Patent GB-1288774-A (GB)
Inventor: CRABTREE N
IPC: F16D65/22
Filed: 1969-04-03
Abstract: 1288774 Actuators; locking GIRLING Ltd 3 April 1970 [3 April 1969 13 June 1969 3 July 1969] 17692/69 30190/69 and 33624/69 Heading F2F An hydraulic slave wheel cylinder for a vehicle internal shoe drum brake comprises a pair of opposed thrust members which are urged apart in use by hydraulic braking pressure to apply the brake shoes to the drum, a strut of adjustable length acting between the thrust members, and an auxiliary fluid pressure actuated motor incorporated in the cylinder having one operative condition in which the strut is free to extend and retract to follow up movements of the thrust members, and another operative condition in which the strut is locked in an extended position, the arrangement being such that, in use the strut can be locked in an extended condition to hold the thrust members apart against brake shoe return forces, after the actuating braking pressure has been released. As shown, the slave cylinder comprises a body 1 housing opposed annular pistons 2, 3 subject to hydraulic pressure in a chamber 4 partly defined by a tubular member 10 extending in sliding sealed relation through the pistons. Tappets 7, 8 engaging respective brake shoes are operable by the pistons, a ring 11 and a resilient overload device comprising a pack 13 of Bellville spring washers, being disposed between the piston 3 and tappet 8. The shaft 14 and the ring 11 are connected by a reversible screwthread, the shaft having a head 18 the radial face 19 of which forms a clutch face for co-operation with the adjacent inner face of the tappet 7 and is engageable by the piston 21 of the auxiliary fluid pressure motor; the head is urged into frictional contact by a spring 16. In operation, the chamber 22 is pressurized so that the piston 21 holds clutch face 19 clear of the tappet 7, and after the chamber 4 has been pressurized to apply the brake shoes to the drum, to prevent release of the brakes the chamber 22 is relieved of fluid pressure whereby the relative rotation between the shaft 14 and ring 11 causes the strut to extend and the clutch to be engaged to prevent return of the brake shoes by the shoe return springs. Release of the parking condition of the brakes is effected by the admission of pressure fluid to the chamber 22 to allow the strut to contract. The overload device limits the thrust forces which can be transmitted back to the strut, and release of the locked strut may be effected by removal of a pin 26. In modifications, the member 10 is (a) formed as an extension integral with or secured to the piston 2, and the spring 16 is situated between the tappet 8 and the adjacent end of the shaft 14, (b) passes through a floating intermediate piston which provides separate fluid pressure chambers for the pistons 2, 3 and means are provided for detecting fluid leakage due to failure of the seals of the intermediate piston, (c) formed by integral extensions of the pistons 2, 3 and which abut, in the contracted position of the slave cylinder, within the axial extent of the intermediate piston, and (d) is secured to the piston 3, passes through an internal fixed wall of the slave cylinder and is internally screwed to engage the shaft 14 to dispense with the ring 11.