Patent GB-1508613-A (GB)
Title: Fluid pressure servomotors
IPC: B60T13/569
Filed: 1975-03-11
Abstract: 1508613 Servomotors AUTOMOTIVE PRODUCTS Ltd 1 March 1976 [11 March 1975] 9956/75 Heading F2F A vacuum-suspended servomotor operable on a brake master cylinder comprises a piston 18 dividing a casing 11 into two chambers 19, 21 and movable within the casing 11 in response to a pressure difference between chambers 19, 21 the pressure difference being controlled by a valve means 29, 47, 48, 54 responsive to relative movement of an input push-rod 53 and on output push-rod 46, the piston 18 being operably linked to the output push-rod 46 through a lever mechanism 63 which is arranged to react against a pressing 68 attached to the casing 11 and to transmit a greater force to the output push-rod 46 than is transmitted to the lever mechanism 63 by the piston 18. As shown in Fig. 1, the lever mechanism comprises a plurality of levers 63 arranged substantially radially in chamber 21. The radially outer ends of levers 63 abut a flange of a ring 65 secured to a skirt 22 of piston 18, the radially inner ends are trapped between flanges 66, 67 of the pressing 68 and at a point intermediate their ends the levers 63 abut a pressing 62 which is mounted on a stepped valve housing 29 of the valve means. The valve means also includes a valve plunger 47, 48 attached to push-rod 53 and formed with a valve seat 52 co-operable with a valve seal 54 slidable in housing 29-the valve seal 54 is also co-operable with a valve seat 57 in housing 29. When installed on a motor vehicle chamber 19 is connected to a vacuum source, the push-rod 53 is connected to the driver's brake pedal and push-rod 46 is connected to a piston of the master cylinder. In the brakes off position a spring 56 biases the push-rod 53 and valve plunger 47, 48 away from the push-rod 46 and a spring 55 biases seal 54 into contact with seat 52: the chambers 19, 21 are then interconnected by passages 61, 58, 59. When the brake pedal is depressed push-rod 53 53 and valve plunger 47, 48 are moved towards push-rod 46 the seal 54 following this movement under the bias of spring 55 until it contacts valve seat 57 to block off communication between chambers 19, 21, and further movement of push-rod 46 causes valve seat 52 to separate from seal 54 so that atmospheric air enters chamber 21 via passages 58, 59. The pressure difference between chambers 21, 19 moves piston 18 towards the push rod 46, and the biasing force on piston 18 is transmitted through the levers 63 and pressing 62 to the valve housing 29-the biasing force on the valve housing 29 is approximately double that exerted by the piston 18 on the levers 63; the biasing force from valve housing 29 is transmitted to push-rod 46 through a rubber disc 44 the resultant movement of push rod 46 causing seal 54 to contact seat 52 to bring the servomotor to a state of balance. The rubber disc 44 acts as a reaction means-as the servomotor is actuated the disc 44 is caused to be extruded to abut the valve plunger 47, 48 to apply a reaction force to the input push-rod 53. In modifications of the embodiment illustrated in Fig. 1, the levers 63 may be joined integrally or otherwise, and the ring 65 may be replaced by a series of links. Also, the lever mechanism may be disposed in chamber 19 rather than chamber 21. The disc 44 may be replaced by another reaction device, e.g. the valve housing 29 may transmit the piston effort on to the outer ends of a pair of radially disposed levers whose inner ends react on the valve plunger and which apply on effort to the output push-rod at a point intermediate their ends. In the embodiment of Figs. 2, 3 the lever mechanism is modified to provide an increasing lever ratio as the stroke of the piston increases. To this end the levers 163 have roller bearings 172 at their radial outer ends, and roller bearings 171 carried between their ends but offset from the longitudinal centre line of the levers to abut a pressing 162 integral with or carried by housing 129.