US Patent 7028600 Hydraulic radial piston engine

A hydraulic radial piston engine has a lifting ring fixed to the casting, a rotor mounted opposite the lifting ring such that it is rotatable about an axis of rotation, multiple piston seats aligned radially in the rotor relative to the axis of rotation, and multiple pistons each displaceably mounted in a piston seat and which, over at least part of their length, have a non-circular guiding and sealing cross section that is coincident with a non-circular guiding and sealing cross section of the piston seats. Circularly cylindrical rollers are borne by the piston. With a given overall size, a greater swept volume per piston is possible and therefore a greater torque is produceable.