GB2433972A

GB2433972A - A torsional isolation device for isolating torque fluctuations

Info

Publication number: GB2433972A
Application number: GB0600094A
Authority: GB
Inventors: Benoit Meyer; Luc Thevenon; David Jones; Claude Noire; Robert Solaro
Original assignee: Metaldyne International France SAS
Current assignee: Metaldyne International France SAS
Priority date: 2006-01-05
Filing date: 2006-01-05
Publication date: 2007-07-11
Also published as: GB0600094D0; WO2007077414A1

Abstract

A torsional isolation device, such as a pulley, isolates torque fluctuations of a rotary drive shaft such as a crankshaft of an internal combustion engine. The device has an inner member 40 for connection to the drive shaft and an outer pulley rim 24 for connection to a driven member via a drive belt (not shown). An intermediate elastomeric member 52 has a pair of cavities 54 in which stop members 58 are received, the walls 56 of each cavity having a predetermined profile that is designed to have the desired torque response characteristics of the device.

Description

A TORSIONAL ISOLATION DEVICE FOR ISOLATING TORQUE

FLUCTUATIONS

The present invention relates to a torsional isolation device for isolating irregular and undesirable fluctuations in torque and more particularly, but not exclusively, to such a device for isolating fluctuations in the drive torque of a crankshaft of an automotive internal combustion engine.

In internal combustion engines the torque applied to the crankshaft fluctuates as a result of, for example, the periodic firing of the engine pistons that drive the crankshaft, changes in the load and changes in the speed of rotation of the crankshaft. These torque fluctuations may be transferred to the front end accessory drive of the vehicle comprising various auxiliary components that are driven from the crankshaft such as, for example, an alternator, a fan, water pump, or an air conditioning pump etc. The transmission of such fluctuations is undesirable as it may affect operation of the auxiliary components. Moreover, the periodic combustion of the cylinders in the internal combustion engine creates a torque spike that sets up torsional vibrations in the crankshaft. Such vibrations have to be damped in order to protect the crankshaft and I. "associated bearings from damage caused in particular by resonance at the natural frequency of S... * * S...

the crankshaft. S* * * S S

:: Environmental pressures and increases in oil prices have lead to a current trend for hybrid drives for vehicles, comprising a conventional internal combustion engine working in *SS *** cmbination with a motor/generator. Such drives generate less polluting emissions and provide S.....

S S

for improved fuel economy. The motor/generator and internal combustion engine can co-operate in different ways but there are some common characteristics all of which present specific design requirements for a torsional isolation device.

Conventional vehicle drive trains have typically employed a separate alternator and starter motor. The alternator is connected to the engine by a belt and generates electricity to recharge the vehicle battery whereas the starter motor cranks the engine crankshaft at engine start-up. Some drive applications have used combined starter/alternators including, for example, hybrid drives in which the motor/generator provides both functions (and is often referred to as a starter-alternator in this context). Thus in a start-up situation, the motor/generator performs as a motor and rotation is transmitted to the engine crankshaft via the accessory drive. Moreover, in hybrid drives the engine may be configured to switch off automatically when the vehicle is stationary for more than a predetermined period. The motor then automatically restarts the engine when the driver presses the accelerator of the vehicle. Similarly, under other running conditions, the motor/generator acts as a motor and converts the stored electrical energy from the batteries into rotational power to provide a torque boost to the engine (in which circumstance both the engine and the motor provide power to accelerate the vehicle) or to provide a separate drive to the accessories. During certain running conditions the motor/generator acts as a generator whereby it is rotated by via the drive train and, like a conventional alternator, it :. provides electrical charge to the battery for storage.

:* In addition to the above-described operating conditions, a hybrid drive may have a * ** * regenerative braking feature whereby the motor/generator acts as a generator and charges the :: . battery during braking by absorbing energy in order to slow the vehicle down. This limits the waste of energy as heat in brake pads when slowing down by using the generating load as an S..

S S

***5engine brake. In such circumstances the torque loading on the drive train is higher than in conventional systems.

A conventional torsional isolation pulley may not be suitable for use in such hybrid drives or in applications which require a combined starter/alternator as these both require significant torque to be transmitted in both directions. There are several factors that need to be accommodated by a torsional isolation device in a hybrid drive, including the following: the torque loads on a torsional isolation device in a hybrid drive would typically be higher than for a device in a conventional drive as the air-conditioning pump and other ancillary devices remain the same as for a conventional engine but the generating load is higher; during start up the torsional isolation device must be capable of transmitting torque to start the engine which may be around 2.5 times the torque transmission requirements for a device in a conventional engine; the feature of stopping the engine whilst the vehicle is stationary and starting it promptly when the driver wishes to pull away (the stop/start function) means that the drive must be capable of transmitting the high start-up torque rapidly without excessive wind-up; the stop/start function means that the engine will shut down and start up far more 1:'freq1t1y that for a conventional drive and the torsional isolation device will have to pass through its natural or resonant frequency somewhere between start up and idle running speed; :: :;and S..

* :..* the isolation device requires a low stiffness across the torque range outside of the start-up conditions and a torque safety limit.

One example of a conventional torsional isolation device is disclosed in European Patent No. EP80843 1. The device comprises a first member connected to the drive shaft and a second member having a contoured pulley rim to which a V-belt drive is attached. The belt transmits power from the drive shaft to a driven component. The first and second members are interconnected by a torsionally flexible elastic ring which is loaded in shear and effectively absorbs rotational fluctuations in the motion of the drive shaft so that they are not transmitted to the driven component. Torsional vibrations of the shaft to which the device is attached are damped by means of an inertia ring connected to the first member by means of an elastic ring. In normal operation such a device is only loaded in one direction and the ring provides a constant linear spring rate over the whole operational range. A device of this kind provides isolation from torque fluctuations via the elastic ring that would deform equally if the relative rotation of the members was reversed but the same linear spring characteristic would apply over the operational range rendering it unsuitable for the application described above. Moreover, a device of this type has disadvantages in that the first and second members are subject to a relatively large relative rotational displacement during initial loading before the drive torque is transmitted to the load through the torsionally flexible elastic ring.

It is an object of the present invention to obviate or mitigate the aforesaid, and other, disadvantages and to provide for a torsional isolation device suitable for use in a hybrid drive of :::avehicle. *s..

According to a first aspect of the present invention there is provided a torsional isolation * :: :: for isolating torque fluctuations of a rotary drive shaft, the device having an axis of rotation and comprising a first member for connection to the drive shaft, a second member for S.. * S *

* connection to a driven member, the first and second members being arranged for relative angular * displacement in both directions, the first and second members being interconnected by an elastomeric member such that it is subject to shear during said relative angular displacement, the elastomeric member having at least one cavity therein, a torsional isolation device for isolating torque fluctuations of a rotary drive shaft, the device having an axis of rotation and comprising a first member for connection to the drive shaft, and a second member for connection to a driven member, the first and second members being arranged for relative angular displacement in both directions, the first and second members being interconnected by an elastomeric member such that it is subject to shear during said relative angular displacement, the elastomeric member having a cavity therein, the cavity having opposite first and second walls defined by edge surfaces of the elastomeric member, at least one rigid stop member defined on one of the first and second members and projecting into the cavity, the stop member having a first stop surface that each faces said first wall and a second stop surface that faces said second wall such that first stop surface and first wall move towards each other during relative angular displacement of the first and second members in a first direction such that the elastomeric member is compressed at an edge surface and at the same time the second surface and second wall move apart from each other and vice versa during relative angular displacement in the opposite second direction, wherein at least one of the first stop surface and the first wall has a tapered surface that is non-complementary to the other.

This arrangement allows the device to transfer torque from the shaft (e.g. a crankshaft of an internal combustion engine) to the driven members (e.g. a motor/generator) and vice versa. In :: : doing the elastomeric member is compressed by virtue of the interaction of the stop member surface and the cavity wall and undesirable torque fluctuations are isolated. It also ensures that :: after a predetermined relative angular displacement rotation in each direction (which may be a different degree of displacement in each case) the elastomeric member reaches a point where it is incompressible or at least is compressed to a point where it has much higher stiffness and the torque is transmitted without further displacement. It is to be understood that compression of the elastomeric member by virtue of the interaction of the tapered wall and stop surface may only occur in one direction: there may be no such tapered surface (and therefore no such significant change in stiffness of the device) in the other direction The tapered surface may be substantially planar or substantially arcuate or a combination thereof. It may comprise a plurality of planar portions inclined to one another. It can be of any suitable simple plane or complex profile that achieves the desired change in stiffness during compression of the elastomeric member.

The tapered surface may be defined on the wall of the elastomeric member or on a stop face of the stop member.

The first and second stop surfaces and/or the first and second walls may extend in a generally axial or radial direction.

The cavity in the elastomeric member has a portion that penetrates through the elastomeric member in an axial direction.

The stop member may extend in a substantially axial direction.

The elastomeric member may be annular and may be fixed between substantially radially extending surfaces of the first and second members, said surfaces being axially spaced.

The second member may be a pulley member for a drive belt that connects to the driven member or members. The pulley member may have an axially extending rim with an outer surface configured to retain a drive belt. I.

The first member may comprise a radially extending wall and an axially extending * ** * annular rim that is substantially concentric with the pulley rim. It may further comprise a : etaining member that has an axially extending portion on which a radial flange is supported and the elastomeric may be fixed between said radial flange and a radially extending wall of the S..

S S S

::..u11ey.

The rim of the first member supports an annular inertia member and associated damping ring, the ring being disposed between the rim of the first member and the inertia member.

The inertia member may conveniently be disposed in a radial clearance between the rims of the first member and the pulley.

The elastomeric member may be disposed radially inboard of the rim of the first member.

The stop member may be defined on the second member or the first member and may be defined on a radially extending wall of thereof.

The first and second stop surfaces of the stop member may extend substantially in parallel to the axis of rotation.

The walls of the cavity are inclined to the axis of rotation. There may be provided a pair of stop members and cavities, the two stop members being disposed at diametrically opposed locations. However, it is to be understood that the exact number of stop members and cavities and their relative positioning can be varied according to the application.

According to a second aspect of the present invention there is provided a drive assembly comprising an engine with a rotary shaft and a rotary electrical machine that are interconnected via a torsional isolation device according to any preceding claim, such that the engine can drive the rotary electrical machine, the machine operating as a generator, and the machine can operate as a motor to drive the rotary shaft of the engine.

Specific embodiments of the present invention will now be described, by way of example * ::: only, with reference to the accompanying drawings in which: Figure 1 is a schematic block diagram of a hybrid drive for a vehicle incorporating a torsional isolation device in accordance with the present invention; Figure 2 is a front sectioned view of a torsional isolation device in accordance with the S.. * . .

present invention taken along line C-C of figure 4; S.....

S S

Figure 3 is a side sectioned view along line A-A of figure 2; Figure 4 is a sectioned view along line B-B of figure 2; Figure 5 is a sectioned view along line D-D of figure 2; Figure 6 is a view of part of the device shown in section along line D-D of figure 2 when the device is in an equilibrium position; Figure 7 is a view identical to that of figure 6 but showing the device in a configuration where there is relative displacement between its members; and Figure 8 is a view identical to that of figure 6 but showing alternative profiles of an edge of the elastomeric member of the device.

An exemplary hybrid drive for a vehicle is shown in figure 1 in schematic representation and has an engine 10 such as, for example, an internal combustion engine, which drives a crankshaft 12 in rotation. The crankshaft 12 is connected to a motor/generator 14 via a torsional isolation device 16 that permits torque to be transmitted to various front end auxiliary components 18 such as, for example, an air conditioning pump etc, whilst isolating torque fluctuations. The motor/generator 14 in turn drives the vehicle transmission 20 which can also be driven via the crankshaft 12.

The torsional isolation device is shown in figures 2 to 6 has a front face 21 a, a rear face 21b and comprises concentric inner and outer annular members 22, 24 disposed about a rotational axis (x-x'). The inner annular member 22 is designed to connect rigidly to the crankshaft (not shown in figs 2 to 5). The outer annular member 24, disposed radially outboard :: : of the inner member 22, serves as a pulley and is designed to connect to a driven component such as a motor/generator 14 operating as a starter/alternator and the other auxiliary equipment S.

S S

18 of the front end drive. The arrangement is configured such that the engine 10 can drive the **0*55

S

driven equipment 18 including the motor/generator 14 and the motor/generator 14 can drive the engine 10 or be used as a brake.

In the exemplary embodiment depicted, the outer annular member 24 has disc-like wall 26 that extends substantially radially along the rear 21 b of the device and a peripheral pulley rim 28 that extends axially with respect to the rotational axis x-x' across the best part of the overall thickness of the device. An outer surface of the rim 28 is provided with a V-shaped recess configuration 30 designed to receive a flexible drive belt (not shown) having a complementary V-recess configuration. It is to be appreciated that alternative embodiments of the surface of the pulley rim 28 may be adopted. The innermost part 26a of the pulley wall 26 is inclined slightly to a plane normal to the axis of rotation in a direction away from the rear 21 b of the device.

The inner member 22 is substantially disc-shaped with a front wall portion 31 and an axially extending peripheral rim 32 that is received substantially concentrically inboard of the pulley rim 28 in a spaced relationship.

An annular inertia member 34 is supported on the outer surface of the axially extending rim 32 of the inner member 22 via an annular elastic damping member 36 as is well known in the art. This provides a tuned damper that is capable of damping torsional vibrations at the natural frequency of the crankshaft. The inertia member 34 is supported in the radial clearance between the rims 28, 32 of the pulley 24 and inner member 22. In an alternative embodiment (not shown), an inertia member may be disposed in a housing containing a viscous or elasto-viscous fluid, the housing being connected to the inner member.

A radial guide bearing 38 is provided between the pulley rim 28 and the outer surface of * S** the inertia member 34. This may be provided with frictional properties as is known from the ** , : aforementioned patent EP080843 1.

The inner member 22 and pulley 24 are kept in axial alignment by an axial retainer 40 **.

* that has a radially extending inner wall 42 fixed to the front wall portion 31 of the inner member * **** S * 22, an intermediate annular portion 44 that extends axially across most of the thickness of the device and is concentric with the rims 28 and of the pulley 24 and inner member 22, and an outer periphery 46 that flares outwardly and overlies the inclined innermost 26a part of the pulley wall 26. A guide bearing 48 is provided between the pulley wall 26 and axial retainer 40 at this point.

An outwardly extending flange 50 is supported in a fixed relationship on an outer surface of the intermediate portion 44 of the axial retainer 40 at a position opposite and axially spaced from the inner part 26a of the pulley wall 26 but radially inboard of the rim 32 of the inner member 22. The flange 50, like the inner part 26a of the pulley waIl 26, is inclined slightly to a plane that is normal to the rotational axis x-x' at more or less the same angle, but in the opposite direction.

The inner, inclined part 26a of the pulley wall 26 and the flange 50 are joined by an annular elastomeric member 52 that is bonded or otherwise fixed to the respective surfaces 26a, so as to couple the pulley 24 to the inner member 22 via the flange 50 and axial retainer 40.

The elastomeric member 52 has a pair of cut-out cavities 54 defined at diametrically opposed locations. The cavities 54 extend from the outermost surface of the member 52, across approximately half its radial depth and pass all the way through in an axial direction. The cut-out cavities 54 are defined by opposed walls 56 of the elastomeric member 52, the walls 56 being tapered as best seen in figure 5 so that each cavity 54 reduces in cross-sectional area from the front to rear of the device. In the embodiment shown, the taper is provided by a substantially flat planar surface but other alternative profiles may be adopted to provide the same effect as will * * become apparent. The flange 50 is also cut away in the region of each cavity 54 in the * S..

embodiment shown in the figures but it is to be appreciated that this is not essential.

Each cavity 54 receives a stop member 58 that is defined on the radially extending wall 26 of the pulley 24. The stop member 58 is approximately square in cross section, extends most S..

Of the way across the axial dimension of the cavity 54 and has a pair of stop surfaces 60 that face S.....

* respective tapered walls 56 of the elastomeric member.

The surface of the walls 56 of the cavity 54 defined by the elastomeric member 52 may have any suitable profile depending on the required characteristic torque response. In figures 5 and 6 it can be seen that the surface of both of the walls is planar. In an alternative embodiment, shown in figure 8, two examples of different profiles are shown: the wall 56' on the left hand side comprises two planar sections disposed at different angles of taper whereas the wall 56" on the right hand side is substantially arcuate.

When the device of figures 2 to 6 is in an equilibrium position whereby there is no relative angular displacement of the inner and pulley members 22, 24, the stop 58 resides centrally in the cavity 54 (as shown in figure 6) and the opposed stop surfaces 60 of the stop member 58 subtend angles of a and 1 to the respective tapered walls 56 of the elastomeric member 52. The angles are selected according to the required torque transmission characteristics of the device, that is they are dependent on the relative angular displacement at which rapid increase in torque is required (for start-up, boost or braking etc.). When there is relative angular displacement of the inner and pulley members 22, 24, the elastomeric member 52 is loaded in shear between the flange 50 and wall portion 26a. The deformation of the elastomer in this way serves to filter out the cyclical torque irregularities transmitted by the crankshaft so that they are not transmitted to the driven components. At the same time the relative angular displacement causes the stop 58 to move against one of the tapered walls 56 and the elastomeric material begins to be compressed as shown in figure 7. At a certain angle of relative rotation (dependent **en the profile of the tapered wall 56) the stop 58 abuts the full wall of elastomer and the * S **** . connection effectively changes from low stiffness to high stiffness whereupon the torque can be S. * directly from the inner member to the pulley (or vice versa) without any further significant relative rotation. In this situation the elastomer and device are referred to as going S. stiff". S....

When the inner member 22 drives the pulley 24 as would be the case where the internal combustion engine 10 is providing all of the drive power and the motor/generator 14 is operating as an alternator by charging the battery in a conventional manner, the torque load from the driven component(s) 14, 18 will cause the pulley 24 to be displaced anti-clockwise relative to the hub 22 (referring to the orientation of figure 2), albeit that the whole device 16 is turning clockwise (assuming normal practice for automotive internal combustion engines). When the elastomeric member 52 is under shear loading and under compression by the stop 58 it serves to isolate torque fluctuations in the manner described above. During regenerative braking by the motor/generator, the device operates with low stiffness to maintain good isolation until the torque load increases beyond to a certain point where the elastomer goes stiff and the coupling between the two parts of the device is substantially rigid thereby preventing overload.

When the motor/generator 14 operates as an electric motor to provide supplementary drive power to the engine 10, the supplied torque from the electric motor 14 will cause the pulley 24 to move clockwise relative to the inner member 22 (albeit that the whole device 16 continues to rotate clockwise, assuming normal practice for an automotive IC engine) and torque fluctuations are isolated. When the motor/generator 14 operates as a starter for the engine 10, rotation/torque must be transmitted therefrom via the pulley 24 to the inner member 22 via the elastomeric element and the angular displacement of the two reaches a point where the elastomer 52 goes stiff and relatively high torque can be transmitted as the coupling between the two parts of the device is substantially rigid. S.

In an alternative embodiment (not shown) the inner member 22 and pulley 24 are S... * S S...

provided with annular concentric surfaces between which the elastomeric member is bonded. In S. S : this embodiment the stop member (which is connected to one or other of the inner member or pulley) may extend in a substantially radial direction into a cavity defined in the inner or outer S..

**** periphery of the elastomeric member.

S

The stop member 58 may be longer than that shown in the figures and may project through an arcuate slot in the radially extending front wall 31 of the inner member 22. In such a design the ends of the slot provide a limit to the angular displacement between the two principal parts of the device.

In an alternative embodiment, instead of being formed on the pulley 24 the stop member 58 may be formed on the inner member 22 e.g. the rim may be in-turned to define a further axially extending portion substantially parallel to the rim and which projects into the cavity. It could as a further alternative be formed on the flange 50 by means of an axially extending portion projecting from the periphery of thereof.

It will be understood that numerous modifications to the above-described designs may be made without departing from the scope of the invention as defined in the claims. For example, the stop faces 60 of the stop member 58 may be profiled instead of the cavity walls 56 defined by the elastomeric element 52. Moreover, the walls of each cavity may have different profiles to provide different torque response characteristics. Further, the inner annular member 22 may be fitted to the rotary drive shaft by any suitable means such as a key, spline or shrink connection.

The presence of the inertia member 34 is optional and the low friction radial and axial guide bearings may be manufactured from any suitable material.

Although described in relation to a hybrid drive for an internal combustion engine driven vehicle it is to be appreciated that the torsional isolation device of the present invention may be used with any rotary drive shaft.

S * a * S.. S... * S S... S. * * .;

S I * S.. * I I S.' * . a S. I

S

S..... a

Claims

CLAIMS

1. A torsional isolation device for isolating torque fluctuations of a rotary drive shaft, the device having an axis of rotation and comprising a first member for connection to the drive shaft, a second member for connection to a driven member, the first and second members being arranged for relative angular displacement in both directions, the first and second members being interconnected by an elastomeric member such that it is subject to shear during said relative angular displacement, the elastomeric member having at least one cavity therein, a torsional isolation device for isolating torque fluctuations of a rotary drive shaft, the device having an axis of rotation and comprising a first member for connection to the drive shaft, and a second member for connection to a driven member, the first and second members being arranged for relative angular displacement in both directions, the first and second members being interconnected by an elastomeric member such that it is subject to shear during said relative angular displacement, the elastomeric member having a cavity therein, the cavity having opposite first and second walls defined by edge surfaces of the elastomeric member, at least one rigid stop member defined on one of the first and S * S...

second members and projecting into the cavity, the stop member having a first stop S. * : surface that each faces said first wall and a second stop surface that faces said second wall such that first stop surface and first wall move towards each other during relative S., angular displacement of the first and second members in a first direction such that the Se....

elastomeric member is compressed at an edge surface and at the same time the second surface and second wall move apart from each other and vice versa during relative angular displacement in the opposite second direction, wherein at least one of the first stop surface and the first wall has a tapered surface that is non-complementary to the other.

2. A torsional isolation device according to claim 1, wherein the tapered surface is substantially planar.

3. A torsional isolation device according to claim 2, where the tapered surface comprises a plurality of planar portions inclined to one another.

4. A torsional isolation device according to claim 1, wherein the tapered surface is arcuate.

5. A torsional isolation device according to any preceding claim, wherein the tapered surface is defined on the edge surface of the elastomeric member that defines a wall of the cavity.

.., 6. A torsional isolation device according to any preceding claim, wherein the first and second stop surfaces and the first and second walls extend in a generally axial S4 p . .: direction. * *s* * . *

7. A torsional isolation device according to any preceding claim, wherein the cavity penetrates through the elastomeric member in an axial direction.

8. A torsional isolation device according to any preceding claim, wherein the stop member extends in a substantially axial direction.

9. A torsional isolation device according to any preceding claim, wherein the elastomeric element is annular.

10. A torsional isolation device according to any preceding claim, wherein the elastomeric element is fixed between substantially radially extending surfaces of the first and second members.

11. A torsional isolation device according to any preceding claim wherein the second member is a pulley member for a drive belt that connects to the driven member or members.

12. A torsional isolation device according to claim 11, wherein the pulley has an axially extending rim having an outer surface configured to retain a drive belt.

13. A torsional isolation device according to claim 12, wherein the first member comprises a radially extending wall and an axially extending annular rim that is substantially concentric with the pulley rim. p.. a. S. p

: 14. A torsional isolation device according to claim 13, where the first member further comprises a retaining member that has an axially extending portion on which a radial SoS flange is supported. osS.S S s

15. A torsional isolation device according to claim 14, wherein the elastomeric member is fixed between said radial flange and a radially extending wall of the pulley.

16. A torsional isolation device according claim any one of claims 13 to 15, wherein the rim of the first member supports an annular inertia member and associated damping ring.

17. A torsional isolation device according to claim 16, wherein the damping ring is disposed between the rim of the first member and the inertia member.

18. A torsional isolation device according to claim 16 or 17, wherein the inertia member is disposed in a radial clearance between the rims of the first member and the pulley.

19. A torsional isolation device according to any one of claims 13 to 18, wherein the elastomeric member is disposed radially inboard of the rim of the first member.

20. A torsional isolation device according to any preceding claim wherein the stop member is defined on the second member. S. * S * S.

21. A torsional isolation device according to claim 20, wherein the stop member is defined on a radially extending wall of the second member. I..

22. A torsional isolation device according to any preceding claim, wherein the first and

S

second stop surfaces extend substantially in parallel to the axis of rotation.

23. A torsional isolation device according to any preceding claim, wherein the surfaces defined by the walls of the cavity are inclined to the axis of rotation.

24. A torsional isolation device according to any preceding claim, wherein there are provided a pair of stop members and cavities.

25. A torsional isolation device according to claim 24, wherein the two stop members are disposed at diametrically opposed locations.

26. A torsional isolation device according to any preceding claim, wherein the tapered surface subtends an angle of less than 900 to the stop surface of the stop member.

27. A torsional isolation device according to any preceding claim, wherein the first and second walls of the (or each) cavity are tapered.

28. A torsional isolation device according to claim 26, wherein the taper on each surface is of a different profile so that the torque transmission characteristics in each direction of relative angular displacement are different.

29. A torsional isolation pulley substantially as hereinbefore described with reference to *...

the accompanying drawings. * * * * * I

* : : 30. A drive assembly comprising an engine with a rotary shaft and a rotary electrical IS.

* : machine that are intercoimected via a torsional isolation device according to any * preceding claim, such that the engine can drive the rotary electrical machine, the machine operating as a generator, and the machine can operate as a motor to drive the rotary shaft of the engine.