Thrust rings for swash plate pumps and motors

In a swash plate pump or motor having a swash plate, a cylinder block and cylinders rotating relatively to said swash plate, pistons in said cylinders moving generally transversely to the swash plate, drive slippers operatively connected to said pistons and a slipper plate fixed to the swash plate against which the slippers bear, the improvement comprising a swivel plate having spaced openings for said slippers on the opposite side of said slippers from the slipper plate, a positive hold down ring connected to the swash plate and bearing on said swivel plate to maintain it at a fixed distance from the slipper plate whereby a fixed running clearance is maintained between the slipper and the slipper plate, said hold down ring having a series of successive controlled wedge angles on the face of the hold down ring adjacent the swivel plate and having oil feed means through the hold down ring and connected to each wedge angle whereby to create a hydrodynamic force holding the swivel plate and hold down ring apart.