Hydraulic Damper Valve

A hydro-pneumatically operated riveting tool, including hydraulically driven gripping means for gripping and pulling the stem of a rivet, thereby to place the rivet, a hydraulic inlet port for supplying hydraulic fluid to drive the gripping means, a sealing region around the inlet port, and return means for returning the gripping means back to a rest position again after placing a rivet;

• the hydraulic supply for the gripping means including a unidirectional flow restriction arranged to provide a restricted flow of hydraulic fluid during pulling of the stem and to provide relatively free flow during return of the gripping means to a rest position, the flow restriction comprising a moveable member located in the hydraulic flow path adjacent the inlet port which is arranged to have limited reciprocal movement in the direction of hydraulic flow towards and away from the inlet port into a sealing and non-sealing position, the moveable member having an inner surface which cooperates with the sealing region around the inlet port to form a seal which substantially prevents flow of hydraulic oil around the periphery of the moveable member when the member is pushed by hydraulic flow into the sealing position, the moveable member including an aperture of relatively small cross-sectional flow area through which hydraulic fluid is permitted to flow when the moveable member is in the sealing position, the moveable member being arranged to diffuse or direct hydraulic flow through the aperture away from components susceptible to erosion which are in the vicinity of the inlet port.