1. Field of the Invention
The present invention relates to variable displacement hydrostatic transmissions and, more specifically, to hydrostatic transmissions utilizing pumps and motors of the radial piston type.
2. Description of the Related Art
Generally, hydrostatic transmissions consist of a variable displacement hydraulic pump unit driven by an internal combustion engine of the vehicle, and a fixed displacement hydraulic motor unit connected to the drive train of the vehicle. A closed fluid loop consisting of first and second conduits fluidly connect the pump with the motor. By increasing the fluid displacement of the pump, the pump drives the motor in a direction consistent with the fluid flow direction in the first and second conduits. The fluid displacement of the pump can be in either direction which changes the flow of fluid in the first and second conduits by creating a difference in pressure between the two conduits. Thus, the motor is caused to rotate in a first and second direction corresponding to forward and reverse directions. The displacement of fluid within the pump is variably controlled by the operator to thereby change the volume of fluid flow within the closed conduit loop between the pump and the motor. Thus, the pump and therefore the output of the motor can variably range from a full reverse position to a neutral position, to a full forward position.
The motor is, in turn, connected through gearing and a differential to the driving axles of the vehicle. Thus, the internal combustion engine imparts rotational motion to the hydrostatic transmission which is translated into variable rotational movement of the axles.
Rotational energy from the internal combustion engine of the vehicle is transferred to the transmission generally via a belt and pulley system. The pulley is connected to an input shaft which extends into the transmission housing. In hydraulic transmissions utilizing radial piston type pumps and motors, the input shaft terminates with an input bevel gear which meshes with a transversely oriented bevel gear connected to the pump cylinder block. The pump cylinder block and the bevel gear are radially disposed about and supported directly on the fluid conduits, known as the pintle, for concentric rotation thereabout. The pump cylinder can be provided with an extending annular hub portion creating a larger bearing surface.
The rotational energy supplied by the internal combustion engine is thus imparted to the pump cylinder block through the input bevel gear of the input shaft and the cylinder bevel gear secured to the pump cylinder block. The input bevel gear, however, produces a load against the cylinder bevel gear, and thus the pump cylinder block, which causes the pump cylinder block and the cylinder bevel gear to depart from concentric rotation relative to and about the pintle. Because of the nonconcentric rotation caused by the imparted load, the pump cylinder block and the cylinder bevel gear can wobble and rock during rotation about the pintle. Such wobbling and rocking creates an undue and uneven wear in both the cylinder block/bevel gear and the outside surface of the pintle thereby resulting in reduced life of the hydrostatic transmission. Additional loading is from the pump cylinder block and pistons which is on the pressure side of the conduit.
It is thus desirable to assure that the bevel gear and associated cylinder block concentrically rotate about the pintle regardless of the magnitude of the loads imparted thereto.