Patent GB-1387807-A (GB)
Title: Vehicle brakes
IPC: F16D66/02
Filed: 1971-01-27
Abstract: 1387807 Indicating wear; shoe-drum brakes; wear adjustment HOPE TECHNICAL DEVELOPEMENTS Ltd 9 Feb 1972 [27 Jan 1971] 3263/71 Heading F2E A device for indicating the operating condition of a vehicle disc or drum brake comprises a first electrical switch 17 spacedly located from a brake lining and being automatically operated to energize an indicator when the lining has worn to a predetermined extent, and a second electrical switch 20 which is automatically operated to energize an indicator when brake adjustment is required. Figure 1 illustrates a drum brake with a back plate 2 carried by an axle housing 1 and incorporating the device; the brake is operated by rotation of an S-cam-shaft 3 which is moved by a piston-rod 4 of an hydraulic or pneumatic brake cylinder through the intermediary of a lever 5 which is provided with a series of holes 6 each of which is engageable by a bolt 7 passing through the forked end of the rod 4 in order to adjust the movement of the rod 4 as the lining is worn down. A ferromagnetic mild steel actuating disc 9 is fitted to the free end of the camshaft 3, and a free end 12 of a plastics support member 10 welded to the axle housing 1 is bifurcated to allow entry of the periphery of the disc 9. To one inner face of the bifurcated end of the support member 10 is fitted a magnetically operated reed relay switch normally held in the off position; to the other inner face is fixed a magnetic element 18 which is operable to close the switch when a cut away segment 13 of the disc 9 begins to enter the bifurcated end. This occurs when the linings need replacing, and the switch will be closed to operate the indicator which may be visual or audible or a combination of both. A ferromagnetic mild steel plate 15 attached to the rod 4 and movable in a slot 14 in the support 10 is operable to allow a magnetic element on one face of the slot 14 to co-operate with a switch on the other face of the slot 14 to close the switch and energize an indicator when sufficient movement of the plate 15 indicative of the need for brake adjustment takes place. In the embodiment of Figs. 2-4 the camshaft 3 carries a split ring 50 which supports a magnetic element M, and a bracket 53 carries a reed-relay type switch 17 operated by element M when the shaft 3 rotates to bring the element M close to the switch 17; and this will occur when the brake lining needs renewing. The brake adjustment indicator operates as before with plate 14 extending between a magnetic element 19 and an associated switch 20. The lining wear switch may alternatively comprise electrical contacts on the axle housing 1 and cam-shaft 3 which engage one another when movement of the shaft 3 is excessive. The brake adjustment switch may comprise contacts on the rod 4 and an insulated bracket. The switches may also be limit switches such as micro-switches. Fig. 5 (not shown) illustrates a circuit arrangement for use with the device and incorporating indicator lamps. Figure 6 (not shown) illustrates a circuit arrangement incorporating means for testing the operation of the indicators and comprising a four-pole changeover switch (184). Reference has been directed by the Comptroller to Specification 1,267,518.