Reciprocating actuator

An actuator is driven in a first direction by equal working fluid pressures acting on different piston areas acting in opposite directions, and is driven in a second direction by applying equal pressures on both sides of one of the piston areas. Working fluid from an inlet port (71), acting in a working fluid chamber (27), on one side of the piston moves a valve plate (36), closing vent ports (33) and opening working fluid discharge ports (32), thereby permitting the working fluid to push the piston (21) in a first direction. Working fluid contained in the working fluid chamber (21) is exhausted from working fluid discharge ports (32) to the outside of the system through a working fluid discharge tube (52). When the movement of piston (21) reaches its limit, a valve plate (36) is engaged by a spring (41), thereby causing simultaneous closing of the working fluid discharge ports (32) and the opening of the vent ports (33). As a result of the opening of the vent ports (33), working fluid pressures in the working fluid chamber (27) and a second working fluid chamber (26) are equalised. The effective piston area facing the second chamber is greater than the effective area facing the first chamber so the direction of piston movement is reversed. The working fluid discharge pipe (52) is fluid-tightly fitted through the piston (21) so as to provide a discharge path of working fluid from the system.