This invention relates to hydraulic actuators and in particular, though not exclusively, to such actuators used as master cylinders in an hydraulic system for the operation of a vehicle clutch.

One of the problems with such 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 to 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, for example by damping the movement of the fluid itself or interposing elastomeric components in the connection between the pedal and fluid. Sometimes both types of damper may be used in the same installation.

It is an object of the present invention to provide a master cylinder which includes an improved form of damping and which is particularly suitable for use with a clutch actuating system.

Thus according to the present invention there is provided a hydraulic master cylinder comprising a housing, a piston in sealed sliding contact with a bore in the housing to define a working chamber ahead of a working surface on the piston, a push rod connected at one end with the piston and at the other end with a pedal-operated linkage, and an elastomeric damper disposed in the thrust path between the other end of the push rod and the working surface of the piston, the master cylinder being characterised in that the damper comprises a body of elastomeric material having one or more clearances or cavities which are normally open when the piston is not actuated by the push rod to provide a first low stiffness connection between the pedal and piston and which are closed when the pedal is actuated to give a second higher stiffness connection in the thrust path from the pedal to the piston. Such an arrangement is particularly suitable for use as a master cylinder in a clutch operating system.

In a preferred arrangement the body of elastomeric material is received in a recess, the body having an outer peripheral zone connected with the remainder of the body by a flexible annular web, this outer peripheral zone contacts a base of the recess and provides by deflection of the web a first low stiffness connection between the piston and the pedal, the actuation of the master cylinder causing the web to deflect to allow a central region in the body of elastomeric material radially within the outer peripheral zone to contact the base of the recess to provide a solid elastomeric connection between the piston and pedal at the second higher stiffness.

The recess may be formed in a first part of the piston which provides the working surface of the piston and the body of elastomeric material is compressed in the recess between a second portion of the piston connected with the push rod and the first part of the piston.

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 is an external view of a master cylinder in accordance with the present invention shown partly in section;

Figure 2 shows, on a larger scale, the sectioned portion of figure 1 , and

Figures 3, 4 and 5 show the progressive compression of an elastomeric damping block used in the master cylinder of figure 1 as the master cylinder is actuated.

Referring to the drawings, the master cylinder 10 has a body 11 with a bore 12 in which a piston 13 is in sealed sliding cooperation. Piston 13 is displaced within bore 12 by a push rod 14 which has a ball joint connection 15 with an associated clutch operating pedal (not shown).

The master cylinder has an inlet 16 connected with a reservoir and an outlet 17 connected by a conduit (not shown) to an associated slave cylinder for operation of a vehicle clutch.

Piston 13 comprises a first part 18 which carries a seal 19 which contacts bore 12 (and which provides a working surface which compresses the fluid to the left of piston part 18 as viewed in the drawings) and a second part 20 provided with a part spherical concave recess 21 which cooperates with a part spherical convex surface 22 formed on a head portion 23 of push rod 14. The part spherical surfaces 21 and 22 permitting limited articulation of the push rod 14 relative to the piston 18,20. The push rod 14 is retained within body 11 by a split ring 24.

The piston part 18 includes a recess 25 within which an elastomeric damping block 26 is located. A projection 27 on the second piston part 20 also extends into recess 25 so that the elastomeric body 26 is confined between the two piston parts 18 and 20.

Elastomeric damping block 26 has an outer annular peripheral zone 28 which is connected with the main body portion 29 by a flexible annular web 30 thus defining an annular cavity 31 behind the outer peripheral zone 28.

Radially within the outer peripheral zone 28 is a central zone 32. The elastomeric body is attached to projection 27 by a radially inwardly extending flange portion 33 which engages a groove 34 in projection 27.

Figure 2 shows the master cylinder in the unactuated condition in which the operators foot is not resting on the associated clutch operating pedal. This condition is also shown in figure 3 which shows half of the elastomeric block 26 and the cooperating portions 18 and 20 of the piston. In this condition the central portion 32 of the elastomeric body is spaced (see clearance "x") from the bottom of recess 25. If the operator now rests his foot on the pedal without intending to depress the pedal, as many operators do, the force applied to the pedal by the weight of his foot will cause annular web 30 to flex thus providing a first low stiffness connection between the pedal and piston part 18 thereby providing good damping of any vibrations in the actuating fluid.

If the operator now decides to actuate the clutch pedal the additional force applied to the push rod 14 causes the central portion 32 elastomeric body to contact the base of the recess 25 (see figure 4) so that there is now a solid rubber higher stiffness connection between the piston part 20 and the piston part 18. As the pedal pressure further increases the annular cavity 31 is closed (see figure 5).

The elastomeric body 26 thus provides a first low level of stiffness when the driver is simply resting his foot on the pedal and the annular web 30 is flexing to provide good isolation of the operator's foot from any vibrations in the fluid and a second higher level of stiffness when the central portion 32 of the body contacts the base of the recess 25 and the operator is actively depressing the pedal.