Patent GB-1314100-A (GB)
Title: Disc brakes
IPC: F16D65/18
Filed: 1970-12-18
Abstract: 1314100 Disc brakes; dual hydraulic systems warning devices SUMITOMO ELECTRIC INDUSTRIES Ltd 18 Dec 1970 [22 Dec 1969 8 April 1970] 60202/70 Headings F2E and F2F A spot-type disc brake includes a floating caliper 4 having legs 3 and 3<SP>1</SP> and in the leg 3 are provided two pistons 6, 7 operable respectively by hydraulic pressures in each of two hydraulic systems and a snap ring 19 fittedinto a groove 31 in the first piston 6 which snap ring 19 functions to allow only limited relative movement of the two pistons 6, 7. As shown in Fig. 1 the second piston 7 has an end port 13 with a flange 30 which is inserted into a chamber 9 and is held therein by the snap ring 19. Hydraulic pressure from the first system is applied to the first piston 6 via an inlet 8 and pressure from the second system is applied to the second piston 7 via an inlet 11. If both systems are in order the first piston 6 is moved to the left to apply a friction pad 2 against a disc 1: the second piston 7 remains stationary because of the balance of hydraulic pressures in the two systems and the friction pad 2<SP>1</SP> is applied to the disc 1 by reaction pressures in chambers 10 and 12 on cylinder walls 14, 15 respectively. If the first hydraulic system fails the second piston 7 is moved to the left to thereby move the first piston 6 to apply the pad 2. If the pressure in the second hydraulic system fails the pressure in the chamber 10 moves the piston 7 to the right such distance as is allowed by the snap ring 19 and the pad 2 is applied by the piston 6 and the pad 2<SP>1</SP> by reaction on the cylinder wall 14. The linkage between the two pistons 6, 7 in the form of the snap ring 19 prevents the capacity of the chambers 10, 12 increasing unduly in the event of failures of the systems which would result in a greatly increased pedal stroke of the tandem master cylinder which operates the systems. The end port 13 of the second piston 7 is spherical and a gap is provided between the flange 30 and the snap ring 31 to allow the second piston 7 to be inclined with respect to the first piston 6. It is envisaged that a higher pressure may be applied to the second piston 7 than the first piston 6 but a compression spring 16 may be used to produce the same effect. The disc brake as described may be used for the front wheels of a vehicle with only one braking system being used for the back wheels, and to prevent skidding if one of the systems fails the crosssectional area of the second piston 7 is 40-50% that of the first piston 6 so that braking on the front wheels when this failure occurs becomes about half of that when both systems are in order. Seal rings 18, 18<SP>1</SP> are provided for the second piston 7 and any leakage around these rings 18, 18<SP>1</SP> is detected by a pressure sensitive transducer or semi-conductor; the transducer 21 of, Fig. 4 (not shown), has a hole (22) through which leaking fluid is applied to a spring-biased piston (23) to bring it into contact with an electrode (26) and thereby light a warning lamp. In the embodiment of Fig. 2 a guide piston 32 is fixed to the leg 3 and pressure is applied to the second piston 7 through an inlet 11 in this piston 32. A snap ring 19 bridges annular grooves 31, 33 in the pistons 6, 7 respectively and serves as a mechanical linkage therebetween. In the embodiment of, Fig. 3 (not shown), which is a variation of Fig. 1 the end part (13) of piston (7) is the same cross-sectional shape as the rest of the piston (7).