Cam follower piston

Claim 1

A piston for a hydrostatic machine comprising: a freely rotating cam follower ball for following a cam surface in a hydrostatic machine by reciprocatory motion of said cam follower ball in a cylinder bore of the machine, a piston body for reciprocatory travel along a cylinder bore, said piston body having at one end a working face to be acted on by the working fluid of a hydrostatic machine, and at the other end a bowl-shaped seating for said ball, said piston body exposing a portion of said ball protruding from said piston body to engage a cam surface, said piston body having an aperture extending from said seating to said working face of said piston body, an insert of low-friction synthetic plastics material positioned between said cam follower ball and said seating in a position at the rear of said ball to accept the bearing load of said ball when engaging a cam surface, said insert having an internal bearing surface for said ball and an external surface mating with and supported by said seating, said insert having an extension which extends into said aperture in said piston body and locates said insert in said piston body, said insert having an aperture extending from said bearing surface through said extension of said insert to said working face of said piston body to allow passage of working fluid to said bearing surface for lubrication purposes and for providing hydrostatic support for said cam follower ball, said piston body encompassing said ball by a portion of said piston body which extends from said seating beyond the centre plane of the cam follower ball, said piston body having a lip extending around the entire perimeter of said bowl-shaped seating and projecting inwardly towards said cam follower ball for engaging said synthetic plastics insert.