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Open loop and closed loop circuits.

Open-loop: Pump-inlet and motor-return (via the directional valve) are connected to the

hydraulic tank.The term loop applies to feedback; the more correct term is open versus closed

"circuit".

Closed-loop: Motor-return is connected directly to the pump-inlet. To keep up pressure on

the low pressure side, the circuits have a charge pump (a small gearpump) that supplies

cooled and filtered oil to the low pressure side. Closed-loop circuits are generally used forhydrostatic transmissions in mobile applications.  Advantages: No directional valve and better

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response, the circuit can work with higher pressure. The pump swivel angle covers both

 positive and negative flow direction. Disadvantages: The pump cannot be utilized for any

other hydraulic function in an easy way and cooling can be a problem due to limited exchange

of oil flow. High power closed loop systems generally must have a 'flush-valve' assembled in

the circuit in order to exchange much more flow than the basic leakage flow from the pump

and the motor, for increased cooling and filtering. The flush valve is normally integrated inthe motor housing to get a cooling effect for the oil that is rotating in the motorhousing itself.

The losses in the motor housing from rotating effects and losses in the ballbearings can be

considerable as motorspeeds will reach 4000-5000 rev/min or even more at max vehicle

speed. The leakage flow as well as the extra flush flow must be supplied by the charge pump.

Large charge pumps is thus very important if the transmission is designed for high pressures

and high motor speeds. High oil temperatures, is usually a major problem when using

hydrostatic transmissions at high vehicle speeds for longer periods, for instance when

transporting the machine from one work place to the other. High oiltemperatures for long

 periods will drastically reduce the life time for the transmission. To keep down the oil

temperature, the system pressure during transport must be lowered, meaning that the

minimum displacement for the motor must be limited to a reasonable value. Circuit pressuresduring transport around 200-250 bar is recommended.

Closed loop systems in mobile equipment are generally used for the transmission as an

alternative to mechanical and hydrodynamic (converter) transmissions. The advantage is a

stepless gear ratio ('hydrostatic' gear ratio) and a more flexible control of the gear ratio

depending on the load and operating conditions. The hydrostatic transmission is generally

limited to around 200 kW max. power as the total cost gets too high at higher power

compared to a hydrodynamic transmission. Large wheel loaders for instance and heavy

machines are therefore usually equipped with converter transmissions. Recent technical

achievements for the converter transmissions have improved the efficiency and developments

in the software have also improved the characteristics, for example selectable gear shifting

 programs during operation and more gear steps, giving them characteristics close to the

hydrostatic transmission.

Hydrostatic transmissions for earth moving machines, such as for tractor loaders, are often

equipped with a separate 'Inch pedal' that is used to temporarily increase the diesel engine rpm

while reducing the vehicle speed in order to increase the available hydraulic power output for

the working hydraulics at low speeds and increase the tractive effort. The function is similar

to stalling a converter gearbox at high engine rpm. The Inch-function affects the preset

characteristics for the 'hydrostatic' gear ratio versus diesel engine rpm.