Patent GB-1414942-A (GB)
Title: Internatal shoe-drum brake
IPC: F16D65/54
Filed: 1974-01-11
Abstract: 1414942 Internal shoe-drum brakes; wear adjustment SOC ANON DBA 11 Jan 1974 [2 Feb 1973] 01467/74 Heading F2E An internal shoe drum brake with shoes 12, 14 has one pair of adjacent shoe ends operable by an hydraulic brake cylinder 30 and the other pair of adjacent shoe ends urged into floating contact with a fulcrum block 32 by a spring 34; an adjusting lever 36 is pivoted on shoe 12 near the wheel cylinder 30 and has a toothed sector 40 co-operating with a toothed sector 46 of a pawl 42 pivoted on shoe 12 near the fulcrum block 32, the toothed sector 40 co-operating with toothed sector 46 by the action of a return lever 50 on a cam surface 48 of the pawl 42, the return lever 50 being pivoted on shoe 12 and co-operating with the fulcrum block 32. A rigid rod 66 provided between shoes 12, 14 has a slot 74 which receives the adjusting lever 36 and the web 26 of shoe 12. A projection 80 on the rod 66 is received within an aperture 82 in the adjusting lever 36-the projection 80 is normally a predetermined distance from the inner edge 84 of the aperture 82 so that the rod 66 and lever 36 are connected with a predetermined amount of play. A spring 70 acts between the rod 66 and the web 28 of shoe 14 which is received in a slot 68. When the cylinder 30 is pressurized to move the shoes 12, 14 apart the rod 66 remains in contact with the shoe 14 by the action of spring 70 and follows the movement of this shoe 14. At the same time the shoe 12 entrains the adjusting lever 36 in the opposite direction-the shoes 12, 14 and lever 50 are now anchored on the fulcrum block 32, the return lever 50 cannot pivot and the pawl 42 is kept in contact with the toothed sector 40 on the adjusting lever 36 which is thereby locked in position on shoe 12. If during brake actuation projection 80 contacts edge 84 the rod 66 is made to follow the motion of the shoe 12 and the spring 70 is extended. When hydraulic pressure ceases to be applied to cylinder 30, the pressure fluid does not leave the cylinder immediately and for a moment the shoes 12, 14 and return lever 50 remain anchored to the fulcrum block 32. Any deformation of the brake components which has occurred during braking now diminishes and is absorbed by contraction of the spring 70. When the pressure in the cylinder 30 is low enough, the shoes 12, 14 and return lever 50 are in floating contact with the fulcrum block 32, and the return lever 50 can now swivel and the pawl 42 is released. The spring 70 which is now extended only by a distance corresponding to the wear on the brake linings 20, 22, can therefore bring the end 72 of slot 68 in rod 66 back into contact with shoe 14 and the rod 66 pivots lever 36 a distance corresponding to one tooth in the co-operating toothed sectors 40, 46 to adjust for such wear. The distances from the pivot 52 of return lever 50 to its ends 54, 56 must be accurately determined. In the embodiment of Figs. 1-3 this is achieved by providing a portion 86 of reduced cross-section between the pivot 52 and the end 54, which portion 86 deforms permanently the first time the brake is applied after assembly, so that the return lever 50 is adjusted automatically. In the variant of Fig. 4 (not shown) the portion (186) of reduced cross-section is provided between the pivot (152) and the end (156) of the lever 50. The end 56 of the lever 50 co-operating with the fulcrum block 32 is formed with a convex surface, whose radius of curvature is greater than the distance separating the end 56 from the pivot 52 and not less than the radius of curvature of that surface of shoe 12 co-operating with block 32. The return lever 50 and the pawl 42 comprise respective lugs 60, 58 which enter respective apertures 64, 62 in the flange 16 of shoe 12.