U.S. patent application number 10/124732 was filed with the patent office on 2003-01-23 for constant velocity universal joint.
Invention is credited to Azuma, Kazuhiro, Ikei, Katsuyuki, Kobayashi, Masazumi, Kura, Hisaaki, Nakagawa, Tohru, Tanimoto, Isamu.
Application Number | 20030017877 10/124732 |
Document ID | / |
Family ID | 26614098 |
Filed Date | 2003-01-23 |
United States Patent
Application |
20030017877 |
Kind Code |
A1 |
Kobayashi, Masazumi ; et
al. |
January 23, 2003 |
Constant velocity universal joint
Abstract
A novel constant velocity universal joint with eight ball is
provided for realizing the improved durability of the joint under
high-loaded conditions and the suppressed variations of lifetimes
of the joints. The universal joint has an outer joint member having
eight curved track grooves axially extending along an inner
circumferential surface with a spherical shape, an inner joint
member having eight curved track grooves axially extending along an
outer circumferential surface with a spherical shape, eight balls
respectively arranged in eight ball tracks provided as pairs of the
track grooves of the outer joint member and the track grooves of
the inner joint member, and a cage having pockets for respectively
holding the balls. The center of each track groove of the outer
joint member is axially displaced a predetermined distance from the
spherical center of the inner circumferential surface. The center
of each track groove of the inner joint member is axially displaced
the same distance from the spherical center of the outer
circumferential surface in the direction opposite to that of the
outer joint member. A PCD gap in the ball track is in the range of
5 .mu.m to 50 .mu.m.
Inventors: |
Kobayashi, Masazumi;
(Shizuoka-ken, JP) ; Azuma, Kazuhiro;
(Shizuoka-ken, JP) ; Nakagawa, Tohru;
(Shizuoka-ken, JP) ; Tanimoto, Isamu;
(Shizuoka-ken, JP) ; Ikei, Katsuyuki;
(Shizuoka-ken, JP) ; Kura, Hisaaki; (Shizuoka-ken,
JP) |
Correspondence
Address: |
ARENT FOX KINTNER PLOTKIN & KAHN
1050 CONNECTICUT AVENUE, N.W.
SUITE 400
WASHINGTON
DC
20036
US
|
Family ID: |
26614098 |
Appl. No.: |
10/124732 |
Filed: |
April 18, 2002 |
Current U.S.
Class: |
464/145 |
Current CPC
Class: |
F16D 3/2245 20130101;
F16D 3/2237 20130101; Y10S 464/906 20130101; F16D 2003/22309
20130101 |
Class at
Publication: |
464/145 |
International
Class: |
F16D 003/50 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 24, 2001 |
JP |
2001-126166 |
Jun 25, 2001 |
JP |
2001-191553 |
Claims
What is claimed is:
1. A constant velocity universal joint, comprising: an outer joint
member having eight curved track grooves axially extending along an
inner circumferential surface with a spherical shape; an inner
joint member having eight curved track grooves axially extending
along an outer circumferential surface with a spherical shape;
eight balls respectively arranged in eight ball tracks provided as
pairs of the track grooves of the outer joint member and the track
grooves of the inner joint member; and a cage having pockets for
respectively holding the balls, a center of each track groove of
the outer joint member being axially displaced a predetermined
distance from a spherical center of the inner circumferential
surface, a center of each track groove of the inner joint member
being axially displaced the same distance from a spherical center
of the outer circumferential surface in a direction opposite to
that of the outer joint member, wherein a PCD gap in the ball track
is in a range of 5 .mu.m to 50 .mu.m.
2. The constant velocity universal joint according to claim 1,
wherein each of the track grooves of the outer joint member and the
inner joint member has a bottom with a linear shape provided as a
straight portion.
3. The constant velocity universal joint according to claim 1 or
claim 2, wherein a gap between the pocket of the cage and the ball
in an axial direction is in a range of -30 .mu.m to +10 .mu.m.
4. The constant velocity universal joint according to claim 1 or
claim 2, wherein a gap between the cage and the outer joint member
or the inner joint member in a radial direction is in a range of 20
.mu.m to 100 .mu.m.
5. A constant velocity universal joint, comprising: an outer joint
member having eight curved track grooves axially extending along an
inner circumferential surface with a spherical shape; an inner
joint member having eight curved track grooves axially extending
along an outer circumferential surface with a spherical shape;
eight balls respectively arranged in eight ball tracks provided as
pairs of the track grooves of the outer joint member and the track
grooves of the inner joint member; and a cage having pockets for
respectively holding the balls, a center of each track groove of
the outer joint member being axially displaced a predetermined
distance from a spherical center of the inner circumferential
surface, a center of each track groove of the inner joint member
being axially displaced the same distance from a spherical center
of the outer circumferential surface in a direction opposite to
that of the outer joint member, wherein a ratio (F/PCR) of an
amount (F) of a displacement and a length (PCR) of a line segment
between the center of the track groove of the outer joint member or
the center of the track groove of the inner joint member and the
center of the ball is in a range of
0.069.ltoreq.F/PCR.ltoreq.0.121, and a contact angle of each of the
track grooves of the outer joint member and the inner joint member
and the corresponding ball is 37.degree. or less.
6. The constant velocity universal joint according to claim 5,
wherein each of the track grooves of the outer joint member and the
inner joint member has a bottom with a linear shape provided as a
straight portion.
7. The constant velocity universal joint according to claim 5 or
claim 6, wherein the lower limit of the contact angle of each of
the track grooves of the outer joint member and the inner joint
member and the corresponding ball is defined such that the pressure
on the contact surface at the time of loading the basic torque
becomes 2.7 Gpa or less.
8. The constant velocity universal joint according to claim 5 or
claim 6, wherein the contact angle of each of the track grooves of
the outer joint member and the inner joint member and the
corresponding ball is in the range of 29.degree. to 37.degree..
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to fixed-type constant
velocity universal joints to be used in power transmission systems
of automobiles and various kinds of industrial machines, each of
which only allows an operational angle displacement between a shaft
on the power-transmitting side and a shaft on the driven side.
[0003] 2. Description of the Related Art
[0004] Fixed-type constant velocity universal joints (e.g., Rzeppa
constant velocity universal joints well known as Birfield universal
joints: BJs) have been used for the connection between drive shafts
or the like in automobiles. Typically, the conventional fixed-type
constant velocity universal joint (hereinafter, simply referred to
as the conventional universal joint) comprises: an outer joint
member in which curved track grooves are axially formed on its
inner circumferential surface with a spherical shape; an inner
joint member in which curved track grooves are axially formed on
its outer circumferential surface with a spherical shape; a
plurality of balls respectively arranged in ball tracks provided as
pairs of track grooves of the inner and outer joint members; and a
cage having pockets for respectively holding these balls. More
specifically, the plurality of balls are arranged in their
respective pockets of the cage at regular intervals in a
circumferential direction of the joint.
[0005] The center of the track groove of the outer joint member is
displaced a predetermined distance from the spherical center of the
inner circumferential surface. On the other hand, the center of the
track groove of the inner joint member is displaced the same
distance from the spherical center of the outer circumferential
surface in the direction opposite to that of the outer joint
member. Here, such a displacement is referred to as a track offset.
Therefore, the ball track formed between the outer joint member and
the inner joint member is shaped like a wedge opened toward one end
of the joint in the axial direction. In addition, the spherical
center of the inner circumferential surface of the outer joint
member and the spherical center of the outer circumferential
surface of the inner joint member are located within a common
plane, i.e., the central plane of the joint that includes the
center of each ball.
[0006] In the conventional universal joint, the constant velocity
of the joint can be ensured because each of the ball held in the
pocket of the cage can be always located in a plane that bisects
any operational angle when there is an angular displacement between
the outer joint member and the inner joint member. Here, the
operational angle refers to an angle formed by a rotational axis of
the outer joint member and a rotational axis of the inner joint
member.
[0007] In recent years, there may be cases where a wheel base is
lengthened from the viewpoint of improving the safety of automobile
in the event of a crash. In this case, however, there is a need to
increase a steering angle for front wheels by providing the
universal joint with a higher operation angle. For filling the need
for such a higher-angle, there is provided another conventional
universal joint, i.e., an undercut-free type universal joint (UJ)
in which the track grooves on the opening side of the outer joint
member are shaped so as to be in parallel with the axial direction
of the joint. In this kind of the universal joint, there is no
under cut formed on either of the outer joint member or the inner
joint member so that a higher operation angle can be attained.
[0008] In each kind of the conventional universal joints (BJ, UJ),
it is very important to define how to fill a gap between a pitch
circle diameter of the track groove of the outer joint member and a
pitch circle diameter of the track groove of the inner joint member
(hereinafter, such a gap is referred to as a PCD gap). If the PCD
gap is too small, it becomes difficult to insert balls into the
respective ball tracks and each of the balls is then difficult to
roll smoothly as the binding force upon the ball increases. During
the rotation of the universal joint, the rolling movement of the
ball is caused with sliding contact between the ball and the ball
track. As a result of frictional heating, the temperature in the
joint increases and hence the lifetime of the joint decreases. If
such a gap is too large, on the other hand, the characteristic
features of the joint (such as NVH, durability) can be affected by
the generation of slapping sounds between the pocket and the ball,
or more the increase in vibrations of the joint.
[0009] Especially, under high-load conditions, the contact ellipse
between the ball and the track groove runs off the track groove.
Under such circumstances, it could be chipped from such a portion,
resulting in flaking. If the PCD gap is small, it is effective to
prevent the contact ellipse from running off the track groove. On
the other hand, if such a gap is substantially larger than usual,
the ball contact point approaches the PCD gap and hence the contact
ellipse becomes easy to run off the track groove.
[0010] In each of the conventional universal joints (BJ, UJ),
furthermore, it is very important to determine the ratio (F/PCR)
between the amount of track offset (F) and the length (PCR) of a
straight line segment between the center of the track groove in the
outer or inner joint member and the center of the ball. In Japanese
Patent Laid-Open Publication No. Hei. 9-317784 (1997), for example,
there is disclosed an appropriate range of F/PCR (i.e.,
0.069.ltoreq.F/PCR.ltoreq.0.121). In this case, the depth of the
track groove becomes shallow if the amount of track offset (F) is
too large. Thus, the contact ellipse between the ball and the track
groove tends to run off the track groove, causing the decrease in
allowable load torque at a higher operational angle region. In
addition, columns of the cage become narrow, so that the strength
of the cage can be also decreased. On the contrary, if the amount
of track offset (F) is too small, the track load increases. It
causes the decrease in the durability of the joint in addition to
lowering of the maximum operational angle.
[0011] Typically, the conventional universal joint has six balls
(hereinafter, such a joint will be also referred to as a six-ball
joint). However, there is an alternative universal joint having
eight balls for the purpose of realizing a smaller and lighter
version of the joint while retaining at least the same properties
(e.g., strength, load capacity, and durability) as those of the
joint using six balls. Here, the universal joint having eight balls
will be also referred to as an eight-ball joint. The eight-ball
joint has its own basic configuration different from that of the
six-ball joint. In this case, the set value of the PCD gap may be
peculiar to such a configuration. The amount of track offset (F)
and the length (PCD) of a straight line segment between the center
of the track groove of the outer or inner joint member and the
center of the ball are also adjusted such that the ratio F/PCR can
be defined within the above appropriate range. It is noted that the
depth of the track groove of the eight-ball joint is smaller than
that of the six-ball joint as the diameter of each of the eight
balls is smaller than that of the six balls. In the eight-ball
joint, the PCD gap exerts a very large influence on the durability
of the joint. Consequently, there is a limit to improve the
durability of the eight-ball joint by means of adjusting the above
F/PCR in the appropriate range.
SUMMARY OF THE INVENTION
[0012] It is an object of the present invention is to provide an
eight-ball joint with the improved durability under high-loaded
conditions, which minimize the variations in lifetimes of the
joints.
[0013] According to a fist aspect of the present invention, there
is provided a constant velocity universal joint comprising: an
outer joint member having eight curved track grooves axially
extending along an inner circumferential surface with a spherical
shape; an inner joint member having eight curved track grooves
axially extending along an outer circumferential surface with a
spherical shape; eight balls respectively arranged in eight ball
tracks provided as pairs of the track grooves of the outer joint
member and the track grooves of the inner joint member; and a cage
having pockets for respectively holding the balls, where a center
of each track groove of the outer joint member is axially displaced
a predetermined distance from a spherical center of the inner
circumferential surface, while a center of each track groove of the
inner joint member is axially displaced the same distance from a
spherical center of the outer circumferential surface in a
direction opposite to that of the outer joint member, wherein a PCD
gap in the ball track is in a range of 5 .mu.m to 50 .mu.m. Here,
the "PCD gap" refers to the difference between a pitch circle
diameter of the track groove of the outer joint member and a pitch
circle diameter of the track groove of the inner joint member. The
present invention can be applied to a constant velocity universal
joint with each of track grooves of the outer and inner joint
members having a linear bottom as a straight portion.
[0014] According to the first aspect of the present invention, the
PCD gap in the ball track is set in the range 5 .mu.m to 50 .mu.m,
so that under high-load conditions the contact ellipse between the
ball and the track groove is difficult to run off the track groove.
Therefore, it becomes possible to easily prevent the generation of
chipping or flaking, allowing the increase in the durability and
also stabilizing the variations in life of the universal joints.
Especially, in the case of the eight-ball joint, the durability can
be significantly improved by the PCD gap because of a smaller
diameter of the ball that allows a relatively shallow depth of the
track groove.
[0015] In the first aspect of the present invention, it is
preferable to make a gap between the pocket of the cage and the
ball in an axial direction so as to be in a range of -30 .mu.m to
+10 .mu.m. There is a wearing down of the wall of the pocket of the
cage in the axial direction because of the contact between the ball
and the wall, so that the gap in the axial direction may
excessively increase as long as it is continuously used, except at
the early stages. Furthermore, the dimensions of each structural
components of the universal joint have unavoidable dimensional
variations within manufacturing tolerance.
[0016] In the eight-ball joint in accordance with the first aspect
of the present invention, the abrasion loss of the pocket is
comparably small as the load to be applied on the pocket with
respect to one ball is smaller than that of the six-ball joint.
Therefore, the smooth rolling of the ball can be ensured with
reducing the binding force of the ball in the pocket. That is, the
gap in the axial direction is positively shifted with respect to
that of the six-ball joint (a gap in the axial direction of -50
.mu.m to -10 .mu.m). Thus, the gap in the axial direction in
accordance with the present invention may be in the range of -30
.mu.m to +10 .mu.m, preferably in the range of -10 .mu.m to +10
.mu.m, for preventing the increase in temperature of the inside of
the joint by lowering the amount of heat liberated from the contact
between the ball and the pocket. As a result, the durability of the
joint can be increased.
[0017] In the above configuration of the universal joint, the gap
between the cage and the outer joint member or between the cage and
the inner joint member in a radial direction of the joint may be in
a range of 20 .mu.m to 100 .mu.m. Therefore, the operating
characteristics between the cage and the outer joint member or
between the cage and the inner joint member can be improved. In
addition, the generation of slapping sounds between the cage and
the outer joint member or between the cage and the inner joint
member can be prevented. Furthermore, the increase in vibrations of
the joint can be also prevented.
[0018] In a second aspect of the present invention, there is
provided a constant velocity universal joint; comprising: an outer
joint member having eight curved track grooves axially extending
along an inner circumferential surface with a spherical shape; an
inner joint member having eight curved track grooves axially
extending along an outer circumferential surface with a spherical
shape; eight balls respectively arranged in eight ball tracks
provided as pairs of the track grooves of the outer joint member
and the track grooves of the inner joint member; and a cage having
pockets for respectively holding the balls, where a center of each
track groove of the outer joint member is axially displaced a
predetermined distance from a spherical center of the inner
circumferential surface, while a center of each track groove of the
inner joint member is axially displaced the same distance from a
spherical center of the outer circumferential surface in a
direction opposite to that of the outer joint member, wherein a
ratio (F/PCR) of an amount (F) of displacement (i.e., the amount of
track offset) and a length (PCR) of a line segment between the
center of the track groove of the outer joint member or the center
of the track groove of the inner joint member and a center of the
ball is in a range of 0.069.ltoreq.F/PCR.ltoreq.0.121, and a
contact angle of each of the track grooves of the outer joint
member and the inner joint member and the corresponding ball is
37.degree. or less. Also, the present invention is applicable to a
constant velocity universal joint where each of the track grooves
of the outer joint member and the inner joint member has a bottom
with a linear shape provided as a straight portion.
[0019] The second aspect of the present invention is predicted on
the universal joint being designed to have an appropriate ratio
(F/PCR) of the amount (F) of the displacement (i.e., the amount of
track offset) and the length (PCR) of a line segment between the
center of the track groove of the outer joint member or the center
of the track groove of the inner joint member and the center of the
ball in the range of 0.069.ltoreq.F/PCR.ltoreq.0.121, as disclosed
in Japanese Patent Laid-Open Publication No. Hei. 9-317784 (1997).
Thus, the present invention is constructed such that a contact
angle of each of the track grooves of the outer joint member and
the inner joint member and the corresponding ball is 37.degree. or
less and the ratio F/PCR is appropriately defined in the above
range. Therefore, the contact ellipse between the ball and the
track groove becomes difficult to run off the track groove. In
addition, there is no clearance of the ball in the ball track at an
operational angle less than a normal angle to be applied in the
automobile or the like, avoiding the generation of slapping
sounds.
[0020] By narrowing the contact angle between the track groove and
the ball, therefore, the pressure on the contact surface between
the track groove and the ball can be increased. However, if the
contact angle becomes too small, the durability of the joint may be
decreased. For that reason, the lower limit of the contact angle
between the track groove and the ball may be defined such that the
pressure on the contact surface at the time of loading the basic
torque becomes 2.7 GPa or less. Consequently, the durability of the
universal joint of the present invention is equal to or more
excellent than that of the conventional universal joint. Here, the
term "basic torque" means torque that allows a lifetime of 1,500
hours at 100 rpm with a torque value defined from the contact
stress (Herz's stress) between the track groove and the ball (basic
torque used for the calculation of lifetime of the universal
joint).
[0021] Here, it is preferable to define the contact angle between
the track groove and the ball in a range of 29.degree. to
37.degree.. Therefore, when the contact angle between the track
groove and the ball is 29.degree. or more, the pressure on the
contact surface between the track groove and the ball can be held
under control. Consequently, the durability of the universal joint
of the present invention is equal to or more excellent than that of
the conventional universal joint.
[0022] The nature, principle, and utility of the invention will
become more apparent from the following detailed description when
read in conjunction with the accompanying drawings in which like
parts are designated by like reference numerals or characters.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] In the accompanying drawings:
[0024] FIG. 1 is a cross sectional view taken along the line A-A in
FIG. 2 or the line B-B in FIG. 3 for illustrating a constant
velocity universal joint as one preferred embodiment of the present
invention;
[0025] FIG. 2 is a cross sectional view of the Rzeppa constant
velocity universal joint (BJ) as one embodiment of the present
invention;
[0026] FIG. 3 is a cross sectional view of the undercut-free
constant velocity universal joint (UJ) as another preferred
embodiment of the present invention;
[0027] FIG. 4 is a graph that illustrates the results of the
endurance test under high-loaded conditions with respect to the PCD
gap;
[0028] FIG. 5 is a graph that illustrates the results of the
endurance test with respect to the gap in the pocket of the cage in
the axial direction;
[0029] FIG. 6 is an enlarged cross sectional view of the main part
of the constant velocity universal joint shown in FIG. 3 for
explaining the track offset angle;
[0030] FIG. 7 is an enlarged cross sectional view of the main part
of the constant velocity universal joint shown in FIG. 3 for
explaining the contact angle between the track groove and the ball;
and
[0031] FIG. 8 is an enlarged cross sectional view of the main part
of the constant velocity universal joint shown in FIG. 3 for
explaining the wedge angle.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] FIG. 2 illustrates a Rzeppa fixed-type constant velocity
universal joint (BJ) and FIG. 3 illustrates an undercut-free
fixed-type constant velocity universal joint (UJ), respectively.
The cross sectional view taken along the line A-A in FIG. 2 and the
cross sectional view taken along the line B-B in FIG. 3 are
identical to each other, so that FIG. 1 is provided as their common
cross sectional view.
[0033] The universal joint (BJ) shown in FIG. 2 as a first
embodiment of the present invention comprises an outer joint member
1 having eight curved track grooves 1b axially extending along an
inner circumferential surface 1a with a spherical shape; an inner
joint member 2 having eight curved track grooves 2b axially
extending along an outer circumferential surface 2a with a
spherical shape and also having a serrated fitting portion 2c
between the end of a middle shaft part 5 of a drive shaft and the
inner circumferential surface 1a, eight balls 3 arranged in
respective eight ball tracks formed as pairs of the track grooves
1b of the outer joint member 1 and the track grooves 2b of the
inner joint member 2, and a cage 4 having pockets 4c for
respectively holding the balls 3 at regular intervals in a
circumferential direction.
[0034] The center O.sub.1 of the track groove 1b of the outer joint
member 1 is axially displaced a predetermined distance F from the
spherical center of the inner circumferential surface 1a, while the
center O.sub.2 of the track groove 2b of the inner joint member 2
is axially and oppositely (i.e., in the figure, the center O.sub.1
is the opening side f the joint, and the center O.sub.2 is the
recess side of the joint) displaced the same distance F from the
spherical center of the outer circumferential surface 2a (i.e.,
track offset). Therefore, the ball track formed between the outer
joint member 1 and the inner joint member 2 is shaped like a wedge
opened to one side in the axial direction (i.e., in the figure, the
opening side of the joint).
[0035] Both the spherical center of an outer circumferential
surface 4a of the cage 4 and the spherical center of the inner
circumferential surface 1a of the outer joint member 1 to be
provided as a guide surface of the outer circumferential surface 4a
of the cage 4 are located within a central plane O of the joint
including the center O.sub.3 of the ball 3. In addition, both the
spherical center of an inner circumferential surface 4b of the cage
4 and the spherical center of the outer circumferential surface 2a
of the inner joint member 2 to be provided as a guide surface of
the inner circumferential surface 4b of the cage 4 are located
within the central plane O. Therefore, the amount of track offset F
in the center O.sub.1 of the track groove 1b in the outer joint
member 1 corresponds to the distance from the center O.sub.1 to the
central plane O of the joint in the axial direction. On the other
hand, the amount of track offset F of the center O.sub.2 of the
track groove 2b in the inner joint member 2 corresponds to the
distance from the center O.sub.2 to the central plane O of the
joint and is equal to the amount of track offset F in the center
O.sub.1 of the track groove 1b.
[0036] In this embodiment, furthermore, the spherical centers of
the outer circumferential surface 4a and the inner circumferential
surface 4b of the cage 4 and the center O.sub.3 of the ball 3 are
arranged in a line on a common plane perpendicular to the axial
direction. Alternatively, these centers may be oppositely displaced
the same distance in the axial direction with respect to the center
O.sub.3 of the ball 3.
[0037] The universal joint (UJ) shown in FIG. 3 is another
embodiment of the present invention and includes the same
structural components except of straight portions U1, U2. That is,
the straight portion U1 is formed as a liner bottom portion in
cross section of the track groove 1b of the outer joint member 1,
while the straight portion U2 is formed as a linear bottom portion
in cross section of the track groove 2b of the inner joint member
2. Therefore, the explanations of the same structural components as
those in FIG. 2 will be omitted in the following description. The
straight portions U1, U2 formed on the track grooves 1b, 2b of the
outer joint member 1 and the inner joint member 2 in the universal
joint (UJ) of the present embodiment allow the maximum operational
angle larger than that of the universal joint (BJ) shown in FIG. 2.
In each of the universal joints (BJ, UJ) shown in FIGS. 2 and 3,
when the outer joint member 1 and the inner joint member 2 undergo
angular displacements with respect to each other, the constant
velocity of the joint can be ensured as each of the ball held in
the pocket 4c of the cage 4 can be always located in a plane that
bisects any operational angle.
[0038] In each of the universal joints (BJ, UJ) shown in FIGS. 2
and 3, furthermore, a PCD gap with respect to the ball track shown
in FIG. 1, i.e., the difference between a pitch circle diameter
PCD.sub.OUT of the track groove 1b of the outer joint member 1 and
a pitch circle diameter PCD.sub.IN of the track groove 2b of the
inner joint member 2, is defined within the range of 5 .mu.m to 50
.mu.m. If the PCD gap becomes smaller than 5 .mu.m, it becomes
difficult to place the balls 3 into the pairs of track grooves 1b,
2b and to maintain the stable operation of the joint. If the PCD
gap becomes larger than 50 .mu.m, then the improvement in the
durability of the joint becomes difficult as the contact ellipse
between the ball 3 and the track grooves 1b, 2b tends to run off
the track grooves 1b, 2b.
[0039] As shown in FIGS. 2 and 3, furthermore, the gap between the
pocket 4c of the cage 4 and the ball 3 in the axial direction,
i.e., the difference between the axial dimension (D.sub.P) of the
pocket 3 and the diameter (D.sub.B) of the ball 3, is defined
within the range of -30 .mu.m to +10 .mu.m, preferably of -10 .mu.m
to +10 .mu.m. If the interference (negative gap) between the pocket
4c of the cage 4 and the ball 3 is smaller than -30 .mu.m, the
binding force of the ball 3 increases to prevent a smooth rolling
movement of the ball 3. During the rotation of the universal joint,
therefore, the rolling movement of the ball 3 is caused with
sliding contact between the ball 3 and the track grooves 1b, 2b. As
a result of frictional heating, the temperature in the joint
increases and hence the lifetime of the joint decreases. On the
other hand, if the clearance (positive gap) between the pocket 4c
of the cage 4 and the ball 3 is larger than +10 .mu.m, undesired
effects for the joint performance can be generated. For example,
slapping sounds can be generated between the pocket 4c and the ball
3, or more vibrations can be generated by the joint.
[0040] Furthermore, as shown in FIG. 1, the gap between the cage 4
and the outer joint member 1 in the radial direction (i.e., the
difference between the inner diameter (D.sub.O) of the outer joint
member 1 and the external diameter (D.sub.K1) of the cage 4) is
defined within the range of 20 .mu.m to 100 .mu.m. On the other
hand, the gap between the cage 4 and the inner joint member 2 in
the radial direction (i.e., the difference between the inner
diameter (D.sub.K2) of the cage 4 and the outer diameter (D.sub.I)
of the inner joint member 2) is defined within the range of 20 to
100 .mu.m. If each of the gaps in the radial direction is smaller
than 20 .mu.m, the operating characteristics between the cage 4 and
the outer joint member 1 or between the cage 4 and the inner joint
member 2 becomes worse. On the other hand, if each of the gaps in
the radial direction is larger than 100 .mu.m, undesired effects
for the joint performance can be generated. For example, slapping
sounds can be generated between the cage 4 and the outer joint
member 1 or between the cage 4 and the inner joint member 2, or
more vibrations can be generated by the joint.
[0041] For evaluating the above PCD gap, an endurance test under
high-loaded conditions was performed on the Rzeppa constant
velocity universal joint (BJ) having eight balls as shown in FIG.
2. The results obtained from such a test are listed in FIG. 4. The
test was performed using two universal joints A, B as test articles
for evaluating each of the different PCD gaps. That is, each of the
test joints A, B was driven under the conditions in which load
torque T=726 N.multidot.m, the number of revolutions N=230 rpm, and
operational angle .theta.=6 deg. Subsequently, the time that
elapsed before generating any problem in the test article was
measured.
[0042] As is evident from the results shown in FIG. 4, each of the
universal joints with PCD gaps of 30 .mu.m and 50 .mu.m ensured the
stable durability. On the other hand, each of those with the PCD
gaps of 70 .mu.m and 100 .mu.m met the desired spec but expressed
variations in lifetimes. In addition, the universal joint with the
PCD gap of 120 .mu.m was the limit to meet the desired spec.
Furthermore, it was difficult to meet the desired spec when the
universal joint with the PCD gap 130 .mu.m was tested.
[0043] For evaluating the gap between the pocket 4c of the cage 4
and the ball 3 in the axial direction, an endurance test was
performed on the Rzeppa universal joint (BJ) shown in FIG. 2. The
results obtained from such a test are listed in FIG. 5. The test
was performed using two universal joints A, B as test articles for
evaluating each of the joints having different gaps in the axial
direction. That is, each of the test joints A, B was driven under
the conditions in which load torque T=186 N.multidot.m, the number
of revolutions N=1700 rpm, and operational angle .theta.=6 deg.
Subsequently, the time that elapsed before generating any problem
in the test article was measured.
[0044] As is evident from the results shown in FIG. 5, each of the
universal joints having the gaps of -20 .mu.m and -30.mu.m in the
radial direction ensured the stable durability. On the other hand,
the universal joint having the gap of -45 .mu.m in the radial
direction showed poor durability and variations in lifetimes.
[0045] In each of the universal joints shown in FIGS. 2 and 3, the
length (PCR) of a line segment between the center O.sub.1 of the
track groove 1b of the outer joint member 1 and the center O.sub.3
of the ball 3 is equal to the length (PCR) of a line segment
between the center O.sub.2 of the track groove 2b of the inner
joint member 2 and the center O.sub.3 of the ball 3. In the
universal joint shown in FIG. 3, an offset angle which the line
segment between the center O.sub.1 of the track groove 1b of the
outer joint member 1 and the center O.sub.3 of the ball 3 forms
with a line segment between the center O of the joint and the
center O.sub.3 of the ball 3 is defined as .beta.. Similarly, an
offset angle which the line segment between the center O.sub.2 of
the track groove 2b of the inner joint member 2 and the center
O.sub.3 of the ball 3 forms with the line segment between the
center O of the joint and the center O.sub.3 of the ball 3 is also
defined as .beta.. Thus, both offset angles are equal to each
other.
[0046] The amount of track offset (F) of each of the track grooves
1b, 2b of the outer and inner joint members 1, 2 is defined such
that the ratio F/PCR is in the range of
0.069.ltoreq.F/PCR.ltoreq.0.121, where the PCR is the length of a
line segment between the center O.sub.1 of the track groove 1b of
the outer joint member 1 or the center O.sub.2 of the track groove
2b of the inner joint member 2 and the center O.sub.3 of the ball
3. Consequently, the allowable load torque, the durability, the
maximum operational angle, and the cage strength can be ensured,
and also the track load can be decreased. In this embodiment, the
ratio F/PCR is defined as F/PCR=0.104 (or 0.1038), which is
substantially lower than the typical F/PCR value (i.e., F/PCR=0.14)
of the conventional six-ball joint.
[0047] As described above, the universal joint of the present
embodiment has eight balls. Comparing with the six-ball joint,
therefore, the load to be applied on one ball in the universal
joint of the present embodiment forms a small portion of the total
amount of load. It means that the diameter of each ball 3 of the
present embodiment can be smaller than that of the conventional
six-ball joint. FIG. 7 is a partially enlarged cross sectional view
of part of the universal joint including the outer joint member 1,
the inner joint member 2, and the ball 3. As shown in the figure,
the track groove 1b is axially formed on the inner circumferential
surface 1a of the outer joint member 1, while the track groove 2b
is axially formed on the outer circumferential surface 2a of the
inner joint member 2. Each of these track grooves 1b, 2b is shaped
like Gothic arch in its lateral cross section.
[0048] Thus, the ball 3 touches the track groove 1b of the outer
joint member 1 at two points C.sub.11, C.sub.12 and also touches
the track groove 2b of the inner joint member 2 at two points
C.sub.21, C.sub.22. Here, a contact angle is defined as an angle
.alpha. formed by a first line segment passing through the center
O.sub.3 of the ball 3 and the center O of the joint and a second
line segment passing through one of the contact points C.sub.11,
C.sub.12, C.sub.21, and C.sub.22 between the ball 3. As shown in
the figure, the contact angles a formed of the line segments
between the contact points C.sub.11, C.sub.12, C.sub.21, C.sub.22
and the center O.sub.3 of the ball 3 are equal to each other. In
other words, the contact angles a at contact points C.sub.11,
C.sub.12, C.sub.21, C.sub.22 are equal to each other and defined
within the range of 29.degree. to 37.degree.. It is noted that the
contact angle .alpha. (29.degree. to 37.degree.) of the present
eight-ball joint is comparatively smaller than the contact angle
(37.degree. to 45.degree.) of the conventional six-ball joint.
[0049] FIG. 8 is provided for illustrating a wedge angle 2.tau. of
the present universal joint. The wedge angle 2.tau. is an angle
provided for initiating the flip of the wedge in the present
universal joint. In other words, the wedge angle is formed by the
common normal H.sub.1 at the contact point C.sub.1 between the ball
3 and the track groove 1b of the outer joint member 1 and the
common normal H.sub.2 at the contact point C.sub.2 between the ball
3 and the track groove 2b.
[0050] As shown in FIG. 8, the contact point C.sub.1 between the
track groove 1b of the outer joint member 1 and the ball 3 is
positioned with an inclination by the curved track groove 1b at an
angle of .tau. with respect to the center O of the joint passing
through the center O.sub.3 of the ball 3. Also, the contact point
C.sub.2 between the track groove 2b of the inner joint member 2 and
the ball 3 is positioned with an inclination by the curved track
groove 2b at an angle of .tau. with respect to the center O of the
joint passing through the center O.sub.3 of the ball 3. Therefore,
the wedge angle 2.tau. is provided by adding both angles .tau.
together. If the operational angle of the joint increases, the
wedge angle 2.tau. decreases with a certain phase and finally the
wedge can be flipped over.
[0051] If the amount of track offset decreases, then the wedge
angle 2.tau. decreases. In this case, however, it allows the ball 3
to move freely in the ball track at the time of flipping the wedge
over, raising the possibility of generating slapping sounds. For
avoiding such a possibility, the contact angle .alpha. between each
of the track grooves 1b, 2b and the ball 3 may be defined as
37.degree. or less as described above to set the wedge angle 2.tau.
to at least a normal angle (generally at 9.degree.) or more to be
applied in the automobile or the like.
[0052] The following table 1 provides a summary of the relationship
among the ratio F/PCR, the contact angle .alpha., and the wedge
angle 2.tau.. As described above, F denotes the amount of track
offset and PCR denotes the length of a line segment between the
center O.sub.1 of the track groove 1b of the outer joint member 1
or the center O.sub.2 of the track groove 2b of the inner joint
member 2 and the center O.sub.3 of the ball 3.
1TABLE 1 Ratio of offset amount to PCR Contact angle Wedge angle
F/PCR .alpha. 2 .tau. 0.069 45.degree. 8.degree. 37.degree.
10.degree. 29.degree. 14.degree. 0.121 45.degree. 14.degree.
37.degree. 18.degree. 29.degree. 25.degree.
[0053] As is evident from the above table, it is found that the
wedge angle 2.tau. increases as the contact angle .alpha.
decreases. Also, the wedge angle 2.tau. increases with an
increasing amount of offset F. Therefore, the ratio (F/PCR) of the
amount of track offset F and the length of the line segment between
the center O.sub.1 of the track groove 1b of the outer joint member
1 or the center O.sub.2 of the track groove 2b of the inner joint
member 2 and the center O.sub.3 of the ball 3 should be set within
the range of 0.069.ltoreq.F/PCR.ltoreq.0.121 in addition to provide
a contact angle .alpha. of 37.degree. or less, which is formed by
the ball 3 and each of the track grooves 1b, 2b. As a result, the
contact ellipse between the ball 3 and the track groove 1b or 2b
becomes difficult to run off the track groove 1b or 2b, and
substantially no clearance between the ball 3 and the ball track
can be found at the contact angle .alpha. equal to or less than the
normal angle of the automobile or the like. Therefore, the
generation of slapping sounds can be prevented.
[0054] If the contact angle .alpha. formed by the ball 3 and each
of the track grooves 1b, 2b decreases, simultaneously the distance
from the contact point to the corner (i.e., up to the connection
part of the track groove and the inner diameter) increases. As a
result, the allowable load torque can be increased. In this case,
however, the pressure on the contact surface between the track
groove 1b or 2b and the ball 3 increases. If the contact angle
.alpha. is too small, the joint can be resulted in poor durability.
Therefore, the contact angle .alpha. between the ball 3 and each
the track grooves 1b, 2b should be defined such that the pressure
on the contact surface at the time of loading the basic torque is
2.7 GPa or less. Such a configuration of the universal joint allows
the durability which is equal to or more than the durability of the
conventional universal joint. According to the present embodiment,
the contact angle .alpha. between the ball 3 and each of the track
grooves 1b, 2b of the outer and inner joint members 1, 2 is set to
29.degree. or more, so that the pressure on the contact surface
between the track groove 1b or 2b and the ball 3 can be suppressed,
allowing the same or more excellent durability compared with that
of the conventional one.
[0055] While there has been described what are at present
considered to be preferred embodiments of the invention, it will be
understood that various modifications may be made thereto, and it
is intended that the appended claims cover all such modifications
as fall within the true spirit and scope of the invention.
* * * * *