In the mechanical processing industry, hydraulic transmission is widely used, such as various semi-automatic hydraulic transmission lathes. During the use of these machine tools, faults such as impact and crawling often occur. When diagnosing and repairing, the cause of the fault cannot be found on the hydraulic control components, leading to difficulties in maintenance work. And ultimately, such faults are often found to be caused by severe wear of the cylinder piston sealing element on the executing components. This phenomenon is particularly common with O-ring seals.

Example 1: During the use of a CB3463-1 program-controlled hexagonal turret semi-automatic lathe, a fault occurred where the feed speed of the turret tool holder could not be adjusted. Even after the shut-off valve 2 was closed, there was still crawling and forward thrust, and the machine could not be used normally until it reached the bottom of the oil cylinder.

Inspection and maintenance: All hydraulic control components, especially the speed control components, were checked and no problems were found. Partial speed control valves have been replaced, but the fault remains unresolved. After dismantling and inspecting the oil cylinder, it was found that the piston "O" - shaped sealing ring was severely worn, causing the hydraulic oil in the two chambers of the cylinder to flow through each other. Analyzing the reason, due to the wear of the "O" - shaped seal ring, the gap between the rod chamber and the rodless chamber of the oil cylinder increases and communicates with each other. When the speed control valve 3 or the shut-off valve 2 is closed, it is equivalent to cutting off the return oil circuit of the rod chamber of the oil cylinder. In theory, when pressure oil enters the rodless chamber of the oil cylinder, a thrust force F is formed on the piston, causing the piston to move towards the rodless chamber. This is because the return oil in the rodless chamber of the oil cylinder is shut off, and the oil in the chamber quickly forms back pressure, causing the two ends of the piston to be balanced and stationary. At this time, F is equal to F, while the pressure of the rodless oil varies depending on the force area on the piston, that is, P is>P is. In practice, due to the gap formed between the two chambers of the oil cylinder, the pressure oil in the cylinder chamber tends to flow from the high-pressure area to the low-pressure area. As soon as the flow occurs, the pressure P in the chamber decreases, but P does not increase. The thrust F acting on both ends of the piston is not greater than F, and the piston loses balance and moves towards the rod chamber to find a new balance point. In this way, the piston moves until it is obstructed by external force before stopping. The occurrence of the above phenomenon has had a great impact on the maintenance work of the machine tool. After replacing the "O" - shaped sealing ring of the oil cylinder piston, the machine tool fault was eliminated and the operation was restored to normal.

Example 2: During the operation of a CE7120 hydraulic copying lathe, there was a frequent occurrence of tool punching and waste cutting due to the downward rotation of the copying tool holder and longitudinal feed cutting, causing the machine tool to malfunction.

Testing and maintenance: Check that all hydraulic control components are functioning properly, and the maintenance work has entered a blind spot for a while. Dismantling and inspecting the longitudinal oil cylinder of the tool holder, it was found that the outer circle of the piston oil seal "O" ring had been ground flat. At the same time, it was found that the size of the piston outer circle did not meet the requirements, with a diameter direction smaller than the standard size of 0.8mm. The gap between the piston and the oil cylinder body was large, resulting in oil leakage between the two chambers of the longitudinal oil cylinder. During maintenance, the problem with the machine tool punch was resolved after replacing the qualified piston and O-ring seal. The reason for this is that at the moment when the profiling tool holder descends into place, the entire tool holder is subjected to an impact force F;, After the force is decomposed, there is a horizontal thrust F; At the same time, the solenoid valve 34E1-25B works on the piston rod of the longitudinal oil cylinder. The rod chamber of the longitudinal oil cylinder is connected to the pressure oil, and the rodless chamber is connected to the return oil circuit of the speed control valve. The force F on the rod chamber of the piston is completed by two parts, one is F; 1. The other is F1=A1P1, where A1 is the force area of the rodless cavity, then F=F; 1+F1. Under normal circumstances, the piston begins to move towards the rodless chamber under the action of force F. Due to the incompressibility of hydraulic oil, the hydraulic oil pressure in the rodless chamber suddenly increases, creating back pressure on the piston. The force on the piston is balanced and the cutting speed is smoothly adjusted according to the speed control valve. At that moment, there was a% F; The effect of 1 is that the pressure P2 in the longitudinal cylinder chamber will be greater than P1. In practice, due to the gap between the two chambers of the oil cylinder, at the moment when P2>P1, the hydraulic oil tends to flow from the rodless chamber in the high-pressure area to the rodless chamber in the low-pressure area. Once the flow occurs, P2 decreases, P1 increases, F1 increases, and the force on the piston is unbalanced. The piston drives the tool holder to quickly move towards the rodless chamber. And after a moment, the horizontal thrust F acting on the longitudinal oil cylinder piston due to the end of the downward movement of the profiling tool holder; 1 disappears on its own, the rapid movement of the piston ends, the tool holder moves steadily, and the punching phenomenon ends.