Patent GB-1206984-A (GB)
Title: Improvements in or relating to fluid pressure operated servo-devices
Inventor: PARSONS DAVID
IPC: B60T13/573
Filed: 1968-01-12
Abstract: 1,206,984. Brake-system servomotors. AUTOMOTIVE PRODUCTS CO. Ltd. 5 Dec., 1968 [12 Jan., 1968], No. 1964/68. Heading F2F. In a brake servo device where the fluid pressure differential across a movable wall is established by the operation of a valve controlled by a pedal or other member and wherein lever means are provided to transmit reaction force from the movable wall to the pedal &c., there is provided, between the movable wall and the braking mechanism, a resilient connecting member which transmits the braking effort directly to the braking system independently of the lever means until the effort has reached a value sufficient to bring the shoes or pads into engagement with the drums or discs, the resilient connecting member yielding as the effort increases so as ultimately to bring the lever means into operation. In operation of the embodiment shown, actuation of the brake pedal (not illustrated) moves push-rod 41 and plunger 43 to the left, whereby levers 32 are angularly displaced about their radially outer ends. At the same time a resilient valve-seating disc 48 is moved, seating on shoulder 53 and thereby disconnecting from one another a front chamber 15 and a rear chamber 16 (previously connected by passages 54 and 55) and immediately thereafter rim 51 moves away from disc 48, thereby connecting chamber 16 to atmosphere through the clearance between disc 48 and push-rod 41. The degree of movement of plunger 43 necessary to open this connection to atmosphere is insufficient to bring the levers 32 into contact with an annular bulge 39 on a plate 35. The atmosphere connection creates in the rear chamber 16 a pressure higher than that in the front chamber 15, which is connected to suction at 46, this pressure differential moving disc 11 forwardly. The thrust is transmitted through a resilient ring 34, located in a recess 31 in disc 11, and through annular plate 35 to a front push-rod 29 operative on the piston of a liquid pressure master cylinder (not illustrated) connected to the brakes. The thrust tends to compress the resilient ring 34, but its resistance to compression is such that, until the brake shoes or pads are in engagement with the drums or discs and are exerting at least a small braking force, the ring does not deform sufficiently to bring the bulge 39 on plate 35 into contact with the levers 32. Once such contact is established, thrust is transmitted to the pushrod 29 and the master cylinder piston through the levers 32, and a reaction thrust, the proportion of which to the thrust exerted by the disc 11 depends on the respective radial distances between, on the one hand, the outer ends of levers 32 and bulge 39 and, on the other hand, the outer and inner ends of levers 32, is transmitted through plunger 43 and push-rod 41 to the brake pedal. Thus no reaction force is applied to the brake pedal until the brakes begin to produce a retarding effect, and the effort required to initiate the braking operation is very small, so that the "feel" of the brake pedal is progressive throughout a braking operation. Modifications.-The resilient ring 34 may be replaced by a coiled compression spring, a leafspring, or a stack of Belleville washers; the rigid plate 35, instead of being formed with the annular bulge 39, may be generally flat but have an annular rib or a series of circumferentiallyspaced projections to contact the levers 32. The servo device, instead of being the vacuumsuspended type, may have chambers 15 and 16 connected to atmosphere, compressed air being admitted to chamber 16 to operate the servomotor; or the rear chamber 16 may be connected to atmosphere, the valve being arranged to connect the front chamber 15 either to the rear chamber or to a source of suction.