Patent GB-1439330-A (GB)
Title: Vehicle brakes
IPC: F16D65/56
Filed: 1972-09-01
Abstract: 1439330 Hydraulic actuators for spot-type disc brakes GIRLING Ltd 31 Aug 1973 [1 Sept 1972] 40533/72 Heading F2E An hydraulic actuator 14 for a spot-type disc brake, the actuator comprising a first component 13 and a second component 15, is provided with an anti-knock-back device 19; the construction of device 19, and its relationship with the actuator components, are set out below, As shown, a brake pad assembly 11 of a spottype disc brake is directly operated by a piston 15, while the indirectly-operated pad 12 is secured to a caliper 13 slidably mounted on a fixed bracket 10. For operation, hydraulic fluid is applied to space 18 on the right-hand side of piston 15. On brake release, piston 15 retracts into blind bore 16 only as far as is necessary to relieve the pressure at the friction faces of the pads 11, 12: the piston is normally prevented from further retraction by the friction developed at the seal 17. However, as a result of disc deflection due to cornering, for example, the piston could be knocked back further because fluid can be displaced from space 18 through the master cylinder (not shown) to the reservoir when the brake is not applied. The anti-knock-back device 19 prevents this, and comprises a strut 20, a collar 21 and a split ring 22 retained by its own resilience in frictional engagement with the strut. The strut 20 is slidably sealed at 29 to piston 15, and is urged by a compression spring 25, acting between a split ring 26 lodged in caliper 13 and a washer 24 captive on the strut, towards the base 28 of blind bore 16. The split ring 22 has an external conical surface co-operating with an internal conical surface of collar 21; the split ring also co-operates with a shoulder 31 formed in piston 15; the axial movement thus permitted to the split ring 22 determines the brake slack, and can be pre-adjusted by screwing collar 21 in or out. In operation, when the brake is first applied, piston 15 is moved to the left relative to the caliper, but strut 20 is constrained by spring 25 to move first of all with the caliper. At a predetermined relatively-low pressure, the spring force is overcome by the fluid pressure, which thereupon urges strut 20 to the left until the conical surface on ring 22 engages that on collar 21, the resulting wedging action preventing sliding of the strut within the ring 22. Thereafter, the piston and strut move together. The converse happens on brake release. If wear takes place while the brake is applied, the piston 15 will not retract so far into bore 16 on release before the brake pressure has fallen to the above-mentioned predetermined value, so strut 20 is moved to the right by spring 25. Due to brake wear, ring 22 engages shoulder 31 before the end of the strut engages base 28 of bore 16, but spring 25 is strong enough to force strut 20 the remaining distance, drawing it through ring 22 a distance corresponding to the lining wear. As brake pressure is finally dissipated, piston 15 further retracts into bore 16 until the conical surface on collar 21 engages that on ring 22. Ring 22 is thereby urged more positively into engagement with strut 20, and knockback is prevented. For re-setting, a cup-shaped member 33 is fitted over the free end of strut 20 and may be used to push ring 22 to the right along strut 20: piston 15 cannot simply be pushed back into the bore because of the above-described wedging action. In Fig. 2 (not shown), base 28 is provided at the end of a separate fluid bore-that of a second piston which acts on the caliper, the caliper in this case being in the form of a plate like yoke.