Patent GB-1107918-A (GB)
Title: Improvements in or relating to internal shoe drum brakes for vehicles
Inventor: CAIRNS WALLACE MARTIN JOHN
IPC: B60T8/18
Filed: 1965-09-28
Abstract: 1,107,918. Internal shoe-drum brakes. AUTOMOTIVE PRODUCTS CO. Ltd. 14 Sept., 1966 [28 Sept., 1965], No. 41188/65. Headings F2E and F2F. In an internal shoe-drum brake for motor vehicles the maximum braking torque which can be applied is adapted to be varied automatically in dependence upon the load supported by the vehicle wheels. As shown in Fig. 1, the internal shoe-drum brake is adapted to be actuated by a double-ended liquid pressure wheel brake cylinder 11, the other ends of the brake shoes 27, 28 being mounted on a shoe abutment housing 12 comprising two spaced parallel side members 13 connected to each other by a bridge piece 14, a sleeve 15 having right and left hand screw threads respectively at its two ends forming an abutment which extends through the housing 12 and is adapted to be rotated automatically by a pawl 65 which co-operates with ratchet teeth 64 on a flange on the sleeve 15 to provide automatic adjustment of the brake during reverse movement of the vehicle. The brake may be applied by mechanical expanding means (not shown) in order to apply the brake for parking purposes. Secured to the exterior of the back plate 10 of the brake is a bracket 44 which carries a cylindrical casing 45 divided into two chambers 46 and 47 by a flexible diaphragm 48. The upper chamber 46 of the casing 45 is connected by a conduit 3 to a cylinder 54 fixed to the body of a vehicle on which the brake is mounted and a piston 56 slidable in the cylinder 54 and is urged inwardly of the cylinder 54 by compression spring 57 taking its abutment on the mounting of the vehicle wheel with which the brake is associated. The chamber 54, conduit 53 and chamber 46 are filled with liquid and a restricting device 59 is arranged in the conduit 53 to damp out rapid fluctuations of pressure in the liquid due to road shocks. In operation, when the brake shoes 27 and 28 are applied by means of the wheel brake cylinder 11 the leading shoe 27 urges the sleeve member 15 to the right such that a shoulder 36 on the sleeve member abuts against a lever member 37 pivoted at 40 (Fig. 3) and urges the portion 39 of the lever member 37 upwardly to actuate a stem 51 secured to the diaphragm 48 in a direction opposite to the force exerted by the liquid pressure on the diaphragm 48. So long as the torque reaction of the shoe 27 is insufficient to overcome that force which is dependent upon the vehicle load, the arm 41 of the lever member 37 is held by the pressure on the diaphragm 48 against the backplate 10 and the brake acts in the normal leading-trailing shoe drum brake. Should the torque reaction become sufficient to overcome the force acting on the diaphragm 48 the lever member 37 moves about its fulcrum to allow the shoes 27, 28 the wheel cylinder pistons 16, 17 and the sleeve 15 all to move round in the direction of drum rotation until a flange 18 on the wheel cylinder piston 16 engages a stop 33 mounted on the brake backplate (Fig. 4). When this engagement takes place the torque exerted by the leading shoe 27 cannot be further increased by increasing the applying pressure and as the force opposing the movement of the lever member 37 varies with the load supported by the wheel on which the brake is mounted the maximum degree of braking which can be applied by the leading shoe depends on that load. The screw threads on the tappets 19 and 21 are preferably of different pitch to compensate for greater wear of the lining of the leading shoe 27.