Patent GB-1497074-A (GB)
Title: Internal shoedrum brakes of the duo-servo type
IPC: F16D51/20
Filed: 1974-06-05
Abstract: 1497074 Internal shoe-drum brakes; wear adjustment GIRLING Ltd 21 May 1975 [5 June 1974] 24861/74 Heading F2E A duo-servo internal shoe drum brake has an actuator 5 between the actuated ends of shoes 1, 2 and a thrust-transmitting strut assembly 6 between the opposite ends of the shoes 1, 2, the strut assembly 6 being slidably guided in a fixed housing 7 to transmit thrust between the shoes 1, 2 and comprising an outer sleeve 8 having screw-threaded engagement with an inner member 9 the sleeve 8 having angularly spaced axially extending teeth 14 meshing with a worm 13 which is rotatable to rotate the sleeve 8 to increase the effective length of the strut assembly 6 and thereby adjust for wear of the linings 4 of the shoes 1, 2. The inner member 10 acts directly on shoe 1 and the sleeve 8 acts on shoe 2 via a tappet 9, the member 10 and tappet 9 having angled abutment surfaces 50 engaging complementary surfaces on the shoe ends and being held against rotation thereby. In the application of brake by actuator 5 (which as shown is of the wedge expander type but may be an hydraulic actuator), the primary shoe, depending on drum rotation, is carried round with the drum with a servo-action and transmits through assembly 6 a thrust to the secondary shoe which anchors on a drag-taking abutment, conveniently the housing of actuator 5. Rotation of the worm 13 for wear adjustment is by means of a rotatable drum 24 and a ratchet assembly 21, 27, 30, the drum 24 being caused to rotate by the inner cable 43 of an inextensible cable assembly 42 wound around it. The inner cable 43 is slidably enclosed within a flexible casing 44 and carries at one end a thickening 45 acting on an abutment plate 40, the adjacent end of the casing 44 acting on a further abutment plate 41-the plates 40, 41 enclose opposite ends of a central helical coiled portion 36 of a shoe return spring 35 having inextensible portions 37. The other end of casing 44 is received in a sleeve 46 in housing 7, and the other end of the cable 43 carries an abutment plate 47 on which acts a compression spring 49. When the brake is applied the cable 43 and casing 44 move relative to each other in opposite directions and the drum 24 is rotated with respect to a ratchet clutch plate 27 of the ratchet assembly and against the force of spring 49. When the relative movement between drum 24 and ratchet plate 27 exceeds the spacing between interengaging teeth 28 thereof, the plate 27 is displaced against the force of a plate spring 30 to permit the drum 24 to rotate relative to it over one tooth pitch. Upon brake release the energy stored in spring 49 applies to the drum 24 a force of sufficient magnitude to rotate worm 13 through plate 27 and thereby sleeve 8. To replace the brake shoes the effective length of the strut assembly 6 is reduced. This is achieved by urging a re-set member 17 inwardly to engage a blade 16 in a slot 15 in the worm 13, simultaneously axially displacing a rod 22 extending through the worm 13 to urge a bridge member 21 away from drum 24 against the force of a spring 18. Thereafter member 17 can be rotated to rotate worm 13 and sleeve 8.