CA1096755A

CA1096755A - Rotatable sleeve rack

Info

Publication number: CA1096755A
Application number: CA299,038A
Authority: CA
Inventors: Frederick J. Adams
Original assignee: TRW Steering Systems Ltd
Current assignee: TRW Steering Systems Ltd
Priority date: 1977-03-15
Filing date: 1978-03-14
Publication date: 1981-03-03
Also published as: AR213757A1; BR7801547A; DK117878A; ES467816A1; NL7802851A; FR2384172B1; IT1107114B; YU61578A; IT7867551A0; FR2384172A1; SE443754B; SU888808A3; DE2809221A1; BE864870A; SE7803022L; JPS53133835A; JPS6033704B2; ZA781395B; LU79225A1; PL205328A1

Abstract

ROTATABLE SLEEVE RACK

ABSTRACT OF THE DISCLOSURE
An improved variable ratio steering apparatus is used to effect turning movement of steerable wheels of a vehicle. The apparatus includes a longitudinally extending force transmitting member or bar which is connected with the steerable vehicle wheels. A drive member is rotatably mounted on the bar. In order to effect axial movement of the bar and turning movement of the vehicle wheels, a pinion gear is disposed in meshing engagement with an array of helical rack gear teeth on the drive member. A cam track is also provided on the drive member. This cam track is coextensive with the array of rack gear teeth. The cam track cooperates with a follower to effect rotational movement of the drive member as the drive member and force transmitting bar are moved axially. This rotational movement of the drive member causes an interaction between the pinion and rack gear teeth to vary the rate of axial movement of the drive member and the force transmitting bar. The follower advantageously engages the cam track at a location which is axially aligned with the location where the pinion gear is disposed in meshing engagement with the rack gear teeth. This tends to minimize deflection of the components of the steering apparatus. Although an apparatus constructed in accordance with the present invention is advantageously used to turn the steerable wheels of a vehicle, it is also contemplated that the apparatus can be used in other environments.

Description

~a~7ss BACKGROUND OF THE INVENTION
This invention relates generally to a force transmit-ting apparatus and more specifically to a force transmittlng apparatus which is advantageously utilized to effect turning movement of a steerable wheel of a vehicle.
A known variable ratio steering apparatus is disclosed in Canadian Patent Application Serial No. 294,635 filed January 10, 1978 by Frederick John Adams and Ralph Malcolm Lehman and entitled "Improvements In Or Relating To Rack And Pinion Assemblies". The steering apparatus disclosed in that application includes a pinion having teeth which mesh with helical gear teeth on a rack bar. The rack bar is displaced longitudinally relative to a housing upon rotation of the pinion.
A cam track is disposed on the rack bar and is engaged by a follower which is connected with the housing. The cam track and follower cooperate to effect rotation of the rack bar upon longitudinal movement of the rack bar. This rotation of the rack bar causes the rack gear teeth and pinion gear teeth to interact in such a manner as to vary the rate of axial movement of the rack bar.
Although the steering apparatus disclosed in the aforementioned application is believed to be generally satisfact-ory in its construction and mode of operatlon, it is believed that after extended usage, ball joints at opposite ends of the rack bar may tend to wear. This wear of the ball joints is, to some extent at the least, induced by the rotation of the rack bar.
In addition, the cam track of the steering apparatus disclosed in the aforementioned application is axially offset from the rack gear teeth. Therefore, the rack bar must have a t~

-2-~0~6755 minimum length which is at least twice as great as the length of the array of rack gear teeth. It is believed that this relatively long length may be objectionable if the steering apparatus is used with relatively small vehi-cles. In addition, by having the cam track axially offset from the rack gear teeth, the rack gear is subjected to both torsional and bending forces when the rack gear is rotated by the cam follower.
The present invention provides a variable ratio gear assembly com-prising a pinion rotatably mounted in a housing and in meshing engagement with a rack of a longitudinally extending rack member, said rack member being rotatably mounted with respect to the housing and being longitudinally dis-placeable in response to rotation of the pinion; a yoke member engaging with the rack member to support it with the rack in meshing engagement with the pinion; control means for imparting controlled rotation to the rack member relative to the housing during longitudinal displacement of the rack member so that the ratio of the gear is varied, and wherein the rack teeth extend part way only around the circumference of the rack member to an extent that they maintain engagement with the pinion throughout the intended rotational movement of the rack member during longitudinal displacement thereof and the non-toothed portion of the rack member which is co-extensive longitudinally with the rack provides a slideway which said yoke member straddles and slid-ably engages to restrain displacement of the rack member in the direction of the axis about which the pinion rotates.
According to another aspect of the invention there is provided an apparatus for use in transmitting force with variable ratio gearing, said ap-paratus comprising a housing, a rotatable drive member mounted in the housing, said rotatable drive member being displaceable along its axis of rotation and relative to the housing; a rack gear comprising an array of helically formed teeth disposed on said drive member; a rotatable pinion gear mounted in the housing and engaging with the rack gear to effect axial displacement of said drive member upon rotation of the pinion gear; a cam track disposed 10"67SS

on said drive member, at least a portion of said cam track being co-extensive with said rack gear in the direction of the axis of rotation of said drive member; follower means disposed in engagement with said cam track for effect-ing rotational movement of said drive member relative to said pinion gear upon axial displacement of said drive member; each of said rack gear teeth extending only part way around the periphery of the drive member to an extent that the rack gear maintains engagement with the pinion gear throughout the intended rotational movement of the drive member; a slideway disposed on said drive member to be substantially co-extensive with said rack gear and being provided by the part periphery of the drive member in~o which said rack gear teeth do not peripherally extend, and support means for at least partially supporting said drive member, said support means being mounted by the housing and slidably engaging the slideway substantially in a plane which extends radially of the drive member and which includes the region of engagement between the rack gear and the pinion gear.
Although an apparatus constructed in accordance with the present invention is advantageously utilized to turn the steerable wheels of a vehi-cle, it is contemplated that the apparatus could be utilized in other envi-ronments if desired. For example, the apparatus could be used to control the movement of a traverse slide of a machine tool.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other features of the present invention will become more apparent upon a consideration of the following description taken in con-nection with the accompanying drawings wherein:
Figure 1 is a fragmentary sectional view of a variable ratio steer-ing apparatus constructed in accordance with the present invention;
Figure 2 is a fragmentary sectional view of a portion of the steer-ing apparatus of Figure 1 and illustrating the relationship between an array of rack gear teeth and a cam track disposed on a rotatable drive member;
Figure 3 is a sectional view, taken generally along the line 3-3 of ~Q~6755 Figure 2, and illustrating the relationship between the drive member and a pinion gear which is disposed in meshing engagement with the rack gear teeth, a cam follower which engages the cam track, and a support member which pres-ses the rack gear teeth into engagement with the pinion gear;
DESCRIPTION OF SPECIFIC PREFERRED
EMBODIMENTS OF THE INVENTION
A variable ratio steering apparatus 10 constructed in accordance with the present invention is illustrated in Figure 1. The steering appara-tus 10 is connected with a pair of steerable wheels (not shown) of a vehicle in a well known manner by a suitable linkage. This linkage includes a tie rod 12 connected with one of the steerable vehicle wheels and a second tie rod 14 connected with the other steerable wheel. The two tie rods 12 and 14 are connected with opposite ends of a longitudinally extending force trans-mitting member or bar 16 by a pair of ball joints 18 and 20. Upon axial move-ment of the bar 16 relative to a housing 24, the steerable wheels of the ve-hicle are turned to effect a desired steering action.
A tubular drive member or sleeve 28 is rotatably mounted on a central portion of the bar 16 and can be rotated without rotating the bar 16. To this end a bearing assembly 30 is disposed between a cylindrical inner sur-face 32 of the drive member 28 and the bar 16. The bearing assembly 30 co-operates with a needle bearing assembly 34 to rotatably support the drive member 28 on the bar 16 in a coaxial relationship with the bar. The bearing assembly 30 is effective to hold the drive member 28 against axial movement relative to the bar 16.
A longitudinally extending array 38 of helical rack gear teeth 40 is formed on the drive member 28 (see Figures 1 and 2). The rack gear teeth 40 extend only part way around the cylindrical outer surface of the drive member 28 and are disposed in meshing engagement with a rotatable pinion gear 42. Upon rotation of the pinion gear 42 about its central axis in response to turning of a steering wheel, the pinion gear 42 and rack gear teeth 40 co-lQ"6755 operate to cause the drive member 28 and bar 16 to be moved along their com-mon central axis 46. This effects turning movement of the steerable vehicle wheels in a known manner.
It is contemplated that during parking and other low speed operations in which the vehicle wheels are sharply turned, it may be desirable to de-crease the effort required to turn the vehicle wheels by increasing the steer-ing gear ratio. In order to increase the steering gear ratio, the drive member 28 is rotated about its central axis 46. This is accomplished by the interaction between a cam track 50 (Figure 2) and a follower 52 ~Figure 3).
Rotation of the drive member 28 causes the helical rack gear teeth 40 to slide relative to the helical teeth on the pinion gear 42. This slid-ing action displaces the drive member 28 longitudinally relative to the hous-ing 24. This longitudinal movement of the drive member 28 is in a direction opposite to the direction in which the drive member is being moved relative to the housing 24 by the pinion gear 42 to thereby increase the steering gear ratio. If the pinion gear 42 is rotated in a counterclockwise direction as viewed in Figure 2, the drive member 28 and bar 16 move toward the right (as viewed in Figure 2). During this rightward movement, the follower 52 engages a downwardly (as viewed in Figure 2) curving end portion 54 of the cam track 50. This rotates the drive member 28 upwardly (as viewed in Figure 2) or in a clockwise direction (as viewed in Figure 3).
The helical rack gear teeth 40 extend at an acute angle to the lon-gitudinal central axis 46 of the drive member 28. Therefore, rotation of the drive member 28 about its central axis 46 causes the flanks on the rack gear teeth to effect a sliding or camming action against the flanks of the helical teeth of the pinion gear 42. This camming action moves the drive member toward the left (as viewed in Figure 2) with a screw type action. This re-sults in a reduction in the rate of rightward (as viewed in Figure 2) move-ment of the drive member 28 and bar 16 to thereby effect an increase in the -mechanical advantage provided by the steering gear assembly.

10C~6755 Similarly, the pinion 42 is rotated in a clockwise direction (as viewed in Figure 2) to effect movement of the drive member 28 and bar 16 toward the left (as viewed in Figure 2). As this occurs, an arcuately up-wardly curving end portion 58 of the cam track 50 engages the cam follower 52 to rotate the drive member 28 downwardly (as viewed in Figure 2). The resulting interaction between the helical rack and pinion gear teeth reduces the rate of leftward (as viewed in Figure 2) movement of the drive member 28 and bar 16.

It should be noted that the cam track 50 has a central portion 62 which extends parallel to the longitudinal axis 46 of the drive member 28.

Therefore when the cam follower 50 is engaging the central portion 62 of the cam track 50, the cam follower and cam track are ineffective to cause rota-tion of the drive member 28 relative to the housing 24. Therefore the steer-ing ratio remains constant at this time.
In accordance with another feature of the present invention, the cam track 50 is axially coextensive with the array 38 of rack gear teeth 40.
Thus, the oppositely curving end portions 54 and 58 of the cam track 50 are disposed adjacent to axially opposite ends of the array 38 of rack gear teeth.

By having the length of the cam track 50 coextensive with the axial length of the array 38 of rack gear teeth 40, the overall length of the drive member 28 is minimized. It is believed that this feature will be particularly advanta-geous when the steering apparatus 10 is utilized in association with a rela-tively small or compact vehicle.
In order to maintain solid meshing engagement between the pinion gear 42 and the array 38 of rack gear teeth 40 during rotation of the drive member 28, the array 38 of rack gear teeth has a curving configuration which is the same as the curving configuration of the cam track 50. Thus, a lon-gitudinally extending edge portion 66 of the array 38 of rack gear teeth has a central portion which extends parallel to the straight central portion 62 of the cam track 50. The opposite ends of the edge portion 66 of the array l~q6755 38 of rack gear teeth curve in opposite directions in the same manner as do the opposite end portions 54 and 58 of the cam track 50. Accordingly, the edge portion 66 of the array 38 of rack gear teeth is parallel to the longi-tudinal central axis of the cam track 50. The opposite edge portion of the array 38 of rack gear teeth (not shown) has the same configuration as the edge portion 66. Therefore the array 38 of rack gear teeth has a substantial-ly constant circumferential extent of about 140 about the drive member 28.
In accordance with still another feature of the present invention, the cam follower 52 engages the cam track 50 at a location which is axially aligned with the location at which the pinion gear 42 meshingly engages the rack gear teeth 40 (see Figure 3). By having the location at which the cam follower 52 engages the cam track 50 axially aligned with the area where the pinion gear 42 meshingly cngages the rack gear teeth, the application of torsional loads to the drive member 28 is minimized. In addition, sidewise loads and bending moments applied to the drive member 28 are minimized. If the cam follower 52 engaged the cam track 50 at a location which was axially offset from the location where the pinion gear 42 meshes with the rack gear teeth 40, the forces applied to the drive member 28 by the cam follower 50 would be offset from the forces applied to the drive member by the interaction between the rack and pinion gear teeth. By having these forces aligned with each other so that there are no bending moments tending to twist the drive member about a transverse axis, the loading applied to the support structure for the drive member 28 and the bar 16 is minimized.
The rack gear teeth 40 and the drive member 28 are continuously ~-pressed into meshing engagement with the pinion gear 42 under the influence of a support yoke 70. The support yoke 70 is pressed upwardly against a lower side of the drive member 28 under the influence of a biasing spring 72 (see Figure 1). It should be noted that the support yoke 70 engages the drive member 28 at a location which is directly opposite from the location where the pinion gear 42 meshingly engages the rack gear teeth 40 so that the gear 10~6755 tooth forces which tend to separate the rack and pinion gears are offset by the support yoke. The cam follower 52 engages the cam track 50 at a location between the support yoke 70 and the area of meshing engagement between the rack and pinion gears.
In Figures 1-3, an apparatus constructed in accordance with the present invention is utilized to effect turning movement of the steerable wheels of a vehicle. ~owever, it is contemplated that the apparatus could be utilized in environments other than in association with the steerable wheels of a vehicle. For instance, it is contemplated that the apparatus could be utilized to move the traverse slide of a machine tool relative to the bed of the machine tool.

_g_

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A variable ratio gear assembly comprising a pinion rotatably mounted in a housing and in meshing engagement with a rack of a longitudin-ally extending rack member, said rack member being rotatably mounted with respect to the housing and being longitudinally displaceable in response to rotation of the pinion; a yoke member engaging with the rack member to support it with the rack in meshing engagement with the pinion; control means for imparting controlled rotation to the rack member relative to the housing during longitudinal displacement of the rack member so that the ratio of the gear is varied, and wherein the rack teeth extend part way only around the circumference of the rack member to an extent that they maintain engagement with the pinion throughout the intended rotational movement of the rack member during longitudinal displacement thereof and the non-toothed portion of the rack member which is co-extensive longitudinally with the rack provides a slideway which said yoke member straddles and slidably en-gages to restrain displacement of the rack member in the direction of the axis about which the pinion rotates.

2. A gear assembly as claimed in claim 1 in which the rack member is a sleeve of a rack assembly having a longitudinally extending bar on which the sleeve is rotatably mounted and relative to which the sleeve is restrained from longitudinal displacement, and wherein said rack assembly is longitudinally displaceable in response to rotation of the pinion and the control means imparts controlled rotation to the sleeve relative to the pinion and to the bar during longitudinal displacement of the rack assembly so that the ratio of the gear is varied.

3. A gear assembly as claimed in claim 1 in which the rack member is longitudinally displaceable in response to rotation of the pinion through a first region and through at least one further region which extends from the first region, and wherein said control means substantially restrains the rack member from rotating relative to the pinion during its displace-ment through said first region so that the ratio of the gear is substant-ially constant and imparts controlled rotation to the rack member relative to the pinion during its displacement through the or a said further region so that the ratio of the gear is varied as compared with the substantially constant ratio during displacement of the rack member through the first reg-ion.

4. A gear assembly as claimed in claim 3 in which the first region is a central region of longitudinal displacement of the rack member and two said further regions are provided which extend one from each end of said central region.

5. A gear assembly as claimed in claim 1, 2 or 3 wherein the yoke member is located between the rack member and a housing of the assembly to engage the slideway of the rack member at a position substantially opposite to the pinion.

6. A gear assembly as claimed in claim 3 in which the control means comprises a co-operating longitudinally extending track and a track follower one of which track and track follower is mounted to be secured against long-itudinal displacement in the assembly and the other of which is located on the rack member so that the track follower is displaced relatively along the track during longitudinal displacement of the rack member and the rack member is caused to be rotated in accordance with the path of the track.

7. A gear assembly as claimed in claim 6 wherein the first region of the track is substantially rectilinear and parallel to the axis of the rack member and at least one said further region of the track communicating with said first region extends longitudinally and in a direction other than parallel to the axis of the rack member.

8. A gear assembly as claimed in claim 7 wherein the, or at least one, said further region of the track is of substantially helical form.

9. A gear assembly as claimed in claim 6 in which the track is located on the rack member and extends longitudinally substantially over the same portion of the rack member as that on which the rack is located.

10. A gear assembly as claimed in claim 9 in which the track is a groove in the slideway of the rack member and the track follower engages within said groove.

11. A gear assembly as claimed in claim 10 wherein the track follower is rotatably mounted to roll along said groove during relative longitudinal displacement between the rack member and the track follower.

12. A gear assembly as claimed in either claim 10 or claim 11 wherein the track follower is mounted to engage the groove substantially in a plane which extends perpendicularly relative to the axis of the rack member and which plane includes the position of engagement between the rack and pinion teeth.

13. A steering gear when incorporating a gear assembly as claimed in any one of the preceding claims 1 to 3.

14. An apparatus for use in transmitting force with variable ratio gearing, said apparatus comprising a housing, a rotatable drive member mounted in the housing, said rotatable drive member being displaceable along its axis of rotation and relative to the housing; a rack gear comprising an array of helically formed teeth disposed on said drive member; a rotatable pinion gear mounted in the housing and engaging with the rack gear to effect axial displacement of said drive member upon rotation of the pinion gear;
a cam track disposed on said drive member, at least a portion of said cam track being co-extensive with said rack gear in the direction of the axis of rotation of said drive member; follower means disposed in engagement with said cam track for effecting rotational movement of said drive member relative to said pinion gear upon axial displacement of said drive member;
each of said rack gear teeth extending only part way around the periphery of the drive member to an extent that the rack gear maintains engagement with the pinion gear throughout the intended rotational movement of the drive member; a slideway disposed on said drive member to be substantially co-extensive with said rack gear and being provided by the part periphery of the drive member into which said rack gear teeth do not peripherally extend, and support means for at least partially supporting said drive mem-ber, said support means being mounted by the housing and slidably engaging the slideway substantially in a plane which extends radially of the drive member and which includes the region of engagement between the rack gear and the pinion gear.