GB2155367A

GB2155367A - Forming ball bearing tracks

Info

Publication number: GB2155367A
Application number: GB08506364A
Authority: GB
Inventors: Daniel Wesley Hazebrook
Original assignee: GKN Automotive Components Inc
Current assignee: GKN Driveline North America Inc
Priority date: 1984-03-12
Filing date: 1985-03-12
Publication date: 1985-09-25
Also published as: GB2155367B; ES541159A0; KR890000213B1; AU567376B2; DE3508487C2; JPS60186334A; KR850007018A; AU3545484A; IT8567247A1; IT1184902B; FR2560948A1; GB8506364D0; DE3508487A1; CA1241532A; BR8406297A; ES8606052A1; IT8567247A0; MX171215B

Abstract

A method for forming a precision ball track in a part, especially an outer or inner race member of a constant velocity ratio universal joint, wherein the part is initially formed with a relief groove 32 in the base of the ball track so that when the ball is subsequently ground to a predetermined precision shape, the relief groove reduces the wear on the grinding wheel used during the grinding operation. <IMAGE>

Description

SPECIFICATION Production of precision ball tracks in parts.

This invention relates to a method for the provision of precision ball tracks in parts. More particularly, the invention relates to the production of precision ball tracks in the inner or outer races of universal joints.

There are many mechanical parts having ball tracks therein for engagement with bearing balls.

There are many ways of production of such ball tracks, including formation of a ball track with the part when the part is forged or cast; grinding, machining or cold forming a track into a rough workpiece; or cold forming or forging an initially roughly dimensioned track during an early manufacturing process for the part and subsequently grinding or machining such a ball track into the required precision shape. The last mentioned method is the most common when a precisely determined shape for the ball track is required, since accurate dimensions are achieved without significant waste of material. Further, such a method causes less wear on the grinding or machining tool than is the case if the ball track is ground or machined in a workpiece not having an initially formed ball track therein.

Precision ball tracks are required, for example, in the inner and outer race members of universal joints of the type in which torque is transmitted between such members by balls disposed in ball tracks in the members. In one form of such universal joint, often referred to as a Rzeppa joint from its first disclosure in US Patents 2046584 and 1665280 of Rzeppa, the ball tracks are disposed as meridians in opposed spherical surfaces of the inner and outer race members. Such a joint has a constant velocity ratio (homokinetic) property, i.e.

when the joint is articulated the joint does not introduce any cyclical variations into the velocity of rotation of the joint members. For this reason, such universal joints have become popular for use in front wheel drive motor vehicles.

A typical Rzeppa joint requires six equally circumferentially spaced precision ball tracks to be provided in each of the joint members. The tracks are initially formed in the inner and outer joint race members when such members are forged or cast, and are subsequently machined or ground to produce the desired precision shape for each of the ball tracks. During such grinding or machining operation, the tool being used is subject to wear, and requires periodic replacement in order to maintain the accuracy of the operation, while replacement is expensive in terms of the cost of the tool and of the loss of operating time during replacement.

When the initial shape of a ball track is formed in such a part by a forming or forging operation, e.g.

cold forming, the metal of the part is displaced to a substantial extent, particularly in the region of the base of the ball track. Resistance to formation of the track is greatest here. Thus some relief in the region of the base of the ball track would be advantageous in order to facilitate the forming process. Further, when a ball track is subsequently to be ground to its final configuration, the grinding tool is usually rotated about an axis which extends outwardly from the base of the ball track. Thus, the portion of the grinding tool in the region of the ball track base is moving comparatively slowly, and encounters the greatest resistance. This causes rapid deterioration of this part of the grinding tool, requiring frequent replacement thereof and a large amount of down time.Friction also creates heat which may generate microscopic cracks in the part, requiring careful inspection. A noticeable amount of scrap results from the detection of such cracks, increasing manufacturing costs per part, and there is also the danger of flaws remaining undetected.

Accordingly, it is the object of the present invention to provide for the overcoming or reduction of such disadvantages.

According to the present invention, we provide a method of providing a precision ball track in a part, comprising the consequtive steps of: forming said part with a ball track therein and with a relief groove extending along said ball track; and grinding or machining said formed ball track to a predetermined shape to produce said precision ball track.

Preferably the relief groove extends along the base of the ball track.

The method provided by the invention results in less wear of the tool used for grinding or machining the ball track, as compared to prior art methods. An operation such as a cold forming operation for providing the initial ball track shape in the part is facilitated, and microscopic cracks are less likely to be introduced into the part. The provision of the relief groove reduces the wear on the machining or grinding tool by reducing the surface area engaged by the tool, in the area of greatest resistance to machining or grinding. Furthermore, the relief groove enhances metal flow at the base of the ball track during the forming operation.

The part preferably is an inner or an outer race member of a universal joint, having a plurality of ball engaging tracks of which at least one, and preferably all, is provided by such method. The tracks may be disposed as meridians in the race member of the universal joint.

The invention also provides a part, particularly an inner or outer race member of a universal joint, made by the method aforesaid.

The invention will now be described by way of example with reference to the accompanying drawings, of which: Figure 1 is a section through a constant velocity universal joint made by the method of the present invention; Figure 2 is a section on the line 2-2 of Figure 2; Figure 3 is an enlarged section of part of the universal joint.

Referring now to the drawings, and more particularly to Figures 1 and 2 thereof, a constant velocity ratio universal joint 10 of the Rzeppa type is illustrated. The principle of such joints is well known.

The joint 10 includes an outer race member 12 having an integral shaft 14 extending in a first direction therefrom. A spherical cavity 16 is formed in the outer race member 12. The spherical cavity 16 opens in a direction opposite the first direction.

As shown in Figures 1 and 2, circumferentially spaced first tracks 18 are formed in the interior of the outer race member 12 for receiving a series of bearing balls 20. The tracks 18 are disposed as meridians in the member 12.

An inner race member 22 is provided within the spherical cavity 16 of the outer race member 12.

The inner race member 22 is mounted to a shaft 24 by means of cooperating splines 26. The inner race member 22 is provided with an outer spherical surface 28 having a diameter substantially smaller than the diameter of the spherical cavity 16 of the outer race member 12.

Circumferentially spaced second tracks 30 are formed as meridians in the outer spherical surface~ 28 of the inner race member 22.

Each of the first tracks 18 is aligned with one of the second tracks 30 and receives therebetween one of the bearing balls 20, to permit angular movement of the shaft 24 connected with the inner race member 22 relative to the integral shaft 14 of the outer race member 12. The bearing balls 20 transfer rotational torque between the race members of the joint. A cage 42 with inner and outer part spherical surface is provided between the outer spherical surface 28 of the inner race member 22 and the spherical cavity 16 of the outer race member 12 and has aperturues to receive the bearing balls 20 in known manner.

According to prior art methods, the inner and outer race members 22 and 12 are formed by a casting or forging process, e.g. cold forming, and the first and second tracks 18 and 30 are formed, respectively, therein by subsequent grinding or machining operations using grinding or machining tools. Such operations result in a considerable amount of wear on the grinding or machining tools, requiring frequent replacement thereof.

Thus, the production machinery used to produce the inner and outer race members requires substantial down time and provides a low production rate. However, according to the present invention, the inner and outer race members 22 and 12 are modified so as to reduce the amount of wear on the machining tool, thus increasing the production rate and reducing the down time of the production machinery.

Each of the first tracks 18 is provided with a recess 32 extending along the length thereof. Similarly, each of the second tracks 30 is provided with a recess 34 therealong. The recesses 32 and 34, provided during the initial casting or forging of the race members, provide relief grooves to reduce the amount of wear on the grinding or machining tool used to grind or machine the first and second tracks 18 and 30 to predetermined precision shapes.

More particularly, as shown in Figure 3 with respect to one first track 18a, the recess 32 is formed at the base of the track since the greatest grinding or machining resistance force is experienced at the base of the track. Thus, the grinding or machining tool used in a finishing operation on the track 18a will experience substantially less wear than is the case if the recess 32 is omitted.

The recess 32 preferably has a substantially greater width w than its depth d, relative to the track 18a so as to provide a substantial lessening of the wear on the machining or grinding tool used, without substantially weakening the outer race member 12. For example, the width w of the recess 32 may be two to four times the dimension of the depth d. Furthermore, the recess 32 extends circumferentially about a portion of the track 18a such as to define an angle theta 0 of arc at the centre of curvature of the track as it is viewed in transverse section. The preselected angle theta is chosen such as to minimise the wear on the machining or grinding tool yet permit a sufficient remaining surface for the contact between the track 18a and the bearing ball 20 disposed therein.In the example illustrated, the angle theta is approximately twenty degrees though it may range from approximately ten degrees to approximately thirty degrees.

In the method of the invention, the outer race member 12 initially has the track 18a formed therein with a surface 36 having a radius rl. The subsequent grinding or machining operation produces a surface varying within preselected tolerances between a surface 38 having a minimum radius r2 and a surface 40 having a maximum radius r3. The depth d of the recess 32 is selected so as to exceed the difference between the maximum ground radius r3 and the initial formed radius r1, so that the grinding or machining tool never encounters the base 44 of the recess 32.

The remaining first tracks and second tracks of the inner and outer race members, respectively, are formed in an analogous manner to that described above. The grinding or machining operation may be performed by a single pass or multiple passes of the appropriate tool.

It should be noted that the method according to the present invention is more advantagously applied to the outer race member 12 than it is to the inner race member 22. The first tracks 18 for the outer race member 12 are typically proportioned, relative to the bearing balls 20, such that the bearing balls engage a surface of the track generally not including the base of the track. This is done so as to maximise the amount of bearing surface contact between the bearing balls 20 and the outer race member 12. Thus, the removal of a portion of the first tracks 18, by provision of a recess 32 in each of the tracks, removes a portion of the first track not experiencing any load. Thus, the method according to the present invention, when applied to the manufacture of an outer race member 12 of a constant velocity universal joint 10, increases the effective life of the grinding or machining tool used by removing that portion of the formed surface 36 of the first tracks 18 that offers the greatest resistance to the machining or grinding tool without effecting the functional engagement between the bearing ball 20 and the first track 18 associated therewith.

In contrast, the second tracks 30 of the inner race member 22 are typically proportioned in a manner providing substantial surface contact adjacent the base of the tracks. Accordingly, when an inner race member 22 is made according to the method of the present invention, as described above, the area of greatest contact between the bearing ball 20 and the second track 30 associated therewith will be on either side of the recess 34.

Therefore, for some applications, only the outer race member 12 would be formed according to the method of the present invention while the inner race member 22 would be formed according to prior art methods. Alternatively, the recess 34 formed at locations other than the base of the second track. For example, two recesses, not shown in the drawing, may be formed in each of the second tracks 30, the two recesses being disposed a predetermined angular distance away from the base of the track on opposite sides of such base.

In the example above described, the ball tracks in the joint race members are of circular shape in cross section. In practice such tracks in universal joints are often of "gothic" arch shape in transverse cross section, with the track having a clearly defined apex not in contact with the ball therein.

The invention is also applicable to the manufacture of parts with tracks of such form. Further, the invention is applicable to joint race members wherein the ball tracks are of forms other than the meridian curves described e.g. straight tracks parallel to the rotational axes of the respective members skewed helically relative thereto.

It will further be appreciated that a race member of constant velocity universal joint is one example only of a part having a ball track which may be made by the present method.

Claims

1. A method of providing a precision ball track in a part, comprising the consecutive steps of: forming said part with a ball track therein and with a relief groove extending along said ball track; and grinding or machining said formed ball track to a predetermined shape to produce said precision ball track.

2. A method according to Claim 1 wherein said relief groove extends along the base of said ball track.

3. A method according to Claim 1 or Claim 2 wherein said part comprises an inner race member of a universal joint having a plurality of ball-engaging tracks, of which at least one is provided by said method.

4. A method according to Claim 1 or Claim 2 wherein said part comprises an outer race member of a universal joint having a plurality of ball-engaging tracks of which at least one is provided by said method.

5. A method according to Claim 3 or Claim 4 wherein said tracks are disposed as meridians in said race member.

6. A method according to any one of the preceding claims wherein the or each said relief groove has a width substantially larger than its depth relative to said formed ball track.

7. A method according to any one of the preceding claims wherein the or each said relief groove has a depth substantially larger than the depth of grinding or machining during said grinding or machining step.

8. A method according to any one of the preceding claims wherein the or each said relief groove has a width defining between ten and thirty degrees of arc at the centre of curvature of said formed ball track, as viewed in transverse cross section.

9. A method substantially as hereinbefore described with reference to the accompanying drawings.

10. A part made by a method according to any one of the preceding claims.

11. An inner or outer member for a constant velocity ratio universal joint, substantially as hereinbefore described with reference to the accompanying drawings.