US20020119894A1

US20020119894A1 - Tripod type constant velocity joint and grease for the joint

Info

Publication number: US20020119894A1
Application number: US09/512,217
Authority: US
Inventors: Kazuhiko Yoshida
Original assignee: NTN Corp
Current assignee: NTN Corp
Priority date: 1999-03-11
Filing date: 2000-02-24
Publication date: 2002-08-29
Also published as: JP2000256694A; FR2791355B1; FR2791355A1

Abstract

In order to reduce vibration and improve durability at a low cost through the reduction of induced thrust, such a grease is used that has SRV friction coefficient μ_hunder a contact pressure of 2176 MPa in a range from 0.060 to 0.080 and decreasing rate of SRV friction coefficient μ₁under a contact pressure of 3954 MPa with respect to the friction coefficient μ_h, namely (μ_h−μ₁)/μ_h×100%, in a range from 40 to 60%. The outer joint member 1 is filled with this grease.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a tripod type constant velocity joint used in the power transmission apparatuses of automobiles or various industrial machines, and to a grease used in the joint.

A tripod type constant velocity joint has a

tripod member

2 that has three trunnions 2 a that project in the radial direction and are disposed at equal intervals along the circumference as shown in FIG. 7. Each of the trunnions 2 a has a roller 3 rotatably mounted thereon via a needle roller 4, the rollers 3 being housed in three grooves 1 a formed at equal intervals on the inner circumference of an outer joint member 1. Each of the rollers 3 is guided by a roller guide surface 1 a 1 of the corresponding track groove 1 a, whereby to be capable of moving along the track groove 1 a while rotating about the trunnion 2 a.

As the

roller

3 rolls over the roller guide surface 1 a 1 while rotating about the axis of the trunnion 2 a, the outer joint member 1 and the tripod member 2 are allowed to undergo relative axial displacement with respect to each other, namely relative displacement in the axial direction (plunging) between two shafts to be coupled. At the same time, each of the trunnions 2 a is allowed to make displacement in the axial direction with respect to the roller guide surface 1 a 1 as the phase of rotation changes when rotation torque is transmitted while the outer joint member 1 and the tripod member 2 maintain a predetermined operation angle θ.

Tripod type constant velocity joints of this type are applied to drive shafts, propeller shafts and the like of front-wheel drive vehicles and 4-wheel drive vehicles, because of less sliding movement and less sliding resistance.

In the tripod type constant velocity joint described above, the

roller

3 and the track groove 1 a are disposed in an obliquely crossing arrangement when rotation torque is transmitted while the outer joint member 1 and the tripod member 2 maintain the operation angle θ. As a result, a thrust force is induced in the axial direction when the roller 3 moves in the track groove 1 a in the axial direction of the outer joint member 1. The induced thrust that is characteristic of the tripod type constant velocity joint may cause mechanical vibration and, particularly in the case of an automobile, may increase noise and lower NVH (noise vibration harshness) characteristic. Thus various measures have been proposed for the reduction of induced thrust. The measures for the reduction of induced thrust include the replacement of the single roller 3 shown in FIG. 7 with a dual roller arrangement, and modification of the shape and material of mechanical parts. However, such measures require additional investment on the existing production facilities, thus leading to such drawbacks as significant increase in the manufacturing cost and increased weight. Also as the automobiles tend to have increasing output power, joints are required to have increasingly higher durability, but measures taken in the prior art to improve the durability inevitably lead to such disadvantages as increased cost and weight. Accordingly, an object of the present invention is to provide a tripod type constant velocity joint and a grease used in the joint capable of achieving lower vibration through decreasing the induced thrust and improving the durability, at a lower cost.

SUMMARY OF THE INVENTION

The present inventors focused on the condition of lubrication between the

roller

3 and the roller guide surface 1 a 1 as a factor that determines the durability and induced thrust. Satisfactory lubrication in this interface is expected to make the rolling motion of the roller 3 smoother, thus causing favorable effects on the durability and induced thrust.

A grease film of lower strength generally results in metal contact due to breakage of the grease film, having adverse effects on the generation of induced thrust and durability. In a tripod type constant velocity joint, in particular, slippage between the

roller

3 and the roller guide surface 1 a 1 increases significantly when operating with a large operation angle (approximately θ=10° to 25°), thus making the grease film more likely to break in such contact areas. Therefore, a grease used in the tripod type constant velocity joint is desired to have grease film strength high enough to reliably prevent the grease film from breaking during operation with a high operation angle.

When the grease film has high strength, on the other hand, initial compatibility of the grease with the contact surface becomes lower. Lower initial compatibility of the grease causes a delay in smoothing of the rolling surface and concentration of stress in higher portions on the surface, thus giving rise to a possibility of flaking of the contact surface or other failures due to the generation and propagation of cracks, eventually resulting in shorter service life of the joint. Therefore, the grease is desired to have a high film strength which is, however, not too high.

Grease film strength and initial compatibility are of conflicting nature with each other, as described above, and it is difficult to satisfy the requirements for both properties at the same time.

Based on the consideration described above, the inventors conducted friction tests on various kinds of grease in order to determine optimum grease properties, particularly friction coefficient μ.

Through this investigation, it has been found that grease film strength and initial compatibility can be well balanced and best effects can be achieved in decreasing the induced thrust and improving the durability by using a grease that has SRV friction coefficient μ _hunder a contact pressure of 2176 MPa in a range from 0.060 to 0.080 and decreasing rate of SRV friction coefficient μ₁under a pressure of 3954 MPa with respect to the friction coefficient μ_hin a range from 40 to 60%, the decreasing rate being represented by the following formula (1):

(μ_h−μ₁)/μ_h×100(%) (1).

It has also been verified that a grease which shows a wear amount of 10 mg or lower in Fafnir friction oxidation test is effective in reducing the variation of induced thrust and achieving less vibration, even when operating with a large operation angle θ.

It will be more advantageous, for reducing the induced thrust and improve the durability, to add 0.3 to 1.5 wt % of molybdenum disulfide to such a grease as described above, or to apply shot peening treatment to the inner circumference of the outer joint member.

According to the present invention, as described above, since the friction coefficient of the grease is optimized, satisfactory lubrication can be maintained between the roller and the roller guide surface, and reduction of vibration through the reduction of induced thrust and improvement of durability can be achieved. Moreover, the increase in the cost and weight can be avoided since it suffices to change only the grease without need to change the shapes or materials of other parts.

With a grease that shows the wear amount of 10 mg or lower in Fafnir friction oxidation test, it is made possible to improve the fretting resistance while making a flat profile of induced thrust and reducing the magnitude of the thrust.

When 0.3 to 1.5 wt % of molybdenum disulfide is added to the grease, initial compatibility of the grease is improved and therefore the induced thrust can be decreased and the durability can be improved further. In this case, it may be also possible to eliminate the finish grinding operation in the manufacturing process of the outer joint member.

When shot peening treatment is applied to the inner circumference of the outer joint member, further reduction of vibration and elongation of service life can be achieved because of the improved effect of preventing fretting and other reason.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings: [0018]
FIG. 1A is a sectional view of a tripod type constant velocity joint in the axial direction, and FIG. 1B is a sectional view thereof in the radial direction: [0019]
FIG. 2 is a side view schematically showing a setup for SRV friction test: [0020]
FIG. 3 shows a test result obtained with the SRV friction test: [0021]
FIG. 4 shows results of durability and NVH characteristic tests: [0022]
FIG. 5 is an exploded perspective view schematically showing a Fafnir friction oxidation tester: [0023]
FIG. 6 shows the results of Fafnir friction oxidation test: and [0024]
FIG. 7 is a sectional view of the tripod type constant velocity joint in the axial direction disposed with an operation angle. [0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiment of the present invention will now be described below with reference to FIG. 1A through FIG. 7. [0026]
As shown in FIG. 1A and FIG. 1B, the tripod type constant velocity joint of the present invention comprises an [0027] outer joint member 1 having a shape of substantially bottomed cylinder, a tripod member 2 that has three trunnions 2 a, disposed at equal intervals along the circumference and projecting in the radial direction, and is inserted in the outer joint member 1, and ring-shaped rollers 3 mounted rotatably on the respective trunnions 2 a of the tripod member 2 via rolling elements 4 such as needle rollers. Provided on the inner circumference of the outer joint member 1 at equal intervals in the circumferential direction are three track grooves 1 a extending in the axial direction of the outer joint member 1, with the rollers 3 mounted on the respective trunnions 2 a being housed in the corresponding track grooves 1 a. Wall surfaces on both sides of each track groove 1 a serve as roller guide surfaces 1 a 1 that make fitting contact with outer circumference surface 3 a of the roller 3, the roller guide surfaces 1 a 1 extending in the axial direction of the outer joint member 1. In this embodiment, a case is described where the roller guide surface 1 a 1 has a concave cylindrical surface and the outer circumference surface 3 a of the roller 3 is formed in a convex spherical surface with the center lying on the axial line of the trunnion 2 a. Coupled with the tripod member 2 on the inner circumference thereof by means of serration or the like is a shaft 5.
The tripod type constant velocity joint is normally made by inserting a tripod assembly, comprising the rolling elements [0028] 4 and the rollers 3 assembled on the tripod member 2, inside the outer joint member 1, and filling the inside of the joint (for example, the inside of the outer joint member 1) with a grease. Fitted between the outer joint member 1 and the shaft 5 is an elastic boot not shown in the drawing, for the purpose of preventing the grease from leaking and protection against external impact.
As described previously, the grease used in the tripod type constant velocity joint is desired to have grease film strength high enough to reliably prevent the grease film from breaking during operation with a large operation angle. However, a grease having excessively high grease film strength cannot be used for the reason of balance with the initial compatibility. [0029]
Based on the consideration described above, a friction test described below was conducted to determine the grease properties that allow it to achieve a good balance between the grease film strength and the initial compatibility. Based on the assumption that grease film strength could be evaluated by the load dependence of friction coefficient μ of the grease, grease film strength was evaluated by the decreasing rate of the friction coefficient as the load was increased at constant steps in this test. It can be said that a low decreasing rate of friction coefficient indicates low load dependence of friction coefficient and hence high grease film strength, and a high decreasing rate of friction coefficient indicates high load dependence and low grease film strength. [0030]
The friction test was conducted in conformity with the SRV test procedure specified in ASTM D5706 and D5707. In the procedure of this test, as outlined in FIG. 2, a [0031] ball 11 is placed on a disk 10 coated with one of various greases (13 kinds of grease in this embodiment), a vertical load P and horizontal vibration of amplitude A are applied to the ball 11, while measuring the friction coefficient μ.
First, at the room temperature, contact pressure between the [0032] disk 10 and the ball 11 was set to 2176 MPa (load P=100 N). Under this conditions, vibration of amplitude A=1 mm and frequency 15 Hz was applied to the ball 11 and friction coefficient μ_hwas measured for each kind of grease. Then the contact pressure was increased to 3954 MPa (load P=600 N) and friction coefficient μ₁was measured under the same conditions as the above (room temperature, A=1 mm and frequency 15 Hz). Decreasing rate of μ₁with respect to the friction coefficient μ_hwas determined by the following formula (1):
(μ_h−μ₁)/μ_h×100(%) (1).
With μ[0033] _hplotted along the abscissa and the calculated decreasing rate plotted along the ordinate, it can be seen that there is a substantially proportional relationship between μ_hand the decreasing rate as shown in FIG. 3. This implies that the higher the friction coefficient μ_hof a grease, the greater the decreasing rate of the friction coefficient and hence the lower the grease film strength.
Then service life test and measurement of induced thrust were conducted on a sample tripod type constant velocity joint filled with each kind of grease. The service life test was conducted under the conditions of rotation torque 834 Nm, operation angle θ=6° and rotational speed 230 rpm, and the induced thrust was measured under the conditions of rotation torque 294 Nm, operation angle θ=6° and rotational speed 150 rpm. Results of both tests are shown in FIG. 4 (‘NVH’ in the drawing indicates the result of induced thrust measurement). Regions A through C in FIG. 4 represent the regions of the graph shown in FIG. 3 divided by lines of μ[0034] _h=0.060 and 0.08 and the lines of decreasing rates of 40% and 60%.
As can be seen from FIG. 4, greases belonging to region B show satisfactory performance in both the service life and NVH characteristic. Greases belonging to region A, on the other hand, has satisfactory NVH characteristic but not in durability, while greases belonging to region C show poor performance in both durability and NVH characteristic. This may be because a grease belonging to region A shows good NVH characteristic due to high grease film strength but shows low durability due to poor initial compatibility, while a grease belonging to region C shows good initial compatibility but the low grease film strength makes the film more likely to break, resulting in a condition of lubrication so poor that cannot be covered by the good initial compatibility. Based on the forgoing discussion, a grease used in the tripod type constant velocity joint is preferably one that belongs to region B, namely that has SRV friction coefficient μ[0035] _hunder a contact pressure of 2176 MPa (load 100 N) in a range from 0.060 to 0.080 inclusive and decreasing rate (formula (1)) of SRV friction coefficient μ₁under a contact pressure of 3954 MPa with respect to the friction coefficient μ_hin a range from 40 to 60%.
While there is sliding friction working between the [0036] roller 3 and the roller guide surface 1 a 1 as mentioned previously, variation in the induced thrust is expected to increase as the amount of fretting wear in the contact surface increases. Accordingly, fretting resistance was evaluated on various greases by using a Fafnir friction oxidation tester.
The Fafnir friction oxidation test was conducted on such an arrangement as shown in FIG. 5 where [0037] roller bearings 13, 14 filled with 1 gram of a sample grease are fixed on a bearing holder 15 a of a vibration stage 15, a shaft 21 is inserted through an upper chuck 12, the bearings 13, 14, the vibration stage 15, a lower chuck 16, a spring guide 17, a spring 18, a spacer 19 and a washer 20 with the tip of the shaft 21 being screwed into a bolt 22, and a predetermined load (550 lb) is applied to the bearings 13,14 with the spring 18 by adjusting the tightening of the bolt 22. Under this condition, the vibration stage 15 is connected to a motor via a connecting rod not shown. After running the motor to oscillate the vibration stage 15 (angular amplitude of 0.21 radian at frequency of 30 Hz) for a predetermined period of time, fretting resistance of the sample grease (6 kinds in this embodiment) is evaluated in terms of the total weight loss of the bearings 13, 14. While the procedure of fretting resistance evaluation test specified in ASTM employs W-5/8 (made in U.S.A.) for the bearings 13, 14 with the test being continued for 22 hours, bearing 51204 made in Japan and test period of two hours are employed in the test of the present invention.
Then variation of the induced thrust was measured on a sample of the tripod type constant velocity joint filled with each kind of grease. The measuring conditions were rotation torque of 294 Nm, operation angle θ of 6° and rotational speed of 150 rpm. Induced thrust was measured at one-minute intervals during five minutes of operation, and the difference between maximum and minimum values of the measurements was taken as the variation of induced thrust. The test results are shown in FIG. 6. [0038]
FIG. 6 shows that the variation of induced thrust increases significantly when the wear amount measured in Fafnir friction oxidation test increases beyond 10 mg. Thus a grease that shows wear not greater than 10 mg in Fafnir friction oxidation test should be used. This makes it possible to obtain flat characteristic of induced thrust even under a large operation angle θ, thus improving the NVH characteristic. [0039]
The outer [0040] joint member 1 of the tripod type constant velocity joint is typically made of a steel such as carbon steel that is formed into a predetermined configuration by cold forging or the like and subjected to heat treatment such as induction hardening in order to ensure the required levels of strength, durability and wear resistance, followed by grinding of portions that require high precision thereby finishing the part to the predetermined dimensions and completing the product. Recently, studies have been made on the possibility of eliminating the grinding operation in the final stage of the manufacturing process, for the purpose of reducing the manufacturing cost. When the grinding operation is omitted, it is anticipated that the accuracy of the track groove 1 a of the outer joint member 1 would decrease with the surface roughness increasing, leading to poorer characteristic of induced thrust and lower durability. When 0.3 to 1.5 wt % of molybdenum disulfide is added to the grease described above, initial compatibility of the grease will be improved and therefore NVH characteristic and the durability can be improved. Thus it is made possible to eliminate the grinding operation. It goes without saying that MoS₂may be added also in the case of applying grinding operation, in which case NVH characteristic and the durability can be improved further. The reason for limiting the amount of MoS₂to be added in the range from 0.3 to 1.5 wt % is that addition of less than 0.3 wt % cannot achieve significant effect of reducing the vibration and, when the amount of addition is more than 1.5 wt %, the effect of reducing the vibration reaches a plateau and, in worst case, MoS₂powder exerts abrasive action on the rolling surface, eventually leading to increased wear and shorter service life.
The inner circumference surface of the outer [0041] joint member 1 that has been induction hardened may also be subjected to shot peening treatment. When this is done, as the surface is hardened and residual compressive stress is generated, fretting can be prevented and further reduction of vibration and elongation of service life can be achieved. It is a common practice to apply phosphoric acid coating treatment (Bonderite treatment) to the outer joint member 1 before applying the forging operation with the result of a hard and brittle oxide coat being formed on the surface as P is concentrated through the subsequent process of hardening, whereas applying shot peening removes the oxide coat and makes it possible to mitigate the wearing of the rolling surface. Also it is made possible to form microscopic pools for the grease out of microscopic dimples formed on the surface, thus reducing the frictional force of the rolling surface.
The present invention can be applied to greases of various compositions regardless of the type of base oil (mineral oil, poly-α-olefin, diester, etc.) and the type of the thickener (lithium-based, urea-based, etc.). Although FIG. 1 shows a case where a single roller is used, the grease of the present invention may also be used for a dual roller assembly comprising an inner roller and an outer roller with the outer roller adapted to be capable of swinging. [0042]
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. [0043]

Claims

What is claimed is:

1. A grease for a tripod type constant velocity joint having SRV friction coefficient μ_hunder a contact pressure of 2176 MPa in a range from 0.060 to 0.080 and decreasing rate of SRV friction coefficient μ₁under a contact pressure of 3954 MPa with respect to the friction coefficient μ_hin a range from 40 to 60%, the decreasing rate being represented by the following formula (1):

(μ_h−μ₁)/μ_h×100(%) (1).

2. A grease for a tripod type constant velocity joint according to claim 1, wherein said grease shows a wear amount of 10 mg or lower in Fafnir friction oxidation test.

3. A grease for s tripod type constant velocity joint according to claim 1, wherein 0.3 to 1.5 wt % of molybdenum disulfide is added thereto.

4. A tripod type constant velocity joint comprising an outer joint member having track grooves formed in the axial direction at equal intervals on an inner circumference thereof and having roller guide surfaces extending in the axial direction on both sides of each track groove, a tripod member having trunnions projecting in the radial direction and disposed at equal intervals along the circumference, rollers mounted rotatably on the respective trunnions and a grease filled the inside of the joint, the rollers being guided on the respective roller guide surfaces of the track grooves, wherein the grease has SRV friction coefficient μ_hunder a contact pressure of 2176 MPa in a range from 0.060 to 0.080 and decreasing rate of SRV friction coefficient μ₁under a contact pressure of 3954 MPa with respect to the friction coefficient μ_hin a range from 40 to 60%, said decreasing rate being represented by the following formula (1):

(μ_h−μ₁)/μ_h×100(%) (1).

5. A tripod type constant velocity joint according to claim 4, wherein the grease shows a wear amount of 10 mg or lower in Fafnir friction oxidation test.

6. A tripod type constant velocity joint according to claim 4, wherein 0.3 to 1.5 wt % of molybdenum disulfide is added to said grease.

7. A tripod type constant velocity joint according to one of claims 4 through 6, wherein the inner circumference surface of the outer joint member is subjected to shot peening treatment.