Basic design for a hydraulic drive in which the fluid circulates from the pump to the actuator and from there straight back into the suction line of the pump. The main difference between this and an "open circuit" is that the hydraulic pump in a closed circuit is fed directly by the hydraulic fluid returning from the actuator (hydraulic motor). This effectively "clamps" the pump from both the suction side and pressure side. The leakage that occurs at the pump and the individual components in the form of escaping leakage oilis compensated for/supplemented by an additional low-pressure charge pump. As in such systems a very small volume of oil is circulated (e.g. approx. 8 l at 300 kW) the oil must be continuously renewed and cooled. For this purpose, the charge pump also operates as a flushing pump.
The closed circuit is particularly advantageous where the load pushes (e.g. vehicles) or pulls (e.g. cranes). In these phases, the hydraulic motor becomes a pump, which in turn is supported by the drive motor (e.g. combustion engine), making it possible to brake with a closed circuit. No valves are necessary in the power flow to control the motor.
Alternative: open circuit
Closed circuit