WO1993021468A1

WO1993021468A1 - Improvements in or relating to drag/damper devices

Info

Publication number: WO1993021468A1
Application number: PCT/GB1993/000746
Authority: WO
Inventors: Richard Arthur Lindsay
Original assignee: Vinten Group Plc
Priority date: 1992-04-08
Filing date: 1993-04-08
Publication date: 1993-10-28
Also published as: AU3900993A; GB9207649D0

Abstract

The disclosure relates to a rotary drag/damper device for controlling rotation of one rotary member (10) with respect to another (11; 13) about a first axis of rotation. The device includes a rotary viscous coupling (40) which slips when subjected to a torque with a resistance to slippage which rises progressively with respect to speed of rotation and a rotary friction coupling (61, 70, 43) arranged in series with the viscous coupling and which transmits torque without slippage in a torque transmitting direction up to a certain level at above which the friction coupling slips with the resistance which does not change significantly with speed. The coupling includes an arrangement (40, 47, 50) to generate slippage in the friction coupling in a direction transverse to the torque transmitting direction with slippage in the viscous coupling to provide a smooth transition when the torque rises above said certain level which causes slip of the friction coupling in the torque transmitting direction.

Description

IMPROVEMENTS IN OR RELATING TO DRAG/DAMPER DEVICES

This invention relates to a drag/damper device for providing a coupling having a selected angular speed/torque characteristic between two relatively rotatable members. The invention is of particular application to TV/video camera mountings such as pan and tilt heads.

Devices which produce drag or damping by means of either friction or viscous shear between relatively moving elements are known. The former typically comprise a friction clutch, attached to one element and contacting the relatively moving element under pressure, such that energy is dissipated by the resulting sliding contact. The magnitude of the damping forces is dependent upon the friction contact pressure and may be readily varied.

The torque/speed characteristic of a friction damper may be modified by the choice of friction materials and/or the provision of a fluid at the friction surface to provide boundary lubrication of the sliding contact in which case the damping force may rise with increasing speed from an initial stationary position and thereafter remain substantially constant irrespective of speed.

Viscous dampers typically comprise multiple interleaved plates alternately attached to each of the relatively moving elements, each plate being separated from adjacent plates by a small gap occupied by a viscous fluid. Relative movement of the plates causes viscous shear to take place in the fluid. The damping force generated tends to increase substantially linearly with speed but at rest is zero. A particular and important application of drag/damper devices is in pan and tilt heads for cameras, especially television^" cameras. For that application, it is especially important to avoid

"jerking" when making very slow movements from rest, for the torque to be substantially zero at near zero speed. On the other hand, for increasingly fast camera movement above a certain speed, it is advantageous for the torque not to increase with speed once a certain resistance to movement has been reached.

In applications such as camera mountings, where the purpose of the damping device is to enhance the smoothness and control of manual movements, the following characteristics are therefore desirable.

1. The rate of change of the damping/speed relationship should tend to zero or close to zero at high speed to permit relatively fast movements to be accomplished without unreasonable restraint, whilst providing adequate damping control at low speed.

For fine control at the commencement of movement and during reversal of movement, the unit should produce zero damping at rest and a high rate of change of damping with speed at low speeds.

In our U.K. patent application No. 9114051.7 we describe and illustrate a drag/damper device providing a coupling having a selected angular speed/torque characteristic between two relatively rotatable members comprising a central shaft fixed to one member and a housing surrounding the shaft and fixed to the other member. The housing accommodates a first set of spaced plates fixed to the shaft, a second set of spaced plates fixed to the housing, and a third set of spaced plates floatingly interleaved with the plates of the first and second sets. A viscous fluid in which the plates are immersed fills the housing and the spacings between the plates of the first and third sets and between the second and third sets are chosen to provide a combined speed/torque characteristic wherein there is zero torque at rest but which gives torque increasing substantially linearly with speed up to a given speed and provides substantially constant torque above said given speed. In essence the arrangement provides the desirable characteristics of the two damper types whilst masking the undesirable characteristics. In the preferred arrangement, one unit (defined by the coupling between the first and third sets of plates) is constructed so as to comprise predominantly a frictional damper, whilst the other unit, defined by the coupling between the second and third sets of plates, is constructed so as to comprise predominantly a viscous fluid damper. Thus the individual speed/torque characteristics of the two units combined together to result in an overall characteristic in which the torque is zero at zero speed (because the viscous fluid damping predominates at low speed) but torque is substantially constant above a given speed above which the frictional damping predominates.

It is an object of the present invention to provide a more practical arrangement for achieving a drag/damper device combining both frictional and viscous damping than hitherto disclosed. - A -

This invention provides a rotary drag/damper device for controlling rotation of one rotary member with respect to another member about a first axis of rotation, the device comprising a rotary viscous - coupling which slips when subjected to a torque with a resistance to slippage which rises progressively with respect to speed of rotation, a rotary friction coupling arranged in series with the viscous coupling and which transmits torque without slippage in a torque transmitting direction up to a certain level above which the friction coupling slips with a resistance which does not change significantly with speed, and means to generate slippage in the friction coupling in a direction transverse to the torque transmitting direction with slippage in the viscous coupling at torques below said certain level and thereby to provide a smooth transition when the torque rises above said level causing slip in the . torque transmitting direction of the friction coupling.

Preferably the rotary viscous coupling is mounted with its axis of rotation eccentric to said first axis of rotation one part of the coupling being driven by said one rotary member and the other part of the coupling engaging said friction coupling, whereby rotation of the rotary member causes the viscous coupling to precess around the first axis and the second part of the coupling moves transversely with respect to the friction coupling to provide slippage with respect to the friction coupling in the transverse direction when there is no slippage in the torque transmitting direction.

For example the rotary viscous coupling may be provided between two relatively rotatable components one of which is drivably connected to said one rotary member and the other of which is rotatable with respect to the said one component about said eccentric axis.

More specifically said other component of the viscous coupling may provide one element of the friction coupling.

In one particular embodiment of the invention the friction coupling may comprise an axially facing annular friction ring mounted on a non-rotating component and engaging an oppositely facing surface of said other component of the viscous coupling.

More particularly the friction coupling may comprise a pair of annular friction rings mounted on oppositely facing parts of the non-rotating component and the other component of the viscous coupling may have axially directed faces on either side thereof which extend between and are engaged by the friction rings.

Preferably resilient means are provided for biassing said oppositely facing parts of the component towards one another to provide the requisite engagement force between the friction rings and component of the viscous coupling.

Further means may be provided for adjusting the engagement force of the friction rings with the viscous coupling.

In any of the above arrangements said one component of the viscous coupling may comprise a rotor having a first set of annular fins and the second component comprises a stator having a second set of annular fins meshing with the first fins of the rotor, with a viscous fluid between them, the stator being part of an annular housing enclosing the rotor and providing the axially directed face or faces with which the annular friction ring or rings engage.

In any of the above arrangements a further drag/damper device comprising a viscous coupling may be provided in parallel with the first above-mentioned drag/damper device to provide a resistance to rotation which rises with speed even when the torque imposed has exceeded said certain level and the friction coupling of the first drag/damper device is in slip mode.

In the case where the friction coupling comprises an annular friction ring or rings, the ring or rings may have an axis or axes offset with respect to the axis of the viscous coupling so that, when the ring or rings slip, they track over an area of the surface with which they engage greater in width than the width of the ring or rings.

The invention also provides a rotary drag/damper device for controlling rotation of one rotary member with respect to another member , the device comprising a rotary viscous coupling which slips when subjected to a torque with a resistance to slippage which rises progressively with respect to speed of rotation, and a rotary friction coupling arranged in series with the viscous coupling and which transmits torque without slippage in a torque transmitting direction up to a certain level above which the friction coupling slips with a resistance which does not change significantly with speed, wherein the device includes a wheel driveable through said rotary viscous coupling, the wheel engaging an annular track through said frictional coupling, such that the wheel has a rolling engagement with the track up to a certain torque level above which the wheel slips with respect to the track, the rolling engagement of the wheel prior to slippage providing a relatively smooth transition between rolling and slippage.

Preferably the annular track is formed around the inner side of an annular member with which the wheel engages, the wheel being mounted to rotate about an axis which precesses around the axis of the track as the wheel rotates around the track.

It is also preferred that the wheel is mounted for rotation on a hub and the viscous coupling is disposed between the hub and wheel. Means may be provided for biassing the hub towards the track to provide a positive engagement between the wheel and track.

The following is a description of some specific embodiments of the invention, reference being made to the accompanying drawings in which:-

Figure 1 is a perspective view of a pan/tilt head for mounting a video/T.V./film camera on a tripod stand and embodying drag/damper devices for controlling pan and tilt movement respectively;

Figure 2 is a cross sectional view through the head illustrating the drag/damper device for controlling pan movement of the head; Figure 3 is a cross ectional view through the head showing the drag/damper device for controlling tilt movement of the head;

Figure 4 is a diagramatic view of an alternative form of drag/damper device;

Figure 5 is a diagrammatic illustration of a further modification; and

Figure 6 is a sectional view of yet a further arrangement.

Referring firstly to Figure l of the drawings, there is shown a pan/tilt head indicated generally by the reference numeral 10 suitable for supporting a T.V./ video/cinematograph camera on a tripod comprising a main body 11 having a base 12 which is mounted for rotation about a vertical axis in a conventional levelling bowl 13 at the upper end of a tripod indicated generally at 14. A camera support platform 15 is mounted on the body 11 for tilting movement about a horizontal axis, the platform having a conventional dove-tail section slot and camera mounting plate 16 to receive a camera which is not shown.

The body contains drag/damper devices for providing resistance to pan and tilt movements of the head which will be described in detail below. The device for resisting pan movement has a control 17 for varying the drag provided by the device and a pan lock 18 is provided for locking the head in any selected position of adjustment. Likewise the drag/damper device for resisting tilt movement has a control 19 for varying drag in the tilt mode and a lock operated by control 20 for locking the head in a selected position of tilt. The head embodies a counterbalancing mechanism so that, when a camera is in situ, the head is automatically balanced in any position throught its range of tilt so that the camera can be left at any angle to which it is set without having to lock off the tilt control. The counterbalance mechanism is adjusted by a control indicated at 21 to cater for different camera weights and centres of gravity above the platform 16.

The head has left and right hand mountings 22 on either side to which an arm (not shown) may be secured for controlling pan and tilt movement of the head by the camera operator. A further feature of the head is the provision of a part spherical spirit level 23 mounted in a partially recessed housing 24 in the base 12 to assist in levelling the head.

Reference is now made to Figure 2 of the drawings which illustrates the drag/damper device for controlling pan movement of the head in greater detail.

The base 12 of the head has an under body 30 in the form of an inverted hemisphere to be received and secured in the levelling bowl 13 of the tripod. The under body has a peripheral rim 31 to which the base 12 of the head 11 is secured by bolts 32.

The base 12 of the head is formed with a central opening 12a the peripheral wall of which has a rebate 33 to provide a seat for a bearing 34 having a vertical axis indicated at 35. The lower part of the under body 30 is formed with a similar rebate 36 in which a bearing 37 is mounted concentric with the axis 35. The base 12 has a downwardly projeting hub 38 thereon which is supported in the bearings 34 and 37 for rotation of the head about the vertical axis 35 to provide pan movement of the head.

An annular viscous coupling indicated generally at 40 encircles and is mounted on the hub 38. The viscous coupling comprises an inner member or rotor 41 fixed to the hub 38 and having a multiplicity of upstanding spaced annular fins 42. The coupling further comprises an outer casing or stator 43 which encloses the inner member and has upper and lower walls 44, 45 formed with bearing surfaces 46 around their inner peripheries which engage with corresponding bearing surfaces 47 on the hub 38 above and below the member 41. The upper and lower walls 44, 45 are formed with overlapping flanges at their outer peripheries which are secured together by bolts 48. The outer casing thus provides a cavity in which the inner member 41 is free to rotate. The inner side of the upper wall 44 of the casing is formed with a multiplicity of downwardly.projecting annular fins 49 which interleave with working clearances between the upstanding fins 42 on the member 41. The clearances between the.interleaving annular fins 42 and 49 are filled with a viscous fluid which provides a resistance to relative rotation of the inner member 41 with respect to the outer casing 43 which rises substantially linearly with relative speed. The axis of rotation of the. viscous coupling as defined by the bearing surfaces 47 on the hub is indicated at 50 and is slightly eccentric with respect to the vertical axis 35 about which the head rotates to cause the viscous coupling to precess around axis 35 with rotation of the hub 38 for a purpose described in greater detail below. Rotation of the outer casing 43 of the viscous damper is constrained by friction brakes engaging the upper and lower surfaces of the outer casing which will now be described. The base 12 of the head overlies the upper side 44 of the casing 43 and is formed with an annular groove 60 in which a rectangular section friction ring 61 is mounted to engage the opposing upper face of the casing 43. The axis of the groove 60 is offset from the axis 35 as can be seen by comparison between the left and right hand sides of Figure 2 so that when the friction resistance provided by the ring 61 is overcome and the outer casing 43 turns with respect to the base, the friction ring 61 will track across the surface of the casing between the positions indicated in full line and dotted line on the left hand side of Figure 2 rather than simply slide around an annular path on the casing equivalent to its own width.

A diaphragm spring plate 65 supports a disc 67 on the underside of the casing 43. The upper side of the disc 67 is formed with an annular groove 69 similar to the groove 60 and also offset from the axis 35 for the same purpose and an annular friction ring 70 is mounted in the groove to engage the underside of the casing 43.

The diaphragm plate 65 is rigidly fixed to the disc 67 at its outer periphery and to the under body 30 at its inner periphery to provide back lash free axial movement of plate 65.

The frictional drag provided by the friction rings may be varied by means of a collar 75 mounted on the hub and engaging an annular bearing 76 mounted adjacent the inner periphery of the plate 65. The collar 75 is connected through an axially movable spindle 77 extending through the hub 38 to an adjustment mechanism 78 located at the upper end of the hub and adjusted by the aforesaid control knob 17. By operating the control .in a direction to draw the spindle 77 upwardly, the plate 67 is raised . through the collar 75 and bearing 76 to increase the clamping force of the friction rings 61 and 70 on the upper and lower sides of the outer casing 43 and thereby increase the torque required to break the frictional grip of the friction rings on the outer casing. Likewise lowering the central spindle 77 relaxes the frictional grip of the rings 61 and 70 on the outer casing.

The frictional brake provided by the engagement of the friction rings 61 and 70 with the outer casing 43 and the viscous coupling provided between the outer casing 43 and inner member 41 are, in effect, arranged in series with one another. When the camera carrying head is rotated about the axis 35, the movement is resisted initially by the viscous coupling with the inner member 41 of the coupling rotating with respect to the outer casing 43. As the inner member of the coupling is rotated, the coupling as a whole precesses around the axis 35 to cause a small movement between the outer casing and the friction rings 61 and 70 in the radial direction to minimize "stiction" between the rings and surfaces of the outer casing with which they engage.

The resistance provided by the viscous coupling increases, e.g. substantially linearly, with the relative speed of rotation between the inner coupling member 41 and outer casing 43. The outer casing 43 is held against rotation by the engagement of the friction rings 61, 70 but there eventually comes a point when the force required to hold the outer casing exceeds the resistance offered by the friction rings and the outer casing then starts to slide with respect to the rings. As indicated earlier, the outer casing is caused to move radially with respect to the friction rings as soon as the inner member of the viscous coupling starts to rotate and so effective "stiction" between the rings and casing is diminished to ensure a smooth transition as the outer . casing starts to rotate rather than a surge or jerk in its movement as the force overcomes the frictional grip of the rings.

Unlike the viscous coupling, the frictional rings provide a substantially constant force resisting rotation of the outer casing. Thus, as the head is rotated from a rest position, initial resistance to rotation of the head rises with the speed of rotation of the head under the control of the viscous coupling. Eventually the frictional resistance of the friction rings 61, 70 holding the outer casing stationary is overcome and the casing starts to rotate with respect to the friction rings. The arrangement then provides a substantially constant resistance to rotation provided by the friction rings whatever the speed of rotation of the head reaches.

The frictional resistance can, as indicated above, be readily adjusted as required by varying the clamping force of the friction rings on the outer casing through the inner spindle 77 and the control knob 17. The aforesaid brake 20 for locking the head against rotation is coupled to a conventional locking mechanism which will not be described or illustrated in detail.

Figure 3 of the drawings shows the application of a similar mechanism to the tilt movement of the head and like parts have been allotted the same reference numerals.

Referring now to Figure 4 of the drawings, there is shown a further form of drag/damper device embodying a viscous coupling and frictional brake located in series with one another for controlling pan or tilt movement of a camera mounting head.

The version of the drag/damper device illustrated provides control for pan movement of the camera and similar arrangement is also used for control of tilt movement. The device comprises a non-rotating housing 100 of upwardly open cup-shaped form and adapted to be secured in the head of a tripod. A rebate 101 is formed around the inner periphery of the rim 102 of the cup to receive a bearing 103 in which a hub 104 formed at the lower end of the base 12 of the pan/tilt head is received to support the head for rotation about a vertical axis indicated at 105. The hub 104 is formed with a downwardly projecting spigot 106 having a threaded lower end 107 on which a collar 108 is engaged which is supported for rotation in a further bearing 109 mounted in the base of the housing 100.

A rotary viscous coupling 110 is provided in the housing 100 comprising an inner hub 111 encircling the spigot 106 with clearance to allow radial play. The hub 111 has at its upper end a radial flange 112 which is located adjacent the underside of the hub 104. A rotary driving connection is provided between the flange 112 and hub 104 in the form of radially extending splines 113 housed between radial slots

114, 115 in the flange and hub to allow the flange to float radially whilst providing a rotary drive.

The viscous coupling further comprises a hollow outer casing 116 which is supported on the hub 111 by means of spaced bearings 117, 118. The hub 111 between the bearings 117 and 118 has a plurality of radially outwardly extending fins 119 secured thereto and the outer casing 116 has a corresponding set of radially inwardly extending fins 120 which interleave with clearances between the fins 119. A viscous fluid fills the clearances between the respective fins to provide a degree of resistance to relative rotation between the fins and thereby between the outer casing and hub of the viscous coupling which varies with the relative speed of the components of the coupling.

The outer periphery of the casing 116 has an annular tyre 121 formed from a frictional material to run on the inner surface of the peripheral wall of the base 100.

The spigot 106 is hollow as indicated at 130 and has a side opening 131 in which a ball 132 is located to project from and engage in a recess 134 in the inner peripheral surface of hub 111. A lever 135 is pivotably mounted on a pivot pin 136 in the spigot and one end of the lever below the pivot pin bears against the ball 132. Above the pivot pin the lever is engaged by a radially extending plunger 137 having a screw threaded portion 138 engaging in a screw threaded bore 139 in the hub 104. The arrangement enables the outer periphery 121 of the viscous coupling to be pressed against the inner face of the housing. The contact pressure can be varied by adjusting the plunger to vary the force applied to the viscous coupling through the lever/ball thereby to vary the torque at which the coupling will slip with respect to the housing.

When the camera carrying head is rotated from rest, initial movement of the head will cause the hub of the viscous coupling to rotate with respect to the outer casing against the resistance created between the respective fins of the casing and hub and at the same time the outer casing of the coupling will precess around the inner periphery of the base. At low speeds, the turning force on the outer casing is matched by the frictional force between the tyre 121 and the inner periphery of the base so that the casing simple precesses around the base. However, as the speed of turning of the head rises, so the force required to constrain rotation of the outer casing 116 rises until this overcomes the frictional resistance between the tyre and the inner periphery of the base and the tyre will then skid or slip around the inner periphery of the base. Because the tyre is already rotating with respect to the base, a very smooth transition between location and skid is achieved. Thus the arrangement provides a two-phase control of the pan movement of the head in a first phase of which the movement is controlled by the viscous damper alone so that as the speed of turning of the head rises, so the resistance to rotation rises. In the second phase, the resistance to skidding of the outer casing around the inner periphery of the base is overcome and the outer casing simply skids around the inner periphery constrained only by the frictional drag between the tyre 121 and the inner periphery of the base providing a substantially constant resistance to rotation whatever the speed of rotation the outer casing reaches.

Figure 5 of the drawings shows a further arrangement similar to that of Figure 2 or Figure 3 with an additional drag/damper device comprising a viscous coupling indicated at 140 between member 41 of the first viscous coupling and base 12 of the head. The further drag-damper device is thereby arranged in parallel with the first drag/damper device 40, 61 and 70, i.e. the drag/damper device of Figure 2 or Figure 3, so that a degree of increasing resistance to movement is provided even after the resistance to slip of the first drag/damper device 40, 61 and 70 has been overcome.

Reference is now made to Figure 6 of the drawings in which a further form of drag/damper device is shown which may be used in place of the drag/damper devices of Figs. 2 and 3 of the drawings for controlling pan/tilt movement of the camera mounting head. The arrangement comprises a housing indicated generally at 150 comprising a fixed circular base 151 on which an upstanding annular side wall 152 is mounted spaced inwardly of the periphery of the base. The housing is completed by a cover 153 having a top wall 154 extending over the side wall 152 and a dependent skirt 155. The cover is mounted for relative rotation with respect to the base 151 about an axis indicated at 156 by an annular bearing race 157 interposed between the lower periphery of the skirt 155 and the outer periphery of the base 151.

A viscous coupling similar to the coupling described with reference to Figures 2 and 3 is disposed within the housing comprising a rotor 41 having spaced annular labyrinth fins 42 and a stator 43 having corresponding spaced annular labyrinth- fins 49. Rotational input to the rotor is effected through the cover 153, a hub 158 mounted at the center of the cover and a flexible disk spring 159, the inner periphery of which is secured to the hub and the outer periphery of which is secured to the rotor.

The viscous coupling is enclosed in a telescopically expandible casing indicated generally at 160 comprising a lower part 161 with which the stator 143 is formed integral, an upper part 162 and an intermediate annular wall 163 telescopically engaged between the upper and lower parts of the casing.

A number of spaced radial slots 164 are formed in the lower part of the casing in which rocker arms 165 are mounted with sufficient clearance to enable the arms to tilt in vertical planes as depicted. The outer ends of the rocker arms engage under the lower end of the annular wall 163 and the inner ends of the arms are engaged by a thrust race assembly 166.

The rotor 41 of the viscous coupling has a hub 168 at its inner periphery formed with an internal screw-thread in which a lead screw 169 engages. The lead screw is connected to a spindle 170 mounted for rotation in the hub 158 of the top wall of the casing 153 so that rotation of the spindle moves the hub 168 and thereby the rotor 41 of the viscous coupling up and down in the housing to vary the degree by which the fins or labyrinths of the rotor and stator of the coupling are interleaved to vary the degree of viscous drag provided by the coupling. A compression ring 167 acts between the hub 168 and thrust race assembly.

The upper periphery of the wall 152 of the housing has an inwardly overhanging annular plate 175 and the upper end of the annular part 163 of the casing has a channel section carrier 176 secured thereto in which a friction ring 177 is mounted to engage the underside of the plate 175. A multiple friction ring arrangement is provided between the lower part 161 of the casing and the base of the housing comprising a first friction ring 178 mounted on the underside of the casing 161 and engaging a plate 179 mounted on the peripheral wall 152 of the housing. A further plate 180 is mounted on the underside of the casing 161 and has upper and lower friction rings 181 and 182 bonded thereto to engage respectively the underside of plate 179 and the upper face of the base 151 to provide the frictional drag arrangement of the coupling as aforesaid.

The compression spring 167 acting through the rocker arms 165 serves to force the lower part of the casing 161 downwardly and the intermediate wall of the casing 163 upwardly to hold the friction rings 177, 178, 181 and 182 in contact with their respective sliding surfaces.

As indicated above, rotation of the spindle 170 raises and lowers the rotor of the viscous coupling to vary the viscous drag provided by the coupling. At the same time, the compression spring 167 is compressed/relaxed to vary the loading applied to the friction rings with their respective surfaces and thereby vary the torque at which the friction rings slip.

The arrangement thus maintains the relationship of the torques between the viscous elements and friction elements throughout the range of adjustment. Thus, the overall characteristics are maintained and only the amplitude of the clamping force of the friction elements is varied.

As before, the friction rings are concentric with an axis 185 which is offset from the axis about which the rings rotate so that the ring slide laterally over their respective engaging surface to prevent "stiction" causing a sharp transition between non-slip and slip when the torque applied overcomes the resistance through rotation provided by the friction rings.

Claims

1. A rotary drag/damper device for controlling rotation of one rotary member with respect to another member about a first axis of rotation, the device comprising a rotary viscous coupling which slips when subjected to a torque with a resistance to slippage which rises progressively with respect to speed of rotation, a rotary friction coupling arranged in series with the viscous coupling and which transmits torque without slippage in a torque transmitting direction up to a certain level above which the friction coupling slips with a resistance which does not change significantly with speed, and means to generate slippage in the friction coupling in a direction transverse to the torque transmitting direction with slippage in the viscous coupling at torques below said certain level and thereby to provide a smooth transition when the torque rises above said level causing slip in the torque transmitting direction of the friction coupling.

2. A drag/damper device as claimed in Claim 1, wherein the rotary viscous coupling is mounted with its axis of rotation eccentric to said first axis of rotation one part of the coupling being driven by said one rotary member and the other part of the coupling engaging said friction coupling, whereby rotation of the rotary member causes the viscous coupling to precess around the first axis and the second part of the coupling moves transversely with respect to the friction coupling to provide slippage with respect to the friction coupling in the transverse direction when there is no slippage in the torque transmitting direction.

3. A drag/damper device as claimed in Claim 2, wherein the rotary viscous coupling is provided between two relatively rotatable components one of which is drivably connected to said one rotary member and the other of which is rotatable with respect to the said one component about said eccentric axis.

4. A drag/damper device as claimed in Claim 3, wherein said other component of the viscous coupling provides one element of the friction coupling.

5. A drag/damper device as claimed in Claim 4, wherein the friction coupling comprises an axially facing annular friction ring mounted on a non-rotating component and engaging an oppositely facing surface of said other component of the viscous coupling.

6. A drag/damper device as claimed in Claim 5, wherein the friction coupling comprises a pair of annular friction rings mounted on oppositely facing parts of the non-rotating component and the other component of the viscous coupling has axially directed faces on either side thereof which extend between and are engaged by the friction rings.

7. A drag/damper device as claimed in Claim 6, wherein resilient means are provided for biassing said oppositely facing parts of the component towards one another to provide the requisite engagement force between the friction rings and component of the viscous coupling.

8. A drag/damper device as claimed in Claim 7, wherein means are provided for adjusting the engagement force of the friction rings with the viscous coupling.

9. A drag/damper device as claimed in any of Claims 3 to 7, wherein said one component of the viscous coupling comprises a rotor having a first set of annular fins and the second component comprises a stator having a second set of annular fins meshing with the first fins of the rotor, with a viscous fluid between them, the stator being part of an annular housing enclosing the rotor and providing the axially directed face or faces with which the annular friction ring or rings engage.

10. A drag/damper device as claimed in Claim 9, wherein means are provided for the viscous coupling for adjusting the spacing of the rotor and stator to vary the extent of overlap of the respective sets of fins on the rotor and stator and thereby vary the viscous drag effected by viscous coupling.

11. A drag/damper device as claimed in Claim 10, wherein said means for adjusting the spacing of the rotor/starter of the viscous coupling also adjusts the pre-load force on the frictional coupling so that the two vary together.

12. A drag/damper device as claimed in any of the preceding claims, wherein a further drag/damper device comprising a viscous coupling is provided in parallel with the first above-mentioned drag/damper device to provide a resistance to rotation which rises with speed even when the torque imposed has exceeded said certain level and the friction coupling of the first drag/damper device is in slip mode.

13. A drag/damper device as claimed in any of the preceding claims, and in the case where the friction coupling comprises an annular friction ring or rings, wherein the ring or rings has an axis or axes offset with respect to the axis of the viscous coupling so that, when the ring or rings slip, they track over an area of the surface with which they engage greater in width than the width of the ring or rings.

14. A rotary drag/damper device for controlling rotation of one rotary member with respect to another member, the device comprising a rotary viscous coupling which slips when subjected to a torque with a resistance to slippage which rises progressively with respect to speed of rotation, and a rotary friction coupling arranged in series with the viscous coupling and which transmits torque without slippage in a torque transmitting direction up to a certain level above which the friction coupling slips with a resistance which does not change significantly with speed, wherein the device includes a wheel driveable through said rotary viscous coupling, the wheel engaging an annular track through said friction coupling, such that the wheel has a rolling engagement with the track up to a certain torque level above which the wheel slips with respect to the track, the rolling engagement of the wheel prior to slippage providing a relatively smooth transition between rolling and slippage.

15. A drag/damper device as claimed in

Claim 14, wherein the annular track is formed around the inner side of an annular member with which the wheel engages, the wheel being mounted to rotate about an axis which precesses around the axis of the track as the wheel rotates around the track.

16. A drag/damper device as claimed in Claim 14 or Claim 15, wherein the wheel is mounted for rotation on a hub and the viscous coupling is disposed between the hub and wheel.

17. A drag/damper device as claimed in

Claim 16, wherein means are provided for biassing the hub towards the track to provide a positive engagement between the wheel and track.

17. A rotary drag/damper device substantially as described, with reference to and as illustrated in Figures l and 2, 3, 4 or 5 of the accompanying drawings.