ABSTRACT
A hydrostatic transmission is simply a pump and motor connected in a circuit. If the pump receives fluid from the reservoir, passes it through the motor, and returns it to the reservoir, the design is identified as an open-circuit transmission. Transmissions that circulate fluid in a continuous loop between the pump and motor are identified as closed-circuit transmissions. Typically, mechanical transmissions have efficiencies of 95%, whereas hydrostatic transmissions have a maximum efficiency of around 85%.
Hydrostatic transmissions are used to increase vehicle maneuverability. Also, they decouple ground speed from engine speed, thus allowing the machine to move slowly while providing maximum hydraulic power to other functions. Before a hydrostatic transmission is chosen over a mechanical transmission, a study is done to ensure that the advantages yield an increase in vehicle productivity to offset the lower energy efficiency.
Sensing motor speed and using this information to control pump displacement is known as feedback. When feedback is included, the transmission is identified as a closed-control-loop transmission.
