Hydraulic cylinder assemblies

#The invention relates to hydraulic wheel cylinder assemblies for internal shoe drum brakes and in particular to brakes of the kind in which the wheel cylinder assembly is interposed between one pair of adjacent ends of a pair of brake shoes and acts as a spacing strut for said ends and a mechanical expander and fixed abutment means are interposed between the other pair of adjacent ends of the shoes.

#A drum brake of the kind referred to is described in British Patent Specification No. 1,513,740. It has leading-and-trailing shoe performance in the hydraulic actuating mode, giving stable and consistent performance in both directions of drum rotation, with duo-servo shoe performance in the mechanical mode, giving a high torque restraint in both directions of parking purposes. However, whilst the wheel cylinder assembly shown in this patent specification performs satisfactorily it has a disadvantage concerning the installation of the brake on a motor vehicle. Space and other restraints sometimes require the brake to be mounted with the shoes disposed either side of the horizontal centreline. In this arrangement the weight of the shoes tends to cause the wheel cylinder pistons and the shoes to fall under the combined effects of vibration and gravity, despite the presence of friction created by the shoe hold-down springs, and so cause brake drag.

#According to the invention there is provided a hydraulic wheel cylinder assembly for an internal shoe drum brake and comprising a cylinder body having a through bore therein, a pair of piston assemblies slidable in said bore, an abutment member interposed between the piston assemblies and an inlet port in the body opening into the bore for supplying hydraulic pressure to urge the piston assemblies away from each other, each piston assembly having an adjustable variable length tappet assembly for transmitting a thrust to an adjacent brake shoe and comprising a rotatable screw-threaded component interengaged with a non-rotatable screw-threaded component, the abutment member being axially slidable in said bore in either direction against a spring centering device.

#Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, of which:

#FIG. 1 is an elevation of an internal shoe drum brake assembly incorporating one embodiment of a hydraulic wheel cylinder assembly according to the invention;

#FIG. 2 is a cross-sectional elevation on the line II--II in FIG. 1;

#The drum brake shown in FIGS. 1 to 4, includes a backplate 11 and a pair of brake shoes 12 and 13 spaced slightly from the backplate by adjustable steady pins 14. The usual hold down spring assemblies 15 are provided.

#Between one pair of adjacent ends of the brake shoes 12 and 13 there is a mechanical expander 16 (FIG. 3) which comprises a wedge 17 operating between rollers 18 carried by tappets 18. The tappets are a sliding fit in the expander body 21 and each has a head which provides an abutting shoulder with the body 21 to react the load of the usual pull-off springs 22 and also brake drag loads as will be described later.

#The wedge 17 is connected to a pull-rod 23 of a spring actuator assembly 24. A diaphragm 25 subject to air pressure at port 26 normally keeps the expander 16 in the inoperative state shown in FIG. 3 against the load of coil springs 27 which act through a dished plate 28 against a cup-shaped plate 29. The springs 27 react against a housing 31 which is retained on a support tube 32. When the air pressure at port 26 is released the springs 27 act through plate 28 onto a head 33 on the pull-rod 23, pulling the wedge 17 to urge the tappets 19 apart.