What is the principle of hydrostatic transmission

Fluid transmission

Fluid transmission are hydraulic gears for stepped or stepless speed and torque conversion. The output speed is almost always lower than the input speed.

Hydrodynamic transmission component (half left) as part of a work machine drive train for a construction machine

Hydrodynamic hydraulic transmissions work with low pressure and high oil flow and allow a compact design (see also Föttinger principle). Hydrostatic hydraulic transmissions work with very high oil pressures and low oil flows. The drive and output can be connected spatially separated by hoses.


Hydrostatic transmission

In technology, a hydrostatic transmission is the connection between a hydraulic pump and one or more hydraulic motors. Such gears can have a distance of up to several meters between the pump and motor parts. The ratio can be continuously adjusted by continuously changing the oil flow between the hydraulic pump and the hydraulic motor by conveying or changing the displacement.

In hydrostatic gears, the pump and motor components are usually constructed the same apart from the minor details: the pump housing is arranged axially to the drive shaft of the pump. Inside there is a ring with circularly arranged cylinder bores into which finger pistons are inserted in the same arrangement. These pistons are axially connected to a sliding disk that rotates with the housing, whereby in adjustable machines the angle of the disk in relation to the ring with the piston (swash plate machine) or the angle of the ring in relation to the disk (bent axis machine) can be changed. Such machines are generally referred to as finger piston pumps or motors (also known as axial piston pumps / motors).

As long as the angle between the disc and the ring with the piston is 0 °, no oil is pumped. If you bend the piston side or the disk (depending on the design) to the side, the pistons, triggered by the inclined position of the disk or the ring with the pistons on the inside of the bend, enter the pump housing and exit on the other side. This results in a pressure side and a suction side, which are separated from one another by a type of cylinder head. Thus, the pump starts to deliver when it rotates. The pump is connected by pressure lines to the similarly constructed hydraulic motor, which converts the hydraulic energy (pressure, volume flow) back into mechanical energy (i.e. a rotary movement). By changing the articulation angle and the articulation direction on the pump, the delivery rate and delivery direction and thus the ratio and direction of travel can be controlled.

Hydrodynamic transmission

Torque converter (cutaway model) Porsche Museum Stuttgart

Fluid coupling

With a fluid coupling (also Föttinger coupling according to Hermann Föttinger), the pump and turbine are combined in one housing (see also Föttinger principle). The pump wheel sits directly on the shaft and is enclosed by the turbine. The pump sucks the oil on the shaft and throws it outwards directly into the turbine blades. For a fluid coupling, the parts mentioned are already all necessary parts. The oil is sucked in through the pump wheel and accelerated to the outside. The impulse is transmitted to the turbine wheel, then the oil collides with the housing and runs back inside to the pump wheel or into the oil sump.

The fluid coupling does not convert the torque, it only couples. A slip occurs, which is energetically converted into heat, so that there is a speed difference between input and output.

Torque converter

In order to achieve a torque conversion, a diffuser is integrated into the housing (see also Föttinger principle). It essentially consists of blades to catch the oil emerging from the turbine. The oil from the turbine has a radial movement component due to the inertia and a tangential movement component due to the speed of the turbine. The diffuser diverts the oil and accelerates it towards the pump. To do this, it stands still, it is usually supported by a freewheel. The oil re-enters the pump with residual kinetic energy. With this arrangement, a speed-torque conversion up to a maximum of 1: 8 can be achieved. The efficiency drops sharply with decreasing or increasing speed ratio, which is why converter lock-up clutches are mostly used in vehicle transmissions, which switch off the slip and thus greatly improve the efficiency.


Road vehicles

Fluid transmission is used in road vehicles, especially in the field of automatic transmissions and in the commercial vehicle sector.

Hydrostatic power transmissions

Hydrostatic power transmissions are mainly used for auxiliary drives with low power or as travel drives in construction machines (e.g. excavators or wheel loaders), industrial trucks and agricultural machines (e.g. combine harvesters). The pump and hydraulic motors can be arranged separately in a convenient place here. One disadvantage is the poor efficiency, which is why they are rarely used in modern tractors. Exceptions are power-split transmissions in which only part of the drive power is transmitted hydrostatically.

Hydrodynamic power transmissions

Hydrodynamic power transmissions are found in automatic transmissions, where they replace the clutch (fluid coupling) or the entire transmission (torque converter). There are also mixed designs between mechanical and hydrodynamic gear designs, such as B. the differential converter transmission (DIWA) from the manufacturer Voith, in which the engine power is transmitted partly mechanically and partly hydrodynamically at the same time, whereby the proportion of the two types of transmission varies with the speed (when starting up, a high proportion of hydrodynamic transmission, with increasing power the proportion increases mechanical power transmission).


In buses and other heavy commercial vehicles, a retarder is often installed as a virtually wear-free brake in order to reduce wear on the friction brakes and to increase safety by means of a second, independent brake system. A retarder is a special type of fluid coupling in which the turbine wheel is stationary. The entire kinetic energy of the pump wheel is converted into heat and transferred to the hydraulic medium, which has to be cooled. The regulation takes place by filling and emptying the pump room. The hydraulic medium in vehicles is usually oil. Recently, the vehicle's cooling water has also been used, for example at Aquatarder from Voith, which is also part of the PritarderBrake system from MAN.

Rail vehicles


In the case of powerful diesel locomotives, hydrodynamic power transmission is one of two possible power transmission methods in addition to diesel-electric power transmission, since purely mechanical (manual) transmissions are not available for the transmission of such high drive powers. Compared to the diesel-electric variant, the hydrodynamic power transmission offers the advantage of a weight saving that is bought at the price of a lower acceleration capacity. An example of a locomotive with hydraulic power transmission is the Deutsche Bahn class 218.


In the field of rail vehicle technology, the term hydrodynamic brake (also called H brake) is common for the retarder. The mode of operation corresponds to the retarder also used in road vehicles, but the components must be designed to be correspondingly more powerful in order to be able to apply the required braking forces; When designing the cooling systems of locomotives equipped with a hydrodynamic brake, the large amount of heat to be dissipated must be taken into account. Known vehicles with hydrodynamic brakes are z. B. the locomotives of the class 218 or the railcars of the class 612 of Deutsche Bahn.


In industry, fluid couplings are used as a starting aid, as a control coupling for stepless speed control and as a brake (retarder). Mineral oil or water are usually used as the operating medium.


Turbo couplings with constant filling are used to gently accelerate large masses such as crushers. Due to the slowly building up circulatory flow between the pump and turbine wheel as a result of the accelerating equipment after starting the drive machine, the same is gently accelerated to the nominal speed. However, there is always a slip between the drive machine and the driven machine. If an infinitely variable speed adjustment is required during operation, a turbo coupling with a variable filling quantity (turbo regulating coupling) is used. Here, the clutch filling can be changed, for example, using a scoop tube and thus the torque transmission or the speed can be controlled. For high-speed machines such as compressors or feedwater pumps, geared variable-speed couplings, turbo couplings with upstream or downstream gearboxes in the same housing are used. With geared variable speed clutches, stepless speed control on prime movers with an output of up to 80 MW is possible.


Industrial retarders with variable filling quantities are used, for example, as test bench brakes.

See also


  • Hermann Roloff, Wilhelm Matek: Machine elements. 19th edition. Vieweg + Teubner, Wiesbaden 2009, ISBN 978-3-8348-0689-5.