This invention relates to torsional vibration dampers (hereinafter referred to as torsional vibration dampers of the kind specified) which comprise an input member for connection with a vehicle engine and an output member for connection with a vehicle transmission, the members being mounted for limited relative rotation about a common axis against a damping means to damp torsional vibrations in the engine/transmission. 
Such torsional vibration dampers are often in the form of a twin mass flywheel in which the input and output members comprise input and output flywheel masses mounted for relative rotation via a support bearing acting between the masses. 
Alternatively the input and output members can be of relatively small mass as shown, for example, in the Figures of the Applicant""s earlier British patent application No.  98 03046.3. 
The damping means of such torsional vibration dampers can take a wide range of forms, for example, the damping means may comprise one or more of the following:
one or more circumferentially acting compression springs;
one or more circumferentially acting elastomeric compression blocks;
one or more friction devices;
one or more hydraulic damping devices, and
one or more bob-weights connecting linkages (which generate speed dependent damping) connected between the members. 
Such torsional vibration dampers therefore tend to be relatively complex devices which consist of a relatively large number of individual parts and are therefore relatively expensive to manufacture. 
There is also a requirement to provide an axially compact torsinal vibration damper. 
It is an objective of the present invention to provide a torsional vibration damper of the kind specified with fewer individual parts which is therefore easier and cheaper to manufacture. 
It is a further objective of the present invention to provide a torsional vibration damper of the kind specified which is of an axially compact design. 
Thus according to one aspect of the present invention in a torsional vibration damper of the kind specified the damping means comprises one or more circumferentially acting compression springs, the or each spring acting between a first abutment formed as an integral part if the input member and a second abutment formed as an integral part of the output member. 
Such a construction is particularly advantageous when the torsional vibration damper is a twin mass flywheel with the input mass formed as a single piece sheet metal pressing with integral first spring abutments and the output mass is a cast component with integrally cast second spring abutments. 
The construction is also particularly compact in an axial sense which is an important feature of twin mass flywheels for use in congested engine compartments particularly when the engine is disposed transversely relative to the vehicle. 
The torsional vibration damper may also include one or more elastomeric springs or blocks which are subjected to compression in end zones of the relative rotation of the input and output members. 
In accordance with a further aspect of the present invention such elastomeric springs or blocks are mounted on one of the input or output members between first circumferentially facing abutments formed on or carried by said member and are acted upon by further circumferentially facing abutments formed on or carried by the other member.  The elastomeric springs or blocks may also be located radially by a radially outer abutment formed on one of the input or output members.  In a preferred and particularly convenient construction both the first circumferentially facing abutments and the radial abutment of each elastomeric spring or block are formed integrally on the input member which may conveniently be formed as a single piece pressing. 
Each elastomeric spring or block may be supported by a sheet metal casing member which sits in a window in the input or output member which supports the block or spring. 
In accordance with a further aspect of the present invention in a torsional vibration damper of the kind specified which includes circumferentially acting compression springs, the circumferentially acting compression springs are each supported at a radially inner location by a support member (typically of general channel configuration) to avoid fouling adjacent elastomeric springs or blocks which are located radially inwardly of the compression springs.  The spring support members deflect the compression springs from their natural straight configuration to an accurate form which bridges the elastomeric springs or blocks. 
In a particularly convenient arrangement, in a vibration damper employing bob-weight connecting linkages, the compression spring support member may rest at one end on the radially outer abutment associated with each elastomeric springs or block and may be also fastened at the other end to a pivot pin of an adjacent associated bob-weight connecting linkages. 
In accordance with a further aspect of the present invention a torsional vibration damper of the type specified may also be provided with a friction damping device whose frictional damping effect varies with the amount of relative rotation of the input and output members of the damper. 
For example, such a rotation dependent damping device may comprise a friction member which is carried by the input or output member and biased into contact with a surface on the other of the input or output members to provide frictional damping.  The surface against which the friction member is biased may be in the form of a cam surface orientated with respect to the axis of relative rotation of the input and output members so that the contact pressure of the friction member on the surface increases with relative rotation between the input and output members.  The friction member may be arranged to contact the surface only in the last end portion of the relative rotation.  In a particularly convenient arrangement the member which is biased into contact with the surface may also act as a stop which co-operates with abutments on the other of the input or output members to limit the relative rotation between the input and output members. 
In accordance with a still further aspect of the present invention, in a torsional vibration damper which includes bob-weight connecting linkages, the bob-weights may each be pivotally mounted on one of the input and output members by a cantilevered pivot pin.  Each bob-weight may be free to move axially to a limited extent on its cantilevered pin. 
The use of cantilevered pivot pins further reduces the axial dimensions of the torsional vibration damper. 
Each linkage may be completed by a single flexible link pivoted at one end on the associated bob-weight and at its other end on the other of the input or output members.  The single flexible link may conveniently be located on the input member side of the bob-weight. 
In a further construction the cantilevered pivot pins and the main support bearing which supports the input and output members for relative rotation are both retained in position by a common retaining member. 
The bob-weight may be mounted on the cantilevered pin via a bearing bush which is a press fit in the bob-weight.  Similarly the flexible link may be pivoted to the bob-weight via a second bush which is a press fit in the bob-weight and a rivet which carries its own collar and which extends through the second bush. 
The flexible link may be pivoted on the other of the input or output members via a stud or other fixing on which the link is sandwiched between a flange formed on a sleeve surrounding the stud or other fixing and a washer. 
In accordance with a still further aspect of the present invention a twin mass flywheel of the kind specified includes a friction damping device whose friction damping effect varies with the speed of rotation of the flywheel. 
The damping effect may be arranged to decrease (or increase) with the speed of rotation depending on the operating characteristics required from the flywheel.  For example, the friction damping device may comprise a friction block supported on one flywheel mass which is biased into rubbing contact with the other flywheel mass and is disposed so that, as the speed or rotation of the flywheel mass increases, the centrifugal effect on the friction block tends to reduce the contact pressure of the block on the other flywheel thus reducing the friction force generated. 
The variation in friction forces of the above friction block arrangement can also be made angularly dependent by arranging the block to make contact with circumferentially ramped surfaces on the other flywheel mass. 
In accordance with a still further aspect of the present invention in a torsional vibration damper of the kind specified which includes bob-weight linkages and circumferentially acting compression springs interconnecting the input and output members the total permitted relative rotation of the input and output members can be increased by connecting the linkages with the input or output member which supports the compression springs radially inboard of the compression springs thus allowing longer linkages to be employed. 
In yet a further alternative construction the damping means may comprise a single plate friction damper in which the single friction plate is biased against the input or output member or a component carried thereby by a belleville spring or other axially acting spring member which acts against the other of the input or output members.  This simple instruction again saves axial space. 
The present invention also provides a method of assembly of a torsional vibration damper of the kind specified which includes one or more cantilevered pin mounted bob-weight connecting linkages, said method including the steps of:
assembling one or more bob-weight connecting linkages;
pivotally connecting one end of the or each linkage to one of the input or output members;
inserting respective locating pin through a respective locating aperture in said one of the input or output members and into a first cantilevered pivot pin bore at the other end of the or each linkage through which the cantilevered pivot pin is to extend;
completing the assembly of the remainder of the torsional vibration damper onto the input and output members;
placing the other of the input and output members over said one member with second cantilever pin bore(s) in said other member in line with the locating Pin(s), and
inserting the cantilevered pin(s) into said first bore(s) thus displacing said locating pin(s) from said first bore(s) and connecting the or each linkage with said other member. 
Preferably the cantilevered pin(s) are inserted into the second cantilevered pin bore(s) prior to the placing of the other member over said one member and the locating pin(s) are displaced from the first bore(s) as the other member is lowered onto said one member. 
In such an arrangement the method preferably includes the further step of securing to said other member a common retaining member for the main support bearing and the cantilevered pin(s) prior to placing the other member over said one member. 
Such an arrangement necessitates the mounting of the main support bearing on the other of said input or output members prior to placing said other member over said one member. 
The invention also provides a torsional vibration damper assembled by the above method in which one of the input or output members includes a locating pin aperture in axial alignment with the or each cantilevered pivot pin.