Created on:2021-07-05 11:09

Hydraulic fault diagnosis and maintenance of die casting machine

Hydraulic fault diagnosis and maintenance of die casting machine

(1) Fault overview

The IP-750 die casting machine is a 750t die casting machine made in Italy. The hydraulic pump used is the double vane pump of Vickers company in the United States. The working medium is water glycol, and its allowable viscosity is 35 × About 10-6m2 / S (at 38 ℃).

The die casting machine stops automatically when it works at night. There are many reasons for the automatic shutdown of IP-750 die casting machine, such as insufficient oil level, vacuum suction of hydraulic pump and overload of motor. Ultra high pressure, etc. At that time, the operator on duty forced to eliminate the equipment alarm (this is allowed by the equipment). Restart the hydraulic pump, only two or three seconds, the equipment stopped. At this time, check the oil level, and find that the oil level is low, but it is not enough to generate alarm shutdown. Considering that the start-up of the hydraulic pump and the charging of the accumulator in the hydraulic system will further reduce the liquid level, the oil tank is filled with about 200L oil, and then the hydraulic pump is started. The equipment still stops after two or three seconds, indicating that the fault has not been eliminated. When checking the pressure relay at the oil suction port of the hydraulic pump, it was found that it acted immediately after the hydraulic pump was started, and the pressure relay was a negative pressure relay. Therefore, it was preliminarily determined that the negative pressure relay acted because the oil suction resistance of the hydraulic pump was too large, resulting in the shutdown.

(2) Fault analysis

The causes of excessive oil suction resistance of hydraulic pump are as follows:

① The problems of the oil filter include the insufficient flow capacity of the oil filter design, the insufficient flow capacity caused by the blockage of the oil filter and the incomplete opening of the oil inlet switch of the oil filter;

② The problems of oil suction pipe include too small design of oil suction pipe and deformation of oil suction pipe (hose) in the working process of hydraulic system;

③ The oil viscosity is too high;

④ Hydraulic pump installation position problems, including the hydraulic pump installation position from the tank liquid level is too high or liquid level drop too large caused by the relative position becomes larger.

IP-750 die casting machine is a mature product, and this machine has been used for several years. The possibility of failure caused by improper design can be eliminated, so what remains is the reason of use. Including oil filter blockage, oil viscosity is too large. The oil suction pipe used in this machine is a hard pipe. The deformation of the oil suction pipe can be eliminated. At the same time, the oil suction port of the hydraulic pump is below the liquid level, so the liquid level problem can also be eliminated. When the oil filter was removed, it was found that the oil filter was seriously blocked. After cleaning, the hydraulic pump was installed and started again. It was found that the negative pressure relay acted after four seconds, and the equipment stopped. So remove the negative pressure relay, forced start (this is illegal operation), hydraulic pump continuous operation, but the noise is very big, forced to stop.

From the analysis of the phenomenon, it is obvious that the cavitation phenomenon is caused by the excessive suction resistance of the hydraulic pump. As the oil filter has been cleaned, it is likely to be the problem of oil viscosity. There is no viscometer on site, so it is difficult to accurately determine the oil viscosity at the moment. The viscosity of crude oil is 50 × 10-6mm2 / S (at 40 ℃), which has exceeded the allowable range of die casting machine. At the same time, due to the lower temperature, the viscosity further increases, so it is more difficult for the hydraulic pump to absorb oil. The reasons for low oil temperature are as follows:

① The environmental temperature is low (about - 3 ℃);

② The shutdown time is long. It takes about two hours from cleaning the oil filter to re installing it. At this time, the cooling system of the oil tank is closed and the temperature of the cooling water is very low;

③ Add 200L new oil, the temperature is the ambient temperature.

In view of the above situation, considering that the working fluid of the IP-750 die casting machine is water glycol, add 70L distilled water to the oil tank to reduce its viscosity (water glycol liquid will evaporate after a long time of use, and the viscosity will rise. In order to reduce its viscosity, directly add distilled water), heat the oil, and then start the hydraulic pump. The hydraulic pump works continuously, but the pressure is low, It is about 3Mpa. In normal operation, the high pressure is 14MPa and the low pressure is 5MPa. It is estimated that water and water glycol are not fully mixed. Low pressure circulation to mix, but an hour later, the pressure still did not rise, shutdown inspection. Analyze the oil source of IP-750 die casting machine, as shown in Figure Z.

At this time, the pressure gauge 8 of the oil source reflects 3Mpa. After the change-over valve 10 acts, the oil source pressure is still 3Mpa, which is lower than the adjustment pressure (5MPa) of the relief valve 5 of the low-pressure pump, that is, the low-pressure relief valve 11 of the high-pressure pump is adjusted to 5.5mpa. If there is a problem with the relief valve, it should be the high pressure relief valve 9. If the relief valve 9 fails, the oil from both the low pressure pump 3 and the high pressure pump 4 can be removed from 9, but the high pressure is not reflected in Table 8. Therefore, check relief valve 9 and find that everything is normal. After installation, the pressure on table 8 is still not up when the machine is started. According to the oil source diagram, if the high-pressure pump 4 is damaged, the above fault may also be caused. At this time, the oil from low-pressure pump 3 can return to the oil tank through valve 7 and pump 4, so that the oil source pressure is lower than the low-pressure pump pressure adjusted by relief valve 5. Check the high-pressure pump. After disassembling the pump, it is found that the spline shaft connecting the high-pressure pump and the low-pressure pump is broken, that is to say, only the low-pressure pump is working. When the high-pressure pump was further disassembled, it can be seen that the two locating pins connecting the stator and the pump body were broken, and a piece between the two adjacent blades of the rotor was broken and stuck in the stator coil together with the two blades. At this time, the two blades and the rotor should have formed high and low pressure closed areas respectively, but due to the fracture of the rotor, the high and low pressure areas of the high pressure vane pump 4 appear oil string, and the oil from the low pressure pump 3 can flow back to the oil tank through the valve 7 and the pump 4, so that the pressure reflected in Table 8 is lower than the set pressure of the relief valve 5. After replacing the high-pressure pump and spline shaft, the system pressure returned to normal and the equipment worked normally.

(3) Summary

Through the analysis of the above reasons, it can be seen that the IP-750 die casting machine started to stop because of the blockage of the oil filter, and the negative pressure increased when the hydraulic pump was sucking oil, which caused the negative pressure relay 2 to stop. Later, due to the long shutdown time, low ambient temperature and high viscosity of the original water glycol, the hydraulic pump had greater oil suction resistance, resulting in shutdown. The removal of negative pressure relay and frequent start-up of hydraulic pump in case of too high negative pressure lead to cavitation on the suction side of hydraulic pump, which leads to the fracture of high pressure pump. After the oil pipe is filled with water, the hydraulic pump starts normally. After reaching a certain speed, the broken rotor block and two adjacent blades are thrown out together due to the effect of core separation force, and they are suddenly stuck in the stator coil. The rotor has a great impact on the stator coil, and the stator coil locating pin is broken. In this way, the stator coil is driven to rotate by the spline shaft. When the stator coil is suddenly stuck in the pump body, the spline shaft is also broken.

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