Pump controlled systems (primary control)

Hydrostatic transmissions connect the actuator directly to the supply pump with­out using any valves for restrictive metering, the control of velocity being made by the displacement of the pump, and, in the case of motors, additionally by the motor displacement. The flow from the actuator is returned to the pump inlet thus avoiding the need for a large capacity boost pump.

The pressure level rises to that required to drive the actuator against the load. Consequently, the pump output flow can only be used to drive a single actuator or multiple actuators that are constrained to move at the same velocity (e.g. coupled motors and actuators attached rigidly to the same moving component)
Figure 1 shows the circuit for a hydrostatic transmission used to drive a hydraulic motor that includes:

• The provision of boost flow to make up for the external losses from the pump and motor. The check valves connect the boost input to the low pressure (unloaded) side of the loop. This applies for both the pump driving the motor and, for overrunning conditions, when the motor is driving the pump (e.g. winch lowering).
• Cross line relief valves to prevent excessive pressures. The flow is passed to the low pressure side in order to maintain the flow into the pump inlet.
• The extraction of fluid from the loop using a purge valve to provide increased cooling. This flow needs to be controlled as it has to be made up from the boost flow.
• Variable motor displacement control for systems requiring a higher speed range at reduced torque.

Motor brake circuit

The operation of spring loaded brakes is incorporated into the hydraulic system (e.g. winches, swing drives) by directing system pressure to the brake actuator as shown in Figure 2.

The pressure required to release the brake actuators needs to be higher than the boost pressure. Often it is necessary to fit a reducing valve to limit the maxi­mum pressure at the actuators if they have a lower rated pressure than the system.

Linear actuator transmissions

For linear actuator systems using equal area actuators the circuit is similar in principal to that for rotary systems as can be seen from Figure 3 which shows a basic circuit.

Motor controlled systems (secondary control)
Secondary control systems operate at a constant supply pressure that can be pro­vided by a pressure compensated pump the motor displacement being controlled so as to maintain constant speed in a closed loop system as shown in Figure 4. The use of secondary control provides some advantages over the conven­tional hydrostatic system which include:

• The storage of energy in the accumulator from regenerative (e.g.
  overrunning) loads.
• Accuracy and dynamic performance.
• Use with multiple motors (ring main systems).