GB1604243A

GB1604243A - Rack bar and pinion assemblies

Info

Publication number: GB1604243A
Application number: GB1829777A
Authority: GB
Original assignee: TRW Steering Systems Ltd
Current assignee: TRW Steering Systems Ltd
Priority date: 1978-05-23
Filing date: 1978-05-23
Publication date: 1981-12-02

Description

(54) IMPROVEMENTS IN OR RELATING TO RACK BAR AND PINION ASSEMBLIES (71) We, CAM GEARS LIMITED of 45 Wilbury Way, Hitchin, Hertfordshire, SG4 OTU, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a rack bar and pinion assembly and is particularly concerned with an improved arrangement for the mounting of the rack bar in the assembly.

According to the present invention there is provided a rack and pinion assembly in which the pinion is rotatably mounted in a housing and the rack bar is mounted in the housing and has a rack which is engaged by the pinion so that upon rotation of the pinion the rack bar is displaced longitudinally relative to the housing, and wherein the rack bar is received within a resilient bush located adjacent the region of engagement of the rack with the pinion and through which bush the rack bar is longitudinally displaceable, the bush having a seating surface which slidably abuts the rack bar on the side thereof remote from its rack and the bush being removably mounted in the housing to resiliently bias the rack bar in a sense which urges the rack into engagement with the pinion, the seating surface of the bush comprising a peripheral array of discrete seating portions which extend around the rack bar, and the bush being capable of being rotated relative to the rack bar to index alternative seating portions for sliding abutment with the rac ar.

The housing of the assembly will have a pinion mounting part within which the pinion is rotatably mounted and through which the rack bar is displaceable and may include at least one tubular portion which extends longitudinally from the pinion mounting part and within which tubular portion or portions the rack bar extends.

The tubular portion or portions may be integral with the pinion mounting part or may be in the form of one or more tubes which are secured to the pinion mounting part. The resilient nature of the bush is utilised to urge the rack bar relative to the housing and in a sense which biases the teeth of the rack into engagement with the opinion; it is therefore preferred that the bush is located in the pinion mounting part adjacent to the position where the pinion meshes with the rack. It is envisaged however that the resilient bush, or an additional resilient bush, may be located at a position longitudinally remote from where the pinion meshes with the rack, for example in a tubular extension of the housing. By utilising the resilience of the bush to bias the rack into engagement with the pinion it is believed that spring or resiliently loaded yoke or saddle assemblies which are usually provided for this purpose in the pinion mounting housing part of conventional rack bar and pinion assemblies may be omitted thereby reducing manufacturing costs, assembly track adjustments tolerances required for accurate initial assembly, difficulties which may be encountered in providing lubrication and the relative size of the housing. Furthermore, the resilience of the bush may be used to accommodate shock loads applied laterally to the rack bar, particularly such loads as would reasonably be expected by use of the assembly in a rack and pinion steering gear for a vehicle.

A particular advantage which is derived from the present invention is achieved by having the seating surface of the resilient bush in the form of the peripheral array of seating portions which extend around the rack bar and by having the bush removably mounted in the housing so that, when one or more of the seating portions becomes worn by its sliding engagement with the rack bar, the bush may be withdrawn from its mounting in the housing, rotated relative to the rack bar and re-located in its mounting to present a further seating portion or portions to the rack bar on the side thereof remote from the rack. By this facility the expected life of the bush may be increased considerably by indexing the bush relative to the rack bar so that relatively unworn seating portions are presented to the rack bar as necessary for sliding abutment therewith at the regions where maximum wear on the bush is to be expected.

The bush is preferably formed as an assembly comprising a metallic sleeve within which is retained a block of appropriate resilient material such as polypropylene or neoprene, the block having an aperture through which the rack bar is slidable while the bush is mounted in the housing by the sleeve being located in a complementary seating. Usually the sleeve will be of cylindrical profile which is mounted as a tight push fit in a bore of the housing (if necessary restraining means such as a locking screw, splines or clips can be provided to restrain relative rotation between the sleeve and housing).

Alternatively, the sleeve can be of polygonal external profile such as square so that it is restrained from rotation by a complementary seating within which it may be mounted in the housing. Usually the aperture in the block of resilient material will be of generally circular profile and in a Dreferred construction the seating surface is formed by a peripherally extending array of seating portions each of which is part cylindrical for receiving a substantially cylindrical rack bar; preferably the radius of each part cylindrical seating portion is greater than the radius of the rack bar so that the peripheral surface of the rack bar will slidably abut in substantially longitudinally extending line or band contact with the surface of one or more of the seating portions. With the radius of each part cylindrical seating portion the same and greater than the radius of the rack bar such seating portions can have coincident centres of curvature (so that the seating portions will be located in the surface of a notional cylinder which is common to all such portions); preferably however the radii of the seating portions are each struck from different positions with respect to the axis from which the radius of the rack bar is struck (so that such seating portions are not located in the surface of a common notional cylinder as aforementioned) and by so offsetting the centre of curvature of each seating portion it is possible to determine the locations at which the seating portions are contacted by the rack bar to positions, say, in the circumferential centres of two adjacent seating portions. Conveniently, a longitudinally extending groove is provided in the resilient material between adjacent seating portions in the array to facilitate lubrication. Alternatively, the aperture may be of polygonal profile to provide substantially flat seating portions of which one or more such portions are intended to sliding abut the rack bar on the side thereof remote from the rack; for example the aperture may be square, a diagonal of which is located to lie in a plane which is perpendicular to the plane of the rack and which plane includes the longitudinal axis of the rack bar so that a cylindrical portion of the rack bar on its side remote from the rack slidably abuts in line or band contact two adjacent flat seating portions in the bearing which two portions are subjected to the major load and wear from the rack bar (and thereby permitting the other two portions to be indexed to the load bearing positions when necessary).

One embodiment of the present invention as applied to a rack and pinion assembly for a vehicle steering gear will now be described, by way of example only, with reference to the accompanying illustrative drawings, in which: Figure 1 is a part section through part of the steering gear and particularly illustrates the mounting and location of the resilient bearing bush; Figure 2 is a perspective view of the resilient bush incorporated in the assembly; and Figures 3 and 4 illustrate alternative forms of line or band contact which can be provided between the bush of Figure 2 and the rack bar.

The steering gear assembly comprises a longitudinally extending rack bar 1 of generally cylindrical form with a longitudinal axis la and which has machined thereon a rack 2. The teeth of the rack 2 engage with the teeth of a pinion 3 rotatably mounted by bearings 4 and 5 in a pinion mounting part 6 so that upon rotation of the pinion 3 the rack bar 1 is displaced longitudinally relative to and through the mounting part 6. The pinion mounting part 6 forms part of a housing which includes a tubular member (part of which is shown at 7) within which extends the rack bar 1 and which is secured to the mounting part 6. In use as a vehicle steering gear, the pinion 3 is coupled for rotation by a steering column and the rack bar 1 is coupled in conventional manner to provide a steering output to road wheels, for example through universal joints at the ends of the rack bar through which the latter is connected to a conventional steering linkage by tie rods.

Housed within the pinion mounting part 6 is a resilient bush 8 of generally annular configuration within which the rack bar is received and through which it is longitudinally displaceable. The bush 8 is formed by a generally annular block of resilient material 9 located within a cylindrical metal sleeve 10. The block 9 is retained by radially inwardly extending flanges 11 and 12 in the sleeve and the latter is received as a tight push fit within a complementary bore 13 of the pinion mounting part 6.

The inner peripheral face of the bush 8 is in the form of a seating surface comprising a peripheral array of four symmetrically disposed seating portions 14 each of which is of part cylindrical profile. A longitudinally extending groove 15 is provided between each pair of adjacent seating portions 14 to facilitate lubrication during movement of the rack bar through the bush. The aperture formed by the seating portions 14 receives the rack bar 1 as a close sliding fit and the radius "R" of each portion 14 is slightly greater than the radius "r" of the rack bar 1 in the region of its rack 2. By this latter arrangement the bush 8 may be located in the mounting part 6 so that the cylindrical portion of the rack bar in the region of the rack 2 and on the side of the rack bar remote from the rack 2 will abut in longitudinal sliding manner with one seating portion 14 or with two adjacent seating portions 14 (depending upon the orientation of the bush in the mounting part 6) and in substantially longitudinally extending line or band contact therewith.

With regard to the orientation of the bush 6, it is preferred that a diametrically opposed pair of the grooves 15 are located in a plane which is perpendicular to the plane of the rack 2 and which extends through the longitudinal axis la of the rack bar (as shown in Figures 3 and 4).

The preferred form of line or band contact between the rack bar 1 and the seating portions 14 is that shown in Figure 3 in which the radii R of the four portions 14 are struck from different centres "Teach of which is displaced from the axis la. By so displacing the centre T it will be seen from Figure 3 that a line or band of contact B is achieved between the rack bar 1 and each of two adjacent portions 14 and that such contact regions B are located approximately at the mid part of the circumferential extent of the respective seating portions. In the alternative form of line or band contact shown in Figure 4 the radii R of the portions 14 are struck from a common centre TI which is displaced from the axis la.

Consequently in Figure 4 the seating portions 14 lie in the surface of a common notional cylinder and a line or band of contact B is again achieved between the rack bar 1 and each of two adjacent portions 14; in this case however the contact regions B are located immediately adjacent to, and on opposite sides of, a groove 15 and as such the support afforded to the rack bar is likely to be less firm in the arrangement shown in Figure 4 than in that shown in Figure 3.

Upon fitting the bush 8 in the assembly it is envisaged that the resilience of the material in the block 9 will bias the rack of the rack bar into engagement with the teeth of the pinion while permitting the rack bar to be displaced longitudinally through the assembly. This resilient biasing afforded by the bush 8 also permits the resilient material to accommodate radial shock loading on the rack bar as may be encountered during normal usage of the steering gear. As the bush 8 biases the rack bar into engagement with the pinion the maximum load and wear on the block 9 of resilient material will be taken through the two seating portions 14 which abut the rack bar on the side thereof remote from the rack 2 and as such these two portions 14 will wear until it is desirable that they are replaced. To effect this latter replacement it is merely necessary to withdraw the bush 8 from the bore 13 within which it is mounted, to rotatably index the bush through 1800 and re-locate the bush within the bore 13 to present the two "new" seating portions 14 in abutment with the rack bar on the side thereof remote from the rack. If required, an appropriate retaining device such as a circlip can be used to retain the bush 8 in the bore 13 or a locking screw can be provided which both retains the bush and restrains it from rotating relative to the mounting part 6.

Relative rotation between the bush 8 and the mounting part 6 may alternatively or in addition be restrained by providing appropriate radially extending projections between the bush and mounting part (for example splines or ribs on the bush) which engage in grooves in the bore 13 of the mounting part and which arrangement may be used to facilitate rotatable indexing of the bush in its bore 13 when necessary.

WHAT WE CLAIM IS: 1. A rack and pinion assembly in which the pinion is rotatably mounted in a housing and the rack bar is mounted in the housing and has a rack which is engaged by the pinion so that upon rotation of the pinion the rack bar is displaced longitudinally relative to the housing, and wherein the rack bar is received within a resilient bush located adjacent the region of engagement of the rack with the pinion and through which bush the rack bar is longitudinally displaceable, the bush having a seating surface which slidably abuts the rack bar on the side thereof remote from its rack and the bush being removably mounted in the housing to resiliently bias the rack bar in a sense which urges the rack into engagement with the pinion, the seating surface of the bush comprising a peripheral array of

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Claims

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cylindrical metal sleeve 10. The block 9 is retained by radially inwardly extending flanges 11 and 12 in the sleeve and the latter is received as a tight push fit within a complementary bore 13 of the pinion mounting part 6.

The inner peripheral face of the bush 8 is in the form of a seating surface comprising a peripheral array of four symmetrically disposed seating portions 14 each of which is of part cylindrical profile. A longitudinally extending groove 15 is provided between each pair of adjacent seating portions 14 to facilitate lubrication during movement of the rack bar through the bush. The aperture formed by the seating portions 14 receives the rack bar 1 as a close sliding fit and the radius "R" of each portion 14 is slightly greater than the radius "r" of the rack bar 1 in the region of its rack 2. By this latter arrangement the bush 8 may be located in the mounting part 6 so that the cylindrical portion of the rack bar in the region of the rack 2 and on the side of the rack bar remote from the rack 2 will abut in longitudinal sliding manner with one seating portion 14 or with two adjacent seating portions 14 (depending upon the orientation of the bush in the mounting part 6) and in substantially longitudinally extending line or band contact therewith.

With regard to the orientation of the bush 6, it is preferred that a diametrically opposed pair of the grooves 15 are located in a plane which is perpendicular to the plane of the rack 2 and which extends through the longitudinal axis la of the rack bar (as shown in Figures 3 and 4).

The preferred form of line or band contact between the rack bar 1 and the seating portions 14 is that shown in Figure 3 in which the radii R of the four portions 14 are struck from different centres "Teach of which is displaced from the axis la. By so displacing the centre T it will be seen from Figure 3 that a line or band of contact B is achieved between the rack bar 1 and each of two adjacent portions 14 and that such contact regions B are located approximately at the mid part of the circumferential extent of the respective seating portions. In the alternative form of line or band contact shown in Figure 4 the radii R of the portions

14 are struck from a common centre TI which is displaced from the axis la.

Consequently in Figure 4 the seating portions 14 lie in the surface of a common notional cylinder and a line or band of contact B is again achieved between the rack bar 1 and each of two adjacent portions 14; in this case however the contact regions B are located immediately adjacent to, and on opposite sides of, a groove 15 and as such the support afforded to the rack bar is likely to be less firm in the arrangement shown in Figure 4 than in that shown in Figure 3.

Upon fitting the bush 8 in the assembly it is envisaged that the resilience of the material in the block 9 will bias the rack of the rack bar into engagement with the teeth of the pinion while permitting the rack bar to be displaced longitudinally through the assembly. This resilient biasing afforded by the bush 8 also permits the resilient material to accommodate radial shock loading on the rack bar as may be encountered during normal usage of the steering gear. As the bush 8 biases the rack bar into engagement with the pinion the maximum load and wear on the block 9 of resilient material will be taken through the two seating portions 14 which abut the rack bar on the side thereof remote from the rack 2 and as such these two portions 14 will wear until it is desirable that they are replaced. To effect this latter replacement it is merely necessary to withdraw the bush 8 from the bore 13 within which it is mounted, to rotatably index the bush through 1800 and re-locate the bush within the bore 13 to present the two "new" seating portions 14 in abutment with the rack bar on the side thereof remote from the rack. If required, an appropriate retaining device such as a circlip can be used to retain the bush 8 in the bore 13 or a locking screw can be provided which both retains the bush and restrains it from rotating relative to the mounting part 6.

Relative rotation between the bush 8 and the mounting part 6 may alternatively or in addition be restrained by providing appropriate radially extending projections between the bush and mounting part (for example splines or ribs on the bush) which engage in grooves in the bore 13 of the mounting part and which arrangement may be used to facilitate rotatable indexing of the bush in its bore 13 when necessary.

WHAT WE CLAIM IS: 1. A rack and pinion assembly in which the pinion is rotatably mounted in a housing and the rack bar is mounted in the housing and has a rack which is engaged by the pinion so that upon rotation of the pinion the rack bar is displaced longitudinally relative to the housing, and wherein the rack bar is received within a resilient bush located adjacent the region of engagement of the rack with the pinion and through which bush the rack bar is longitudinally displaceable, the bush having a seating surface which slidably abuts the rack bar on the side thereof remote from its rack and the bush being removably mounted in the housing to resiliently bias the rack bar in a sense which urges the rack into engagement with the pinion, the seating surface of the bush comprising a peripheral array of

discrete seating portions which extend around the rack bar, and the bush being capable of being rotated relative to the rack bar to index alternative seating portions for sliding abutment with the rack bar.
2. An assembly as claimed in claim I in which the housing has a pinion mounting part in which the pinion is rotatably mounted and through which the rack bar is displaceable, and wherein the bush is located in the pinion mounting part adjacent to the position where the pinion meshes with the rack.
3. An assembly as claimed in either claim 1 or claim 2 in which the seating portions have part cylindrical profiles.
4. An assembly as claimed in any one of the preceding claims in which the surface of the rack bar which slidably abuts the seating surface of the bush is substantially cylindrical or part cylindrical.
5. An assembly as claimed in claim 4 when appendant to claim 3 in which the part cylindncal seating portions have radii each of which is greater than the radius of the cylindrical rack bar so that the rack bar stidably abuts at least one of the seating portions substantially in a longitudinally extending line or band of contact.
6. An assembly as claimed in claim 5 in which the radius of the respective seating portions is the same and such radii are struck from different positions with respect to the axis from which the radius of the cylindrical surface of the rack bar is struck.
7. An assembly as claimed in claim 5 in which the radius of the respective seating portions is the same and such radii have coincident centres of curvature so that the seating portions are located in the surface of a notional cylinder which is common to all said portions.
8. An assembly as claimed in any one of the preceding claims in which longitudinally extending grooves are provided in the resilient material of the bush between peripherally adjacent seating portions in the array.
9. An assembly as claimed in claim 8 when appendant to claim 6 in which the bush is orientated, or is capable of being orientated, relative to the rack bar so that a line or band of contact is provided between the cylindrical surface of the rack bar and each of two seating portions and such lines or bands of contact are located substantially at the mid part of the circumferential extent of the respective seating portions.
10. An assembly as claimed in either claim 1 or claim 2 or in claim 4 when appendant to either claims 1 or claim 2 in which the aperture in the bush through which the rack bar is slidable is substantially of polygonal profile to provide substantially flat seating portions.
11. An assembly as claimed in any one of the preceding claims in which the bush is an assembly comprising a metallic sleeve within which is retained a block of resilient material, said block having the aperture through which the rack bar is slidable and the sleeve being located in a complementary seating in the housing.
12. An assembly as claimed in any one of the preceding claims in which the bush has a substantially cylindrical external profile and is mounted in a bore in the housing.
13. An assembly as claimed in claim 12 in which restraining means is provided to restrain relative rotation between the bush and the housing.
14. An assembly as claimed in any one of claims 1 to 11 in which the bush has a substantially polygonal external profile and is restrained from rotation relative to the rack bar by engagement of the polygonally profiled bush with a seating in the housing.
15. An assembly as claimed in any one of the preceding claims in which retaining means is provided for retaining the bush in its mounting in the housing.
16. A rack and pinion assembly substantially as herein described with reference to the accompanying illustrative drawings.
17. A rack and pinion steering gear which includes an assembly as claimed in any one of the preceding claims.