When the control valves for the cylinders are in the NEUTRAL position and an external force acts on one end of the cylinder, the oil pressure increases on the opposite end of the cylinder. The oil pressure also increases in the passage of the line relief valve that is connected to the cylinder. The line relief valve relieves the high pressure. The line relief valves limit the circuit pressure to the specified pressure settings.
ReferenceRefer to Testing and Adjusting, "Specifications" for the line relief valve pressure settings.
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| Illustration 1 | g03599502 |
Line relief valve (CLOSED position) (1) Passage (2) Valve (3) Valve (4) Spring chamber (5) Valve (6) Spring (7) Piston (8) Return passage (9) Passage | |
The high pressure between the cylinder and the control valve is transmitted to passage (1) . This pressurizes the line relief valve. The pressure oil flows from passage (1) through passage (9) of piston (7) . The oil then flows into spring chamber (4) . When the oil pressure is lower than the line relief valve pressure setting, valve (5) remains in the CLOSED position by the force of spring (6) . The oil pressure in passage (1) and the oil pressure in spring chamber (4) are equal. The surface area of the right side of valves (2) and (3) is larger than the surface area of the left side. The force on the right side of valves (2) and (3) is greater than the force on the left side. Valves (2) and (3) are forced to the left. The pressure oil does not flow from passage (1) to passage (8) .
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| Illustration 2 | g03599505 |
Line relief valve (OPEN position) (1) Passage (2) Valve (3) Valve (4) Spring chamber (5) Valve (6) Spring (7) Piston (8) Return passage (9) Passage (10) Valve chamber (11) Passage (12) Passage | |
When the high-pressure oil in passage (1) reaches the line relief valve pressure setting, valve (5) overcomes the force of spring (6) and opens. The high-pressure oil flows from valve chamber (10) , through passage (12) , and to return passage (8) . The pressure now becomes low pressure. The pressure in passage (1) pushes piston (7) to the right until the piston comes in contact with the left end of valve (5) . The oil in passage (1) flows around the end of piston (7) and the oil enters spring chamber (4) . Since the flow around the outside of piston (7) is restricted, the oil in spring chamber (4) becomes low-pressure oil. As a result, valve (3) is pushed to the right. Passage (11) opens. The oil flows from passage (1) to passage (8) .
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| Illustration 3 | g03599507 |
Line relief valve (makeup operation) (1) Passage (2) Valve (3) Valve (4) Spring chamber (8) Return passage (9) Passage (13) Shoulder | |
The line relief valve functions as a makeup valve in the following manner.
When oil from one end of the cylinder is discharged through the line relief valve, a vacuum condition is created on the opposite end of the cylinder. Makeup oil is needed to prevent the vacuum condition in the cylinder.
When the vacuum condition occurs on the end of the cylinder that is connected to passage (1) , a vacuum condition also occurs in spring chamber (4) . The pressure of the oil in passage (8) acts on shoulder (13) of valve (2) . Since a vacuum condition is present in spring chamber (4) , the pressure in spring chamber (4) is lower than the pressure of the return oil in passage (8) . Valves (2) and (3) are pushed to the right by the pressure of the return oil in passage (8) . Return oil flows from passage (8) to passage (1) as makeup oil in order to remove the vacuum condition in the cylinder.