Created on：2021-11-30 15:55

# Truck crane_ Cause analysis and solving measures of broken shaft of gear pump

Truck crane_ Cause analysis and solving measures of broken shaft of gear pump

1. Cause analysis of broken shaft of gear pump

(1) Judgment on the cause of shaft disconnection of the third unit pump. After replacing the standby pump, the test run shows that the outrigger system where the third pump is located works normally, but the swing system does not act, indicating that the sudden shaft break of the pump occurs in the swing hydraulic system. When 13 was opened, it was found that its seal had been damaged, its filter element was sintered metal, and there were signs of blockage, indicating that oil pollution had blocked the oil filter.

Under normal working conditions, the oil output from the third combined pump first passes through pilot sequence valve 14 and then through multi-way valve 15 to swing hydraulic motor 16. If the pilot sequence valve is opened normally, even if the swing hydraulic motor 16 has a large load, the third combined pump 3 will be protected due to the safety valve at the inlet of multi-way valve 15.

Therefore, pilot sequence valve 14 is not opened or opened too slowly, resulting in "open circuit" at the outlet of the third combined pump. It can be seen from figure f that there is no safety valve between the outlet of the third unit pump and the pilot sequence valve 14, so the pilot sequence valve 14 is not opened or not opened in time, so that the oil circuit between the pump outlet and the pilot sequence valve 14 becomes a closed cavity, and the pump outlet pressure rises sharply, resulting in a large load on the third unit pump, resulting in the fracture of the pump shaft.

(2) Pump shaft stress calculation. The stress of gear pump shaft is calculated and analyzed by using material mechanics and gear pump stress formula. Through calculation, as long as the pressure difference between the inlet and outlet of the pump is greater than 20.55mpa, the pump shaft may break.

(3) The reason why the pilot sequence valve cannot be opened. Figure G is the diagram of rotary hydraulic system (see Figure f for serial number marking in the figure). Pilot sequence valve 14 is of internal control and external control type. The pump outlet pressure oil enters the lower chamber of the main valve core through oil filter 13, and then enters the upper chamber of the main valve core, the front chamber of the pilot cone valve and the right side of the ejector rod through the damping hole of the main valve. The pressure oil of accumulator 11 enters the left side of the ejector rod through the damping hole of the pilot valve, and opens the pilot cone valve under the combined action of ejector rod thrust and hydraulic pump outlet oil pressure, After the pilot cone valve is opened, the main valve core is opened and the pilot sequence valve is opened under the action of the pressure at both ends of the damping hole of the main valve.

Because the oil filter 13 is blocked or the pressure of the oil filter 11 is too low, the pilot sequence valve 14 cannot be opened, and the outlet pressure of the third combined pump will increase sharply in a short time, rapidly exceeding 20.55mpa, which is the root cause of shaft breakage.

2. Solutions

(1) Oil pollution monitoring and control. The broken shaft fault of hydraulic pump in the slewing system of truck crane is caused by the blockage of oil filter caused by oil pollution, which makes the pilot sequence valve unable to open normally. The truck crane has been in continuous use for 10 years and has never completely replaced the hydraulic oil or cleaned the oil filter. The hydraulic oil has been seriously aged and a large number of pollutants exist in the system circuit.

Remove the pilot sequence valve and pipeline oil filter, carefully clean the damping hole and filter element, connect the pipeline, and restart the crane. All mechanisms of the crane work normally. At present, the crane has been in continuous use for several months, and there is no failure of hydraulic pump shaft breaking and rotation failure.

Selecting an oil filter with bypass oil circuit or an oil filter with transmitter, regularly cleaning the oil and replacing the filter element in time are effective measures to reduce the failure rate of equipment.

(2) A safety valve is added at the outlet of the hydraulic pump.

In Figure F, the overflow valve in the swing hydraulic motor circuit can protect the hydraulic pump and swing hydraulic motor from overload only when the hydraulic pump is connected with the multi-channel valve and swing hydraulic motor oil circuit, and the overflow valve 6 in the outrigger control valve 4 can protect the hydraulic pump and its oil circuit only when the hydraulic pump is connected with the outrigger oil circuit, The system is not provided with a safety valve at the outlet of the hydraulic pump.

The pump outlet will form a closed dead volume cavity, which will lead to the sharp increase of pump outlet pressure and shaft breaking accident. If an overflow valve is set at the outlet of the third pump to protect the pump, even if the circuit fails, the hydraulic pump can be protected through the overflow valve to prevent the hydraulic pump from breaking the shaft due to high pressure.

In this system, it is only necessary to change the overflow valve 6 in the outrigger control valve 4 to the front of the positioning control valve 5, so that the overflow valve can truly become the safety valve of the third pump, as shown in Fig. H

3. Conclusion

(1) The pollution of the oil led to the blockage of the oil filter or poor oil supply, resulting in the sequence valve not being opened or not being opened in time. Selecting oil filter with bypass oil circuit or oil filter with transmitter, regularly cleaning oil, monitoring and controlling oil pollution status are effective measures to reduce equipment failure rate.

(2) The outlet of the hydraulic pump is not provided with a safety valve, the sequence valve is not opened or not opened in time, resulting in a "open circuit" at the outlet of the hydraulic pump, resulting in a sudden increase in the outlet pressure of the hydraulic pump, which is the direct cause of the broken shaft of the gear pump and the rotation failure of the slewing mechanism in the slewing system of the crane.

(3) An overflow valve must be set at the outlet of the hydraulic pump to limit the maximum pressure at the outlet of the hydraulic pump and form direct safety protection for the hydraulic pump.

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