The hydrostatic transmission offers in- finite control of speed and direction. The operator has complete control
of the system with one lever for starting, stopping, forward motion, or reverse motion.
Control of the variable displacement, axial piston pump is the key to controlling the vehicle. Prime mover
horsepower is transmitted to the pump. When the operator moves the control lever, the swashplate in the
pump is tilted from neutral.
When the variable pump swashplate is tilted, a positive stroke to the pistons is created. This, in turn, at any
given input speed, produces a certain flow from the pump. This flow is transferred through high pressure lines
to the motor. The ratio of the volume of flow from the pump to the displacement of the motor will determine the
speed of the motor output shaft. Moving the control lever to the opposite side of neutral, the flow from the
pump is reversed and the motor output shaft turns in the opposite direction. Speed of the output shaft is
controlled by adjusting the displacement (flow) of the transmission. Load (working pressure) is determined by
the external conditions, (grade, ground conditions, etc.) and this establishes the demand on the system.
Pump and motors are contained in separate housings or may be connected by a common end cap. All valves
required for a closed loop circuit are included in either the pump or motor assemblies. A reservoir, filter, cooler,
and lines complete the circuit.
Fig. 1 illustrates the internal components of a typical Sundstrand heavy-duty hydrostatic transmission. Fig. 2
illustrates the general appearance of the components of a heavy-duty transmission.