Created on:2021-07-08 14:14

Hydraulic fault diagnosis and maintenance of hydraulic press (1)

Hydraulic fault diagnosis and maintenance of hydraulic press (1)

(1) Fault overview

A 400t hydraulic press, due to insufficient pressure, can not meet the work requirements and repair. The maintenance personnel first repaired the related relief valve, and then replaced the new plunger pump, relief valve, electro-hydraulic directional valve, and so on. However, the equipment could not work normally, and finally even the hydraulic cylinder did not move up and down. There are two hydraulic system diagrams in the operation manual of the equipment. Individual configurations are slightly different, and the drawings are different from the actual configuration of the equipment. Therefore, according to the actual configuration at that time, the hydraulic system diagram (Figure T) was drawn for fault analysis.


Start the pumps 6, 4 and 7 successively as required, operate the "return", "empty" and "pressurization" buttons respectively, and the hydraulic cylinder does not move. Then check the electromagnetic directional valves 1, 3 and 2 of the electro-hydraulic directional valve. The solenoid valve can act, but the directional slide valve can not act. It is often in the middle position. As a result, the main pump enters the oil return tank of the directional valve, the system fails to establish pressure, and the hydraulic cylinder does not act.

(2) Improvement and troubleshooting

In order to realize the normal action of the directional valve slide valve, there must be a source of control pressure oil. According to the system analysis, the control pressure oil comes from the bypass output of pump 6. When the pump 6 is turned on and the main pump 7 is not turned on, the output oil of the pump 6 drives the pump 7 to rotate, and the output oil pressure of the pump 6 can be adjusted to about 1.5MPa at will. When the pump 7 is turned on, the output oil of the pump 6 is impulsive, and the load of the pump 7 is reduced. At this time, the oil pressure is about 0.5 ~ 0.7MPa, and the load determines the pressure. Even if the relief valve is adjusted, it is useless. The control oil pressure of the Electro-hydraulic Directional Valve must be about 1.5MPa. Therefore, the system is reformed (Fig. U) and an independent control oil source is established. The control oil is supplied by pump 8, and the pressure is set to 1.5MPa, then the electro-hydraulic directional valve works normally. But the hydraulic cylinder still doesn't work after starting. In order to check the pump 7 and the main relief valve, the pipeline between the pump 7 outlet and the directional valve is cut off, and a pressure gauge is installed. Other factors were excluded and examined separately. Start pump 6 and pump 7, gradually pressurize the relief valve to 5MPa, after 3S, the pressure suddenly drops to 0, and then adjust the relief valve without any response.


If the machine stops and starts again, repeat this phenomenon again. It is concluded that the replacement of the new pump is also a problem (there is a back plate phenomenon). The old pump replaced before was disassembled and repaired, installed and started up. Pump 6 and pump 7 were started up, and the pressure of main relief valve was adjusted freely. Put the shut-off valve at the oil outlet of pump 7 in the open state, start the equipment, and everything works normally.

(3) Summary

During the whole process of diagnosis and repair, people should pay attention to the following problems.

① The inconsistency between the equipment manual and the actual structure configuration of the equipment will bring a lot of inconvenience to the operator and maintenance personnel.

② It is not convenient for the original hydraulic pressure system to check the main pump 7 and the main relief valve. Only when the whole system is pressurized under normal working conditions can the system pressure be reflected on the pressure gauge. If the system does not work properly, especially if there is a problem with the main pump 7 itself, it is difficult to self check. Once the new pump is replaced, there is also a problem, which is even more troublesome for the maintenance personnel.

③ The quality and delivery inspection of hydraulic components are worrying. When the auxiliary control pump is installed, the pressure of the newly installed gear pump (CB20) can only be adjusted to 1MPa, and then it can be adjusted to a higher level, with oil injection and leakage everywhere. Change the same pump again, and the situation is obviously better. The newly installed P port and O port of the electro-hydraulic directional valve are reversely installed without marks, which is puzzling. The quality of the new plunger pump is too poor to maintain the pressure. Maintenance personnel determine equipment faults by comparing new components and parts. Once these new components and parts have problems, the diagnosis process will be more complicated.

Two hydraulic control check valves were removed in the process of equipment repair and transformation. In the lower chamber (return) system of the hydraulic cylinder, the one-way throttle valve is still set according to the original drawing (in actual configuration, there is only throttle valve), which improves the downward stability of the plunger body of the hydraulic cylinder. Since the pump 7 is installed on the ground at one side of the oil tank, if the oil (with stop valve) is led out in the middle of the oil tank and enters the inlet of the pump 7 through the oil filter, the hydraulic pump 6 can be avoided.

In the equipment system, the upper chamber circuit is equipped with a solenoid valve 3. When the upper chamber is unloaded, the lower chamber is filled with oil again. When the hydraulic cylinder changes direction, it is stable and basically without impact (most of the same type hydraulic presses do not have this device).

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