GB2025568A

GB2025568A - Toothed gearing

Info

Publication number: GB2025568A
Application number: GB7829624A
Authority: GB
Original assignee: TRW Steering Systems Ltd
Current assignee: TRW Steering Systems Ltd
Priority date: 1978-07-12
Filing date: 1978-07-12
Publication date: 1980-01-23
Also published as: GB2025568B

Abstract

A variable ratio gear assembly particularly suitable for steering gears and having a helically toothed wheel or pinion (30) which is axially rotatable and in toothed driving engagement with a further toothed member such as a rack bar (27) or a further pinion. One of the toothed members, usually the wheel (30), is axially displaceable relative to the other toothed member, (the rack bar 27) while toothed driving engagement is maintained. Cam means such as cam surfaces (37 and 38) engaging with cam followers (42 and 43) on a roller (39) operatively associated with the wheel (30) is responsive to rotation of the wheel (30) to cause axial displacement of the wheel (30) relative to the rack bar (27) during rotation of the wheel (30) and thereby a variation in the gear ratio which is transmitted through the assembly. The axially displaceable wheel can be driven or an idler wheel. <IMAGE>

Description

SPECIFICATION A variable ratio gear assembly This invention relates to a variable ratio gear assembly.

According to the present invention there is provided a variable ratio gear assembly comprising a toothed wheel member which is axially rotatable and in toothed driving engagement with a further toothed member, one of said toothed members being axially displaceable relative to the other toothed member while said toothed driving engagement is maintained and wherein cam means is provided which is operatively associated with said toothed wheel member and is responsive to rotation of that member to cause displacement of the axially displaceable toothed member relative to said other toothed member during rotation of the toothed wheel member and thereby a variation in the gear ratio which is transmitted through the assembly.

Although the variable ratio gear assembly of the present invention is capable of a wide field of industrial application, for example, in the control of machine tool components, it was primarily developed for use in a steering gear and the invention further provides a steering gear which includes a variable ratio gear assembly as specified in the immediately preceding paragraph and in which the toothed wheel member is rotatable in response to a steering manoeuvre applied to a steering input component and the further toothed member is coupled to a steering linkage which provides a steering output in response to said manoeuvre.

By the present invention the toothed wheel member drives or is driven by its toothed engagement with the further toothed member. The latter component will usually be in the form of a second rotatably mounted toothed wheel member or a rack bar the rack teeth of which engage with the teeth of the wheel member so that, in either case, as the toothed wheel member is rotated axially the further toothed member is displaced rotationally or longitudinally in a direction which extends radially with respect to the forementioned axis. Assuming that there is no axial displacement between the toothed wheel member and the further toothed member while drive is being transmitted between those members, then a predetermined and constant gear ratio will be provided.

However, if axial displacement is effected between the toothed wheel member and the further toothed member whilst drive is being transmitted between those members then the effect of such displacement will be to cause a relative increase or decrease in the gear ratio which is being transmitted between those members as compared with such gear ratio as would be provided in the absence of the aforementioned relative axial displacement. As a consequence, by controlling the relative axial displacement between the toothed wheel member and the further toothed member, a predetermined variation in gear ratio can be provided for the assembly.Such a predetermined variation in gear ratio has been found particularly advantageous in steering gears where it is desirable for steering the road wheels of a vehicle to provide a progressively decreasing gear ratio (that is an increasing mechanical advantage) as the steerable wheels are manoeuvred from their straight head condition to either of their full lock conditions so that the lowest gear ratio is available at or towards the full lock condition of the steerable wheels to assist in parking manoeuvres.

In the present invention control of relative axial displacement between the toothed wheel member and the further toothed member is provided by the cam means which is responsive to rotation of the toothed wheel member. The cam means will usually be arranged so that for each revolution of the toothed wheel member the axially displaceable member will exhibit a complete and predetermined cycle of reciprocal displacement axially whereby, when applied to a vehicle steering gear, the same variation in steering characteristics will be accurately reproduced for each steering manoeuvre from one full lock to the other.

As aforementioned the further toothed member can be in the form of a rack bar and although it is envisaged that such a rack bar can be mounted to be capable of displacement in the direction of the axis of rotation of the toothed wheel member (that is in a direction which extends laterally with respect to the longitudinal extent of the rack bar) it will be more usual, especially in steering gear applications, for the toothed wheel member to be the member which is axially displaceable under control of the cam means. Cam means for effecting axial displacement of the toothed wheel member preferably comprises a cam follower which co-operates with a cam surface and which follower and surface move relatively to each other during rotation of the toothed wheel member to control its axial displacement.In one form of construction the cam surface may simply comprise a cam track and the cam follower a peg or similar component which engages with the track (such a cam track is conveniently machined on the toothed wheel member two rotate therewith relative to the cam follower which latter is conveniently carried by a housing of the assembly) so that by appropriate configuration or profiling of the cam track the desired axial displacement in both senses of axial direction is imparted to the toothed wheel member as a consequence of its rotation. Preferably, however, the cam means comprises a cam surface of a predetermined profile on the toothed wheel member and the cam follower is mounted to react by abutment on the cam surface to control axial displacement of the toothed wheel member during its rotation.In this latter arrangement the cooperation between the cam follower and the cam surface controls displacement of the toothed wheel member in one sense of axial direction and consequently there may be provided a second of cam surface and cam follower in abutment therewith to control displacement of the toothed wheel member in the opposite sense of axial direction; by appropriately profiling and axially inclining the aforementioned cam surfaces a desired cycle of axial displacement may be imparted to the toothed wheel member for each revolution thereof.

The toothed wheel member may provide an input component of the gear assembly and as such is conveniently in the form of a pinion which, in the case of a vehicle steering gear, would be coupled to a steerable component for rotation in response to a steering manoeuvre. In some gear assemblies however, especially for pinion actuated steering gears, it may not be convenient or desirable that such an input pinion component exhibits axial displacement and to alleviate this the axially displaceable toothed wheel member can be in the form of an idler which itself is driven by rotation of a pinion in engagement therewith and in turn drives an output component of the gear assembly (which output component may be in the form of a rack bar or a further pinion).

Embodiments of variable ratio gear assemblies constructed in accordance with the present invention will now be described, by way of example only, with reference to the accompanying illustrative drawings in which: Figures 1 and2 are respectively a plan and a side elevation (in part sections) of a first embodiment of the gear assembly; Figures 3 and 4 are respectively a plan and a side elevation (in part sections) of a second embodiment of the gear assembly.

Figure 5 is a part section through a third embodi mentofthe gear assembly; and Figure 6 is a section taken on the line X-X of the assembly shown in Figure 5.

The gear assembly shown in Figures 1 and 2 has a housing 1 within which is rotatably mounted a helically toothed pinion 2. The pinion 2 has integral shafts 3 and 4 which are respectively mounted in bearings 5 and 6 for rotation of the pinion about an axis 7. The shaft 4 extends from the housing 1 and is splined at 8 to be rotatably driven. The bearings 5 and 6 are ball bearing assemblies respectively comprising outer races 9 and 10 which are secured relative to the housing 1 and inner races 11 and 12 which are rotatable with the shafts 3 and 4 respectively. The inner races 11 and 12 are provided with axially opposed convexly radiussed surfaces 13 and 14 respectively.

Also rotatably mounted in the housing 1 is a wheel member 15 having a circumferential array of helical teeth 16 which engage with the teeth of the pinion 2.

The wheel member 15 is slidably mounted on a cylindrical shaft 17 (the axis 18 of which is parallel to the axis 7) to be capable of both rotation about the shaft 17 and longitudinal displacement relative thereto along the axis 18. The shaft 17 has its ends in the form of cylindrical spigots 19 which are coaxial with an axis 20 and which are received in com plementaryboresinthewall of the housing 1 so that the shaft 17 is itself rotatable in the housing 1. The axis 20 of the spigots 19 is parallel to and displaced from the axis 18 so that the spigots 19 are eccentric with respect to the cylindrical portion of the shaft 17 on which the wheel member 15 is rotatable. Provided in the housing 1 to secure a spigot 19 and thereby the shaft 17 against rotation relative to the housing 1 is a lock screw 21.

The helical teeth 16 of the wheel member 15 engage with a rack of helical teeth 22 on a rack bar 23 which is longitudinally displaceable through the housing 1 in a direction which extends radially relative to the axes 7 and 18. A spring biased yoke 24 is mounted in the housing 1 on the side of the rack bar 23 remote from the wheel member 15 to slidably engage the rack bar and urge its teeth into engage mentwith the teeth 16.

In operation of the gear assembly shown in Figures 1 and 2 the pinion 2 is restrained by its bearings 5 and 6 from displacement along its axis 7 relative to the housing 1. Upon rotation of this piniDn 2 in response to an input through the shaft 4, the rack bar 23 is displaced longitudinally relative to the housing 1 by a drive which is transmitted thereto from the pinion 2 through the idler wheel member 15. If, during such displacement of the rack bar 23, the position of the wheel member 15 along its axis 18 remains unchanged with respect to the pinion 2 then the assembly will provide a constant gear ratio between rotation of the pinion 2 and displacement of the rack bar 23.However, if during rotation of the pinion 2, the wheel member 15 is displaced along its axis 18 on the shaf6 17 and with respect to the pinion 2 whilst the teeth 16 are maintained in engagement with the teeth of a pinion and those of the rack 22, then the rate of longitudinal displacement of the rack bar 23 will, for a constant rate of rotation of the pinion 2, be increased or decreased (depending upon the direction in which the wheel member 15 is axially displaced) so that there is a variation in the mechanical advantage or gear ratio of the assembly.

To provide such a variation in the gear ratio in the embodiment of Figures 1 and 2 the idler wheel member 15 has its axially opposed side faces in the form of concavely radiussed cam surfaces 25 and 26 which are assymmetrical with respect to the axis 18.

These cam surfaces 25 and 26 are axially spaced to be received between the surfaces 13 and 14 of the inner races 11 and 12. The radiussed surfaces 13 and 14 engage the cam surfaces 25 and 26 respectively in abutment and during rotation ofthe pinion 2 and wheel member 15 maintain sliding contact therewith. To achieve the axial displacement of the wheel member 16 along its axis 18 which is required for the required variation in gear ratio, the cam surfaces 25 and 26 are appropriately profiled so that their engagement with the inner races 11 and 12 effects in such axial displacement of the wheel member 15 as is appropriate.

From Figure 2 it will be apparent that several revolutions of the pinion 2 are required to effect one revolution of the wheel member 15. The cam surfaces 25 and 26 are arranged however so that for each revolution of the wheel member 15 that member will exhibit one cycle of reciprocal displacement along its axis 18 all be itthat during such a cycle the rate of axial displacement can increase or decrease in accordance with the profiles of the cam surfaces 25 and 26.

If required the depth of tooth engagement between the wheel member 15 and the rack bar 23 can be adjusted by releasing the lock screw 21 and partially rotating the shaft 17 about the axis 20 of its spigots 19 and with respect to the housing 1 (thereby raising or lowering the wheel member 15 with respect to the rack teeth 22) as appropriate prior to relocking the spigot 19 of the shaft 17 with the screw 21. Rotation of the shaft 17 about axis 20 is conveniently achieved by application of a screw driver to a slot 1 9a provided in the end of one of the spigots.

In the embodiment shown in Figures 3 and 4 a rack bar 27 having helical rack teeth 28 is longitudinally displaceable through a housing 29 within which is rotatably mounted a pinion 30. The pinion 30 has helically formed teeth which engage with the rack teeth 28 so that upon rotation of the pinion the rack is displaced longitudinally relative to the housing 29.

Integrally formed with the pinion 30 are shafts 31 and 32 by which it is mounted for rotation about an axis 33 with respect to the housing. The shaft 31 is mounted in a plane socket bearing 34 in the housing while the shaft 32 extends through a plane bearing from the housing in sealed manner and is splined at 35 to be rotatably driven.

Axially spaced on the pinion 30 and secured relative thereto are two diametrically enlarged portions 35 and 36. The portions 35 and 36 are preferably integral with the pinion and respectively carry axially opposed frusto conical cam surfaces 37 and 38 which are assymmetrical with respect to the pinion axis 33. These cam surfaces 37 and 38 extend around the periphery of the pinion and are tapered to converge towards the axis 33 as they approach each other.

Mounted in the housing 29 adajcentto the pinion 30 is a roller 39 which is born on spigots 41 for rotation about an axis 40 which axis is parallel to axis 33. Formed on the ends of the roller 39 are axially opposed frusto conical surfaces 42 and 43 which are concentric with and symmetrical about the axis 40 and diverge as they approach each other. As is clearly seen in Figure 3 the surfaces 42 and 43 are received within the peripheral recess formed by the spool like configuration presented by the pinion 30 and cam surfaces 37 and 38 so that the surfaces 42 and 43 form cam followers which respectively engage in abutment with the cam 37 and 38.

The roller 39 is mounted in the housing 29 to be restrained against displacement along its axis 40 and relative to the housing. The spool 30 however is mounted in the housing 29 to be capable of displacement relative to the housing along its axis 33 by axial sliding movement of the shafts 31 and 32 in their respective bearings. In this embodiment a variation in gear ratio is achieved by displacing the pinion 30 'along its axis 33 and across the rack bar 27 whilst engagement is maintained between the pinion teeth and the rack teeth; such displacement as is required is effected by appropriate profiling of the cam surfaces 37 and 38 whereby the engagement of these cam surfaces with the frusto conical surfaces 42 and 43 causes, during rotation of the pinion, the pinion 30 to be displaced axially to vary the gear ratio.

Similarly to the arrangement shown in Figure 2, the rack bar 27 in Figure 4 is biased relative to the housing 29 and into engagement with the pinion teeth by a spring loaded yoke 44.

Reverting to the embodiment shown in Figures 1 and 2 it will be seen that displacement of the toothed wheel member 15 in one sense of axial direction is achieved by the axial force exerted on the wheel member 15 by the surface 14 of the inner race 12 (which may be regarded as a cam follower) abutting the cam surface 26 while the displacement of the member 15 in the opposite sense of axial direction is achieved by the axial force exerted on the wheel member 15 by the surface 15 of the inner race 11 (which may also be considered as a cam follower) abutting the cam surface 25.Similarly in the embodiment shown in Figures 3 and 4 axial displacement of the pinion 30 in one sense of direction is effected by the axial reaction which results from abutment between the cam surface 38 and the cam follower surface 43 while displacement of the pinion 13 in the opposite sense of axial direction is effected by the axial reaction which results from abutment between the cam surface 37 and the cam follower surface 42.

In the embodiment of Figures 5 and 6 a helically toothed pinion 45 is mounted in bearings 46 and 47 within a housing 48 to be rotatable about an axis 49. The pinion 45 is restrained by the bearings 46 and 47 from axial displacement relative to the housing 48 and has a shaft 50 which extends from the housing and is splined at 51 to be rotatably driven.

The pinion 45 meshes with a larger diameter helically toothed idler wheel 52 having integral cylindrical spigots 53 by which the idler wheel 52 is mounted for rotation about an axis 54 in the housing 48. The spigots 53 are respectively received in complementary plane bearing sockets 55 in the housing 48 so that the idler wheel can be displaced slidably relative to the housing 48 along its axis 54 (which latter axis is parallel to the axis 49 of the pinion 45).

In meshed engagement with the idler wheel 52 is a second helically toothed pinion 56 mounted in bearings 57 and 58 within the housing 48 to be rotatable about an axis 59 (which latter axis is parallel to the axis 49. The pinion 56 is restrained by the bearings 57 and 58 from axial displacement relative to the housing and has a shaf6 60 which extends from the housing and is splined at 61 to provide an output connection from the gear assembly.

Upon rotation of the pinion 45 in response to an input through the shaft 50 it will be apparent that the pinion 56 is rotated by a drive which is transmitted thereto through the idler wheel 52. If, during rotation of the pinion 45 the position of the idler wheel 52 along its axis 54 remains unchanged with respect to the housing 48 then the assembly will provide a constant gear ratio between the rotation of the pinion 45 and the rotation of the pinion 56. However, if during rotation of the pinion 2 the idler wheel 52 is displaced along its axis 54 whilst its teeth are maintained in engagement with both pinions 45 and 56 it will be appreciated that, for a constant rate of rotation of the pinion 45 there will be a variation in the rate at which a pinion 56 is rotated (depending, in part, upon the axial direction in which the idler wheel 52 is displaced) and as a consequence there will be a variation in the gear ratio of the assembly.

To provide such a variation in gear ratio of the assembly of Figures 5 and 6 the idler wheel 52 has axially opposed and parallel side faces in the form of flat cam surfaces 62 and 63 which extend acutely with respect to the axis 54. In rolling engagement one with each of the cam surfaces 62 and 63 are rollers 64 and 65 respectively. These rollers engage in abutment with their respective cam surfaces at positions where they oppose each other in a direction parallel to the axis 54 and are freely mounted in the housing 48 on shafts 66 to rotate about respective axes which extend radially and perpendicularly with respect to the axis 54 of the idler wheel 52. The rollers 64 and 65 while abutting the cam surfaces 62 and 63 are restrained by their respective shafts 66 from displacement in the direction of the axis 54.

Consequently when the idler wheel 52 is rotatably driven by the pinion 45 to drive the pinion 56, the co-operation between the rollers 64 and 65 and their respective cam surfaces 62 and 63 causes the idler wheel 52 to be displaced along its axis 54 in response to rotation of the pinion 45 and in accordance with the inclinations of the cam surfaces 62 and 63. By providing a predetermined angle of inclination between the cam surfaces 62 and 63 and the axis 54 a desired variation in gear ratio can be achieved for the assembly. From Figure 6 it will be seen that engagement between the roller 65 and the cam surface 63 controls axial displacement of the idler wheel 52 downwardly in that Figure whereas engagement between the roller 64 and the cam surface 62 control axial displacement of the idler wheel upwardly in that Figure. If required the cam surfaces 62 and 63 can have other than flat profiles to provide a required variation in gear ratio of the assembly.

In the application of the previously described and illustrated embodiments of the gear assembly to a steering gear each of the input components (that is the splined ends of the pinion shafts 4,32 and 50) is coupled for rotation by a steerable component such as a vehicle steering wheel while each ofthe output components (that is the rack bars 23 and 27 and the splined end of pinion shaft 60) is connected through an appropriate and conventional form of steering linkage to steerable road wheels of the vehicle.

Claims

CLAIMS:

1. Avariable ratio gear assembly comprising a toothed wheel member which is axially rotatable and in toothed driving engagement with a further toothed member, one of said toothed members being axially displaceable relative to the other toothed member while said toothed driving engagement is maintained and wherein cam means is provided which is operatively associated with said toothed wheel member and is responsive to rotation of that member to cause displacement of the axially displaceable toothed member relative to said other toothed member during rotation of the toothed wheel member and thereby a variation in the gear ratio which is transmitted through the assembly.

2. An assembly as claimed in claim 1 in which the cam means is arranged so that for each revolution of the toothed wheel member the toothed member which is axially displaceable exhibits a complete and predetermined cycle of reciprocal displacement axially.

3. An assembly as claimed in either claim 1 or claim 2 in which the toothed wheel member is the member which is axially displaceabie under control of the cam means.

4. An assembly as claimed in claim 3 in which the cam means comprises a cam surface and a cam follower in co-operation therewith, said cam surface and cam follower being displaceable relative to each other in response to rotation of the toothed wheel member to control axial displacement of the toothed wheel member.

5. An assembly as claimed in claim 4 in which the cam surface is carried by, and rotatable with, the toothed wheel member.

6. An assembly as claimed in either claim 4 or claim 5 in which the cam means comprises two axially opposed cam surfaces and two cam followers which respectively co-operate one with each cam surface, one cam surface and its respectivelyco- operating cam follower being arranged to control displacement of the toothed wheel member in one sense of axial direction and the other cam surface and its respectively co-operating cam follower being arranged to control displacement of the toothed wheel member in the opposite sense of axial direction.

7. An assembly as claimed in any one of claims 4 to 6 in which the toothed wheel member is rotatably mounted in a housing and the cam follower or followers are carried by said housing.

8. An assembly as claimed in claim 7 in which the cam means comprises a cam follower in the form of a roller member carried by said housing and which is in rolling engagement with the, or at least one, cam surface.

9. An assembly as claimed in any one of the preceding claims in which the further toothed member is a second rotatably mounted toothed wheel member.

10. An assembly as claimed in any one of claims 1 to 8 in which the further toothed member is a longitudinally displaceable rack bar having rack teeth with which the toothed wheel member engages.

11. An assembly as claimed in claim 3 or in any? one of claims 4 to 10 when appendantthereto in which the toothed wheel member is an idler which is in toothed engagement with the further toothed member and with a third toothed member so that drive is transmitted between said further and third toothed members through the idler toothed wheel member.

12. An assembly as claimed in claim 11 in which the third toothed member is a rotatably mounted wheel member.

13. An assembly as claimed in claim 11 in which the third toothed member is a longitudinally displaceable rack bar having rack teeth which engage with the toothed wheel member.

14. An assembly as claimed in claim 12 when appendant to claim 5 in which the third toothed member is a pinion which engages the axially displaceable toothed wheel member carrying the cam surface and the cam follower is carried by said pinion.

15. An assembly as claimed in any one of the preceding claims in which the toothed wheel member is mounted for axial rotation by a shaft which is carried by the our a housing.

16. An assembly as claimed in claim 15 when appendantto claim 3 in which the shaft is secured against axial displacement relative to the housing and the toothed wheel member is axially displaceable on the shaft.

17. An assembly as claimed in claim 16 in which the shaft is capable of rotational adjustment about a second axis with respect to the housing, said second axis being parallel to, and eccentric with respect to, the axis of rotation of the toothed wheel member so that by rotational adjustment of the shaft about said second axis an adjustment can be effected in the depth of toothed engagement between the toothed wheel member and the teeth with which it is in meshed engagement.

18. An assembly as claimed in any of the preceding claims in which the teeth of the toothed wheel member and the teeth which are meshed in engagement of that member are of substantially helical formation.

19. A steering gear which inctudesavariable ratio gear assembly as claimed in any one of the preceding claims and in which the toothed wheel member is rotatable in response to a steering manoeuvre applied to a steering input component and the furthertoothed member is coupled to a steering linkage which provides a steering output in response to said manoeuvre.

19. A variable ratio of gear assembly substantially as herein described with reference to Figures 1 and 2 of the accompanying and illustrative drawings.

20. A variable ratio of gear assembly substantially as herein described with reference to Figures 3 and 4 of the accompanying and illustrative drawings.

21. Avariable ratio of gear assembly substantially as herein described with reference to Figures 5 and 6 of the accompanying illustrative drawings.

22. A steering gear which includes a variable ratio gear assembly as claimed in any one of the preceding claims and in which the toothed wheel member is rotatable in response to a steering manoeuvre applied to a steering input component and the further toothed member is coupled to a steering linkage which provides a steering output in response to said manoeuvre.

New claims or amendments to claims filed on 22 Oct 1979 Superceded claims 1 to 22 New or amended claims:

1. A variable ratio gear assembly comprising a housing; a helically toothed wheel member which is axially rotatable in the housing; a further helically toothed member which is displaceable relative to the housing and with which the wheel member is in toothed driving engagement to effect said displacement; said wheel member being axially displaceable relative to the housing and to the further toothed member while maintaining driving engagement therewith; two axially opposed cam surfaces rotatable and axially displaceable in unison with said wheel member, and cam follower means rotatably mounted relative to, and restrained against axial displacement with respect to, the housing; said cam follower means comprising two opposed follower surfaces which are maintained in rolling engagement one with each of said cam surfaces to determine and control, during axial rotation of the wheel member, displacement of the wheel member in both senses of its axial direction and thereby the variations in the gear ratio which is transmitted through the assembly.

2. An assembly as claimed in claim 1 in which the respectively engaging follower surfaces and cam surfaces are arranged so that for each revolution of the toothed wheel member the toothed member which is axially displaceable exhibits a complete and predetermined cycle of reciprocal displacement axially.

3. An assembly as claimed in either claim 1 or claim 2 in which the two axially opposed cam surfaces are located on the toothed wheel member.

4. An assembly as claimed in any one of the preceding claims in which the further toothed member is a second rotatably mounted toothed wheel member.

5. An assembly as claimed in any one of claims 1 to 3 in which the further toothed member is a longitudinally displaceable rack bar having rack teeth with which the toothed wheel member engages.

6. An assembly as claimed in any one of the precedinfg claims in which the toothed wheel member is an idler which is in toothed engagement with the further toothed member and with a third toothed member so that drive is transmitted between said further and third toothed members through the idler toothed wheel member.

7. An assembly as claimed in claim 6 in which the third toothed member is a rotatably mounted wheel member.

8. An assembly as claimed in claim 6 in which the third toothed member is a longitudinally displaceable rack bar having rack teeth which engage with the toothed wheel member.

9. An assembly as claimed in claim 7 in which the third toothed member is a pinion which engages the axially displaceable toothed wheel member and the opposed follower surfaces are carried by said pinion.

10. An assembly as claimed in any one of claims 1 to 9 in which the opposed follower surfaces are carried by the housing.

11. An assembly as claimed in claim 10 in which the opposed follower surfaces are located on roller means carried by the housing to be in rolling engagement with the respective cam surfaces.

12. An assembly as claimed in any one of the preceding claims in which the toothed wheel member is mounted for axial rotation by a shaft which is carried by the housing.

13. An assembly as claimed in claim 12 in which the shaft is secured against axial displacement relative to the housing and the toothed wheel member is axially displaceable on the shaft.

14. An assembly as claimed in claim 13 in which the shaft is capable of rotational adjustment about a second axis with respect to the housing, said second axis being parallel to, and eccentric with respect to, the axis of rotation of the toothed wheel member so that by rotational adjustment ofthe shaft about said second axis an adjustment can be effected in the depth of toothed engagement between the toothed wheel member and the teeth with which it is in meshed engagement.

15. An assembly as claimed in any of the preceding claims in which the teeth of the toothed wheel member and the teeth which are meshed in engagement with that member are of substantially helical formation.

16. A variable ratio of gear assembly substantially as herein described with reference to Figures 1 and 2 of the accompanying and illustrative drawings.

17. A variable ratio of gear assembly substantially as herein described with reference to Figures 3 and 4 of the accompanying and illustrative drawings.

18. A variable ratio of gear assembly substantially as herein described with reference to Figures 5 and 6 of the accompanying illustrative drawings.