GB1427521A - Shoe drum brakes for vehicles - Google Patents

Abstract

1427521 Shoe drum brakes GIRLING Ltd 2 May 1973 [4 May 1972] 20820/72 Heading F2E In an internal shoe drum brake wherein arcuate shoes 1, 2 mounted on a stationary back-plate 4 are separated, by a service actuator 7, at one pair of adjacent ends thereof to bring them into engagement with a rotatable drum and at the other pair of adjacent ends by a mechanical parking actuator 10, and abutments are provided at the actuator 10 to take the braking loads when the brake is applied by the service actuator, and means are provided in or adjacent the service actuator for transmitting servo braking forces from one shoe to the other when the brake is applied by the mechanical parking actuator, the said other end of each shoe has first and second load bearing faces 2a, 3a and 2b, 3b, and the parking actuator incorporates expander means, i.e. tappets 8, 9, acting directly on the load bearing faces 3a, 2a. The abutments, i.e. the opposite ends of the housing of the actuator 10, are independent of the tappets 8, 9 and are engageable by the load bearing faces 2b, 3b when the brake is applied by the service actuator 7 so that braking forces on the shoes are taken directly by the abutments independently of the tappets when the brake is applied by the actuator 7. A force is applied to the shoes by the release of energy stored in the parking actuator upon operation of an actuating member, and the force is applied continuously to urge the shoes into engagement with the drum when the actuating member is operated irrespective of the initial position of the shoes. As shown, the parking actuator comprises a normally retracted plunger 13 acted upon by a spring 16 and which, when released, acts through a wedge 12 and rollers 11, to spread apart the shoes 1, 2. The servo force is applied from one shoe to the other during a parking application of the brake by a strut extending between the shoes or through the pistons 18, 19 of the actuator 7. The latter includes provision for automatic slack adjustment comprising a spindle 20 one end of which is in screw-threaded engagement with the piston 18, the other end of the spindle being a sliding fit in the piston 19 and carrying a drive ring 24 urged by a spring 25 into clutching engagement with a coned surface 26 on the piston 19. During service actuation when the pistons 18, 19 are moved apart more than a predetermined amount the drive ring 24 is moved away from the piston 19 to reduce the clutching force and rotates as the spindle moves axially therethrough thereby automatically increasing the distance between the pistons. In a modification, the parking actuator comprises toggle members (29), (30), Fig. 5 (not shown), in rocking abutment with a recess (31) in the plunger 13 which, for actuation thereof, is connected by a cable or rod extending via a bell-crank lever (37), Fig. 6 (not shown), to either a hand lever working in a gate or a hand lever carrying a pawl working over a toothed segment; the plunger may be released by air pressure or applied by fluid pressure with a reserve or back-up pressure accumulator.