Hydrostatic Drives in Heavy Mobile Machinery–New Concepts and Development Trends

The use of hydrostatic transmission as vehicle drives is primarily motivated by its large range of continuously variable speed, high maneuverability and a possibility to increase the overall efficiency. Consequently an optimally designed system can provide low fuel consumption and thereby low exhaust gas emission. Modern technology involving electro-hydraulic systems, microprocessor control and control theory makes it easier to utilize the advantage of these properties than using traditional hydro-mechanical control.

The requirements on productivity are very high for mobile machinery. High output capacity and a wide velocity range are therefore of great importance. To meet these requirements an ordinary hydrostatic transmission is generally used with a mechanical gearbox connected in series. The main drawback in this concept is the high cost in using a gearbox, which can switch gear ratios without jerks and other disturbances in speed and torque transmission. By joining pumps, motors and fixed gears in smart concepts, it is possible to keep prescribed output and velocity range without the need of a multi-speed mechanical gearbox. Two-motor concept and hydro-mechanical split power transmissions are representing interesting solutions.

The natural control quantities of the transmission are the output shaft speed or torque. By using microprocessor control the flexibility of the control system increases which makes it possible to adapt different control loops for different states of driving, also taking into account simultaneous use of other hydraulic equipment on the vehicle.