Preface
There are two main reasons for hydraulic cylinder leakage, one is internal factors, mainly including whether the design of sealing devices and mechanical structures is reasonable. The second is external factors, which refer to the external conditions during the use of hydraulic cylinders, including the working pressure of the hydraulic cylinder, the speed of the cylinder movement, the temperature of the oil, oil contamination, the roughness of the sliding surface, the clearance between the moving pairs, and the usage time of the sealing device. In most cases, the cause of hydraulic cylinder leakage is not solely due to a single factor, but includes multiple factors. Therefore, by analyzing the various reasons that may cause leakage in the sealing device of hydraulic cylinders and implementing corresponding improvement measures, it is possible to better control the leakage of hydraulic cylinders.
01 Reasons for hydraulic cylinder leakage
In theory, hydraulic leakage can be divided into two situations: internal leakage and external leakage. The internal discharge of hydraulic cylinders affects their technical performance, such as insufficient output, slow speed, and unstable operation. Leakage will lead to environmental pollution and resource waste.
The main reasons for hydraulic cylinder leakage are: the quality of its structure, materials or sealing grooves and sealing contact surfaces, wear and installation of seals, working environment of seals, bending and wear of hydraulic cylinder piston rods, surface strain of hydraulic systems, and welding and manufacturing defects such as pollution of hydraulic cylinders.
(1) The difference between the structure and material of the seal will directly lead to hydraulic cylinder leakage. If the sealing material is too soft, the seal is easily squeezed and damaged in the sealing gap when the hydraulic cylinder is working; If the seal is too hard, it will be difficult to effectively seal.
(2) The sealing groove and sealing contact surface cannot be installed with the required dimensional accuracy according to the work requirements. The surface roughness and shape tolerance of the seal alignment will cause damage to the seal, leading to leakage.
(3) Generally speaking, the sealing components of hydraulic cylinders must have high dimensional accuracy and shape and position accuracy. The wear of sealing components and damage caused by the assembly process are the main causes of hydraulic cylinder leakage.
(4) The working environment of seals and other sealing components under high temperature conditions will accelerate aging.
(5) Piston rods may sometimes experience impact or surface strain during assembly or normal operation, which makes them unable to function properly.
(6) The pollution of hydraulic systems is an undeniable factor in mining environments. The hydraulic oil containing particles acts on the moving surface of the seal, causing grinding effect, which leads to seal failure and leakage.
02 Hydraulic cylinder fault diagnosis
Generally, the diagnosis of hydraulic cylinder leakage accidents mainly adopts the methods of inquiry and observation.
Specifically, 'inquiry' refers to the maintenance personnel asking the operator about the basic information of the faulty machinery. What are the abnormal phenomena of the host; Is failure sudden or gradual; Sudden failures are mainly caused by hydraulic system contamination or unexpected loads, resulting in hydraulic valve or seal failure; Gradually failing, most seals appear due to severe wear and tear. The maintenance personnel mainly personally inspect the working status of the hydraulic system. When the hydraulic cylinder moves to its limit position, the working pressure of the hydraulic system can be observed. Once the working pressure does not reach the specified value and the pressure of the safety valve is not changed by adjusting the pressure, it may be caused by the release of the hydraulic cylinder.
Firstly, when the hydraulic system components are not removed, the construction machinery is installed in place according to the testing operation rules (for example, the stamping leg of the crane is flat), and then the piston of the hydraulic cylinder is operated to the testing position through the operation of the directional valve. The leakage of the hydraulic cylinder can be determined by observing whether hydraulic oil flows out from the nozzle.
By operating the directional valve used to stack the balance valve, the piston of the hydraulic cylinder of the tested hydraulic cylinder is appropriately protruding, and the pressure in the rodless chamber of the hydraulic cylinder is generated by the working device through the load. Then, operate the valve towards the middle, open the hydraulic cylinder inlet and outlet ports c and d, and observe the leakage of oil ports c and d. If hydraulic oil flows out from port c, it indicates that the hydraulic cylinder is leaking; If hydraulic oil flows out from port d, the balance valve will leak.
For the operation of the directional valve of the stacked bidirectional hydraulic lock hydraulic cylinder, the piston of the tested hydraulic cylinder is appropriately extended and supported by the working device to generate pressure in the rodless chamber of the hydraulic cylinder. Then, operate the directional valve to the neutral position, open the hydraulic cylinder inlet and outlet ports e and f, and observe the leakage of ports e and f. If hydraulic oil flows out of the e-port, it indicates that the hydraulic lock is not locked in the direction of the rodless chamber; If the piston rod retracts under load and there is no hydraulic oil flowing out of port e, while there is hydraulic oil flowing out of port f, it indicates that the hydraulic cylinder is leaking and the hydraulic lock is not locked in the direction of the rod chamber; If there is no hydraulic oil flowing out from both ports, it indicates that the bidirectional hydraulic lock is working properly and the hydraulic cylinder is leaking.
03 Hydraulic cylinder disassembly, inspection and maintenance
Once there is a leakage fault in the hydraulic cylinder, it must be disassembled for inspection. Corresponding maintenance methods should be taken to address the causes of the malfunction.
The inspection and repair of seals. Piston seals are the main components to prevent hydraulic cylinder leakage. The lip seal should be checked for lip and lip wear, and the wear amount of the sealing surface should be checked to determine the combination seal, and then determine whether the seal can be used. Additionally, check for any compression damage to the piston and piston rod between the static sealing rings. The sealing of the piston rod should pay attention to the wear of the sealing and supporting rings. Once defects are found in the sealing components and guide bearing rings, they should be replaced according to the structure of the repaired hydraulic cylinder seal, which can minimize the oil film thickness between the seal and the sealing surface. Reduce leakage of seals.
The inspection and maintenance of the inner surface of the cylinder and the piston seal are the main factors causing hydraulic cylinder leakage. If there is a longitudinal tension mark inside the cylinder, even if a new piston seal ring is replaced, the fault cannot be effectively eliminated. The main inspection of the inner surface of the cylinder is to check whether the dimensional and shape tolerances meet the technical requirements, whether there are longitudinal tensile marks, and to measure the depth of the longitudinal tensile marks. In order to adopt corresponding solutions.
(1) When there are trace deformations and shallow stretching marks inside the cylinder, repair the cylinder through strong honing technology. The powerful honing process can repair cylinders that exceed the original tolerance by 215 times. By using a powerful honing machine to hone parts with significant size or shape errors, the tolerance and roughness of the overall size and shape of the cylinder meet technical requirements.
(2) The inner surface of the cylinder is severely worn. When there are deep longitudinal drawing marks, they should be surveyed according to the target and replaced by a professional production factory according to the cylinder manufacturing process. TS311 anti-wear repair agent can also be used to repair the cylinder barrel. TS311 anti-wear repair agent is mainly used for repairing worn and sliding metal parts. During the repair process, use an alloy knife to shave grooves with a depth of more than 1mm on the sliding surface, then clean the groove surface with acetone. Use a cylinder inner diameter contour plate to apply the adjusted TS311 anti friction repair agent to the polished surface, scrape it flat with force, ensure it is compacted, and higher than the inner surface of the cylinder. After solidification, polish to leave a finishing allowance. Finally, grind the cylinder to achieve the required overall size, shape tolerance, and roughness. But the lifespan and reliability of this repair cylinder are not high.
The inspection and maintenance of the relative motion between the piston rod and the guide sleeve are the main causes of leakage. If the chrome plating layer on the surface of the piston rod peels off or shows longitudinal stretching marks due to wear, it will directly lead to seal failure. Therefore, it is important to check whether the surface roughness and shape tolerance of the piston meet the technical requirements. If the bending of the piston should be directly corrected according to requirements or mapped based on physical objects, it should be manufactured by a professional manufacturer. If the surface coating of the piston rod is worn, slips, or partially peels off, grinding coating and chrome plating surface treatment processes can be adopted.
The inspection and maintenance of the buffer valve should focus on checking the wear of the buffer valve core and seat for the hydraulic cylinder of the buffer valve. Once increased wear and seal failure are detected, replacement should be carried out. Abrasives can also be used to repair the valve core and valve seat by grinding them together.
04 Summary
The leakage of hydraulic cylinders during use may be caused by various factors. Under the condition of relatively fixed oil products, the clearance between the sliding surfaces is an important parameter in product design, which is an internal factor for the leakage problem of hydraulic cylinders. Therefore, it needs to be determined theoretically and practically. Based on the analysis of the causes of hydraulic cylinder leakage, corresponding improvement measures and methods are taken and applied in the design and small-scale production of oil cylinders. The use of machinery is only based on ensuring the leakage problem of hydraulic cylinders, in order to ensure the normal use of mechanical hydraulic cylinders.
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