This invention relates to so-called concentric slave cylinders (CSCs), hereinafter referred to as being of the type described, in which an annular piston moves in an annular bore in an annular body which surrounds a gearbox input shaft, the piston carrying a clutch release bearing for the operation of an associated vehicle drive clutch.

It is an object of the present invention to provide an improved form of CSC of the type described.

Thus according to the present invention there is provided a CSC of the type described in which the body and piston are disposed radially outboard of the release bearing. With such an arrangement, when the CSC is in its retracted condition virtually all the release bearing can be accommodated radially inside the body.

Preferably a compression spring acts between the body and the release bearing to apply a constant load on an associated clutch release mechanism (e.g. the conventional diaphragm spring fingers) when the CSC is deactivated and the clutch is engaged. This compression spring can also be housed within the body of the CSC.

Such an arrangement allows the diameter of the body (and hence the piston) to be as large as possible within the available space envelope in the engine compartment of the vehicle. This gives larger available clutch actuation force, makes the piston (and hence the release bearing) more stable and less likely to be displaced relative to the body of the CSC and provides more clearance for the passage of the gearbox input shaft through the CSC. A gearbox input shaft seal can also be carried by the body of the CSC. The outer periphery of the piston preferably encircles the outer periphery of the release bearing so that the bearing can move laterally relative to the piston to provide self-centring of the release bearing. Also the bearing is well protected from the ingress of dirt etc. since it is radially within the body and no additional sealing bellows is required.

The body also preferably includes mounting lugs through which mounting bolts or other fasteners can be inserted to fasten the CSC in its operational position on the vehicle.

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 perspective view of a CSC in accordance with the present invention;

Figure 2 shows an axial section through figure 1 with the CSC in the fully retracted position;

Figure 3 shows an axial section with the CSC in the fully extended position, and

Figure 4 shows , on a larger scale, details of the manner in which the release bearing is held in place in the body of the CSC.

Referring to the drawings the CSC 10 comprises an annular plastics piston 11 slideable in an annular bore 12 formed in an annular plastics body 13. Piston 11 is sealed to body 13 by an annular seal 14. Body 13 is provided with three mounting lugs 15 (two of which are visible in figure 1) having bores 16 through which mounting bolts or other fasteners (not shown) may be inserted for attaching the CSC to the associated vehicle. An inlet 17 is also moulded into body 13 through which pressurised fluid is introduced into the CSC, inlet 17 communicating with the right hand closed end of bore 12. Piston 11 is provided with an outer peripheral flange 18 inside which a release bearing 19 is located (see Figure 4). Release bearing 19 is held within the peripheral flange 18 by a spring washer 20 which grips the inside of the flange 18 and acts against a friction ring 20a which is attached to an outer race 19b of the release bearing. A small radial clearance "x" is provided within flange 18 to allow lateral movement of the release bearing relative to the piston in order that the release bearing can align itself with the rotation axis X-X of the associated vehicle drive clutch by sliding between friction ring 20a and spring washer 20.

The inner race 19a of release bearing 19 contacts the release fingers 21 of a diaphragm spring of the associated vehicle clutch in a conventional manner. This is shown diagrammatically in figures 2 and 3. A compression spring 22 acts between the body 13 and the outer race 19b of the release bearing to maintain a constant load on the release fingers 21 when the clutch is engaged and the CSC is in the fully retracted position as shown in figure 2.

A gearbox input shaft seal 23 is also housed in a recess 24 in the body 13.

The piston is retained in the bore 12 by one or more stops 25 which are ultrasonically welded but otherwise fixed in cut-outs 26 in the free end of the outer wall of body 13. The provision of this stop arrangement prevents the piston disengaging the bore under the action of spring 22 as can be seen in figure 2 due to the interaction between the stops 25 and a radially outward flange 11a provided on piston 11.

As will be appreciated, the clutch is disengaged by pressurising bore 12 via inlet 17 to displace piston 11 from the fully retracted position shown in figure 2 in which the clutch is fully engaged to the release position shown in figure 3 in which the piston has been displaced to the left as shown in figure 3 to move the release fingers 21 in the direction of the arrows A of figure 2. As will be understood, by locating the release bearing 19 radially within the confines of the body 13 the diameter of the operating piston 11 can be maximised within the available envelope in the engine compartment and the other advantages discussed above are also provided. Also, since the body of the CSC is a single piece component, leakage from the CSC is reduced when compared with a CSC in which the inner and outer walls defining bore 12 are provided by separate components which must then be sealed to each other.