The line relief valves are located between each cylinder and the respective control valve. The line relief valves limit the amount of pressure in the circuit of the hydraulic cylinders. Excessive pressure can damage components.
When the control valve for a cylinder is 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 within the circuit of the cylinder line increases. The oil pressure that is created in the circuit of the cylinder line is blocked by the control valve. The line relief valve is used to allow the excessive pressure in the circuit of the cylinder line to be vented to the hydraulic tank. The line relief valves limit the circuit pressure to the specified pressure settings.
Reference: Refer to Testing and Adjusting, "Specifications" for the line relief valve pressure settings.
Each line relief valve contains a makeup valve. As a line relief valve relieves high pressure on one side of the cylinder, the opposite side of the cylinder will create a vacuum condition. When a vacuum condition is created, the makeup function will allow return oil to enter the circuit. The vacuum condition will be eliminated.
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| Illustration 1 | g02140504 |
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Line relief valve (CLOSED position) (1) Passage (2) Piston (3) Valve (4) Valve (5) Spring chamber (6) Valve (7) Spring (8) Return passage (9) Passage (10) Spring (11) Spring | |
The high pressure between the cylinder and the control valve is transmitted to passage (1). This oil pressurizes the line relief valve. The pressure oil flows from passage (1) through passage (9) of piston (2). The oil then flows into spring chamber (5). When the oil pressure is lower than the line relief valve pressure setting, valve (6) remains in the CLOSED position by the force of spring (7). The oil pressure in passage (1) and the oil pressure in spring chamber (5) are equal. Spring (10) and spring (11) force piston (2), valve (3), and valve (4) to the left. The high-pressure oil is not able to flow from passage (1) to return passage (8). The high pressure between the cylinder and the control valve is maintained.
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| Illustration 2 | g02140505 |
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Line relief valve (OPEN position) (1) Passage (2) Piston (4) Valve (5) Spring chamber (6) Valve (7) Spring (8) Return passage (9) Passage (10) Spring (11) Spring (12) Spring chamber (13) Passage (14) Passage | |
As pressure spikes occur in the line between the cylinder and the control valve, the oil pressure is momentarily higher in passage (1) than spring chamber (5). Piston (2) and valve (4) are forced to the right against spring (10) and spring (11). high-pressure oil is able to flow from passage (1) to passage (14), and to return passage (8) which relieves the pressure spike.
high-pressure oil flows through passage (9) into spring chamber (5). Because the high-pressure oil is equal on the left side and the right side of piston (2) and valve (4), spring (10) and spring (11) force piston (2) and valve (4) to the left which closes the flow of oil from passage (1) to return passage (8).
When the high-pressure oil in passage (1) and spring chamber (5) reaches the line relief valve pressure setting, valve (6) overcomes the force of spring (7) and shifts to the right. When valve (6) shifts to the right, high-pressure oil flows from spring chamber (5) to spring chamber (12), to passage (13), and then to return passage (8). The high-pressure oil in passage (1) becomes low pressure. The low-pressure oil is not high enough to overcome the force of spring (7). Valve (6) shifts to the left which stops the flow of oil from spring chamber (5) to spring chamber (12).
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| Illustration 3 | g02140506 |
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Line relief valve (makeup operation) (1) Passage (3) Valve (4) Valve (5) 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 (5). The pressure of the oil in passage (8) acts on shoulder (13) of valve (3). Since a vacuum condition is present in spring chamber (5), the pressure in spring chamber (5) is lower than the pressure of the return oil in passage (8). Valves (3) and (4) are pushed to the right by the pressure of the return oil in passage (8). Return oil flows from passage (8) to passage (1) in order to remove the vacuum condition in the cylinder.