A prime mover, such as an internal combustion engine or the like, can be connected to a hydrostatic transmission to drive a wheel in a light vehicle. A hydrostatic transmission is particularly suitable to provide traction drive for light vehicles such as turf machines, lawn tractors, ride-on lawn mowers, and like devices. A simple usage of hydrostatic transmissions is on zero-turn radius vehicles, including zero-turn radius mowers and tractors.
Generally, a hydrostatic transmission includes a hydraulic pump and a hydraulic motor. The hydraulic pump usually is a piston-type pump including a plurality of reciprocating pistons, which are in fluid communication through hydraulic porting with the hydraulic motor. Rotation of the hydraulic pump against a moveable swash plate creates an axial motion of the pump pistons that forces hydraulic fluid through the hydraulic porting to the hydraulic motor to drive the motor, which allows the transmission output speed to be varied and controlled. The rotation of the hydraulic motor may be used to drive an output shaft, which in turn ultimately drives a wheel axle of a light vehicle of the types described above.
In certain vehicle configurations, the hydrostatic transmission may be configured for full wheel torque by connecting the hydrostatic transmission output directly to a wheel hub without any intervening gear reduction element. Specifically, a pump is driven by the prime mover (engine). The motor typically is of larger displacement (e.g., 20:1) as compared to the pump, which provides speed reduction and torque amplification of the prime mover to drive the wheel. Spool valve driven motors are known in the art for use in various applications, but conventional configurations have proven to be deficient for wheel drives, and hence have not been practical for transmission applications. In conventional spool valve/motor configurations, the spool valve and output shaft are integral components. Side loads on the wheel hub require outboard and inboard bearings. Because the spool valve bore normally serves as the inboard bearing, large side loads may be transferred to the spool valve resulting in significant wearing of the spool valve bore. To accommodate such loads and otherwise provide suitable performance, it remains difficult to provide a compact spool valve motor configuration that is suitable for hydrostatic transmissions applied to light vehicle wheel drives. Accordingly, because of the size constraints associated with light vehicles, it has been difficult to provide a hydrostatic transmission for a full wheel torque configuration that is compact, cost effective, and easy to assemble.