Patent GB-1363408-A (GB)
Title: Electrical devices for detecting or indicating vehicle brake lining wear
IPC: F16D66/02
Filed: 1973-04-28
Abstract: 1363408 Brake wear indicating devices SAB BRAKE REGULATOR CO Ltd 28 April 1973 20330/73 Heading F2E In an electrical device connected to earth (at the top), Fig. 2, for indicating brake-lining wear and including a voltage source 6 having one pole earthed, a control means 7A, a switch 7B controlled by the control means, and an activating means 5 located at least partially within the brake lining (not shown) and normally electrically conductive but becoming non-conductive on excessive lining-wear occurring, and thereby opening the circuit of the device and directly affecting the control means 7A for switching the switch 7B to a "warning" position where an indicator 8 connected through switch 7B to the circuit becomes effective the control means 7A is a relay coil connected between the activating means 5 and earth, and the switch 7B is a relay switch controlled by the relay coil. Preferably switch 7B is spring-loaded to its "warning" (dotted line) position. In operation, an ignition key switch 9 is first closed, whereupon lamp 8 is energized and lights up. As the vehicle engine starts, a transient reset signal is received from the starting system from the right of a diode 12 and energizes relay 7A through (closed) activator connection 5. Energization of relay 7A moves switch 7B to its full line position as a holding switch and thereby extinguishes the lamp (now tested). If a spurious earth occurs, or if lining wear has reached a stage where an embedded wire loop makes contact with the (earthed) brake drum (see below) on brake application, represented by the dotted arrow adjacent to activator 5, Fig. 2, relay 7A is de-energized, switch 7B springs to the left, and lamp 8 lights. On brake release, the temporary earth connection is broken but the lamp remains lit. If now, switch 7B is by-passed by operation of button switch 10, relay 7A will be re-energized, switch 7B will return to full-line position, and the lamp will be, and remain, extinguished. At a further stage of lining wear, when on a brake application the wire loop is broken by frictional contact with the brake drum, represented by the dotted line position of activator 5, relay 7A is again de-energized, causing lighting of lamp 8. However, on this occasion, operation of button switch 10 will not result in the lamp being extinguished, and this serves to differentiate the present condition from the previous one.  In Fig. 3, relay switch 7B is no longer a holding switch, lamp 8 is coloured amber, and a further lamp 14, coloured red, is connected to the circuit through a holding switch 13B controlled by a further relay 13A. In operation, during normal working, both lamps are out. During a temporary earth connection lamp 8 lights; if the earth connection becomes permanent, lamp 8 remains lit. If either a temporary or a permanent open-circuit condition occurs, both lamps light and remain lit. In Fig. 4 (a modified Fig. 3), relay 7A also provides a hold current by having associated with it a further relay switch 7C, and a further connection to the vehicle's starting system through a further diode 12<SP>1</SP> is provided. The only difference from Fig. 3 in operation is that after a temporary earth connection lamp 8 stays lit.  Fig. 1 (not shown) depicts the activator, which consists of a rod of insulating material having a loop of wire (5<SP>1</SP>) embedded therein and forming part of the above circuits; the rod protrudes through a hollow rivet in the backing plate of a drum brake shoe into a recess in the brake lining for eventual engagement by the brake drum.