Created on:2021-11-15 09:32

Cause analysis and Countermeasures of oil leakage on the joint surface of gear pump

Cause analysis and Countermeasures of oil leakage on the joint surface of gear pump

1. Introduction

U pump is a series of axial compensation gear pumps developed by our factory for the hydraulic system of loader, including working pump and steering pump. The design mechanism of this series of pumps is axial tracking compensation of the side plate, and the "8" shaped side plate floats axially in the pump cavity. Due to overcoming the problem of low efficiency after wearing the fixed side plate, it quickly occupied the market after intervening in the main engine, and achieved good economic and social benefits. However, in the later stage of "Three Guarantees", the products were returned in large quantities due to the leakage of the joint surface between the pump body and the pump cover, accounting for about 40% of the returned pump, which not only caused a great burden to the manufacturer, but also brought great inconvenience to the host and customers. The phenomenon of oil leakage on the joint surface soon attracted our attention. We took part of the external return pump and ran on the test bench according to the JB / T 7041-2006 external leakage inspection test plan of gear pump, and found that there was leakage on the joint surface. After dismantling these retraction pumps, it is found that the sealing groove of the joint is not damaged, which is mainly manifested in the following phenomena; crack; Gnawing; No obvious abnormality.

2. Failure cause analysis

(1) Cause analysis. The phenomenon shows that the sealing ring is subjected to force in the groove. The seal between the joint surfaces belongs to static seal. When the two joint end faces are close, the pressure in the groove is generally very small and will not impact the sealing ring. Where does the greater force come from? We analyze; In the working state, the pressure in the gear pump cavity is relatively large, and the screws penetrating the pump body and pump cover are elastically deformed under the action of pressure, forming a gap on the joint surface. The pressure in the cavity enters the groove from the gap to act as the sealing ring. When the pressure in the cavity is relatively large, the elastic deformation of the screw increases, the clearance of the joint surface increases, and the groove pressure is subject to the pressure in the cavity. In the process of loading and unloading materials, the loader directly impacts the sealing ring and makes it invalid. As shown in figure I, it is the clearance between the pump body and the joint surface of the front and rear covers measured in the laboratory under 20MPa. The front rear cover warps and deforms under pressure, and the deformation is the maximum at points c and F, expanding outward by 0.08 and 0.09mm respectively. The clearance between the front cover, the rear cover and the joint of the pump body has been increased by 0.085mm respectively. The actual working condition of the pump on the loader is worse, and its pressure peak can sometimes reach 25MPa, so the gap between the two joint surfaces needs to be increased, and the seal ring is more likely to fail due to long-term non-stop impact. In a short time, even if the sealing ring has not broken or bitten, under such working conditions, the heat generated by friction becomes thinner, the compression becomes smaller, and the Joule effect occurs, resulting in leakage.


(2) Analysis of reducing groove pressure. In order to reduce the pressure and pressure impact in the groove, we opened the a groove as shown in Figure J at the expense of a little efficiency. This method connects the oil inlet and the sealing groove to keep the sealing ring at low pressure and get rid of the influence of high pressure in the cavity, but the effect is still not ideal. The reason: usually, the sealing ring is in an outward stretching state under the action of cavity pressure. The pump often produces negative pressure in the working process, and the atmospheric pressure squeezes the sealing ring inward through the gap of the joint surface. The negative pressure is unstable, resulting in the continuous action of the sealing ring with the groove and a groove, and the phenomenon of gnawing and breaking. The friction between the seal ring and the groove generates heat. Even if the seal ring is not broken, leakage will occur due to the reduction of compression due to Joule effect.


3. Countermeasures

In view of this situation, an improvement method is proposed to improve the environment of the sealing ring. Firstly, multiple reinforcing screws are arranged at the joint, as shown in Figure K, so as to minimize the elastic tensile deformation of the screws and the warpage of the front and rear covers, and minimize the gap between the joint surfaces. After arranging the reinforcing screws, measure points c and f again according to figure I, and the measured outward expansion is 0.03mm and 0.033mm. It can be seen that the gap of the joint has been fully reduced.

After analysis, the sealing ring in the groove is affected by both the inward pressure under negative pressure and the outward pressure in the cavity, that is, the alternating pressure. The improvement method is to protect the sealing ring and add a double retaining ring in the groove. This not only reduces the active area of the sealing ring in the groove, but also forms protection for the sealing ring (see Figure L).

4. Conclusion

The improved gear pump simulates the extreme working conditions on the loader as much as possible on the test bench: the rotating speed is 2500r / min, the pressure is 25MPa, the test temperature is 80 ~ 90 ℃, and runs in accordance with the JB / T 7041-2006 external leakage inspection test plan of gear pump. There is no leakage in the whole process, so it can be safely delivered. After leaving the factory, there were almost no pumps returned due to joint surface leakage during the "Three Guarantees" period, which successfully solved the problem.

This successful improvement reduces the failure rate of gear pump and has strong guiding significance for the design and maintenance of similar products in the future.

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