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| Illustration 1 | g01009888 |
Schematic of Check and Relief Valves (1) Relief valve (2) Line to implement control valve (3) Line to implement control valve (4) Relief valve (5) Check valve (6) Check valve (7) Return to tank passage | |
Combined relief and check valves are installed in the implement control valves for the auxiliary circuits and the coupler tilt. The valves serve a dual purpose. The valves limit the maximum oil pressure in the circuit. The valves also provide makeup oil in order to prevent cavitation.
Note: Check valves that provide makeup oil are also called makeup valves or anticavitation valves.
The valves will open if the pressure in lines (2) or (3) rises above the preset pressure. The relieved pressure will then flow through the open relief valve into tank passage (7) .
The check valves act as anticavitation valves. When the pressure in the supply side of the system falls below the pressure in the hydraulic tank, the check valve in that line opens. Oil at tank pressure then flows from tank passage (7) into the passage in order to prevent cavitation.
For example, the operator moves the control lever in the cab to the TILT FORWARD position. When the forks are moving at a speed that is greater than the oil pressure in line (2) then line (2) will cavitate. The pressure of the oil in line (2) becomes less than the pressure of the oil in return to tank passage (7). Since the pressure of the oil in line (7) is greater than the pressure of the oil in line (2), check valve (5) opens. When the check valve opens, oil in the return to tank line flows into line (2). This oil prevents line (2) from cavitating.
Reference: See Systems Operation/Testing and Adjusting, "Compensating System" for more information about the check and relief valves.