Maintenance of Kalmar DRU Front Lift Weakness

Fault Description
The customer reported that a DRU450 front hoist was slow in lifting and extending the boom, the lifting was weak, the heavy box could not be lifted, the rest of the actions were normal, and no fault codes appeared.
Troubleshooting
The maintenance engineer arrived at the scene, started the locomotive, and when the boom was lifted and extended, the engine throttle was turned to the maximum, and it could be seen directly that the boom was indeed slow.
Enter the diagnostic menu of the Kalmar locomotive control system, check the output signal when the aviation handle is in motion, and find no abnormality; check the working current of each solenoid valve when the boom is in motion, it is also normal, and the electrical fault point is ruled out.
It is initially suspected that the problem is in the hydraulic system, so measure the pressure of each part of the hydraulic system one by one, hoping to find the problem:
First, use a 100bar range hydraulic gauge to measure the servo pressure of the hydraulic system, 35bar, normal;
Then lift the boom and extend the boom to the limit position respectively, measure the maximum working pressure of the 4 hydraulic variable pumps, and the 4 hydraulic pumps are only about 120Bar, which shows that the maximum working pressure of the hydraulic pump is seriously insufficient.
Considering that the probability of four hydraulic variable pumps having problems at the same time is relatively low, it is suspected that a certain signal or action has affected the working conditions of the four hydraulic pumps at the same time. In order to further narrow the scope of the fault, continue to measure the hydraulic pressure of other actions:
Check the steering pressure, 210bar, normal;
Check the spreader oil supply pressure, 220bar, normal;
Check the retracting action pressure of the boom, 220bar, normal.
According to the working chart of the hydraulic pump when the locomotive is moving:
Combined with the on-site pressure measurement, the pressure of the four pumps is low only when the locomotive is lifting the boom and extending the boom, but the pressure is normal when the boom is retracting, the rear wheel is turning, and the spreader is moving. According to the above chart, among the 4 pumps, the right front, right rear, and left front pumps are normal, because only the left rear pump supplies oil for lifting and boom extension alone, and the scope of the fault is narrowed to the left rear pump. But how does the left rear pump cause the working pressure of other pumps to not go up when the boom is lifted or stretched out?
Looking further at the hydraulic schematic diagram, it is found that the load sensing signal LS of the 4 hydraulic variable pumps is sent from the 4 variable pumps to the frame hydraulic main valve, then from the main valve to the servo combined valve, and then from the servo combined valve to the 4 variable variable pumps , and the variable displacement pump has an air exhaust and oil return channel inside, so it is suspected that the LS load sensing oil pipe of the left rear pump is depressurized, causing the load sensing signal pressure of the four pumps to fail to rise when the boom is lifted or extended, and the pump Pressure cannot build up sufficiently.
Use a plug to block the LS load sensing oil pipe leading to the left rear pump, and test run. At this time, when the remaining 3 pumps are lifted or the boom is extended to the bottom, the maximum working pressure of the pump is 220bar, and it returns to normal.
Therefore, it is judged that the internal LS oil passage of the left rear variable oil pump is faulty. First dismantle the pressure regulating valve to check, remove the spare pressure adjusting bolt part of the pressure regulating valve, and find that an internal orifice has fallen off, so it is judged that the pressure is released here.
Resolution
Reinstall the dropped orifice, and take anti-loosening measures. The test run pressure is normal, and the locomotive returns to normal use.
Case Summary
From the repair process of this maintenance case, we concluded that:
When dealing with the failure of the locomotive load handling system, it is possible to distinguish whether it is an electrical failure or a hydraulic failure, so as to narrow down the scope of the failure, save inspection time, and reduce equipment downtime;
When checking hydraulic faults, be sure to prepare various hydraulic gauges and pressure measuring points, and choose a hydraulic gauge with a suitable range according to the standard pressure provided on the hydraulic nameplate of the locomotive;
The hydraulic system is an overall system, and it affects the whole body. If you want to quickly judge the fault point of the hydraulic system, you must understand the hydraulic principle of the locomotive in detail. If necessary, print out the hydraulic drawings in advance for on-site reference.

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