The unloading valve is installed on the upper and lower ends of the main control valve, one on each side (the opposite side of the main relief valve). This valve sets the maximum pressure of the entire hydraulic system when all levers are in the neutral position. At this time, the oil pumped by the main pump returns to the oil tank through the unloading valve. In normal operation, the valve is always closed (the set pressure of the unloading valve is 40kg/cm2).
1. When all the operating handles are in the neutral position (at this time, the PLS pressure = 0kg/cm2)
①The pump pressure gradually increased to 40kg/cm2
②The valve core (1) is pushed down against the force of the spring (2)
③Pressure oil flows back to the fuel tank
④Main pump pressure drops
⑤When the main pump pressure drops below 40kg/cm2, the spring (2) will
The core (1) is pushed back to close the passage between the main pump oil and the oil tank
⑥When the main pump pressure rises again and exceeds 40kg/cm2, repeat ②③④
⑦Main pump pressure is stable at 40kg/cm2
2. When working (the PLS pressure is slightly lower than the main pump pressure)
①Main pump pressure rises
②PLS pressure also rises synchronously
③ The downward thrust generated by the main pump pressure is less than the sum of the upward thrust generated by the PLS pressure + the force of the spring (2)
④The valve core (1) cannot be pushed down (that is, the unloading valve is always closed during normal operation)
⑤Main pump pressure can be maintained
Fault phenomenon: the pressure is lower than the specified value (355kg/cm2) when the main pump overflows
Inspection result: O-ring (3) is damaged
Failure analysis: The PLS pressure directly flows back to the fuel tank through the damaged O-ring, and the main pump pressure on the valve core (1) cannot be supported by the spring force alone, causing the valve core (1) to open downward in advance, and the main pump pressure through the fuel tank Cause the main pump pressure to drop, the system pressure is not enough.
Troubleshooting: Replace with a new O-ring.
To learn more, please refer to: Hand Brake Valves Manufacturers