The unloading valve is installed at the upper and lower ends of the main control valve, and one piece at the upper and lower ends (opposite side of the main relief valve). The valve sets the maximum pressure of the entire hydraulic system when all operating levers are in the neutral position. At this time, the oil pumped out by the main pump returns to the oil tank through the unloading valve. While in normal operation, the valve is always closed (unloading valve set pressure 40kg/cm2).
1. When all operating handles are in the middle position (at this time, PLS pressure = 0kg/cm2)
①The pump pressure gradually rises to 40kg/cm2
②The spool overcomes the spring force and is pushed down
③The pressure oil flows back to the oil tank
④Main pump pressure drop
⑤When the pressure of the main pump drops below 40kg/cm2, the spring pushes the spool back to close the passage between the main pump oil and the oil tank
⑥ When the main pump pressure rises to more than 40kg/cm2 again, repeat ②③④
⑦The main pump pressure is stable at 40kg/cm2
2. When working (the PLS pressure is slightly lower than the main pump pressure at this time)
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 spring force
④The spool cannot be pushed downwards (that is, the unloading valve is always closed during normal operation)
⑤ The main pump pressure is maintained
Fault phenomenon: when the main pump overflows, the pressure is lower than the specified value (355kg/cm2)
Inspection result: O-ring damage
Fault analysis: PLS pressure flows directly back to the fuel tank through the damaged O-ring, and the spring force alone cannot withstand the main pump pressure on the spool, causing the spool to open downward in advance, and the pressure of the main pump to the fuel tank causes the pressure of the main pump to drop. Insufficient system pressure.
Troubleshooting: Replace with a new O-ring.
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