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Water plays an important role in power transmission systems. The inherent cleanliness of water, combined with its long history as a power source, has pushed water back to the hydraulic forefront. Obviously, economic and environmental forces have had an impact in helping water regain its prominence as a hydraulic fluid. Today, engineers are viewing water hydraulics as an interesting technology that can offer significant advantages to solve power transmission and motion control tasks. In order to make use of the advantage of water hydraulics the component and system design must be adapted, especially to the physical behavior of pressurized water. The materials used in the hydraulic components must have high corrosion and cavitation resistance. Poor lubricating properties requires special design of all components with moving parts, such as pumps, motors and valves. The major aim is to discuss and high light new ideas useful for the design of water hydraulic systems.

In mobile vehicles such as earth moving machines, agriculture machines, forest machines, industrial and mining lifters there is a demand for sophisticated performance. Since the requirements on productivity are very high for mobile machinery, high output capacity combined with high overall efficiency over a wide velocity range is therefore of great importance. A traditional solution for drive train in mobile machinery is to use a hydrodynamic transmission (torque converter) in series with a multi-speed shiftable gearbox. For heavy machines, engine power above 150 kW, torque converters have been used exclusively until a few years ago. However, the development of hydrostatic drive concepts and microprocessor control have made it possible to be competitive with converter drives.

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.