SYSTEM / Hydraulic System MAIN CIRCUIT Outline Main pumps (1, 2) and fan pump draw hydraulic oil from the hydraulic oil tank. Main pump 1 and main pump 2 deliver pressure oil to the 4-spool control valve and the 5-spool control valve re- spectively. The fan pump delivers hydraulic oil to the fan motor and derives the fan motor. Delivered pressure oil from the main pump is routed to the motor or cylinder in response to op- eration of the spool in the control valve. Return oil from the motor and/or cylinder flows back to the hydraulic oil tank via the control valve, low-pressure relief valve and oil cooler.
The low-pressure relief valve is provided in the return circuit (between control valve and oil cooler) of main circuit, and keeps a constant pressure in the main circuit. Therefore, actuator operation is improved when cavitation occurs. When oil temperature is low (high viscosity), oil flow resistance increases in the oil cooler, which opens the bypass check valve, so that hydraulic oil returns directly to the hydraulic oil tank. T2-4-14