Radial piston hydraulic machines

Claim 1

A radial piston hydraulic machine comprising a rotor, a plurality of pistons carried in respective cylinders therein, a slipper at the radially outer end of each piston, and a cam track with an internal cylindrical bearing surface with which bearing surfaces of the slippers co-operate, wherein the improvement comprises the provision, on each slipper, of a. a (relieved central area of the) bearing surface with a radius of curvature greater than that of (respect to) the cooperating cylindrical surface of the cam track to provide a clearance between the two surfaces reducing from the centre outwards, b. a central oil supply port providing unrestricted communication between the associated cylinder and said relieved central area to create a pocket of oil between said bearing surfaces. c. from the centre of the bearing surface a radial sequence of a central land, an annular pressure groove, an annular land and an annular pressure relief groove, and d. surface grooving through the central land from the oil supply port to the annular pressure groove.