Piston assembly

Claim 1

A hydraulic device comprising a cylinder block, means supporting said cylinder block for rotation about its central axis, a plurality of longitudinally extending cylinders formed at spaced apart locations in said cylinder block, each of said cylinders having a longitudinal axis extending generally parallel to the central axis of said cylinder block, a thrustplate having a reaction surface extending transversely to the longitudinal axes of said cylinders, a plurality of pistons each of which is disposed in and reciprocatable relative to an associated one of said cylinders and has an inner end portion cooperating with a wall of the associated cylinder to at least partially define a pressure chamber for receiving hydraulic fluid, a plurality of slippers each of which is disposed in engagement with an outer end portion of an associated one of said pistons and has a bearing surface disposed in sliding engagement with said reaction surface on said thrustplate, each of said pistons including a generally cylindrical metallic body having an outer surface disposed in sliding engagement with an inner surface of an associated one of said cylinders, said metallic body including interior surface means for at least partially defining a longitudinally extending cavity having one end formed by said metallic body adjacent to the outer end portion of the piston and a main opening at the inner end portion of the piston, molded polymeric filler means disposed in and substantially filling said cavity, said molded polymeric filler means having a lower mass per unit volume than the metallic material forming said body and a higher bulk modulus than the hydraulic fluid, passage means extending axially through the piston for conducting hydraulic fluid between the inner and outer end portions of the piston, said passage means including first surface means formed by said molded polymeric filler means and second surface means formed by said metallic body, said first surface means extending from the main cavity opening to said inner end of said cavity, said second surface means extending from the inner end of said cavity to the outer end of the piston, plate means at least partially blocking the main opening to said cavity for retaining said molded polymeric filler means in said cavity during reciprocating movement of the piston in the associated one of the cylinders, said plate means including an inner surface disposed in abutting engagement with said molded polymeric filler means and an outer surface defining a portion of one of said pressure chambers, and weld means for connecting said plate means to said metallic body.