si.Mn-based nitride, hardness, the amount of
residual austenite, and the concentration of nitrogen other than
the area percentage of the Si.Mn-based nitride are made constant.
In relation to measurement of area percentage of the Si.Mn-based
nitride, the rolling surface is observed at an accelerated voltage
of 10 kV by use of a field emission scanning electron microscope
(FE-SEM). After photographs of at least three visual fields (see
FIG. 13) have been captured at a 5000 magnification, the
photographs are binarized, and an area percentage is computed by
use of an image analyzer. As shown in FIG. 12, the abrasion
resistance characteristic and the indentation resistance
characteristic are superior as the area percentage of the
Si.Mn-based nitride increases. When the area percentage of the
Si.Mn-based nitride exceeds 1%, an effect noticeably appears.
However, an area percentage of 2% or more is more preferable.
In order to investigate the influence of the area percentage of the
Si.Mn-based nitride on indentation-originated flaking life, a test
was conducted by a thrust life test on condition that lubrication
is contaminated with foreign substances. Table 1 shows components
of materials used in the test; steel type 1 is a material
corresponding to JIS SUJ3; and steel type 2 is a material
corresponding to JIS SUJ2. The materials of Table 1 were machined
into a disk of which diameter is 65 mm and thickness is 6 mm by
turning process. After being subjected to carbonitriding in a gas
mixture of an RX gas, a propane gas, and ammonium at 820 to
900.degree. C. for 2 hours to 10 hours, the disk was subjected to
oil hardening and subsequently to tempering at 160 to 270.degree.
C. for 2 hours. Specimens having various nitrogen concentrations
were formed by changing a processing temperature, a processing
time, and the flow rate of an ammonium gas. After heat treatment,
the surfaces of the specimens were mirror-finished by polishing and
lapping.
TABLE-US-00002 TABLE 1 C Si Mn Cr Present invention Steel type 1
1.01 0.56 1.10 1.10 Comparative Steel type 2 0.99 0.25 0.40 1.49
example
Test conditions are as follows: Test load: 5880N (600 kgf) Number
of revolutions: 1000 min.sup.-1 Lubricating oil: VG68 Hardness of
foreign substance; HV870 Size of foreign substance: 74 to 147 .mu.m
Amount of contaminated foreign substance: 200 PPM
Table 2 shows a relationship among the concentration of nitrogen,
an area percentage of the Si.Mn-based nitride, and life achieved
after contamination. Results of the life test are provided as
ratios on condition that life L10 of a comparative example 1 is
taken as 1.
TABLE-US-00003 TABLE 2 nitrogen area concentration percentage life
ratio present steel type 1 1 0.20 1.12 2.10 invention 2 0.30 1.56
2.20 3 0.42 2.30 3.10 4 0.51 3.51 3.00 5 0.64 3.65 3.20 6 0.89 4.99
3.40 7 1.00 5.77 3.40 8 1.44 7.55 3.50 9 1.78 9.99 3.40 10 2.10
11.00 3.20 comparative steel type 2 1 0.21 0.59 1.00 example 2 0.28
0.77 1.10 3 0.35 0.98 1.10 comparative steel type 1 4 2.10 11.00
3.20 example 5 0.18 0.90 1.20
FIG. 14 shows a relationship between the nitrogen concentrations of
the steel types 1 and 2 and the area percentage of the Si.Mn-based
nitride, and FIG. 15 shows a relation between the area percentage
of the Si.Mn-based nitride and indentation-originated flaking life.
The amount of the Si.Mn-based nitride precipitated is understood to
increase in proportion to the concentration of nitrogen. Moreover,
it is also seen that, when comparison is performed at the same
nitrogen amount, larger amounts of precipitated Si.Mn and longer
life are achieved by the steel doped with larger amounts of Si and
Mn. As in the case of the indentation resistance characteristic and
the abrasion resistance characteristic, life is considerably
elongated when the area percentage of the Si.Mn-based nitride comes
to 1 percent or more and when the nitrogen amount comes to 0.2 mass
%.
In the meantime, when the amount of precipitated Si.Mn-based
nitride becomes excessively large, there arises a drawback of a
decrease in toughness and static strength, as in the case of the
nitrogen concentration. Since toughness and static strength are
required performances for the rolling elements of the rolling
bearing, an excessively-large amount of precipitated Si.Mn-based
nitride is not preferable. FIG. 16 shows results of a Charpy impact
test, and it is seen that a drastic drop arises in toughness when
the area percentage of the Si.Mn-based nitride exceeds 20%.
Accordingly, the upper limit of the area percentage of the
Si.Mn-based nitride in the present invention is 20%, more
preferably 10%.
Nitride whose size exceeds 1 .mu.m does not much contribute to
strength of the material. The material is strengthened when fine
particles of nitride are dispersed. The reason for this is that,
since a precipitation having a smaller particle-to-particle
distance exhibits superior strengthening capability according to a
theory of strengthening precipitation, the particle-to-particle
distance becomes relatively shortened and intensified even at the
same area percentage of the Si.Mn-based nitride when the number of
precipitated particles is large. Specifically, it is better to use
steel having a large Si content and a large Mn content and to
increase the number of fine nitride particles having a mean
particle size of 0.05 .mu.m to 1 .mu.m within the area percentage
of the Si.Mn-based nitride ranging from 1 to 20%. A ratio of the
Si.Mn-based nitride particles measuring 0.05 to 0.50 .mu.m to the
Si.Mn-based nitride having a particle size of 0.05 .mu.m or more is
set to 20% or more in terms of the number of particles, thereby
enabling further intensification.
Specifically, the Si.Mn-based nitride measuring 0.05 .mu.m to 1
.mu.m is preferably 100 or more within an area of 375 .mu.m.sup.2.
A technique for achieving this state is preferably to set a
carbonitriding temperature within a range from 800.degree. C. to
870.degree. C. When the temperature is exceeded, the nitride
becomes bulky, so that the number of fine Si.Mn-based nitride
particles is decreased. Moreover, when the temperature exceeds the
processing temperature, the solubility limit of nitrogen becomes
greater, and hence the amount of nitride becomes smaller, whereby
there may arise a case where a desired area percentage cannot be
obtained. From the beginning of the carbonitriding process, it is
better to take a mixed gas atmosphere consisting of the RX gas, an
enriched gas, and an ammonium gas; to set the CP value to 1.2 or
more; and to set the flow rate of an ammonium gas to at least 1/5
or more of the flow rate of the RX gas. It is desirable that
hardening should be performed at an oil temperature from 60 to
120.degree. C. after carbonitriding. When the temperature is higher
than the temperature range, it may be the case where sufficient
hardness is not obtained. Tempering is performed at a temperature
from 160 to 270.degree. C., and the range of surface hardness is
set to Hv 740 or more, desirably to Hv780 or more. When necessary,
sub-zero treating may also be performed after hardening.
Table 3 shows a relationship between an area percentage of the
Si.Mn-based nitride and the number and life ratio of the
Si.Mn-based nitride measuring 0.05 .mu.m to 1 .mu.m, and FIG. 17
shows, in the form of a graph, a relationship between an area
percentage of the Si.Mn-based nitride and the number and life ratio
of the Si.Mn-based nitride measuring 0.05 .mu.m to 1 .mu.m. As is
obvious from the drawing and the table, a base structure is
strengthened as a result of lob Si.Mn-based nitride particles or
more being dispersed over a measured area of 375 .mu.m.sup.2, and
longer life is achieved despite of contaminated lubrication.
TABLE-US-00004 TABLE 3 area percentage of number of Si.cndot.Mn
Si.cndot.Mn based nitride (%) nitride of 0.05-1 .mu.m life ratio 1
2.05 63 1 2 2.12 94 1.2 3 2.98 105 1.7 4 2.76 123 2.5 5 3.11 145
2.7 6 2.56 162 3 7 2.39 173 3.1
It is desirable that the rolling elements should contain elements
provided below.
[C: 0.3 to 1.2 Mass %]
Carbon is an element which is important for ensuring required
strength and life of steel. When the amount of carbon is too small,
sufficient strength is not attained, and a heat treatment time
required to achieve the depth of a quench-hardened layer required
at the time of carbonitriding, which will be described later,
becomes longer, which in turn results in an increase in the cost of
heat treatment. Therefore, a carbon content is set to 0.3 mass % or
more, preferably 0.5 mass % or more. In order to achieve hardness
of Z=0.18 d, preferably hardness of Z>0.06 d, a carbon content
is preferably 0.95 mass % or more. Moreover, when the carbon
content is too great, macro carbides are generated at the time of
manufacture of steel, which in turn adversely affects a
characteristic of subsequent quenching or a rolling fatigue.
Moreover, a header characteristic may be decreased, to thus incur
an increase in cost. Therefore, the upper limit of the carbon
content is set to 1.2 mass %, preferably 1.10 mass %.
[Si: 0.3 to 2.2 Mass %, Mn: 0.2 to 2.0 Mass %]
As mentioned previously, in order to sufficiently precipitate the
Si.Mn-based nitride, a steel material having a high Si content and
a high Mn content must be used. SUJ2 which is a common bearing
material has 0.25% of Si content and 0.4% of Mn content. Even when
nitrogen is excessively added to the material by carbonitriding,
Si.Mn-based nitride content is low. Therefore, in relation to the
Si content and the Mn content, the following values are taken as
critical values.
[Si Content: 0.3 to 2.2 Mass %]
Si is an element which is necessary to precipitate an Si.Mn-based
nitride. By presence of Si; namely, by addition of 0.3 mass % or
more of Si, Si effectively reacts with Mn, to thus become
noticeably precipitated. The Si content is set preferably to 0.4 to
0.7 mass %.
[Mn Content: 0.3 to 2.0 Mass %]
Mn is an element necessary to precipitate an Si.Mn-based nitride.
By coexistence with Si; namely, addition of 0.3 mass % or more of
Mn, Mn exhibits the property of promoting precipitation of an
Si.Mn-based nitride. Moreover, Mn has the property of stabilizing
austenite. Hence, the Mn content is set to 2.0 mass % or less in
order to prevent occurrence of a problem of the amount of austenite
still remaining after hardening heat treatment increasing more than
necessary. The Mn content is preferably set to 0.9 to 1.15 mass %.
More preferably, an Si/Mn ratio is set to a value of five or less
for reasons provided below.
Contrary to the nitride stemming from tempering, the Si.Mn-based
nitride is formed as a result of nitrogen intruded during
carbonitriding process reacting with Si while taking in Mn within a
domain of austenite. Accordingly, when an additive amount of Mn is
smaller than an additive amount of Si, precipitation of the
Si.Mn-based nitride is not promoted even when nitrogen is
sufficiently diffused. When 0.2 mass % or more of nitrogen is
caused to intrude within the range of the previously-described Si
and Mn additive amount, the amount of precipitation of Si.Mn-based
nitride having an area percentage of 1.0% or more-which is
effective for extension of life and enhancement of the abrasion
resistance characteristic and the seizure resistance
characteristic-can be ensured by setting the Si/Mn ratio to a value
of 5 or less.
[Cr: 0.5 to 2.0 Mass %]
Cr is an element which enhances hardenability and which is used for
forming a carbide; promotes precipitation of a carbide which
strengthens a material; and further miniaturizes the precipitate
further. When the Cr content is less than 0.5 mass %, hardenability
is deteriorated, to thus fail to achieve sufficient hardness or
make a carbide bulky during carbonitriding process. When the Cr
content exceeds 2.0 mass %, a Cr oxide film is formed over the
surface of the material during carbonitriding, thereby hindering
diffusion of carbon and nitrogen. For these reasons, the Cr content
is preferably set to 0.5 mass % to 2.0 mass %; more preferably 0.9
mass % to 1.2 mass %.
At least one type of element selected from Mo, Vi, and V may also
be added, as necessary.
[Mo: 0.2 to 1.2 Mass %]
Mo is an element which enhances hardenability and which is used for
forming a carbonitride, and has the property of promoting
precipitation of a carbide, a carbonitride, and a nitride which
strengthens a material and miniaturizes the precipitates further.
An effect of Mo becomes noticeable when 0.2 mass % or more of Mo is
added. When the Mo content exceeds 1.2 mass %, the effect becomes
saturated, and cost increases. Therefore, the Mo content is
preferably set to 0.2 mass % to 1.2 mass %.
[Ni: 0.5 to 3.0 Mass %]
Ni has the property of enhancing toughness as well as
hardenability, and an effect of Ni becomes noticeable when 0.5 mass
% or more of Ni is added. Ni is an element which stabilizes
austenite. When 3.0 mass % or more of Ni is added, residual
austenite becomes excessive, and the hardness of a core decreases.
Accordingly, the Ni content is preferably set to 0.5 mass % to 3.0
mass %.
[V: 0.5 to 1.5 Mass %]
V has the property of forming a hard carbide or carbonitride by
carbonitriding, thereby enhancing the abrasion resistance
characteristic. This effect becomes noticeable when 0.5 mass % or
more of V is added. When 1.5 mass % or more of V is excessively
added, V combines with solid-solution carbon of the material to
form a carbide, thereby decreasing hardness of the material.
Accordingly, the V content is preferably set to 0.5 mass % to 1.5
mass %.
In the present invention, when the amount of retained austenite on
the raceway surfaces of the inner and outer rings is taken as
.gamma.r.sub.AB and when the amount of retained austenite on the
rolling surface of the rolling elements is taken as .gamma.r.sub.C,
setting of
.gamma.r.sub.AB-15.ltoreq..gamma.r.sub.C.ltoreq..gamma.r.sub.AB+15
(0.ltoreq..gamma.r.sub.AB, .gamma.r.sub.C.ltoreq.50) is preferable.
A unit of the amount of retained austenite is vol. %.
As mentioned previously, as the amount of residual austenite
becomes smaller, the indentation resistance characteristic and the
abrasion resistance characteristic are enhanced. In the meantime,
it has become evident that, as the amount of retained austenite on
the surface becomes greater, the flaking life is extended.
Specifically, when consideration is given primarily to rolling
elements, the indentation resistance characteristic and the
abrasion resistance characteristic of the rolling elements are
enhanced with a decrease in the amount of austenite on the surface
of the rolling elements. Although the life of the raceway ring is
extended, the life of the rolling elements decreases. Accordingly,
although austenite is present on the rolling elements in amount
optimum for rendering the life of the bearing longest, the optimum
range of residual austenite varies according to the amount of
retained austenite on the raceway ring. When the amount of retained
austenite on the raceway ring is large, the life of the raceway
ring becomes longer, and the indentation resistance characteristic
of the raceway ring decreases. Tangential force acting between the
raceway ring and the rolling elements also becomes greater. Hence,
as compared to enhancement of the indentation resistance
characteristic and the abrasion resistance characteristic of the
raceway ring, extension of the life of the rolling elements becomes
required more.
Therefore, when the amount of retained austenite on the raceway
ring is large, the amount of retained austenite on the rolling
elements must also be increased. Specifically, the range of the
amount (.gamma.r.sub.C) of retained austenite on the rolling
elements for achieving the longer life of the bearing varies
according to the amount (.gamma.r.sub.AB) of retained austenite on
the raceway ring, and hence setting of
.gamma.r.sub.AB-15.ltoreq..gamma.r.sub.C.ltoreq..gamma.r.sub.AB+15
(0.ltoreq..gamma.r.sub.AB, .gamma.r.sub.C.ltoreq.50) is preferable.
When the amount of residual austenite is too large, the hardness is
reduced, thereby deteriorating the indentation resistance
characteristic and the abrasion resistance characteristic as well
as dimensional stability for the case where the bearing is used at
high temperatures. Therefore, the upper limit for the amount of
residual austenite is set to 50 vol. %.
At least one of the inner and outer rings is preferably formed from
high-carbon chromium bearing steel; for instance, SUJ2 or SUJ3
specified by Japanese Industrial Standard JIS G4805. Since the
quality of the high-carbon chromium bearing steel, including an
index of cleanliness of steel, is considerably stable, the raceway
ring formed from the high-carbon chromium bearing steel is less
vulnerable to internally-originated flaking which originates from
inclusions and the like, and the sufficient life of the rolling
bearing can be ensured. Moreover, since the material is the
high-carbon chromium steel, the hardness of the raceway ring from
its surface to core can be made high by appropriately quenching and
tempering the steel. In the present invention, the quality of the
high-carbon chromium bearing steel is preferably of a level
(bearing quality) satisfying a cleanliness regulation stipulated by
Japanese industrial Standard JIS G4805. In consideration of a
balance between the life and cost of the overall bearing, or the
like, use of SUJ2 is preferable because of superior ease of
machining achieved when the steel is taken as a raw material, ease
of machining achieved after heat treatment of the steel, low cost
of the raw material, and the like.
Moreover, it is preferable to form a hardened surface layer section
on the raceway surface of the raceway ring by subjecting the
raceway ring to heat treatment including carburizing and
carbonitriding. Specifically, the hardness of the surface layer
section formed on the raceway surface is preferably HRC58 or more,
and the hardness of an inner core of the surface layer section is
preferably HRC56 or more. Further, both the hardness of the surface
layer section and the hardness of the core are preferably HRC60 or
more. However, when the hardness is excessively large, toughness
decreases to raise the fear of occurrence of cracking. Accordingly,
the hardness of the surface layer section is preferably HRC66 or
less and more preferably HRC64 or less. Moreover, the hardness of
the core is preferably HCR64 or less. The surface layer section
used herein designates an area extending from a surface to a depth
of 200 .mu.m.
As mentioned previously, it has become evident that, as the surface
nitrogen concentration becomes higher, the indentation resistance
characteristic and the abrasion resistance characteristic of the
material are enhanced. However, the present inventors have further
found that, even when the concentration of nitrogen is of the same
level, the indentation resistance characteristic changes according
to the status of presence of nitrogen in the material. Nitrogen is
present in the form of a nitride as well as in the form of a
precipitated nitride. When a material containing large amounts of
Si and Mn is subjected to nitriding or carbonitriding, nitrogen
precipitated on the surface in the form of an Si.Mn-based nitride
becomes greater in amount, even at the same concentration of
nitrogen, than nitrogen which is present in the form of a solid
solution in a material. Therefore, the indentation resistance
characteristic is enhanced as a result of an increase in the
amounts of Si and Mn in a raw material; especially, when Si and Mn
are of 1.0 mass % or more, the indentation resistance
characteristic is enhanced conspicuously. The reason for this is
that the indentation resistance characteristic is enhanced further
even at the same level of the nitrogen concentration when Si and Mn
are present in the form of an Si.Mn-based nitride having higher
hardness rather than when nitrogen is present in the form of a
solid solution in a base structure.
FIG. 18 shows results of the indentation resistance characteristic
tests analogous to that mentioned above which were performed while
quantities of Si and Mn in samples were changed. The nitrogen
concentration is essentially constant at about 0.3 mass %. As
illustrated, the indentation resistance characteristic is enhanced
with an increase in the quantities of Si and Mn in a raw material.
When the amounts of Si and Mn exceed 1.0 mass %, the indentation
resistance characteristic is enhanced noticeably. Therefore, in
order to make a raw material less vulnerable to indentation, it is
better to set the amounts of Si and Mn in a raw material to 1.0
mass % or more.
FIG. 19 shows example results of analysis of components of the
Si.Mn-based nitride.
EXAMPLES
Although the present invention will be further described hereunder
by examples and comparative examples, the present invention is not
limited to these examples.
(First Test)
A life test was carried out in lubrication contaminated with
foreign substances by using a conical roller bearing L44649/610
(the diameter of a rolling element d=5.44 mm) as a test bearing
after excessive contact pressure of 4000 MPa was exerted on the
bearing once. Test conditions are as follows: Test load: Fr=12 kN,
Fa=3.5 kN Number of revolutions: 3000 min.sup.-1 Lubricating oil:
VG68 Hardness of foreign substance: HV870 Size of foreign
substance: 74 to 134 .mu.m Amount of contaminated foreign
substance: 0.1 g
High-carbon chromium bearing steel (SUJ2) was used for inner and
outer rings of the test bearing, and the bearing was subjected to
carbonitriding for 1 to 3 hours at 830 to 850.degree. C. in an
atmosphere consisting of an RX gas, an enriched gas, and an
ammonium gas. Subsequently, the bearing was subjected to tempering
at 180 to 240.degree. C., whereby three types of bearings: one type
of bearing including about 10 vol. % of retained austenite on the
raceway surfaces of the inner and outer rings; another type of
bearing including about 20 vol. % of retained austenite on the
raceway surfaces of the inner and outer rings; and the other type
of bearing including about 30 vol. % of retained austenite on the
raceway surfaces of the inner and outer rings.
Materials including contents (remainders include iron and
inevitable impurities) and surface properties shown in Table 4 were
used for the rolling elements. First, a wire containing the
components shown in the table was formed through header machining
and rough grinding; was subjected to carbonitriding quenching (at
830.degree. C. for 5 to 20 hours in an atmosphere consisting of the
RX gas, the enriched gas, and the ammonium gas); and was subjected
to tempering heat treatment at 180 to 270.degree. C. and processing
pertaining to post-processes. The electron probe micro analyzer
(EPMA) was used for measuring the nitrogen amount in the surface of
the rolling elements, to thus have performed quantitative analysis.
Further, the amount of retained austenite on the surface layer was
measured by X-ray diffraction. In either case, the surface of the
rolling elements was directly subjected to analysis and
measurement. In relation to measurement of area percentage of the
Si.Mn-based nitride, the rolling surface was observed at an
accelerated voltage of 10 kV by use of the field emission scanning
electron microscope (FE-SEM). After capture of photographs of at
least three visual fields (see FIG. 13) at a 5000 magnification,
the photographs were binarized, and an area percentage was computed
by use of the image analyzer. A value of 0.045 d and a value of
0.18 d were measured in relation to the surface hardness of the
rolling elements.
Table 4 shows results of life tests of the respective bearings of
the examples and the comparative examples. The life tests were
performed twelve times for each of the test bearings, thereby
studying a life time lasting until occurrence of flaking, preparing
a Weibull plot, and determining life L10 from a result of the
Weibull distribution. The thus-determined life is taken as a life
value. Life is provided in the form of a value of a ratio on
condition that the life of a first comparative example having the
shortest is taken as one.
FIG. 20 shows a relationship between the amount of retained
austenite on the rolling surface of the rolling elements and a life
ratio, which is acquired when the residual austenite on the raceway
surface of the raceway ring is 10, 20, and 30 vol. %. As the amount
of retained austenite on the raceway surface of the raceway ring
becomes greater, the bearing tends to exhibit longer life. However,
the life is dependent on the amount of retained austenite on the
raceway surface of the rolling elements. As a result of the amount
of residual austenite on the rolling elements being defined so as
to fall within the range of the present invention, the entire
bearing achieves longer life. When the amount of residual austenite
on the rolling elements is less than the range of the present
invention, all of the rolling elements become broken. When the
amount of residual austenite is greater than the range of the
present invention, all of the raceway rings become broken. It is
understood that, by controlling the amount of austenite within the
range of the present invention, the life of the rolling elements
and the life of the raceway ring are extended in a balanced manner
and that the life of the entire bearing can be extended.
As described in Patent Document 1, a result showing that an
increase in the amount of residual austenite leads to extension of
life in the environment of lubrication contaminated with foreign
substances is also acquired even in this test result. However, a
mere increase in the amount of residual austenite is not
sufficient, and life can be extended by defining the amount of
residual austenite on the partner member as described in connection
with the present example. Moreover, even when life cannot be
extended by increasing the amount of residual austenite for reasons
of costs or operating conditions, life can be extended by defining
a range where life is extended effectively.
TABLE-US-00005 TABLE 4 Amount of Number retained Amount of Nitrogen
Area of austenite retained Surface concen- percentage Si.cndot.Mn
on austenite hardness Hardness tration on of nitride based raceway
on rolling of rolling HV at Hardness surface of on surface nitride
surface of surface roll- Element Z = HV at rolling of rolling in
raceway of rolling ing Si/ HV 0.045 d Z = 0.18 d element element
375 ring element life C Si Mn Cr Mn (kgf/mm.sup.2) (kgf/mm.sup.2)
(kgf/mm.sup.2) mass % (%) .mu.m.sup.2 (vol. %) (vol. %) ratio
example 1 1.01 0.56 1.10 1.10 0.51 795 777 734 0.37 2.02 140 10 10
2.5 example 2 0.30 0.60 1.00 1.10 0.60 769 653 405 0.40 2.35 139 10
10 2.1 example 3 1.20 0.50 0.90 1.00 0.56 790 779 752 0.40 2.51 141
10 10 2.5 example 4 1.01 0.30 0.70 1.00 0.43 786 765 733 0.40 1.80
138 10 10 2.2 example 5 0.99 2.20 1.00 0.93 2.20 810 772 726 0.45
2.95 145 10 10 2.6 example 6 0.98 0.50 0.30 0.95 1.67 780 761 735
0.47 1.92 143 10 10 2.1 example 7 1.03 1.00 2.00 0.92 0.50 822 776
736 0.44 3.21 149 10 10 2.5 example 8 1.10 0.40 1.15 1.20 0.35 824
754 747 0.42 3.46 155 10 10 2.6 example 9 0.89 0.70 0.90 0.90 0.78
826 738 693 0.46 3.66 166 10 10 2.7 example 10 1.06 0.40 0.90 1.00
0.44 818 782 731 0.43 3.20 162 10 10 2.6 example 11 1.07 0.40 1.15
1.10 0.35 830 776 740 0.40 3.54 168 10 10 2.7 example 12 1.00 0.50
0.99 0.50 0.51 750 742 733 0.20 1.04 105 10 10 2.0 example 13 1.01
0.56 1.10 1.10 0.51 830 789 735 0.50 5.10 180 10 10 2.9 example 14
1.01 0.56 1.10 1.10 0.51 844 795 732 1.12 9.82 192 10 10 3.0
example 15 1.10 2.00 1.80 2.00 1.11 853 797 739 1.49 15.46 231 10
10 3.1 example 16 1.10 2.00 1.80 2.00 1.11 859 806 737 1.95 19.60
256 10 10 3.1 example 17 1.01 0.56 1.10 1.10 0.51 821 765 705 0.45
3.57 154 10 0 2.5 example 18 1.01 0.56 1.10 1.10 0.51 826 773 718
0.45 3.57 154 10 5 2.7 example 19 1.01 0.56 1.10 1.10 0.51 832 787
730 0.45 3.57 154 10 10 2.8 example 20 1.01 0.56 1.10 1.10 0.51 838
792 744 0.45 3.57 154 10 15 2.7 example 21 1.01 0.56 1.10 1.10 0.51
845 799 753 0.45 3.57 154 10 20 2.6 example 22 1.01 0.56 1.10 1.10
0.51 847 805 761 0.45 3.57 154 10 25 2.4 example 23 1.01 0.56 1.10
1.10 0.51 849 804 764 0.45 3.57 154 10 30 1.8 example 24 1.01 0.56
1.10 1.10 0.51 848 810 767 0.45 3.57 154 10 35 1.7 example 25 1.01
0.56 1.10 1.10 0.51 821 765 705 0.45 3.57 154 20 0 2.6 example 26
1.01 0.56 1.10 1.10 0.51 826 773 718 0.45 3.57 154 20 5 4.3 example
27 1.01 0.56 1.10 1.10 0.51 832 787 730 0.45 3.57 154 20 10 4.4
example 28 1.01 0.56 1.10 1.10 0.51 845 799 753 0.45 3.57 154 20 20
4.5 example 29 1.01 0.56 1.10 1.10 0.51 849 804 764 0.45 3.57 154
20 30 4.3 example 30 1.01 0.56 1.10 1.10 0.51 848 810 767 0.45 3.57
154 20 35 4.1 example 31 1.01 0.56 1.10 1.10 0.51 851 816 769 0.45
3.57 154 20 40 2.5 example 32 1.01 0.56 1.10 1.10 0.51 855 815 771
0.45 3.57 154 20 45 2.4 example 33 1.01 0.56 1.10 1.10 0.51 826 773
718 0.45 3.57 154 30 5 3.0 example 34 1.01 0.56 1.10 1.10 0.51 832
787 730 0.45 3.57 154 30 10 3.2 example 35 1.01 0.56 1.10 1.10 0.51
838 792 744 0.45 3.57 154 30 15 5.6 example 36 1.01 0.56 1.10 1.10
0.51 845 799 753 0.45 3.57 154 30 20 5.9 example 37 1.01 0.56 1.10
1.10 0.51 849 804 764 0.45 3.57 154 30 30 6.1 example 38 1.01 0.56
1.10 1.10 0.51 851 816 769 0.45 3.57 154 30 40 5.8 example 39 1.01
0.56 1.10 1.10 0.51 855 815 771 0.45 3.57 154 30 45 5.7 example 40
1.01 0.56 1.10 1.10 0.51 853 813 770 0.45 3.57 154 30 50 3.2
example 41 1.01 0.56 1.10 1.10 0.51 848 815 768 0.45 3.57 154 30 55
2.9 comparative 0.28 0.60 1.00 1.10 0.60 731 624 398 0.42 4.23 140
10 10 1.0 example 1 comparative 0.99 0.25 0.40 1.49 0.63 777 761
744 0.21 0.59 85 10 10 1.1 example 2 comparative 1.00 0.40 0.25
1.50 1.60 779 748 728 0.30 0.65 97 10 10 1.3 example 3 comparative
1.01 0.56 1.10 1.10 0.51 780 759 738 0.18 0.90 45 10 10 1.1 example
4 comparative 0.99 2.00 0.30 1.20 6.67 772 761 747 0.42 0.78 98 10
10 1.2 example 5
(Second Test)
Various steels were subjected to carbonitriding in a gas mixture
consisting of the RX gas, the propane gas, the ammonium gas at 820
to 870.degree. C. for 2 to 10 hours; oil hardening; and tempering
at 160 to 270.degree. C. for 2 hours. At that time, steels of
examples 42-54 and steels of comparative examples 6-16 shown in
Table 5 were manufactured by changing the heat treatment time, the
heat treatment temperature, and the flow rate of the ammonium gas.
Rolling elements for a JIS6206 deep groove ball bearing were
manufactured from the steels, and raceway rings were also
manufactured from SUJ2. The life test was conducted under the
following conditions. Test load: 6223N (635 kgf) Number of
revolutions: 3000 min.sup.-1 Lubricating oil: VG68 Hardness of
foreign substance: Hv590 Size of foreign substance: 74 to 147 .mu.m
Amount of contaminated foreign substance: 200 ppm
Table 5 shows, in relation to each of the steels, chemical
components, Si/Mn ratio, the concentration of nitrogen, an area
percentage of Si.Mn-based nitride, and the number and life of
Si.Mn-based nitride of 0.05 .mu.m to 1 .mu.m. Life is shown as a
ratio on condition that the life L10 of a comparative example 6
(corresponding to SUJ2) is taken as 1.
TABLE-US-00006 TABLE 5 area chemical composition ratio nitrogen
percentage number of rolling (mass %) of concentration of nitride
of life C Si Mn Cr Si/Mn (mass %) nitride (%) 0.05-1 .mu.m ratio
remark example 42 1.01 0.56 1.10 1.10 0.51 0.35 2.04 138 2.0
central value of SUJ3 example 43 1.01 0.30 0.70 1.00 0.43 0.38 2.21
120 2.2 Si lower limit example 44 0.99 2.20 1.00 0.93 2.20 0.45
2.61 150 2.3 Si upper limit example 45 0.98 0.50 0.30 0.95 1.67
0.53 3.06 167 2.5 Mn lower limit example 46 1.03 1.00 2.00 0.92
0.50 0.61 3.52 153 2.5 Mn upper limit example 47 1.00 0.50 0.99
0.50 0.51 0.20 1.04 102 2.6 N % area percentage lower limit example
48 1.10 1.98 0.89 2.00 2.22 1.90 9.55 187 3.1 N % area percentage
lower limit example 49 0.90 1.45 0.71 1.05 2.04 1.01 5.80 180 2.9
example 50 1.20 0.58 0.78 1.02 0.74 0.55 3.18 168 2.9 example 51
1.10 0.40 1.15 1.20 0.35 0.60 3.46 171 2.9 SUJ3 Si lower limit
example 52 0.89 0.70 0.90 0.90 0.78 0.48 2.78 145 2.5 SUJ3 Si upper
limit example 53 1.06 0.40 0.90 1.00 0.44 0.33 1.92 143 2.6 SUJ3 Mn
lower limit example 54 1.07 0.40 1.15 1.10 0.35 0.40 2.21 126 2.3
SUJ3 Mn upper limit comparative 0.99 0.25 0.40 1.49 0.63 0.21 0.59
98 1.0 outlier Si (SUJ2) example 6 comparative 1.00 0.40 0.25 1.50
1.60 0.30 0.65 74 1.3 outlier Mn example 7 comparative 0.99 2.00
0.30 1.20 6.67 0.42 0.78 53 1.2 outlier ratio example 8 comparative
0.98 2.00 0.40 1.00 5.00 0.46 0.80 33 1.1 outlier ratio example 9
comparative 1.01 0.56 1.10 1.10 0.51 0.18 0.90 41 1.1 outlier
nitrogen example 10 comparative l.00 0.80 0.30 1.45 2.67 0.43 2.45
157 2.3 example 11 comparative 1.10 1.05 0.30 1.11 3.50 0.45 2.56
143 2.5 example 12 comparative 0.90 1.00 1.20 0.90 0.83 0.50 2.94
129 2.5 example 13 comparative 1.20 1.21 0.31 1.51 3.90 0.34 2.02
110 2.6 example 14 comparative 1.10 1.20 0.40 2.00 3.00 0.38 2.25
177 2.7 example 15 comparative 0.89 1.73 0.41 1.00 4.22 0.50 2.84
151 2.6 example 16
As is clear from Table 5, in the case of the embodiments where the
steels falling within the range of the present invention are used
and where the number of the Si.Mn-based nitride particles having a
nitrogen concentration of 0.2 to 2.0 mass %, an Si.Mn-based nitride
area percentage of 1 to 10%, and a size of 0.05 to 1 .mu.m is 100
or more, a life-extending effect is greater than that found in the
comparative examples. FIG. 21 shows, in the form of a graph, a
relationship between the Si/Mn ratio and the area percentage of the
Si.Mn-based nitride in the table. Although the eighth and ninth
comparative examples use the steels falling within the range of the
present invention and also adopt the nitrogen concentration set to
0.2 mass % or more, the Mn content is smaller than the Si content,
and the amount of deposited Si.Mn-based nitride is an area
percentage of less than one percent. As is obvious from FIG. 21,
precipitation of the Si.Mn-based nitride can be promoted by setting
the Si/Mn ratio to a value of five or less.
(Third Test)
A conical roller bearing (bearing number L44649/610) was prepared.
As shown in Table 6, in the examples 67 to 100 and the comparative
examples 18, 19, the inner and outer rings were formed from
high-carbon chromium bearing steel (SUJ2) and subjected to heat
treatment including carbonitriding, and carburizing or quenching
and tempering. During carbonitriding, the rings were held at 830 to
850.degree. C. for 1 to 3 hours in an atmosphere consisting of the
RX gas, the enriched gas, and the ammonium gas. During carburizing,
the rings were held at 830 to 850.degree. C. for 1 to 3 hours in an
atmosphere consisting of the RX gas and the enriched gas. During
quenching, the rings were held at 830 to 850.degree. C. for 1 hour
in an atmosphere of the RX gas and then subjected to oil cooling.
Further, during tempering, the rings were left to cool after held
at 180 to 240.degree. C. By such a heat treatment, the amount of
retained austenite on the raceway surfaces of the inner and outer
rings is set to 10, 20 and 30 vol. %.
In the meantime, in the seventeenth comparative example, the inner
and outer rings were formed from wp case-hardened steel SCr420 and
subjected to heat treatment carburizing and tempering. During
carburizing, the rings were held at 920 to 950.degree. C. for 3 to
8 hours in an atmosphere consisting of the RX gas and the enriched
gas and then subjected to oil cooling. During tempering, the rings
were held at 180 to 240.degree. C. and subsequently left to
cool.
The steels having the compositions shown in Table 6 were used for
the rolling elements, and member having the shape of a conical
roller were manufactured from a steel wire by header machining and
rough grinding. The members were subjected to carbonitrided
quenching at 830.degree. C. for 5 to 20 hours in an atmosphere
consisting of the RX gas, the enriched gas, and the ammonium gas,
and subsequently to tempering at 180 to 270.degree. C. The members
were subjected to processing pertaining to post-processes, such as
finishing, whereby rolling elements were obtained.
In relation to the conical roller bearings mentioned above, there
were measured the surface hardness HRC (the hardness of the surface
layer section) of the raceway surface of the raceway ring, the
hardness HRC of the core of the raceway ring (core hardness), the
amount of retained austenite on the raceway surface of the raceway
ring, the surface hardness Hv (the hardness of the surface layer
section) of the rolling surface of the rolling element, the amount
of retained austenite on the rolling surface of the rolling
element, the concentration of nitrogen on the surface layer section
of the rolling element, and the amount of the Si.Mn-based nitride
deposited on the surface layer section of the rolling element (an
area percentage).
The concentration of nitrogen was measured by use of the electron
probe micro analyzer (EPMA). Further, the amount of retained
austenite on the surface layer was measured by X-ray diffraction.
In either case, the surface of the rolling elements was directly
subjected to analysis and measurement. The amount of the
Si.Mn-based nitride was measured by use of the field emission
scanning electron microscope (FE-SEM). Specifically, the rolling
surface was observed at an accelerated voltage of 10 kV;
photographs of at least three visual fields at a 5000
magnification; the photographs were binarized; and the amount of
the Si.Mn-based nitride was computed in the form of an area
percentage by use of the image analyzer. Hardness was measured by a
hardness meter. Results are provided in Table 6.
The conical roller bearings were subjected to the life test and an
excessive static load-carrying test. The life test was conducted by
rotating the conical roller bearings in the environment of
lubrication contaminated with foreign substances and under the
following conditions. A rotation time elapsing until flaking arises
in the raceway surface of the raceway ring or the rolling surface
of the rolling elements is taken as life. Twelve bearings were
tested for one type of bearing, and a Weibull plot was prepared.
Life L10 was determined from a Weibull distribution of results, and
the thus-determined life was taken as life. Table 6 shows results,
but the results are provided as relative values acquired when the
life of the eighteenth comparative example having the shortest is
taken as one. Radial load: 12 kN Axial load: 3.5 kN Rotational
speed: 3000 min.sup.-1 Lubricant: lubricating oil whose ISO
viscosity grade is ISO VG68 (contaminated with 200 ppm fine
particles having hardness of Hv870 and a particle size of 74 to 134
.mu.m)
The excessive static load-carrying test was conducted by exerting
32 kN of radial load on the conical roller bearing analogous to
that used in the life test for 30 seconds, thereby causing
permanent deformation in the raceway ring and the conical roller.
After removal of the load, the permanent deformation occurred in
the inner ring and the permanent deformation occurred in the center
of the conical roller were measured. A sum of the amounts of
permanent deformation of both the inner ring and the conical roller
was computed, and the thus-computed sum was taken as the amount of
permanent deformation in the conical roller bearing. The amount of
permanent deformation was measured by use of Form Talysurf
manufactured by Talor Hobson Ltd. Table 6 shows results, but the
results are provided as relative values acquired when the value of
the seventeenth comparative example exhibiting the greatest amount
of permanent deformation is taken as one.
TABLE-US-00007 TABLE 6 raceway ring (1) rolling element (2) amount
of contents in steel (mass %) heat surface core surface nitrogen
amount permanent C Si Mn Cr treatment hardness hardness
.gamma..sub.R .gamma..sub.R hardne- ss concentrate of nitride life
deformation example 55 1.01 0.56 1.10 1.10 carbonitride 64 62 10 10
795 0.37 2.02 5.0 - 0.29 example 56 0.30 0.60 1.00 1.10
carbonitride 64 62 10 10 769 0.40 2.35 4.2 - 0.31 example 57 1.20
0.50 0.90 1.00 carbonitride 64 62 10 10 790 0.40 2.51 5.0 - 0.32
example 58 1.01 0.30 0.70 1.00 carbonitride 64 62 10 10 786 0.40
1.80 4.4 - 0.31 example 59 0.99 2.20 1.00 0.93 carbonitride 64 62
10 10 810 0.45 2.95 5.2 - 0.28 example 60 0.98 0.50 0.30 0.95
carbonitride 64 62 10 10 780 0.47 1.92 4.2 - 0.30 example 61 1.03
1.00 2.00 0.92 carbonitride 64 62 10 10 822 0.44 3.21 5.0 - 0.28
example 62 1.10 0.40 1.15 1.20 carbonitride 64 62 10 10 824 0.42
3.46 5.2 - 0.26 example 63 0.89 0.70 0.90 0.90 carbonitride 64 62
10 10 826 0.46 3.66 5.4 - 0.26 example 64 1.06 0.40 0.90 1.00
carbonitride 64 62 10 10 818 0.43 3.20 5.2 - 0.26 example 65 1.07
0.40 1.15 1.10 carbonitride 64 62 10 10 830 0.40 3.54 5.4 - 0.25
example 66 1.00 0.50 0.99 0.50 carbonitride 64 62 10 10 750 0.20
1.04 4.0 - 0.34 example 67 1.01 0.56 1.10 1.10 carbonitride 64 62
10 10 830 0.50 5.10 5.8 - 0.22 example 68 1.01 0.56 1.10 1.10
carbonitride 64 62 10 10 844 1.12 9.82 6.0 - 0.21 example 69 1.10
2.00 1.80 2.00 carbonitride 64 62 10 10 853 1.49 15.46 6.2- 0.21
example 70 1.10 2.00 1.80 2.00 carbonitride 64 62 10 10 859 1.95
19.60 6.2- 0.20 example 71 1.01 0.56 1.10 1.10 carbonitride 64 62
10 0 821 0.45 3.57 5.0 0- .27 example 72 1.01 0.56 1.10 1.10
carbonitride 64 62 10 5 826 0.45 3.57 5.4 0- .26 example 73 1.01
0.56 1.10 1.10 carbonitride 64 62 10 10 832 0.45 3.57 5.6 - 0.25
example 74 1.01 0.56 1.10 1.10 carbonitride 64 62 10 15 838 0.45
3.57 5.4 - 0.26 example 75 1.01 0.56 1.10 1.10 carbonitride 64 62
10 20 845 0.45 3.57 5.2 - 0.26 example 76 1.01 0.56 1.10 1.10
carbonitride 64 62 10 25 847 0.45 3.57 4.8 - 0.24 example 77 1.01
0.56 1.10 1.10 carbonitride 64 62 10 30 849 0.45 3.57 3.6 - 0.23
example 78 1.01 0.56 1.10 1.10 carbonitride 64 62 10 35 848 0.45
3.57 3.4 - 0.23 example 79 1.01 0.56 1.10 1.10 carbonitride 64 62
20 0 821 0.45 3.57 5.2 0- .28 example 80 1.01 0.56 1.10 1.10
carbonitride 64 62 20 5 826 0.45 3.57 8.6 0- .25 example 81 1.01
0.56 1.10 1.10 carbonitride 64 62 20 10 832 0.45 3.57 8.8 - 0.25
example 82 1.01 0.56 1.10 1.10 carbonitride 64 62 20 20 838 0.45
3.57 9.0 - 0.26 example 83 1.01 0.56 1.10 1.10 carbonitride 64 62
20 30 845 0.45 3.57 8.6 - 0.22 example 84 1.01 0.56 1.10 1.10
carbonitride 64 62 20 35 847 0.45 3.57 8.2 - 0.22 example 85 1.01
0.56 1.10 1.10 carbonitride 64 62 20 40 849 0.45 3.57 5.0 - 0.21
example 86 1.01 0.56 1.10 1.10 carbonitride 64 62 20 45 848 0.45
3.57 4.8 - 0.21 example 87 1.01 0.56 1.10 1.10 carbonitride 64 62
30 5 832 0.45 3.57 6.0 0- .24 example 88 1.01 0.56 1.10 1.10
carbonitride 64 62 30 10 838 0.45 3.57 6.4 - 0.23 example 89 1.01
0.56 1.10 1.10 carbonitride 64 62 30 15 845 0.45 3.57 11.2- 0.23
example 90 1.01 0.56 1.10 1.10 carbonitride 64 62 30 20 847 0.45
3.57 11.8- 0.23 example 91 1.01 0.56 1.10 1.10 carbonitride 64 62
30 30 849 0.45 3.57 12.2- 0.22 example 92 1.01 0.56 1.10 1.10
carbonitride 64 62 30 40 848 0.45 3.57 11.6- 0.22 example 93 1.01
0.56 1.10 1.10 carbonitride 64 62 30 45 847 0.45 3.57 11.4- 0.23
example 94 1.01 0.56 1.10 1.10 carbonitride 64 62 30 50 849 0.45
3.57 6.4 - 0.24 example 95 1.01 0.56 1.10 1.10 carbonitride 64 62
30 55 848 0.45 3.57 5.8 - 0.22 example 96 1.01 0.56 1.10 1.10
carburize 63 62 10 15 795 0.37 2.02 5.0 0.4- 4 example 97 1.01 0.56
1.10 1.10 quench 62 62 10 10 795 0.37 2.02 4.8 0.53 example 98 0.28
0.60 1.00 1.10 carbonitride 64 62 10 10 760 0.40 3.80 2.4 - 0.28
example 99 0.99 0.25 0.40 1.49 carbonitride 64 62 10 10 777 0.32
1.05 2.6 - 0.27 example 1.00 0.40 0.25 1.50 carbonitride 64 62 10
10 779 0.41 1.20 2.6 0.2- 6 100 comparative 1.01 0.56 1.10 1.10
carburize 57 55 10 10 795 0.37 2.02 3.0 1.- 00 example 17
comparative 0.99 0.35 0.40 1.49 carbonitride 64 62 10 10 761 0.19
0.52 1.0- 0.34 example 18 comparative 0.99 0.30 0.40 1.49
carbonitride 64 62 10 10 754 0.20 0.87 1.2- 0.32 example 19 (1)
Surface hardness of a raceway surface and core hardness are
Rockwell hardness. .gamma..sub.R designates the amount of retained
austenite on a surface layer section, and a unit is vol. %. (2)
Surface hardness of a rolling surface is Vickers hardness Hv.
.gamma..sub.R designates the amount of retained austenite on a
surface layer section, and a unit is vol. %. A unit of nitrogen
concentration is mass %. A unit of nitride amount (area percentage)
is percent.
(Fourth Test)
The life test was conducted in the environment of lubrication
contaminated with foreign substances by use of a conical roller
bearing assigned bearing number L44649/610. Test conditions are as
follows: Test load: radial load Fr=12 kN, axial load Fa=3.5 kN
Number of revolutions: 3000 min.sup.-1 Lubricating oil: VG68
Hardness of foreign substance: Hv870 Size of foreign substance: 74
to 134 .mu.m Amount of contaminated foreign substance: 0.1 g
High-carbon chromium bearing steel (SUJ2) or chromium steel
(SCr420) was used for the inner and outer rings, and steel having
chemical components corresponding to SUJ3 was used for the rolling
element except the amounts of Si+Mn. In relation to heat treatment,
materials corresponding to SUJ2 and SUJ3 were quenched in the
atmosphere of the RX gas at 830 to 850.degree. C. or subjected to
carburizing or carbonitriding at 830 to 850.degree. C. in the
atmosphere consisting of the RX gas+the enriched gas+the ammonium
gas (the ammonium gas is used for carbonitriding) for 1 hour to 20
hours. The materials were subsequently tempered at 180 to
240.degree. C. After undergoing carburizing or carbonitriding at
850 to 900.degree. C., SCr240 was subjected to secondary quenching
at 800 to 850.degree. C. and then to tempering at 150 to
200.degree. C.
Table 7 shows the quality of raceway rings and rolling elements,
which were used in the test, and results of the life test. The
electron probe micro analyzer (EPMA) was used for measuring the
surface nitrogen concentration in the raceway surface and the
rolling surface, thereby performing quantitative analysis. The life
tests were performed twelve times (n=12) for each of the test
bearings, thereby studying a life time lasting until occurrence of
stripping, preparing a Weibull plot, and determining life L10 from
a result of the Weibull distribution. The thus-determined life is
taken as a life value. Life is provided in the form of a value of a
ratio on condition that the life of the comparative example 20
having the shortest is taken as 1.
TABLE-US-00008 TABLE 7 area percentage of surface nitrogen Si + Mn
amount of concentration material based Si + Mn (mass %) of nitride
of of rolling raceway rolling raceway rolling element life ring
element ring element (mass %) ratio example 101 0 0.6 SUJ2 3.1 0.5
3 example 102 0 0.5 SUJ2 2.7 0.5 3.1 example 103 0 0.4 SUJ2 2.6 0.5
2.9 example 104 0 0.35 SUJ2 2.4 0.5 2.8 example 105 0 0.3 SUJ2 2.3
0.5 2.2 example 106 0 0.25 SUJ2 2.1 0.5 2.2 example 107 0 0.2 SUJ2
2.0 0.5 2.1 example 108 0 0.5 SUJ2 2.9 0.7 3.1 example 109 0 0.5
SUJ2 3.2 0.9 3.2 example 110 0 0.5 SUJ2 3.8 1 4.4 example 111 0 0.5
SUJ2 4.1 1.2 4.5 example 112 0 0.5 SUJ2 4.5 1.6 4.7 example 113 0
0.5 SUJ2 5.1 2 4.7 example 114 0.01 0.5 SUJ2 4.5 1.6 4.5 example
115 0.03 0.5 SUJ2 4.5 1.6 4.4 example 116 0.05 0.5 SUJ2 4.5 1.6 4.2
example 117 0 0.5 SCr420 4.5 1.6 5.1 example 118 0.01 0.5 SCr420
4.5 1.6 4.8 example 119 0.03 0.5 SCr420 4.5 1.6 4.6 example 120
0.05 0.5 SCr420 4.5 1.6 4.5 comparative 0 0 SUJ2 0.0 0.5 1 example
20 comparative 0 0.1 SUJ2 0.9 0.5 1.2 example 21 comparative 0 0.15
SUJ2 1.6 0.5 1.2 example 22
As is clear from Table 7, when the raceway ring made from SUJ2 is
compared with the raceway ring made from SCr420, greater effects
are yielded in the case of the raceway ring made from SCr420. The
conceivable reason for this is that, when SCr420 is used for the
raceway ring, the raceway ring is vulnerable to indentation and the
rolling element becomes less susceptible to indentation, because
the core hardness of SCr420 is softer than the core hardness of
SUJ2, whereby the life-extension effect is yielded.
In relation to the tests, there provided examples where the
high-carbon chromium bearing steel (SUJ2) or the chromium steel
(SCr420) was applied to the raceway ring; steel including 1.0 mass
% of raw material carbon was applied to the rolling element; and
where the steels were subjected to quenching, tempering, or
carburizing or carbonitriding. However, so long as the surface
hardness of a completed raceway surface and the surface hardness of
a completed rolling surface are greater than HRC55 and fall within
the range of the present invention, analogous effects are
yielded.
Although the present invention has been described in detail or by
reference to specific embodiments, it is manifest to those skilled
in the art that the present invention is susceptible to various
alterations or modifications without departing the spirit and scope
of the present invention.
The present patent application is based on Japanese Patent
Application (JP-A-2006-148497) which was filed on May 29, 2006 and
which is based on Japanese Patent Application (JP-2006-140111)
filed on May 19, 2006; Japanese Patent Application (JP-2006-150375)
filed on May 30, 2006; Japanese Patent Application (JP-2007-107250)
filed on Apr. 16, 2007; and Japanese Patent Application
(JP-2007-112995) filed on Apr. 23, 2007, and their contents are
hereby incorporated by reference.
* * * * *