GB2036893A - A Speed Control Device - Google Patents

Abstract

Plates (19, 16) with opposed annular grooves (22, 21) forming a toroidal track for rollers (23) are mounted for rotation with concentric shafts (6, 8) which emerge from one end of a fixed housing (1). The plate (19) is capable of axial displacement (through splines 20) relative to its shaft (6) by spring biasing (32). The plate (16) is capable of axial displacement (through splines 17) relative to its shaft (8) by fluid pressure in chamber (38) controlled by a valve (41) actuated by a governor (42) driven by the shaft (8). Axial displacement of plate (16) causes carriers (27) for the roller (23) to pivot about (28) to change the transmission ratio. The splines (17, 20) are helical so that the plates (16, 19) are urged axially into driving engagement with the rollers (23) by torque which is imparted to the plates (16, 19) by their driving engagement with the rollers (23). In a modification the centrifugal governor (42) and valve (41) are omitted and the chamber (38) is responsive to fluid pressure derived directly from a variable delivery pump driven from the shaft (8). <IMAGE>

Description

SPECIFICATION A Speed Control Device This invention relates to a speed control device.

More particularly, the invention concerns a speed control device which comprises co-axially mounted driven and driving plates, the plates having opposed faces each of which carries an annular groove co-axial with the plates, the grooves being opposed to form a generally toroidal track; a power transmitting roller mounted and running within the track which roller is rotatable about an axis which lies substantially in a plane containing the axis of rotation of the plates and engages within each of the opposed grooves so that on rotation of the driving plate in one direction power is transmitted therefrom, through the roller, to rotate the driven plate in the opposite direction and in which at least one of the two plates is axially displaceable relative to a housing of the device and means is provided for biasing the plates axially towards each other and into engagement with the power transmitting roller. A device of this kind will hereinafter be referred to as of "the kind specified".

A device of the kind specified is the subject of our U.K. Patent Specification No. 1,469,776 and our co-pending U.K. Patent Application No.

44743/78. In our aforementioned prior published Patent and typically for a device of the kind specified the driven and driving plates are coupled to rotate with respective first and second housing parts through one of which a drive input is applied to the device while the other of which provides apower take-off-usually the input is applied to the device and the take-off from the device is by way of V pulley grooves on the respective housing parts which are intended to be engaged by V belts. In such a known structure the housing parts and their respectively associated plates are rotatably mounted on a central shaft which extends from the device and serves to provide a mounting by which the device can be anchored to an appropriate base or frame whilst the housing parts are free to rotate relative to the base or frame.In many instances it is undesirable to provide a device in which relatively large exposed housing parts may be rotating at high speed, particularly in the engine compartment of a motor vehicle where such housing parts are unlikely to be guarded and also where the mounting for the device may consist of a central shaft which is bolted or otherwise secured to a frame in cantilevered fashion (which is undesirable from the point of view of both the lack of stability which the mounting is likely to provide and the considerable strains to which the cantilevered mounting is likely to be subjected during use) and it is an object of the present invention to provide a device of the kind specified and by which the aforementioned disadvantages associated with hitherto proposed devices can be alleviated.

According to the present invention there is provided a speed control device of the kind specified in which the housing is non-rotatable and the driving and driven plates are respectively mounted for rotation with co-axial input and output shafts which shafts extend from the housing. By the present invention the housing can be non-rotatably mounted to a frame or base by bolts or otherwise for optimum stability and in a manner which does not impart strain on any of the moveable components in the device while the input and output shafts which emerge from the housing can rotate relative to the frame or base and are readily available for coupling to desired input drives and take-offs. Preferably the input and output shafts are concentric and are rotatably mounted one within the other.By this latter concentric arrangement for the shafts a compact structure may be achieved in which both shafts emerge from a common side of the housing.

Both the input and output shafts will usually be restrained against axial displacement relative to the housing and in such case the or each plate which is axially displaceable relative to the housing will also be mounted to be capable of axial displacement relative to the input or output shaft as the case may be with which it is mounted for rotation.Axial displacement between the input or output shaft and its respective plate will usually be through an axially extending track and track follower such as co-operating splines between the plate and its shaft so that the plate can be displaced axially with respect to its associated shaft whilst rotatable drive is transmitted through the co-operating track and track follower from that shaft to the plate or vice versa. it is preferred that the, or at least one, axially displaceable plate is coupled to the input or output shaft as the case may be for rotation therewith by means of a helical track and track follower which engages with the track, said track and track follower being arranged so that during axial displacement of the plate which is associated therewith relative to the shaft with which it is rotatable and in a direction towards the power transmitting roller, that plate exhibits a rotary/axial motion relative to the shaft.

This latter rotary/axial motion is determined by the helical track and is in a sense which will cause the plate to be urged into driving engagement with the roller by torque which is imparted to the plate by its frictional engagement with the roller.

For convenience of manufacture the helical track will usually be machined as helical splines on the shaft which are engaged by complementary helical splines on the appropriate plate.

Similarly to the device in our U.K. Patent No.

1,469,776, it is preferred that in the device of the present invention the roller is mounted on a carrier which carrier is pivotally mounted about an axis which is substantially normal to the plane which contains the axis of rotation of the plates and is off-set from the centre of that roller so that, on axial displacement of the plates, the carrier can pivot to displace radially relative to the axis of rotation of the plates the position of engagement between the roller and the respective grooves to effect a change in ratio of angular velocity at which the driving plate and its input shaft are driven relative to the driven plate and its output shaft.In such an arrangement it is further preferred that the driving and driven plates are biased towards each other and into engagement with the roller by the admission of fluid under pressure into an expansible chamber associated with one of the plates. Either the driven or driving plate can be displaceable in response to fluid under pressure as aforementioned while the other plate is preferably spring biased relative to the housing to accommodate axial displacement of that plate effected by pivotal movement of the carrier and in sympathy with axial movement of the plate which is displaceable by fluid pressure.

The pressure of fluid to which the expansible chamber is subjected can be variable in accordance with the delivery from a variable output pump or similar device so that the plate will be axially displaced in accordance with the increase or decrease of pressure from the pump as required. Valve means can be provided to control the admission of fluid pressure to, and the exhausting of fluid pressure from, the expansible chamber, particularly when the source of fluid is at substantially constant pressure.The valve means is preferably responsive to the speed of rotation of either the input or output shaft so that such valve means will be controlled in accordance with variations in rotational speed of that shaft to cause an adjustment in fluid pressure in the expansible chamber which results in the plate associated with that chamber being axially displaced to vary the ratio at which drive is transmitted through the device.Automatic control for the valve means can be provided by a governor, conveniently one which is centrifugally operated, which is responsive to the speed of rotation of one or other of the input or output shafts so that when that speed varies from a predetermined value the governor reacts to adjust the valve means in a sense which controls fluid pressure in the expansible chamber and adjusts the transmission ratio through the device to maintain the aforementioned rotational speed of the governed shaft substantially constant.

Preferably the governor is responsive to the speed of rotation of the output shaft to control the fluid pressure in an expansible chamber the expansion and contraction of which chamber effects in axial displacement of the driven plate relative to the output shaft to vary the transmission ratio through the device as aforementioned. It will be appreciated that axial displacement of the plates can be controlled other than by fluid pressure, for example such axial displacement can be achieved by a wholly mechanical system such as the centrifugal governors disclosed in our aforementioned U.K. Patent, by an electrical solenoid system or by a combination of fluid pressure, mechanical or electrical techniques.

One embodiment of a speed control device 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 a side elevation of the device in part section in which variations in gear ratio are controlled by hydraulic fluid pressure (an hydraulic system for which is shown schematically); Figure 2 is a sectional end elevation of the device shown in Figure 1 and particularly illustrates mountings for the power transmission rollers, and Figure 3 is a similar view to that shown in Figure 1 and illustrates a modification of the device (together with a modified hydraulic system which is shown schematically).

The speed control device of Figures 1 and 2 has a housing 1 which is intended to be firmly and nonrotatably mounted by bolts 2 and brackets 3 to a fixed frame or base (not shown). Mounted by roller bearings 4 in the housing 1 and for rotation about an axis 5 is a tubular input shaft 6 having an outer end 7 which extends in sealed manner from the housing. Mounted within the tubular input shaft 6 and extending through the bore thereof is an output shaft 8 having an outer end 9 which extends in sealed manner through the tubular shaft 6 and from the housing 1. The output shaft 8 is mounted by a ball race 10 within the bore of the tubular input shaft 6 and by a ball race 11 at its inner end 12 within the housing 1 to be concentric with the input shaft 6 and capable of rotation about axis 5 relative to that shaft and to the housing 1.At its inner end 13 the tubular input shaft 6 is radially supported on the output shaft 8 by roller bearings 14.

Mounted on the output shaft 8 by roller bearings 1 5 is a generally annular plate 1 6. The plate 1 6 is coupled for rotation with the output shaft 8 by co-operating axially extending splines 1 7 which permit relative axial sliding movement between the plate 16 and the shaft 8. Also mounted on the output shaft 8 by roller bearings 18 is a second plate 19 of generally annular form.

The plate 19 is axially slidable along the shaft 8 and is coupled for rotation with the input shaft 6 for rotation therewith by co-operating axially extending splines 20 which permit relative axial displacement between the plate 1 9 and the shaft 6. The input shaft 6 and output shaft 8 are restrained, conveniently by their respective bearings, from axial displacement relative to the housing 1 while the annular plates 1 6 and 19 are both capable of axial displacement relative to the shafts and to the housing while being coupled for rotation with the output shaft 8 and input shaft 6 respectively.

The opposing faces of the plates 16 and 19 are each provied with an annular groove 21 and 22 respectively, these grooves being of part circular section. The grooves 21 and 22 are co-axial with their respective plates and are axially opposed to form a generally toroidal track. The faces of these grooves 21 and 22 and thus the plates 1 6 and 19 are held apart by a circumferentially disposed array of three symmetrically arranged power transmission rollers 23. Each roller 23 rotates about an axis 24 contained in a plane which includes the axis of rotation 5 of the plates 1 6 and 19 and the rollers 23 impart by friction means a torque from the plate 19 to the plate 16 so that the latter is rotated in the opposite sense of direction to the plate 19. Each roller 23 is rotatably mounted by bearings 25 on a shaft 26 carried by a carrier 27.The carrier 27 is pivotally mounted by a pin 28 (see Figure 2) in bearings 29 in a mounting ring 30 which is secured with respect to the housing 2. Each carrier 27 together with its roller 23 is pivotable about the axis 31 of its respective pin 28 so that the axis of rotation 24 of the roller 23 can swing either side of the vertical in Figure 1. The axis 31 extends substantially perpendicularly relative to the plane which includes the axis of rotation 24 of its respective roller 23 and the axis 5 of the shaft 6 and 8.

The plate 1 9 is biased axially relative to the housing 1 and the shaft 8 by an array of spring units 32 which are disposed circumferentially around the axis 5. The spring units 32 react between the housing part 1 and an annular thrust plate 33 which latter is axially displaceable in response to spring pressure. Roller thrust bearings 34 are interposed between the thrust plate 33 and the plate 1 9 to alleviate friction between these components as the plate 19 will rotate relative to the thrust plate 33.

Located within the housing 1 and on the side of the plate 16 axially remote from the plate 19 is a generally annular piston plate 35 which is displaceable in the direction of axis 5 relative to the housing 1. The radially inner peripheral edge of the piston plate 35 is axially slidable in sealed manner on a tubular shaft part 36 carried by an internal flange of the housing 1. The radially outer peripheral edge of the piston plate 35 is slidable in the direction of axis 5 and in sealed manner over a cylindrical bore part 37 formed by the internal wall of the housing 1. Formed between the piston plate 35 and the housing 1 and on the side of the piston plate axially remote from the rollers 23 is a piston chamber 38 which is intended to be subjected to fluid under pressure to bias the piston plate 35 axially towards the plate 19.Interposed axially between the piston plate 35 and the annular plate 16 is an annular thrust plate 39 carrying roller thrust bearings 40.

During operation of the device the annular plate 1 6 is intended to rotate relative to the piston plate 35 but to be displaced axially in unison with that plate and the thrust bearings 40 alleviate friction between the relatively rotating plates 1 6 and 35 whilst transmitting axial thrust between these plates during their displacement in either sense of direction along the axis 5. Consequently, when the chamber 38 is subjected to fluid pressure to bias the piston plate 35 axially towards the rollers 23 the plate 16 is biased into engagement with the rollers 23 against the biasing effect of the spring units 32.

Hydraulic fluid pressure within the piston chamber 38 is controlled by valve means 41 which valve means is itself controlled by a centrifugal governor 42 responsive to the speed of rotation of the output shaft 8. The valve means 41 comprises a spool 43 which is axially slidable in a blind bore 44 formed in the housing 1. The spool 43 is biased relative to the housing 1 by a spring 45 to a condition in which an annular recess 46 of the spool opens communication between a fluid pressure inlet port 47 and a port 48 in which latter is in constant communication with the piston chamber 38. The inlet port 47 communicates by way of line 49 with the output from a constant delivery pump 50 drawing fluid from a hydraulic reservoir 51.The speed control device of the present embodiment is particularly suitable for use in a motor vehicle and the pump 50 will usually be driven by the vehicle engine.

Axial displacement of the spool 43 against the biasing of its spring 45 (that is leftwardly in Figure 1) closes communication between the annular recess 46 and the inlet port 47 and opens communication between the recess 46 and a fluid outlet port 52 which latter communicates by way of a return line 53 with the reservoir 51.

Displacement of the spool 43 against its spring biasing 45 is controlled by the centrifugal governor 42 which comprises ramp surfaces 54 which are inclined relative to the axis 5 and are carried by a flange 55 which is secured to the inner end 12 of the output shaft 8 for rotation therewith. Located on the ramp surfaces 54 and captured between those surfaces and a retaining plate 56 are balls 57 circumferentially disposed around the axis 5. The retaining plate 56 is carried by a guide peg 58 which is co-axial with the axis 5 and is displaceable together with the retaining plate along that axis. One end of the peg 58 is axially slidably received in a blind bore 59 provided in the inner end 12 of the output shaft while the other end of the peg abuts the spool 43.

Operation of the governor 42 is effected, as aforementioned, by rotation of the output shaft 8 relative to the housing 1 and as the speed of such rotation progressively increases the balls 57 will eventually be displaced radially outwardly relative to the axis 5 by the centrifugal force to which they are subjected thereby causing the peg 58 to be displaced axially relative to the shaft 8 against the spool 43 thus displacing the spool leftwardly in Figure 1 and against the biasing of its spring 45 to open or further open communication between the piston chamber 38 and the outlet port 52.

Conversely, when the output shaft 8 is stationary or its speed of rotation decreases sufficiently, the balls 57 are displaced over the ramp surfaces 54 and to or towards their radially innermost condition by the biasing effect of spring 45 acting through the spool 43 and peg 58 on the retaining plate 56. The cavity within which the centrifugal governor 42 is located is sealed from the piston chamber 38 and is preferably in constant communication (by way of a passage 58) with the outlet port 52.

We will now consider operation of the speed control device commencing from the condition in which the device is illustrated. The input shaft 6 is conveniently rotatably driven through a V pulley 59 by a V-belt and it will be assumed that the input shaft is driven at variable speed (say from the crank shaft of the vehicle engine) and also that the pump 50 is operative. Rotation of the input shaft 6 is transmitted through the splined connection 20 to rotate the plate 1 9 which causes the plate 1 6 to be rotated in a contra direction as previously mentioned.With the rollers 23 in the position shown whereby the radius R2 with respect to the axis 5 at which the roller engages the groove 21 is greater than the radius R1 with respect to the axis 5 at which the roller engages the groove 22, the speed transmission ratio between the plates 1 9 and 1 6 will be at a minimum (while mechanical advantage for the drive which is transmitted will be at a maximum) so that the speed of rotation of the plate 1 6 is less than that of the plate 1 9-this can be advantageous particularly when starting the vehicle engine since the load presented by the speed control system to the vehicle engine is at a minimum.Upon rotation of the plate 1 6 the output shaft 8 is rotated therewith through the splined connection 1 7 to provide an output which is conveniently taken off through a V pulley 60 (which is connected to the outer end of the output shaft) by a V-belt. The output from the pulley 60 may be used to drive, for example, auxiliary equipment of the vehicle such as a water pump or generator. It will be seen from Figure 1 that since both the input and output shafts 6 and 8 emerge from the housing 1 through a common side of the housing and are concentrically arranged, the pulleys 59 and 60 can be disposed axially adjacent to each other so providing a compact structure which may facilitate coupling of the V belts to the respective pulleys.

If the speed at which the output shaft 8 is rotatably driven is insufficient to displace the balls 57 of the governor from their radially innermost condition as shown, Fluid under pressure from the pump 50 is directed by way of port 47, recess 46 and port 48 into the piston chamber 38 causing the latter to expand and displace the piston plate 35 together with the plate 1 6 rightwardly in Figure 1. This has the effect of pivoting the rollers 23 (in an anti-clockwise direction for the roller shown in Figure 1) about the pin axes 31 of the carriages 27 and displacing the plate 1 9 against its biasing spring unit 32.During such pivotal movement of the rollers 23 the radius R2 is progressively decreased while the radius R1 is increased correspondingly which causes an increase in the speed transmission ratio between the plates 1 9 and 1 6 and thereby a relative increase in the speed of rotation of the plate 1 6 (and thereby a relative decrease in the mechanical advantage through the device). When the speed of rotation of the plate 1 6 and thereby the output shaft 8 attains a predetermined value, for example that at which the aforementioned auxiliary equipn,ent is designed to be driven at substantially constant speed, the governor 42 is arranged to react in the event that the desired constant speed is exceeded.Such reaction by the governor causes the spool 43 to be displaced against its spring biasing 45 and thereby to close off communication between the inlet port 47 and the piston chamber 38 and if necessary to open communication between the piston chamber 38 and the reservoir 51 through the outlet port 52 and the return line 53. Upon opening of the piston chamber 38 to the reservoir the piston plate 35 together with plate 1 6 can be displaced leftwardly in Figure 1 under the biasing effect of the spring units 32 on plate 1 9 (which reacts through the rollers 23) so causing the carriages 27 to pivot about their respective axes 31 and increase the radius R2 while the radius R1 is decreased correspondingly.In this way it will be apparent that by appropriate selection of the biasing springs 32 and 45 and characteristics of the governor 42 the control device can be arranged to provide the output (which is derived from the output shaft 8 by way of pulley 60) with a substantially constant speed of rotation when the speed of rotation at which the input shaft 6 is driven has a value which may be varied within a predetermined range of such values; for example in a practical embodiment it is envisaged that the device may provide a substantially constant output speed for the shaft 8 of 2,000 revolutions per minute (r.p.m.) for a range of input speeds for the shaft 6 from 888 r.p.m. to 4,500 r.p.m.

The splined connections 1 7 and 20 between the plates 1 6 and 1 9 and their respective shafts may be formed by substantially rectilinear splines which extend substantially parallel with the axis 5. Preferably however one or both of these splined connections is formed by co-operating splines which are of substantially helical formation. These helically co-operating splines are orientated with respect to the axis 5 so that, as the plate or plates with which they are associated is axially displaced with respect to the shaft 8, such plate or plates exhibits a screwed motion or rotary motion about the axis 5.This latter axial and rotary motion effected by relative movement between the co-operating splines either or both between the plate 1 6 and shaft 8 or plate 19 and shaft 6 is arranged to be in the same rotational direction as that in which the plate 1 6 or 1 9 associated with the splined connection is driven in engagement with the rollers 23 so that the torque effected on the respective plate or plates 16 and 19 by frictional engagement with the rollers 23 imparts relative rotational movement between the input or output shaft to which the plate is connected as the case may be to urge the plate axially into closer engagement with the rollers 23. In this way the force created between the plates 1 6 and 19 and the rollers 23 may be increased in relation to the magnitude of the torque which is being transmitted through the device. For example, if the input shaft 6 is rotated in a clockwise direction (as viewed from the righthand end in Figure 1) and the splines of the connection 20 have a lefthand helix a force will be generated in the splined connection 20 which urges the annular plate 19 leftwardly in Figure 1 thereby increasing the force between the rollers 23 and the plate 19.Rotation of the input shaft 6 in a clockwise direction as aforementioned causes the annular plate 1 6 to rotate in an anti-clockwise direction and if the co-operating splines of the connection 1 7 also carry a lefthand helix then the annular plate 16 will be urged to the right in Figure 1 thus similarly increasing the force between the rollers 23 and the plate 1 6. From this it will be seen that the torque capacity of the device is increased as the torque loading increases.

In the modification of the device shown in Figure 3 the valve 41, the governor 42 and the constant delivery pump 50 as aforementioned have been omitted and the piston chamber 38 is in constant communication by way of port 61 and passage 62 with the output from a variable delivery pump 63 which draws hydraulic fluid from the reservoir 51. The variable delivery pump 63 also has its output communicating with the reservoir 51 by way of a passage 64 which includes a temperature compensated restrictor 65.The output of the variable delivery pump 63 increases with the speed of the pump and it is this output which is used to expand the piston chamber 38 or to permit the chamber 38 to contract thereby displacing the piston plate 35 and plate 16 against the biasing spring units 32 or permitting the piston plate 35 and plate 1 6 to be dispaced under the effect of the biasing spring units 32 respectively. Further in this modification the tubular shaft 6 (with the pulley 59) is to be regarded and coupled as the output shaft and the shaft 8 (with the pulley 60) is to be regarded and coupled as the input shaft.Preferably the variable delivery pump 63 is arranged to be driven at a speed which is proportional to the rotary speed of the input shaft 8 and indeed the variable delivery pump is conveniently driven through an appropriate linkage (indicated at 66) from the input V pulley 60. By this modified arrangement as the speed of the input shaft 8 progressively increases so does hydraulic pressure in the piston chamber 38. With the input shaft 8 rotating slowly and low pressure output from the variable delivery pump, the rollers 23 and plates 1 6 and 19 will be in the condition shown in Figure 3 (so that the gear has a high speed transmission ratio whereby R2 is greater than R 1 and plate 19 is driven at a greater angular velocity than that of the driving plate 1 6).As the rotary speed of the input shaft 8 is increased so will that of the output shaft 6 (with a corresponding increase in fluid pressure in the piston chamber 38). When the rotary speed of the input shaft 8 attains (and exceeds) a predetermined value, the output from the variable delivery pump 63 is arranged to pressurise the chamber 38 to an extent which displaces the piston plate 35 and plates 1 6 and 1 9 rightwardly in Figure 3 against the biasing of spring units 32-this causes the rollers 23 to pivot about their associated axes 28 to decrease R2 and increase R1 so that the transmission ratio of the gear is reduced and there is a relative reduction in the rotary speed of the output shaft 6. In this way, and by appropriate selection of the axially pressurised area of the piston plate 35 in the chamber 38 and of the spring units 32, the rotary speed of the output shaft 6 can be maintained substantially constant when the rotary speed of the input shaft 8 attains a value which can be variable within a predetermined range of such values. In this modification it will be apparent that the helixes of the splined connections 1 7 and 20 are appropriately orientated to ensure that the plates 1 6 and 1 9 are urged into driving engagement with the rollers 23 by the torque transmission as previously described.

Claims (25)

Claims

1. A speed control device of the kind specified in which the housing is non-rotatable and the driving and driven plates are respectively mounted for rotation with co-axial input and output shafts.

2. A device as claimed in claim 1 in which the input and output shafts extend from the housing.

3. A device as claimed in either claim 1 or claim 2 in which the input and output shafts are concentric and are rotatably mounted at least partly one within the other.

4. A device as claimed in claim 3 when appendant to claim 2 in which the input and output shafts emerge from a common side of the housing.

5. A device as claimed in any one of the preceding claims in which both plates are borne on the same input or output shaft.

6. A device as claimed in any one of the preceding claims in which the input and output shaft are restrained against axial displacement relative to the housing and the or at least one axially displaceable plate is axially displaceable relative to the respective shaft with which it is mounted for rotation.

7. A device as claimed in claim 6 in which the or at least one axially displaceable plate is coupled to its respective shaft for rotation therewith and axial displacement relative thereto through an axially extending track and a track follower which co-operates therewith.

8. A device as claimed in claim 7 in which the track and track follower comprise co-operating splines between the plate and the shaft with which it is rotatable.

9. A device as claimed in either claim 7 of claim 8 in which the track follower co-operates with a helical track which is arranged so that during axial displacement of the plate which is associated therewith, that plate exhibits a screwed (rotary/axial) motion relative to the shaft with which it is rotatable and said motion is in a sense which causes the plate to be urged into driving engagement with the roller by torque which is imparted to that plate by its driving engagement with the roller.

10. A device as claimed in claim 9 when appendant to claim 8 in which the track is formed by helical splines on the shaft and the track follower comprises helical splines on the respective plates which engage with the track in substantially complementary manner.

11. A device as claimed in any one of the preceding claims in which both said plates are axially displaceable, and the roller is mounted on a carrier which carrier is mounted in the housing for pivotal movement about an axis which is substantially normal to a plane which contains the axis of rotation of the plates and is off-set from the centre of that roller so that, on axial displacement of the plates, the carrier can pivot to displace radially relative to the axis of rotation of the plates the positions of engagement between the roller and the respective grooves to effect a change in ratio of the angular velocity at which the driving plate and its input shaft are driven relative to the driven plate and its output shaft.

12. A device as claimed in claim 11 in which the driving and driven plates are biased towards each other and into engagement with the roller by the admission of fluid under pressure into an expansible chamber associated with one of the plates.

13. A device as claimed in claim 12 in which one of said plates is displaceable in response to the admission of fluid under pressure to the expansible chamber which is associated with that plate and the other plate is spring biased relative to the housing to accommodate axial displacement of that other plate effected by pivotal movement of the carrier and in sympathy with axial displacement of said one plate.

14. A device as claimed in either claim 12 or claim 13 in which valve means is provided for controlling the admission of fluid pressure to, and the exhausting of fluid pressure from, the expansible chamber.

15. A device as claimed in claim 14 wherein the valve means is responsive to the speed of rotation of one of said shafts so that such valve means is controlled in accordance with variations in rotational speed of that shaft to cause an adjustment in fluid pressure in the expansible chamber which results in the plate associated with the expansible chamber being axially displaced to vary the ratio at which drive is transmitted through the device.

16. A device as claimed in claim 1 5 in which the valve means is automatically controlled by a governor which is responsive to the speed of rotation of one of said shafts so that when that speed varies from a predetermined value the governor reacts to adjust the valve means in a sense which controls fluid pressure in the expansible chamber and adjusts the drive transmission ratio to maintain the rotational speed of the output shaft substantially constant.

17. A device as claimed in claim 16 wherein the governor is centrifugally operated and is mounted for rotation with the shaft to which it is speed responsive.

18. A device as claimed in any one of claims 14 to 1 7 wherein the valve means is mounted on the housing.

19. A device as claimed in claim 18 when appendant to claim 17 wherein the valve means comprises a spring biased spool which is displaceable relative to the housing and against its biasing to control fluid pressure in the expansible chamber, said spool co-operating with a component of the governor which component is displaceable in response to centrifugal operation of the governor to effect displacement of the spool.

20. A device as claimed in claim 19 wherein said spool and component are displaceable in a direction which is co-axial with the axis of rotation of the shafts.

21. A device as claimed in either claim 12 or claim 13 in which the expansible chamber is in constant communication with a source of fluid pressure which provides a variable pressure output which output is in constant communication with exhaust by way of a restrictor.

22. A device as claimed in claim 21 in which said source provides an output the pressure of which increases and decreases in proportion to an increase or decrease respectively in the speed of rotation of one of said shafts.

23. A device as claimed in claim 22 in which fluid pressure is derived from a variable delivery pump which is responsive to the speed of rotation of one of said shafts so that the output of said pump is proportional to that speed whereby fluid pressure in the expansible chamber is varied automatically to adjust the drive transmission ratio through the device and maintain the rotational speed of the output shaft substantially constant.

24. A device as claimed in claim 23 wherein the pump is coupled to be driven by the shaft to the rotational speed of which it is responsive.

25. A speed control device substantially as herein described with reference to Figures 1 and 2 of the accompanying illustrative drawings.

26: A speed control device as claimed in claim 24 and modified substantially as herein described with reference to Figure 3 of the accompanying illustrative drawings.