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 associated bearings from damage particularly caused by resonance at the natural frequency of the crankshaft as well as to reduce objectionable noise.

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 combination with a motor/generator. Such drives generate less polluting emissions and provide 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 include 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. In a hybrid drive, 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). 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 avoids the waste of energy as heat in brake pads when slowing down by using the generating load as an engine 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 which 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 frequently 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 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. EP808431. 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. Moreover, if the torsionally flexible elastic ring should fail through age or excess loading the intermediate member provides no drive connection between the driving shaft and the driven member. A further disadvantage is that the torsionally flexible elastic ring is loaded in shear as in all devices of this kind. Elastomeric materials exhibit poor physical characteristics when loaded in shear and have a tendency to wear and/or fail.

. 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 a vehicle.

According to a first aspect of the present invention there is provided 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 spaced so as to define an annular cavity, said first member having at least one first substantially radial projection extending into the annular cavity and said second member having at least one second substantially radial projection extending into the annular cavity so as to divide the cavity into at least one first chamber and at least one second chamber defined between said first and second

members and first and second radial projections, each chamber extending in a generally circumferential direction, at least one first elastomeric element disposed in said first chamber and at least one second elastomeric element disposed in said second chamber, the first and second elastomeric elements being arranged in their respective chambers with a radial and/or axial clearance volume such that when compressed between the radial projections they are allowed to deform into said clearance volume, the first element being compressed during relative angular displacement of first and second members in a first circumferential direction and the second element being compressed during relative angular displacement in a second circumferential direction opposite to said first direction, the clearance volumes for the first and second elements being different, whereby a predetermined amount of relative rotation is accommodated in each direction by the deformation of the respective element in said chamber until said clearance volume is occupied whereupon the torque is transmitted from one member to the other more rapidly than before, the different clearance volumes allowing a different amount of relative rotation in each direction.

This arrangement allows a given elastomeric element to deform axially and/or radially into its clearance volumes as the volume of the chamber decreases. Once the elastomeric element has deformed to occupy the clearance volume the stiffness of the device increases significantly so that the torque response profile changes.

The first and second members are preferably spaced in both axial and radial directions to define said cavity.

The first and second elastomeric elements may be of different sizes. For example, they may have different cross-sectional areas and/or may be of different lengths. They may also have different spring characteristics. Alternatively, the first and second elastomeric elements may be substantially the same size and the dimensions of the first and second chambers are different to

provide the different clearance volumes. As a further alternative both elements and both the chambers may be of different sizes.

The clearance volume may be provided by a radial clearance between the elastomeric element and the boundary of its respective chamber. This may be provided radially outboard or radially inboard of the elastomeric member. The clearance volume may alternatively or additionally be provided by an axial clearance between a wall of the chamber and the elastomeric element.

The first and second chambers are preferably elongate. The first and second elastomeric members are preferably elongate and extend between radial projections at each end of their chamber.

There may be provided more than one first and second elastomeric elements and first and second chambers arranged around the device.

There may be more than one elastomeric element provided in each chamber. There may be provided different numbers of elements in the first and second chambers.

The first and second elastomeric elements may be of different materials

Each element is preferably substantially the same general shape as the chamber in which it resides. Alternatively it may be generally straight and pre-deformed so as fit into said chamber.

At least one of the first and second elastomeric elements may have one or more voids in order to reduce the stiffness of the material.

The first member may have a radially extending wall that defines a first wall of each chamber. The second member may have a radially extending wall spaced and opposite from the radially extending wall of the first member and which defines a second wall each chamber. The first member may have a first axially extending annular wall that defines a third wall of each chamber and a second axially extending annular wall that defines a fourth wall of each chamber. The first member may comprise a hub member defining the first axially extending annular wall

and an axial retaining ring fixed to the hub member and defining the second axially extending annular wall, the second axially extending wall extending between the first and second members, whereby said fourth wall of said chamber is substantially concentric with said third wall.

The fact that the third and fourth walls are both defined by the first member means that they do not rotate relative to one another. This is advantageous in that it does not risk the fouling of the elastomeric elements in the chamber should the walls be moving relative to one another.

The first radial projection preferably extends inwardly from said axially extending annular wall of the first member. Alternatively it may extend outwardly from said axially retaining member.

The second member may be in the form of a pulley for a drive belt that connects to the driven member or members. It may have an axially extending rim with an upper surface configured to retain a drive belt. The upper surface may have one or more grooves designed to receive complementary formations on the belt. These may be V-shaped grooves.

The first and second radial projections are preferably in the form of webs, which may be integrally formed with the first and second members. The webs may have a circumferential thickness that is less than their radial depth and the thickness may be less than half the depth

The first member may be in the form of a hub that is concentrically disposed radially inboard on the second member.

There may be provided an annular inertia member connected to the hub via an elastomeric damping member. The annular inertia member may be supported on a radially outer surface of a rim of the hub with the damping member interposed between the two. The inertia member may be disposed in a clearance between the rim of the hub and the rim of the pulley. A radial bearing may be interposed between a radially outer surface of the inertial member and a radially inner surface of the hub.

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 as defined above such that the engine can drive the rotary electrical machine such that it operates as a generator and the machine can operate as a motor to drive the rotary shaft and engine.

According to a third aspect of the present invention there is provided a torsional isolation pulley for isolating torque fluctuations of a rotary drive shaft, the device having an axis of rotation and comprising an inner member for connection to the drive shaft, an outer pulley member having a rim for connection to a driven member via a drive belt, the inner and pulley members being arranged for relative angular displacement in both directions and having an axial spacing so as to define an annular cavity between radially extending walls of the inner and pulley members, said inner member having at least one first substantially radial projection extending into the annular cavity and said pulley member having at least one second substantially radial projection extending into the annular cavity so as to divide the cavity into at least one first chamber and at least one second chamber defined between said inner and pulley members and first and second radial projections, each chamber extending in a generally circumferential direction, at least one first elastomeric element disposed in said first chamber and at least one second elastomeric element disposed in said second chamber, the first and second elastomeric elements being arranged in their respective chambers with a radial and/or axial clearance volume such that when compressed between the radial projections they are allowed to deform into said clearance volume, the first element being compressed during a relative angular displacement of first and second members in a first circumferential direction and the second element being compressed in a relative angular displacement in a second circumferential direction opposite to said first direction, whereby a predetermined amount of relative angular displacement of the inner and pulley members is accommodated in each direction by the deformation of the

respective element in said chamber during its compression until said clearance volume is occupied whereupon the torque is transmitted from one member to the other more rapidly than before.

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 vibration damper in accordance with the present invention;

Figure 2 is a front view of a torsional isolation device in accordance with the present invention;

Figure 3 is a 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 C-C of figure 2;

Figure 6 is a sectioned view along line D-D of figure 3;

Figure 7 is a graph illustrating the torque response characteristics of the device of figures 2 to 6; and

Figures 8 to 11 are sectioned side views of through an upper half of alternative embodiments of the present invention.

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 undesired 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 and comprises concentric inner and outer annular members 22, 24 disposed about a rotational axis (x-x')- The inner annular member 22 is a hub that is designed to connect rigidly to the crankshaft (not shown in figs 2 to 6). The outer annular member 24, disposed radially outboard of the hub, 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 18 of the front end drive. The arrangement is configured such that the engine 10 can drive the motor/generator and the driven equipment 14, 18, or alternatively, the motor/generator 14 can drive the engine 10. Moreover, the motor/generator 14 can be used as a brake to convert kinetic energy of the drive transmission and wheels into electrical energy stored in a battery in what is known as regenerative braking.

In the exemplary embodiment depicted, the outer annular member 24 has a disc-shaped wall 26 that extends radially and a peripheral pulley rim 28 that extends axially with respect to the rotational axis x-x'. 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 hub 22 is generally disc-shaped with a radially extending wall 32 spaced axially from and opposite to the corresponding wall 26 of the pulley 24, and a peripheral annular rim 34 that extends in an axial direction concentric with the pulley rim 28.

The hub 22 and pulley 24 are kept in axial alignment by an axial retainer 36 that has a radially extending wall 38 fixed to the radially extending wall 32 of the hub 22 and an annular portion 40 that extends axially and is concentric with the rims 28 and 34 of the pulley 24 and hub 22.

The hub 22 has one or more webs 42 (two are shown in the example) integral therewith and depending from a radially inward facing surface of the rim 34. In the configuration

illustrated, the webs 42 extend radially inwards such that they would meet if they were extended inwards to the central rotational axis (x-x') of the device. Each web 42 has a width that is significantly shorter than its radial length and an axial dimension such that when the hub 22 is assembled with the other components there remains a small clearance in the axial direction between the web 42 and the radial wall 26 of the pulley 24. Whilst in the embodiment shown the webs 42 are formed integrally with the hub 22, for example from a stamping process or from a moulding process in the case of a polymer based hub, they may be separate components fixed as part of a fabrication/assembly process, such as welding. In an alternative embodiment (not shown), the webs 42 extend radially outwards from the axial retainer 36.

An equivalent number of corresponding webs 44 of similar configuration to webs 42 are formed on a radially inward facing surface of the pulley rim 28. The optimum configuration of the webs 44 on the pulley 24 follows the same principle as for the hub 22 described above. The webs 44 may be formed integrally with the pulley 24, for example from a stamping process or from a moulding process in the case of a polymer based pulley or may be added as part of a fabrication/assembly process, such as welding.

When the hub 22 is position within the pulley 24 and the axial retainer 36 in place, four chambers 46, 48 are formed between the webs 42, 44 of the hub and the pulley, the annular portion 40 of the axial retainer 36 and the radially inward facing surface of the hub rim 34. It is to be appreciated that provided there are two or more, the exact number of chambers 46, 48 is not important to the principle of the present invention. It will be seen from figure 6 that the webs 42, 44 are angularly spaced around the device such that, when the device is unloaded, they define alternate relatively short and relatively long chambers 46, 48. In this example the relatively short chambers 46 are of the same dimension and are diametrically opposed and similarly the relatively long chambers 48 have the same dimensions and are also diametrically opposed.

If the pulley 24 is rotated relative to the hub 22 about the axis x-x', two of the four chambers 46, 48 will decrease in size, while the other two will increase in size. If the pulley 24 is rotated in the opposite direction relative to the hub 22 about the axis x-x' then the chambers 46, 48 that had previously increased in size will decrease in size, while the others will increase.

An elongate elastomeric element 50, 52 of rubber or the like is interposed between webs 42, 44 in each chamber 46, 48. In the example shown, the elastomeric elements are in the shape of sections from an annular ring. As best seen in figure 6, a relatively long element 52 is housed in each of the relatively long chambers 48 and likewise a relatively short element 50 is received in each short chamber 46. In section the elastomeric elements 50, 52 are approximately rectangular in profile with rounded corners. Other forms/shapes of elastomeric element are envisaged such as, for example, a square prism. The elastomeric element 50, 52 within each chamber 46, 48 may also comprise multiple parts, arranged in series or parallel, and/or may be a mixture of different materials, forms/shapes and/or spring rates.

The volume of each chamber 46, 48 is designed to be greater than the volume of the corresponding elastomeric element 50, 52 therein. As best seen in figure 3, there is a radial clearance 54 between each elastic element 50, 52 and the annular portion 40 of the axial retainer 36. In the embodiment shown the elements have a radial clearance at their inner surface only, with the outer surface touching the hub rim. However, it will be appreciated that in another embodiment the radial clearance could be provided, additionally or alternatively, at its outer surface. There is also an axial clearance 56 on each side of the element 50, 52 between it and the radially extending walls 26, 32 of the pulley 24 and hub 22. However, in figure 6 it can be seen that one end of the elastomeric element 50, 52 is in contact with a web 42 of the hub and the other with a web 44 of the pulley. In fact, the elastomeric element 50, 52 may even be slightly compressed between the faces of the respective webs.

The webs 42, 44 in the hub and the pulley and the elastomeric elements 50, 52 combine to form an elastic torque transmission device suitable for isolating small torque fluctuations.

An annular inertia member 58 is supported on the outer surface of the axially extending rim 34 of the hub 22 via an annular elastic damping member 60 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. Alternatively (not shown), an inertia member may be disposed in a housing containing a viscous or elasto-viscous fluid, the housing being connected to the hub.

A radial bearing 62 is provided between the pulley rim 28 and the outer surface of the inertia member 58. This may be similar to the one described in European patent no. 0808431 referred to above. An axial thrust bearing 64 is provided in an axial clearance between the radial wall 26 of the pulley 24 and the inertia member 58 andan additional axial bearing 64 is provided between the pulley 24 and the axial retainer 36.

To prevent excessive factional effects of the elastomeric elements 50, 52, a lubricating material may be applied on one or more surfaces of the elements and/or the adjacent surfaces of the hub, pulley or retainer 22, 24, 36.

When the hub 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 6), albeit that the whole device 16 is turning clockwise (assuming normal practice for automotive internal combustion engines), thus reducing the circumferential distance between adjacent webs 42, 44 defining the longer chambers 48 and compressing the two longer elastomeric elements 52. The material properties and the size of the elastomeric elements 52 are chosen such that during this operation, the torque requirement is transmitted from the drive component (in this case the hub 22) to the driven component (the

pulley 24), and at the same time the pulley 24 and the components 14, 18 it drives are isolated from torque fluctuations of the crankshaft 12. The webs 42, 44 transfer tangential load on the end faces of the elastomeric element 52. The elastic element 52 deforms radially and axially into the clearances 56, 58 provided for this reason.

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 hub 22 (albeit that the whole device 16 continues to rotate clockwise, assuming normal practice for an automotive IC engine), thus reducing the circumferential length of the shorter chambers 46 and compressing the two shorter elastomeric elements 50. The material properties and the size of the elastic elements 50 are chosen such that during this operation the supplementary torque is transmitted from the pulley 24 to the hub 22, and the pulley 24 continues to remain isolated from the torque fluctuations of the engine 10. The webs 42, 44 transfer tangential load on the end faces of the elastomeric elements 50, which deform radially and axially into the clearances 56, 58 provided for this reason.

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 hub 22 via the shorter elastomeric elements 50. The material properties and the size of the shorter elastomeric elements 50 are chosen such that load required to start the engine 10 will cause the size of the chamber formed by the pulley 24, hub 22, axial retainer 36, web 42 and web 46 to be very similar to that of the elastomeric element, that is to say, the clearance volume 56, 58 has all or almost all disappeared. In this condition, the elastomeric element 50 will have no further room into which it can deform. The relatively incompressible nature of the elastomeric element 50 will allow a high stiffness connection between the pulley 24 and the hub 22 thus allowing a positive drive to start the motor

14 without significant relative rotation or "wind-up".

The substantially incompressible nature of rubber in compression serves to reduce the wild oscillations that can be observed on a classic rubber in shear type coupling during the startup phase as the coupling passes through its resonant frequency. This will provide benefit to the starting of the motor as well as extending the life of the coupling.

The edges of the elastic elements are all appropriately profiled to avoid the elastic element becoming pinched between the webs and other rotating components. The elastomeric elements and their respective chambers may take any convenient form provided there is a clearance volume defined between at least one of the chamber types and the respective elements so that the elements have room to deform radially and axially outwardly in their chambers during compression thereby permitting a predetermined amount of relative rotation in at least one direction before the element goes "stiff. The clearance volumes of one set of chambers is different to that of the other chambers so that a different torque characteristic is provided in each direction of relative rotation. The difference clearance may be provided by different lengths or cross-sectional areas of the two types of elastomeric element or alternatively may be provided by different chamber volumes or a combination of any of these possibilities. The elastomeric elements may also have different spring characteristics.

The graph of figure 7 illustrates the torque characteristics of the torsional isolation device described above. The angle of relative rotation is shown on the x-axis whereas the torque transmitted is represented by the y-axis. It can be seen that the characteristic is generally linear and of constant gradient in the principal working area where the device exhibits a relatively low stiffness. At one end of the plot the response follows this characteristic initially for the torque boost phase where the motor supplies additional torque to the transmission but beyond that (represented in the bottom left hand corner of the graph), the first or second elastomeric elements fill the clearance volume of their chamber so as to exhibit high stiffness and a higher magnitude

of torque is transmitted with negligible increase in relative rotation, hence the gradient of the

response plot increases significantly. Similarly at the top right of the plot it can be seen that during the regenerative braking phase the response is initially linear but then changes as the respective elastomeric elements fill their chamber and become stiff such that the torque load can increase rapidly with negligible increase in relative rotation of the pulley and hub.

Figures 8 to 11 show alternative embodiments of the isolation device of the present invention. In figure 8 the inner member 122 has spaced axially extending walls 134, 140, the radially outer one of which forms the rim 134 that supports the inertial member 158 and inner one of which provides an annular inner wall 140 of the chambers 146, 148. The outer pulley member 124 has a second radially extending wall 200 that is concentric with and adjacent to the rim 134. This wall forms the outer wall of the chambers 146, 148 and is optional. The other embodiments of figures 9 to 11 show different configurations of the inner and outer members. In each case, a radial or axial wall marked with the letter X is optional.

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 first annular member 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 is optional and the low friction bearings may be manufactured from any suitable material. Furthermore, the elastomeric elements may be supplemented with mechanical springs.

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.