Analysis of the Causes of Early Piston Rod External Leakage in Hydraulic Cylinders and Improvement Measures

Hydraulic cylinders are important actuating components of construction machinery, and the stability of their quality directly affects the working efficiency and reliability of construction machinery. Through the statistical analysis of the failure rate during the three-guarantee period of loaders, the problem of hydraulic cylinders ranks fourth, among which the external leakage of the piston rod of the hydraulic cylinder is the most prominent quality issue. Although various hydraulic cylinder manufacturers have taken multiple measures to make improvements, the results have not been very significant. By identifying the main causes of early piston rod external leakage and proposing corresponding improvement measures.

Analysis of the causes of early external leakage of the piston rod in Hydraulic cylinders

The traditional opinion holds that the main reasons for the early external leakage of the piston rod in hydraulic cylinders are the failure of the sealing ring and the contamination of the oil. Through disassembly and inspection of the hydraulic cylinders returned to the market due to external leakage of the piston rod, it was indeed found that the sealing rings had varying degrees of wear, scratches, aging and other phenomena. From the actual detection situation of the failure of the hydraulic cylinder sealing rings, the main reasons are as follows

The dust-proof ring has a poor dust-proof effect.

In practical application, people do not pay enough attention to the dust-proof effect of the dust-proof ring of the hydraulic cylinder, which is reflected in the following two aspects:

The structure selection of the dust-proof ring is unreasonable. As shown in Figure 1, with the multi-purpose application of construction machinery, especially for individual users in highly polluting places such as coal mines and mines, where they operate continuously for more than ten hours a day under overload and over-temperature conditions, it is required that the dust-proof rings must have the functions of high pollution resistance and high-temperature resistance. At present, single-lip dust-proof rings can no longer meet the requirements of construction machinery. Judging from the dust-proof rings of the hydraulic cylinders that have been disassembled and inspected, they have basically lost their dust-proof function, allowing contaminants to enter through the piston rod and causing seal wear and failure. ② The dust-proof ring has poor protective measures during the assembly process. This is mainly manifested in the process of painting the entire machine of construction machinery. No protective measures are taken for the dust-proof ring of the hydraulic cylinder, resulting in residual paint at the lip of the dust-proof ring, which has a certain destructive effect on the dust-proof ring and affects the dust-proof effect. The fact that there was paint inside the dust-proof ring of the hydraulic cylinder that was disassembled and inspected can prove this point.

(2) The structure of the combined seal used for the shaft is unreasonable.

At present, the main sealing structure of the piston rod is dust-proof ring + low-pressure seal (Y-shaped ring)+ high-pressure seal (shaft combined seal). The Y-shaped ring has been studied for many years and is a relatively mature structure. With the application of imported Y-shaped rings, there is basically no problem with the quality of low-pressure seals. The imported parts of the shaft combination seal are relatively expensive, while the domestic parts have an unreasonable design Angle of the sealing lip, which is prone to material flow problems at high temperatures, causing changes in the size and shape of the sealing lip, resulting in sealing performance failure, and concentrating the pressure at the low-pressure sealing Y-ring, even leading to early leakage of the piston rod. From the disassembled and inspected hydraulic cylinders, it was found that the above problems do exist.

(3) Scratches on the surface of the piston rod.

During the lifting process of the loader, due to the movement of the bucket, the materials inside the bucket slide off and hit the surface of the extended piston rod, causing tiny pits to appear on the surface of the piston rod. This increases the friction between the sealing ring and the piston rod, leading to the wear and failure of the sealing ring. At present, this has become one of the main problems of early piston rod external leakage in hydraulic cylinders.

(4) The fit clearance between the piston rod and the guide sleeve is not reasonably selected.

According to the design principles of hydraulic cylinders, the fit tolerance between the piston rod and the guide sleeve is H9/f8. However, in practical applications, if the fit between the two is at the minimum tolerance value, due to the long-term overloading of construction machinery, the material expands at high temperatures, which can easily cause a sharp reduction in the oil film on the surface of the piston rod, a decrease in the lubrication effect of the sealing lip, and local temperatures exceeding the allowable temperature of the sealing ring, resulting in high-temperature aging of the sealing ring and loss of sealing effect. Market users have reported that the blackening problem on the surface of the piston rod is caused by the above reasons.

Based on the investigation and analysis of the quality causes of external leakage of hydraulic cylinder piston rods over the past few consecutive years, the above-mentioned issues are the main reasons for the early occurrence of external leakage of piston rods in hydraulic cylinders, accounting for more than 60%.

2. Main measures to reduce the early external leakage of the piston rod in hydraulic cylinders

(1) Select the inner and outer double-lip dust rings. The structure is shown in Figure 2, which enables the dust-proof ring to prevent dirt from entering the hydraulic cylinder through the lip and the bottom of the groove. (2) Add protective measures for the dust-proof ring during the painting process of the hydraulic cylinder and the entire machine. Drawing on the foreign experience in protecting the dust-proof rings of hydraulic cylinders, a specially designed sealing ring should be installed at the lip of the dust-proof ring to effectively isolate the paint from the dust-proof ring. After the painting is completed, the specially designed sealing ring should be removed.

(3) Improve the structure of the combined seal for the shaft. As shown in Figure 3, R1 arc transition is added to the edge of the shaft combined seal and a 7° deflection Angle is added to the sealing lip to prevent the flow problem of the material at the sealing lip and edge at high temperatures. With this improvement, even if the lip material flows, due to the existence of material flow space, the sealing effect of the shaft combination seal will not be reduced. (4) Medium-frequency quenching of the piston rod surface. Through medium-frequency quenching, the hardness of the piston rod base is enhanced, which not only increases the anti-scratch ability of the piston rod surface but also facilitates the reduction of the surface roughness value of the piston rod. In addition, there is an opinion that due to the existence of a hardened layer with a depth of more than 2mm, the entire piston rod forms a cylindrical structure, which can enhance the bending strength of the piston rod.

(5) Take protective measures for the surface of the piston rod. As people have a deeper understanding of the problem of piston rod damage in hydraulic cylinders during the operation of construction machinery, various loader manufacturers have begun to add a protective plate at the piston rod earring for dump hydraulic cylinders with more severe piston rod damage, which can effectively reduce and avoid the risk of piston rod damage.

(6) Reasonably design and select the fit clearance between the piston rod and the guide sleeve. Under the premise of ensuring the safety gap of the sealing ring extrusion, try to make the fit gap between the two have sufficient oil film thickness through the support ring to ensure the lubrication of the sealing lip and avoid abnormal high-temperature conditions caused by friction.