Leakage analysis of automobile water pump circulation pipe

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Automotive industry production requires high efficiency and low cost to meet the needs of market development. With the continuous emergence and development of new materials, polyphenylene sulfide is widely used in engineering plastics with excellent comprehensive performance due to its good forming quality, high production efficiency and stable and reliable mechanical properties. On the parts of the car engine cooling water pump.
Fig. 1 is a structural view of an automobile water pump, and the material of the circulation pipe is polyphenylene sulfide. The pump frequently has a quality problem of intermittent leakage at the junction of the circulation pipe and the casing on the vehicle. In order to grasp the cause of the leakage of the water pump, the failed parts were inspected and tested.

figure 1
1. Pulley 2. Water seal 3. Shaft bearing 4. Impeller 5. Housing 6. Circulating tube

Gas tight test

According to QC/T288.1-2001 "Technical Conditions for Automotive Engine Cooling Water Pumps", the gas-tightness test of the failed water pump is carried out. The technical parameters required for the test are shown in Table 1. The results show that the leakage of the pump is 2.5-6 ml/min, which is within the acceptable range.
Table 1 Pump airtight test parameters

Bench test

Two of the pumps were sent to the laboratory for bench test. The test parameters are shown in Table 2. After 24 hours of continuous operation, no signs of leakage of the circulation tube were found.
Table 2 Test parameters

Dimensional inspection

Remove the water pump from the bench test and check the fit size at room temperature. The relevant dimensional data records are shown in Table 3.
Table 3 Circulation tube and volute fit size
One of the circulating tubes has a size of φ30.045 mm, which is 0.003 mm smaller than the theoretical size. However, the calculated interference is 0.034 mm, and the interference required to meet the design requirements is 0.026 to 0.061 mm. Therefore, this cannot be used as a cause of pump leakage failure.

Low temperature test

The pump was placed in a laboratory freezer for low temperature testing. After standing at -25 ° C for 60 min, the appearance of the product was observed, and it was found that part of the circulation tube was detached from the casing of the water pump.
The original design between the circulation tube and the housing is an interference fit to seal. However, in the low temperature environment, the dimensional shrinkage of the two parts is inconsistent, and the shrinkage of the size of the circulation tube is significantly larger than the shrinkage of the size of the housing. The reason is that there is an excessive difference in the linear expansion coefficients of the two materials, that is, the circulation tube of the polyphenylene sulfide material has a larger linear expansion coefficient than the aluminum alloy material, thereby causing the low temperature drop of the circulation tube. See Table 4 for the coefficient of linear expansion of the material obtained in the material manual.
Therefore, the low temperature environment from normal temperature to -25 ° C can be theoretically calculated, and the amount of interference between the circulation tube and the casing is reduced:
ΔL=30×(50-23)×10-6×[20-(-25)]≈0.036mm
It can be seen that the reduction of the interference at low temperature is much larger than the original design of the interference of 0.026 ~ 0.061mm. Therefore, it is inevitable that the circulation tube will fall off at a low temperature.
Experimental analysis and theoretical calculations show that the reason for the intermittent leakage of the circulation pipe is that the linear expansion coefficient of the circulation pipe is larger than that of the casing. When the pump is operated in a low temperature environment, the shrinkage of the circulation pipe is larger than the shrinkage of the casing size, so that the interference of the fit is insufficient or disappears; when the pump is operated under high temperature, the expansion of the circulation pipe is larger than the expansion of the casing. The combined interference is increased. This is also the failure of the pump low temperature test, and the bench heating test failed to find the root cause of the leak.

solution

An O-ring seal is placed on the outer diameter of the original circulation tube without changing the material configuration of the pump (see Figure 2). Because the compression of the O-ring seal is relatively large, it is less affected by temperature changes, so it can play a good sealing role. The solution is simple and easy to operate, low in cost and reliable in sealing. After the improved water pump passed the low temperature test, the high temperature test and the reliability test, no abnormal phenomena such as leakage were found, and no quality problems of the circulation pipe leakage occurred after being put on the market.

figure 2

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