Hydrostatic transmissions

A hydraulic variable speed transmission comprising a casing, an input shaft, an output shaft, a first hydrostatic pump/motor unit within the casing having at least two similar reaction surfaces supported for coordinated non rotative eccentric movement in radial directions equiangularly spaced abot the unit axis, a second hydraulic pump/motor unit coaxial with the first unit within the casing and having means defining at least two similar equiangularly spaced reaction surfaces supported for rotation together about the axis of the unit, each unit having rotor equipped with radially movable displacement control members engaging the reaction surfaces of that unit and coupled to the input and output shafts respectively, the reaction surface defining means of the second unit being supported by a rotatable member coupled to the input shaft, a stationary commutating valve member establishing two independent paths for fluid flow between the units whereby to coordinate the transfer of fluid in an internal circuit between the pump/motor units, and an external hydraulic circuit including at least one source of pressurized hydraulic fluid, said valve member including means establishing external connections between said independent paths and said external hydraulic circuit whereby pressurized fluid may be transferred bidirectionally between said internal circuit and said external circuit and whereby a difference in the fluid pressure in said two independent paths may be selectively responded to or maintained by said external circuit.