Patent GB-938575-A (GB)
Title: Fluid pressure servomotors
Inventor: HOGANSON EUGENE MORGAN
IPC: B60T13/16
Filed: 1961-12-29
Abstract: 938,575. Fluid-pressure brakes. BENDIX CORPORATION. Dec. 29, 1961, No. 46712/61 Class 103 (1). [Also in Group XXXI] A fluid-pressure servomotor for an hydraulic brake system of a road vehicle comprises a piston B acting on hydraulic fluid in a power chamber 12 and having a larger diameter portion 36 which is arranged within a larger diameter cylinder 14 for servo operation by admission of hydraulic fluid supplied by a pump under control of a valve assembly C to a chamber 38 behind the portion 36, the space in front of portion 36 being connected to a return conduit 54 to a reservoir, and control mechanism, including an applying member D having a lost-motion connection with a valve spool 50 slidable within a valve sleeve 48 of the assembly C, the sleeve 48 being in turn slidable within the casing 14 and the arrangement being such that follow-up action does not normally occur, the pressure in chamber 38 reacting on sleeve 48 to maintain it in contact with a snap ring 126, but should the application force exceed a predetermined maximum or should the pump pressure fail then the applying member moves the sleeve 48 forwards to act directly on the piston B, providing a limit to power operation in the former case and direct manual operation in the latter case. The applying member D comprises a sleeve 110 within which a prestressed spring 114 is arranged on an actuating rod 104, one end of the spring being retained by a shoulder 116 of the rod 104 and the other by a snap ring 122. A plug 112, which carries the sleeve 110 and to which a foot pedal is connected, abuts the rear end of the spring to transmit force via the spring to the rod 104. Rod 104 has a ball end 106 contacting one end of the valve spool 50 which has lands 72, 74 and an intermediate recess connected via drillings 80, 82 to the chamber 38. The sleeve 48 has co-operating lands and recesses so arranged that in the position shown the chamber 38 is exhausted via recess 56 to " Reservoir," whereas when the valve spool is moved forwards the chamber 38 is successively cut off from the reservoir, connected to both " Pump " chamber 58 and " To Power Steering " chamber 60, and finally disconnected from the chamber 60 while retaining connection with chamber 58. The arrangement is for incorporation in a series hydraulic circuit wherein the chamber 60 is connected to the power steering such as to supply fluid thereto so long as the spool 50 is in its normal position, but to cut off the supply of fluid when the spool is moved to its fully actuated position. As pressure builds up in chamber 38 it reacts on the spool 50 to provide " feel " in addition to retaining the sleeve 48 against snap ring 126. The spool returns to a lap position when the reaction force equals the applying force. Should a predetermined applying force be exceeded spring 114 is compressed, allowing lost motion of the sleeve 110 until it abuts sleeve 48, moving the latter forwards to cut off the supply of fluid to chamber 38 and hence limit pressure therein. During movement of the sleeve 48 it is maintained in alignment with the ports for connection to pump, reservoir and power steering by the engagement of longitudinal grooves 64 with balls 66 supported in the casing, the recesses 56, 58, 60 having associated with them grooves 62 to maintain communication with their respective ports. Should the pump supply fail the reaction force ceases so that on application of force to sleeve 110 it moves easily into contact with sleeve 48 against the force of a light spring 102. To prevent too rapid a rise of pressure in chamber 38 when the valve spool is actuated a plug 86 is provided in front of the valve spool having a restricted opening 92 which is by-passed by a one-way ball valve 94 permitting quick release. The fluid in chamber 38 is used to top up the fluid in the main hydraulic system including the power chamber 12, by providing a passage 136 through piston B which communicates with chamber 38 via a one-way ball valve, the ball having a stem engageable with the casing in the fully retracted position of piston B to maintain the valve open in this position of the piston. Another one-way ball valve 128 prevents vacuum developing in chamber 38 when piston B is actuated directly in the event of hydraulic power failure. A spring-loaded valve 20 for maintaining a small pressure in the main hydraulic system is as described in Specification 694,409.