This invention relates to hydraulic concentric slave cylinders (CSCs), hereinafter referred to as being of the type described, which comprise an annular bore in which an annular piston slides in sealed relation to inner and outer walls of the bore to define a working chamber between the piston and a closed end of the bore into which pressurised fluid is admitted via an inlet to displace the piston in the bore.

Such CSCs are well known and are commonly used, for example, to surround a vehicle gearbox input shaft and operate a drive clutch of the vehicle.

One of the problems with clutch operating systems is the damping of the vibrations induced in the pressurised fluid of the system by the associated clutch and vehicle driveline to prevent the transmission of the vehicle operator of unacceptable noise or physical vibrations through the clutch pedal.

Many solutions have been proposed to damp such vibrations in the system fluid but whilst these solutions may operate adequately to damp such vibrations they are often bulky in their design and require additional piping for the connection of the damper to the system.

It is an object of the present invention to provide an improved arrangement for the damping of fluid vibrations in clutch actuating system which uses a CSC of the type described.

Thus according to the present invention there is provided a concentric slave cylinder of the described having a hydraulic damper disposed in the annular bore. Such an arrangement is not only efficient but, since the damper is housed inside the CSC, does not take up any further room in the already crowded engine compartment of the vehicle.

Preferably the damper is located between the cylinder inlet and piston and comprises an annular diaphragm which is deflected in response to pressure fluctuations in the working chamber.

In such an arrangement the diaphragm may be held in spaced relation to the base of a generally U-shaped cross-section annular support member disposed in the annular bore, the support member being closed off by an annular inlet member positioned above the diaphragm in the support member and which is sealed with the annular bore, the inlet member including ports communicating with the cylinder inlet and with the working chamber.

The diaphragm may be held in spaced relation to the base of the generally U- shaped support member by circumferentially spaced blocks formed on the side of the diaphragm facing the base of the support member.

Preferably the outside of the base of the generally U-shaped support member includes circumferentially spaced spacer blocks which contact an end of the annular bore, and the outsides of the sides of the support member also include circumferentially spaced spacer blocks which contact the inner and outer walls of the annular bore to ensure that the support member is fully surrounded by the pressurised fluid of the working chamber to equalise the internal and external pressure acting on the support member.

One embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:-

Figure 1 shows a side view, partly in section, of a CSC for the operation of a vehicle clutch; Figure 2 shows an axial section, on a larger scale of the CSC of figure 1;

Figure 3 shows, on a larger scale, part of the damper built into the CSC of figures 1 and 2;

Figures 4 and 5 show further sectional details of the damper of figure 3;

Figures 6a and 6b show details of part of the damper body;

Figures 7a and 7b show details of a further part of the damper body, and

Figure 8 shows details of the diaphragm of the damper.

Referring to the drawings, a CSC 10 for the operation of a vehicle clutch comprises a plastics body 11 defining an annular bore 12 in which an annular piston 13 is slideable. The piston is sealed to the bore 12 by an annular seal 14 which normally contacts face 15 of piston 13 but is shown in the drawings in an initial position before the CSC has been pressurised for the first time.

Between the seal 14 and the closed end 16 of bore 12 a working chamber 17 is defined into which pressurised fluid is introduced from a stand pipe 18 via a connector 19 as indicated in by arrow X in figure 1. This pressurised fluid is received from a master cylinder (not shown) which is operated by a normal clutch pedal which is depressed by the vehicle operator when he wishes to disengage the clutch.

The CSC is completed by a clutch release bearing 20 which is moved by a flange 21 formed on piston 13. Release bearing 20 operates release fingers 22 of the associated clutch in the conventional manner.

A coil spring 23 acts between housing 11 and release bearing 20 to bias the CSC piston towards its extended condition thus maintaining a pre-set pre-load on the clutch release fingers 22 even when the working chamber 17 of the CSC is not pressurised. Figure 2 shows the piston 13 in its fully extended condition when piston 13 contacts a circlip 11a to limit the piston travel. The CSC is supplied to the vehicle manufacturer in this condition.

A rubber bellows 24 extends between the release bearing 20 and the housing 11 to protect the CSC against the ingress of dirt.

In accordance with the present invention an annular vibration damper 25 is housed in the working chamber 17 of the CSC.

This damper comprises an annular diaphragm 26 held between to the base of a generally U-shaped cross-section support member 27 and an annular inlet member 28 positioned above the diaphragm 26 and partially inserted into the U-shaped cross section support member 27. The inlet member 28 is sealed within bore 12 by O- rings 29 and 30.

As can be seen from figures 6a and 6b the generally U-shaped cross section support member 27 is provided with circumferentially spaced spacing blocks 31 which contact the closed end 16 of bore 12 and with spacer blocks 32 and 33 formed in the outer and inner peripheral walls of support member 27 will contact with the walls of bore 12.

As can be seen from figures 7a and 7b the inlet member 28, which is a press fit into the bore 34 in U-shaped support member 27, includes openings 35 adjacent diaphragm 26 and inner and outer peripheral openings 36 and 37 respectively which open into an internal annular groove 38.

Diaphragm 26 is provided with integrally formed circumferentially spaced blocks 39 (figure 8) formed on the side of the diaphragm facing the base of U-shaped support member 27 to support the portions 39a of the diaphragm between the blocks 39 in spaced relation to the base of support member 27. As will be appreciated from the above, to release the associated clutch, pressurised fluid is introduced into bore 12 adjacent the closed end 16 via the standpipe 18 (see figure 3). This fluid is free to pass down the slots 40 and 41 defined between the spacing blocks 33 and 32 on the U-shaped support member 27 and then, via openings 36 and 37, into annular groove 38 and hence into working chamber 17 adjacent seal 14 thus moving the associated piston 13 to operate the CSC to release the associated clutch.

The pressurised fluid in working chamber 17 also communicates via openings 35 with the space above diaphragm 26 and any vibrations induced in the fluid due to vibration of the release fingers 22 or other parts of the vehicle drive line cause the portions 39a of the diaphragm between the support blocks 39 to flex. This flexing of the diaphragm damps the vibrations induced in the hydraulic fluid thus suppressing noise and the transmission of vibration to the associated clutch operating pedal.

By arranging for the U-shaped support member 27 to be totally surrounded by the pressurised fluid within the bore 12 the stresses on this component are reduced.

The present invention thus provides a simple, efficient and economical way of providing a damper in a CSC of the type described. Also since the damper is fitted totally within the existing working chamber of the CSC no additional room in the already crowded engine compartment of the vehicle is required.