GB2235171A - Power assisted steering assembly with speed-proportional feel - Google Patents

Abstract

Input shaft (7) and a co-axial output shaft (6) are rotatable in response to a steering input and can rotate partially relative to each other to adjust valve parts (8, 10) which control hydraulic fluid to the servomotor. A ball clutch is engageable between the shafts 6 and 7 to restrain said relative rotation. Displacement of the ball (13) to actuate the clutch is controlled by a ball wedge (15) which is biassed against the ball (13) by a spring (16) within the shaft (7). The spring biasing of the ball wedge (15) is adjustable by a rod (19) which is axially displaceable by a screw shaft (23) which is rotatable by a stepper motor (24) in response to input signals derived from a sensor (26) (such as a vehicle speed sensor). In a further embodiment (Fig. 5) the shafts (6) and (7) are displaceable under spring biasing axially relative to each other and the ball clutch (13) is located between profiled surfaces of those shafts to restrain relative rotation between them under spring biasing adjustable by the stepper motor (24). <IMAGE>

Description

TITLE "A power assisted steering assembly" TECHNICAL FIELD & BACKGROUND ART The present invention relates to a power assisted steering assembly of the kind comprising an input member which is rotatable about an axis thereof in response to a steering input, an output member co-axial with the input member and rotatable in response to the steering input to provide a steering output and in which one of said members is capable of partial axial rotation relative to the other member in response to the steering input to adjust valve means for controlling hydraulic fluid flow to a power assistance servomotor of a steering system in which the assembly is to be incorporated.Steering assemblies of the aforementioned kind are well known in the art and typically one of the input and output members has a rotor part wnich is received in a sleeve part of the other member; the valve is formed between said rotor and sleeve parts to be adjustable by relative rotation between the input and output members and control hydraulic fluid flow for power assistance as appropriate in response to the steering input.

It has hitherto been proposed to provide steering assemblies of the kind mentioned with clutch devices which react in response to predetermined signals to engage between the input and output members and restrain relative rotation between those members - thereby restraining adjustment of the valve means and the availability of power assistance. The signals to the clutch device may be derived from a vehicle speed sensor whereby the engagement and dis-engagement of the clutch between the input and output members is responsive to vehicle speed.With this latter arrangement it is usual for the clutch device to react and progressively restrain relative rotation between the input and output members as vehicle speed increases so that a progressively greater steering input torque is required to actuate the valve means and less power assistance is available as the vehicle speed is increased (such steering is usually known as having "speed proportional feel" characteristics).A known form of clutch device comprises one or more balls carried in the input or output member and which are displaceable radially by hydraulic fluid under pressure into engagement with the other member so that relative rotation between the two members is progressively restrained as the fluid pressure on the balls is increased - this latter fluid pressure varies in response to variations in vehicle speed, usually by appropriately varying the speed of an hydraulic pump or controlling the fluid pressure derived from a constant speed pump. An example of a speed proportional feel steering system incorporating a clutch device in which balls thereof are subjected to varying fluid pressure is disclosed in our British Patent Specification No.

1,465,901. The hydraulics necessary to subject the balls in the known clutch device to hydraulic fluid pressure can prove inconvenient in a steering assembly, for example by having to accommodate for hydraulic power losses which may be associated with such devices. It is an object of the present invention to provide a power assisted steering assembly of the kind referred to having a clutch device which is responsive to input signals, typically to provide speed proportional feel, and by which disadvantages associated with clutch devices actuated under hydraulic fluid pressure can be alleviated.

STATEMENT OF INVENTION & ADVANTAGES According to the present invention there is provided a power assisted steering assembly comprising an input member rotatable about an axis thereof in response to a steering input, an output member co-axial with the input member and rotatable in response to said steering input to provide a steering output, one of said members being capable of partial axial rotation relative to the other member; valve means which is adjustable for controlling hydraulic fluid flow to a servo motor for power assistance in response to said relative axial rotation between the input and output members; a ball clutch member engageable between the input and output members; axially displaceable means spring biasing said ball member into engagement with the input and output members to restrain said relative rotation between the two members, and axially adjustable control means responsive to input signals to axially adjust the axially displaceable spring biasing means for varying the axial biasing thereof on the ball member and thereby cause an adjustment in the restraint provided by said ball member to the relative rotation between the input and output members.

By the present invention it is envisaged that the axially displaceable spring biasing means will comprise one or more mechanical springs which react axially to bias the ball member, radially and/or, axially, into engagement with the input and output members and the biasing force of the springs on the ball member is varied by axial adjustment of the control means, effectively to increase or decrease the compression of the springs, so that the ball member is subjected wholly to mechanical forces. The control means for effecting axial adjustment of the biasing for the ball member is preferably electrical such as a stepper motor or solenoid device which responds to appropriate input signals to increase or decrease the spring biasing force.The input signals may be derived from a vehicle speed sensor so that the assembly will provide speed proportional feel or from other sensors whereby the feel presented during use of the assembly will vary, for example, in response to variations in weather conditions or lateral acceleration of the vehicle. It will be realised that where the mechanical clutch device of the present invention is controlled electrically as aforementioned, no hydraulics are required for actuation of the clutch thereby alleviating hydraulic power losses which are usually associated with known clutch devices in which the -ball clutch member is subjected to hydraulic fluid pressure to control its engagement between the input and output members of the assembly.However; it will be appreciated that the control means may be other than electrical, for example it may even comprise a small hydraulic ram which is actuated to provide the required axial adjustment of the spring biasing.

The axially displaceable spring biasing means may comprise a wedge member which is biased axially relative to.

the input and output members and against which wedge the ball member reacts to be displaced radially to engage between the input and output members under a biasing force that is variable by adjustment of the spring biasing on the wedge member, the latter adjustment being effected by the control means. Preferably the ball member is in rolling contact with the wedge member, conveniently by the ball member running on a frusto conical or spherical surface part of the wedge member. The wedge member is conveniently located co-axially within the input or output member with the ball member captured in a radial bore or similar seating provided in the input or output member.

In a further proposal tie input and output members are displaceable axially relative to each other and comprise profiled surfaces between which the ball member is engageable so that as the input and output members are displaced in one or the opposite sense axially relative to each other the ball member is subjected to increasing or decreasing compressive forces (which may be axial and/or radial) between the profiled surfaces to increase or decrease respectively the clutching effect of the ball member between the input and output members. In this latter proposal the axially displaceable spring biasing means serves to bias the input and output members axially relative to each other and the control means adjusts the axial biasing which is provided between the input and output members.

The control means may comprise a rod or similar component which extends through one or both of the input and output members and is coaxial therewith. One end of the rod is reacted upon by a motor, such as the aforementioned stepper motor, which is responsive to the input signals and determines axial displacement of the rod while the opposite end of the rod controls axial adjustment of the biasing means and thereby the biasing force with which the ball member is urged radially to engage between the input and output members. The rod may itself include axial spring biasing and/or control axial adjustment of spring biasing remote therefrom.

Usually the steering assembly will be provided with two or more ball clutch members which are located in a common plane that extends perpendicular to the axis and are spaced circumferentially about said axis to be biased simultaneously for engagement between the input and output members. The or each ball member will usually be restrained for rotation with one of the input and output members and urged radially and/or axially by axial adjustment of the spring biasing means into engagement with a seating in the other of the input and output members.

The latter seating will usually present a concave cam shaped profile to the ball member the geometry of which profile, when reacted upon by the ball member, can serve to rotationally bias the input and output members with respect to each other to a predetermined 'centre" position (for example to a neutral condition of the valve means).

Furthermore, the geometry for the cam profile of the seating may permit the ball member to be displaced therefrom against the spring biasing in response to sufficient torque being provided between the input and output members from the steering input.

Usually the valve means will be of the rotary type where one of the input and output members has a rotor part which is received in a sleeve part of the other member and said rotor and sleeve parts include valve ports and passages which; during relative rotation between the rotor and sleeve parts, control the flow of hydraulic fluid to and from a servo motor (such as a double acting piston and cylinder device) that provides power assistance for a steering system.

The present invention was primarily developed for steering assemblies of the rack and pinion type in which the output member is coupled for co-axial rotation with the pinion. It will be realised however that the invention is not intended to be restricted to steering assemblies of this type, for example the invention may be applied to a worm and sector type gear in which the output member is coupled to rotate a worm screw and displace a sector in toothed driving engagement therewith to provide the steering output.

DRAWINGS Embodiments of power assisted steering assemblies of the rack and pinion type and 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: Figure 1 is an axial section through a first embodiment of the invention in which an axially displaceable spring biased wedge member is located in an input member to bias a ball clutch; Figure 2 is a section of the assembly taken on the line A - A of Figure 1; Figure 3 shows a modification for the wedge biasing of the assembly in Figure 1;; Figure 4 shows a further modification for the wedge biasing of the assembly in Figure 1, Figure 5 is an axial section showing part of a second embodiment of the invention in which input and output members thereof are axially displaceable with respect to each other and are spring biased axially to effect engagement of a ball clutch between those output members, and Figure 6 is an axial section through part of the rack and pinion assembly and illustrates an alternative form of control for effecting axial adjustment of the spring biasing.

DETAILED DESCRIPTION OF DRAWINGS The steering assemblies illustrated comprise a rack and pinion steering gear in which a rack bar 1 is longitudinally displaceable through a housing 2 with teeth of the rack bar engaging teeth of a pinion 3 that is rotatably mounted in the housing 2 by bearings 4. The pinion 3 provides an output member of the assembly whereby the rotation of the pinion in response to a steering input applied from a steering wheel column (not shown) to an input shaft 7 drives the rack bar 1 longitudinally to effect in adjustment of the steering linkages in conventional manner. The pinion 3 has an axis 5 about which it is rotatable and a co-axial pinion shaft 6. The input shaft 7 extends into the housing 2 and is mounted for rotation therein co-axially with the pinion 3. The input shaft 7 has a rotor part 8.Adjacent ends oE the input shaft 7 and pinion shaft 6 interengage through dogs or splines 9 which permit partial relative rotation between those shafts. Coupled by a pin 6A for unified rotation with the pinion and its shaft 6 is a cylindrical sleeve part 10 within which the rotor part 9 is received. The rotor and sleeve parts 8, 10 are provided with co-operating ports and passages and constitute a valve for controlling hydraulic fluid flow to and from a servo motor (such as a double acting piston and cylinder device) associated with the rack bar 1 in known manner.The purpose and operation of the valve incorporated in the rotor part 8 and sleeve part 10 will readily be understood by those familiar with power assisted steering gears whereby partial relative rotation between the shafts 6 and 7 in response to a steering input torque applied to the shaft 7 cases an adjustment in the valve to control the flow of hydraulic fluid to the servo motor so that the servo motor assists in the longitudinal displacement of the rack bar 1 which is intended by rotation of the input shaft 7 (and, through engagement of the dogs or splines 9, by rotation of the pinion 3). Conventionally, a power assisted steering valve is biased to a neutral condition from which it is displaced in response to a steering input torque.It is recognised that in so-called speed proportional feel power assisted steering assemblies, power assistance should be readily available at low vehicle speeds and sllch availability should progressively reduce as vehicle speed increases (possibly to the extent that above a predetermined high speed of the vehicle power assistance will not normally be available in response to the steering input). To provide such speed proportional feel characteristics a mechanical clutch device is provided which is responsive to vehicle speed and engages to react between the input shaft 7 and the pinion shaft 6 to increasingly restrain relative rotation between those shafts as vehicle speed increases (so that a progressively increasing steering input torque has to be applied to the shaft 7 to cause rotational displacement between the rotor part 8 and sleeve part 10 from the neutral condition of the valve and render power assistance available).

In the embodiment of Figures 1 and 2 the input shaft 7 is provided with a co-axial blind-bore 11 which opens into the end thereof adjacent to the pinion shaft 6. Extendinq radially from the bore 11 and through the wall thickness of the shaft 7 are three coplanar and circumferentially spaced bores 12 in each of which is received; as a close sliding fit, a spherical clutch member or ball 13. Located within the bore 11 and on the sides of the balls 13 adjacent to the blind end 14 of the bore is a wedge member in the form of a fourth spherical ball 15. The ball wedge 15 is received within the bore 11 to be axially slidable therein and is biased axially into rolling abutment with the three balls 13 by a compression spring 16 which reacts between the ball wedge 15 and the blind end 14 of the bore.The spring force on the ball wedge 15 and the abutment of that ball with the baii clutches 13 causes the bails 13 to be displaced radially outwardly of the valve rotor part 8 so that with the valve in its aforementioned neutral condition, the three ball citches 13 project from the part 8 and engage in respective concave seats 17 circumferentially spaced in the interior face of the sleeve part which forms an extension of the pinion shaft 6. The three seats 17 are each of concave profile circumferentially of the axis 9 (as shown in Figure 2 the seats 17 are conveniently of part circular profile to correspond wit'n t'ne diameter of the balls 13).

Furthermore t'ne geometry of the seats 17 is such that the engagement of the respective ball clutches 13 therewith will oide a positive centering in a rotational sense between the input shaft 7 and pinion shaft 6 so that t'ne valve rotor part 8 and valve sleeve part 9 are biased to the neutral condition of the valve by the thrust exerted from the biasing spring 16.

The spring 16 will normally give a maximum thrust on the balls 13 which effects in maximum restraint being provided by the clutch device to relative rotation between the valve rotor and sleeve parts and this will correspond with minimum or zero power assistance being provided to a normal steering manoeuvre.

Extending through the pinion 3 and its shaft 6 is a coaxial bore 18 located within which is a two part axially extending thrust rod 19. The rod 19 is axially displaceable relative to the pinion and its two parts are axially connected through a spring loaded lost motion unit 20 located in a counterbore 18A of the shaft 6. One end of the rod 19 projects coaxially from the shaft 6 and between the balls 13 to abut the ball wedge 15 and the other end projects from the pinion 3 to abut a plate 21 of a roller thrust bearing 22 on a screw shaft 23 which is coaxial with the axis 5. The spring unit 20 acts to expand the effective axial length of the rod 19 and thereby ensure that the e ends of the rod 19 are maintained in abutment with the respective ball 15 and thrust plate 21.

Mounted on the housing 2 is an electrical stepper motor 24 which when actuated screws the shaft 23 into or out of a cap 25 and thereby displaces the thc,lst plate 21 axially in one or the opposite sense of direction. During axial displacement oE the screw shaft 23 the rod 19 will be axially displaced (possibly to extend or contract in length through the spring unit 20) by abutment of the thrust plate 21 therewith and it will be seen that suc displacement of the rod will increase or decrease the compressive forces in the spring 16.Movement of the rod 19 leftwardly in Figure 1 will urge the ball 15 against the biasing of spring 1 (effectively to compress that spring) and thereby relieve the thrust on the ball clutches 13 which urges those balls radially outwardly into engagement with the seats 17. Alternatively, displacement of the rod 19 rightwardly in Figure 1 permits greater thrust to be exerted axially from the biasing spring 16 through the ball 15 against the ball clutches 13 and thereby increases the force with which the balls 13 engage in their respective seats i7. The rod 19 and stepper motor 24 therefore control the axial adjustment of the biasing provided by the ball 15 and spring 16 to the balls 13.The stepper motor 24 is actuated in response to signals derived from a vehicle speed sensing device 26 so that the thrust plate 21 is progressively displaced to the right in Figure 1 as vehicle speed increases and to the left as vehicle speed decreases.

In the modification shown in Figure 3 the biasing spring 16 has been omitted and the ball wedge 15 is located to abut the three balls 13 on the side thereof adjacent to the rod 19. The rod 19 again a'o,lts the ball wedge 15 to bias the latter axially within the blind-bore 11 and into engagement with the ball clutches 13 however, in this case the spring biasing force is wholly provided by the spring unit 20 which is incorporated in the rod 19.

Consequently axial displacement of the rod 19 by adjustment of the screw shaft 23 operates in a reverse sense to the Figure 1 arrangement in that displacement of the rod 19 leftwardly in Figure 3 increases the radial thrust on the balls 13 to increasingly restrain relative rotation between the rotor and sleeve parts 8 and 10, and axial displacement of the rod 19 rightwardly in the Figure conversely decreases the radial thrust on the balls 13.

The steering assembly of Figure 4 shows a further modification of the Figure 1 arrangement in ur'nic;i a single part control rod 19 extends from t'ne thrust plate 21 of the bearing 22 and is connected to the thrust plate 21 for axial displacement in unison therewith (and thereby with the screw shaft 23). Conveniently the rod 19 is integral with the thrust plate 21 as shown in Figure 4. The spherical ball wedge 15 of Figures 1 and 3 is replaced in Figure 4 by a part spherical tubular wedge member 15A, the part spherical surface of which is in rolling engagement with the three balls 13 on the side of the balls adjacent to the blind end 14 of t'ne bore 11.The rod 19 projects through both the bore of the wedge 15A and the biasing spring 16 and its end within the bore 11 carries a thrust plate 30. The spring 16 reacts between the plate 30 and the wedge member 15A to bias the part spherical surface thereof axially into engagement with the balls 13. Axial displacement of the rod 19 by rotation of the screw shaft 23 with the stepper motor 24 in one sense of direction (rightwardly in Figure 4) will increase the axial compression of spring 16 and thereby increase the biasing of the balls 13 radially outwardly; conversely, displacement of the rod 19 in the opposite sense of axial direction will decrease the axial biasing of the spring 16 and reduce the force with which the balls 13 are radially biased outwardly to engage between the shafts 5 and 6.

9 thrust roller bearing 22 is provided to alleviate the transmission of excessive rotational force between the pinion 3 and the stepper motor 24. If required this roller bearing can be omitted. Furthermore, it will be appreciated that it is not essential for the wedge members 15 and 15A to present spherical or part spherical surfaces for engagement with the ball clutch members 13 and the surface of the wedge member may be frusto conical.

In the embodiment of the steering assembly shown in Figure 5, where for convenience, the rack and pinion gear and stepper motor for axial control of the rod 19 have been omitted from the drawing (these components may correspond to the arrangement shown in Figure 3), the input shaft 7 is received within the housing 2 to be capable of restricted axial displacement relative thereto and to the pinion and its shaft 6.

The shaft 7 is rotatably mounted in the housing by a thrust bearing 40 and is axially displaceable relative thereto and to a transverse pin 41 which extends diametrically through the shaft 7 in the axial part length thereof which includes the blind-bore 11. The transverse pin can react axially against an inner race 40A of the bearing 40. Located within the bore 11 of the rotor part 8 is a biasing spring 42 which reacts axially between the transverse pin 41 and a thrust plate 45 which is axially slidable in the bore 11. The plate 45 is biased by the spring 42 axially for abutment with a circlip 43 in the end of the bore 11 adjacent to the rod 19.Consequently the shaft 7 is axially biased rightwardly in Figure 5 relative to the housing 2 and the shaft 6 by the reaction of spring 42 between the transverse pin 41 and the thrust plate 45 which biases the latter against the clip 43 and hence urges the shaft 7 rightwardly. The end of the rotor part 8 adjacent to the pinion shaft 6 is provided with a circumferentially spaced array of profiled seats 12A within which are located hall clutch members 46. The balls 46 engage between the rotor pact 8 and the adjacent end of the pinion shaft 6, the latter of which is provided with a part toroidal circle'at track 47 within which the balls are seated. mie biasing of the input shaft 7 provided by t'ne spring 42 and relative to the pinion shaft 6 urges the balls 47 into engagement with t'ne seatings oE those shafts to restrain relative rotation between the rotor part 8 and sleeve part 10.

The end of the rod 19 remote from the stepper motor abuts the thrust plate 45 and it will be seen from Figure 5 that the rod 19 can be displaced axially under control of the stepper motor to compress the spring 42 between the transverse pin 41 and the thrust plate 45. This will reduce the axial biasing of the spring 42 on the input shaft 7 to relieve the force with which the balls 46 are biased to engage between the rotor part 8 and pinion shaft 6.

It will therefore be appreciated that by appropriately controlling the compression of the spring 42 as aforementioned, the stepper motor can control the amount of restraint which is provided to the relative rotation between the rotor and sleeve parts and thereby, the adjustment of the valve from its neutral condition in response to variations in ve'nicle speed.

In the above described examples, the stepper motor 24 is mounted on an axial end of the housing 2. However, such an arrangement may not be convenient and the modification in Figure 6 shows a modification whereby the stepper motor can be re-located. In Figure 6 the end of the rod 19 which projects from the pinion 3 carries a small rack bar 50 and is supported on a bearing 51 within the housing. Engaging with the rack bar 50 is a pinion 52 rotatable to axially displace the rod 19 by drive from a stepper motor, such stepper motor may be attached to a side of the housing 2 or remote therefrom with an appropriate angle drive to the pinion 52, for example through a flexible shaft.

Claims (25)

1. A power assisted steering assembly comprising an input member rotatable about an axis thereof in response to a steering input, an output member co-axial with the input member and rotatable in response to said steering input to provide a steering output, one of said members being capable of partial axial rotation relative to the other member; valve means which is adjustable for controlling hydraulic fluid flow to a servo motor for power assistance in response to said relative axial rotation between the input and output members; a ball clutch member engageable between t'ne input and output members; axially displaceable means spring biasing said ball member into engagement with the input and output members to restrain said relative rotation between the two members, and axially adjustable control means responsive to input signals to axially adjust the axially displaceable spring biasing means for varying t'ne axial biasing thereof on the ball member and thereby cause an adjustment in the restraint provided by said ball member to the relative rotation between the input and output members.

2. An assembly as claimed in claim 1 in which the control means for effecting axial adjustment of t'ne biasing for the ball member comprises an electrical stepper motor or solenoid device which is responsive to said input signals to vary the spring biasing force.

3. An assembly as claimed in either claim 1 or claim 2 in which the control means comprises an electric motor mounted on a housing of the assembly.

4. An assembly as claimed in either claim 1 or claim 2 in which the control means comprises an electric motor mounted remote from a housing of the assembly and coupled to vary the axial biasing of said spring biasing means by way of an angled or flexible drive shaft.

5. 9n assembly as claimed in any one of the preceding claims in which the control means comprises rod means axial displacement of which adjusts the spring biasing means for varying the axial biasing thereof on the ball member.

6. An assembly as claimed in claim 5 in which the rod means comprises two axial part lengths between which is located a spring unit which is axially displaceable with the rod means and biases the part lengths axially relative to each other.

7. An assembly as claimed in claim 6 in which the spring unit biases said part lengths to an elongated condition of the rod means.

8. An assembly as claimed in any one of claims 5 to 7 in which axial displacement of the rod means is effected by rotational adjustment of a threaded shaft, said rotational adjustment being determined in response to the input signals.

9. An assembly as claimed in claim 8 in which the threaded shaft is coaxial with the rod means and its rotation is effected by the or an electrical stepper motor.

10. An assembly as claimed in any one of claims 5 to 9 in which the rod means is axially adjustable by displacement of a rotatable shaft and throug'n a Ellrust bearing by which the rod means and said rotatable shaft are rotatable relative to each other.

11. An assembly as claimed in any one of the preceding claims 5 to 10 in which axial displacement of the rod means is controlled through a rotary toothed gear device.

12. 9n assembly as claimed in claim 11 in which the toothed gear device comprises a rack axially displaceable wit'n tiie rod means and engaging with a rotationally mounted pinion, rotation of which is controlled in response to said input signals to effect axial displacement of the rack.

13. An assembly as claimed in any one of claims 5 to 12 in which the rod means extends co-axially through at least one of said input and output members.

14. An assembly as claimed in any one of the preceding claims in which the axially displaceable spring biasing means comprises a wedge member which is biased axially and against which wedge member the ball member reacts to be displaced radially to engage between the input and output members under a biasing force that is variable by adjustment of the spring biasing on the wedge member with the control means.

15. An assembly as claimed in claim 14 in which the ball member is in rolling contact with the wedge member.

16. An assembly as claimed in either claim 14 or claim 15 in which the wedge member has a spherical surface part with which. the ball member engages in abutment.

17. An assembly as claimed in any one of claims 14 to 16 n which the wedge member is located coaxially within one oE the input any output members and the 'oall member is captured in a seating to be radially displaceable into engagement between the input and output nembers in response to the axial biasing from Lhe wedge member.

18. An assembly as claimed in claim 17 in which one of the input and output members has a rotor part which is received in a sleeve part of the other member and the ball member is captured in a radial bore of the rotor part to be displaced radially by axial biasing from the wedge member and provide clutch engagement between the rotor and sleeve parts.

19. An assembly as claimed in any one of the preceding claims 14 to 18 in which the wedge member is spring biased axially by spring means which reacts between that wedge member and one of said input and output members.

20. An assembly as claimed in any one of the claims 14 to 19 in which the wedge member is spring biased axially by spring means which reacts between said wedge member and the or a rod means which is axially displaceable in response to said input signals.

21. An assembly as claimed in any one of claims 1 to 13 in which the input and output members are displaceable axially relative to each other and comprise profiled surfaces between which the ball member is engageable so that as the input and output members are displaced in one sense axially relative to each other the ball member is subjected to compressive forces between the profiled surfaces to progressively increase the clutching effect of the ball member between the input and output members, and wherein said axially displaceable spring biasing means biases the input and output members axially relative to each other and the control means axially adjusts the axial biasing which is provided between said members.

22. An assembly as claimed in any one of the preceding claims in which the ball member is restrained for rotation with one of the input and output members and is urged by axial adjustment of the spring biasing means into engagement with a seating in the other of said members, said seating comprising a concave shaped profile the geometry of which profile when reacted upon by the ball member serves to rotationally bias the input and output members to a predetermined centre position.

23. An assembly as claimed in claim 22 in which the valve means comprises the or a rotor part on one of said input and output members which is coaxially received in the or a sleeve part of the other of said members, said rotor and sleeve parts including valve ports and passages which comprise the valve means and during relative rotation therebetween are intended to control the flow of hydraulic fluid for the servomotor, and wherein said valve means has a neutral condition which corresponds with said centre position.

24. An assembly as claimed in any one of the-preceding claims in which at least two ball clutch members are provided and located in a common plane that extends perpendicularly to the axis, said ball clutch members being spaced circumferentially about said axis to be biased simultaneously for engagement between the input and output members.

25. A power assisted steering gear as claimed in either claim 23 or claim 24 and having the modification as herein described with reference to Figure 6 of the accompanying illustrative drawings.

25. A power assisted steering assembly substantially as herein described with reference to Figures 1 and 2, Figure 3 or Figure 4 of the accompanying illustrative drawings.

26. A power assisted steering assembly substantially as herein described with reference to Figure 5 of the accompanying illustrative drawings.

27. A power assisted steering gear as claimed in either claim 25 or claim 26 and having the modification as herein described with reference to Figure 6 of the accompanying illustrative drawings.

Amendments to the claims have been filed as follows 1. A power assisted steering assembly comprising an input member rotatable about an axis thereof in response to a steering input, an output member co-axial with the input member and rotatable in response to said steering input to provide a steering output, one of said members being capable of partial axial rotation relative to the other member; valve means which is adjustable for controlling hydraulic fluid flow to a servo motor for power assistance in response to said relative axial rotation between the input and output members; a ball clutch member engageable between the input and output members; axially displaceable means spring biasing said ball member into engagement with the input and output members to restrain said relative rotation between the two members, and control means comprising axially adjustable rod means extending co-axially through at least one of said input and output members, axial displacement of said rod means being responsive to input signals to the control means and serving to axially adjust the axially displaceable spring biasing means for varying the axial biasing thereof on the ball member and thereby cause an adjustment in the restraint provided by said ball member to the relative rotation between the input and output members.

2. An assembly as claimed in claim 1 in which the control means for effecting axial adjustment of the biasing for the ball member further comprises an electrical stepper motor or solenoid device which is responsive to said input signals to axially displace the rod means and vary the spring biasing force.

3. An assembly as claimed in either claim 1 or claim 2 in which the control means further comprises an electric motor mounted on a housing of the assembly.

4. An assembly as claimed in either claim 1 or claim 2 in which the control means further comprises an electric motor mounted remote from a housing of the assembly and coupled to vary the axial biasing of said spring biasing means by way of an angled or flexible drive shaft.

5. An assembly as claimed in any one of the preceding claims in which the rod means comprises two axial part lengths between which is located a spring unit which is axially displaceable with the rod means and biases the part lengths axially relative to each other.

6. An assembly as claimed in claim 5 in which the spring unit biases said part lengths to an elongated condition of the rod means.

7. An assembly as claimed in any one of the preceding claims in which axial displacement of the rod means is effected by rotational adjustment of a threaded shaft, said rotational adjustment being determined in response to the input signals.

8. An assembly as claimed in claim 7 in which the threaded shaft is coaxial with the rod means and its rotation is effected by the or an electrical stepper motor.

9. An assembly as claimed in any one of the preceding claims in which the rod means is axially adjustable by displacement of a rotatable shaft and through a thrust bearing by which the rod means and said rotatable shaft are rotatable relative to each other.

10. An assembly as claimed in any one of the preceding claims in which axial displacement of the rod means is controlled through a rotary toothed gear device.

11. An assembly as claimed in claim 10 in which the toothed gear device comprises a rack axially displaceable with the rod means and engaging with a rotationally mounted pinion, rotation of which is controlled in response to said input signals to effect axial displacement of the rack.

12. An assembly as claimed in any one of the preceding claims in which the axially displaceable spring biasing means comprises a wedge member which is biased axially and against which wedge member the ball member reacts to be displaced radially to engage between the input and output members under a biasing force that is variable by adjustment of the spring biasing on the wedge member with the control means.

13. An assembly as claimed in claim 12 i'n which the ball member is in rolling contact with the wedge member.

14. An assembly as claimed in either claim 12 or claim 13 in which the wedge member has a spherical surface part with which the ball member engages in abutment.

15. An assembly as claimed in any one of claims 12 to 14 in which the wedge member is located coaxially within one of the input and output members and the ball member is captured in a seating to be radially displaceable into engagement between the input and output members in response to the axial biasing from the wedge member.

16. An assembly as claimed in claim 15 in which one of the input and output members has a rotor part which is received in a sleeve part of the other member and the ball member is captured in a radial bore of the rotor part to be displaced radially by axial biasing from the wedge member and provide clutch engagement between the rotor and sleeve parts.

17. An assembly as claimed in any one of the preceding claims 12 to 16 in which the wedge member is spring biased axially by spring means which reacts between that wedge member and one of said input and output members.

18. An assembly as claimed in any one of the claims 12 to 17 in which the wedge member is spring biased axially by spring means which reacts between said wedge member and the or a rod means which is axially displaceable in response to said input signals.

19. An assembly as claimed in any one of claims 1 to 11 in which the input and output members are displaceable axially relative to each other and comprise profiled surfaces between which the ball member is engageable so that as the input and output members are displaced in one sense axially relative to each other the ball member is subjected to compressive forces between the profiled surfaces to progressively increase the clutching effect of the ball member between the input and output members, and wherein said axially displaceable spring biasing means biases the input and output members axially relative to each other and the control means axially adjusts the axial biasing which is provided between said members.

20. An assembly as claimed in any one of the preceding claims in which the ball member is restrained for rotation with one of the input and output members and is urged by axial adjustment of the spring biasing means into engagement with a seating in the other of said members, said seating comprising a concave shaped profile the geometry of which profile when reacted upon by the ball member serves to rotationally bias the input and output members to a predetermined centre position.

21. An assembly as claimed in claim 20 in which the valve means comprises the or a rotor part on one of said input and output members which is coaxially received in the or a sleeve part of the other of said members, said rotor and sleeve parts including valve ports and passages which comprise the valve means and during relative rotation therebetween are intended to control the flow of hydraulic fluid for the servomotor, and wherein said valve means has a neutral condition which corresponds with said centre position.

22. An assembly as claimed in any one of the preceding claims in which at least two ball clutch members are provided and located in a common plane that extends perpendicularly to the axis, said ball clutch members being spaced circumferentially about said axis to be biased simultaneously for engagement between the input and output members.

23. A power assisted steering assembly substantially as herein described with reference to Figures 1 and 2, Figure 3 or Figure 4 of the accompanying illustrative drawings.

24. A power assisted steering assembly substantially as herein described with reference to Figure 5 of the accompanying illustrative drawings.