Hydraulic transmission is widely used in mechanical processing industry, such as various semi-automatic hydraulic transmission lathes. In the process of using these machine tools, faults such as impact and creep often occur. When diagnosing and repairing, the reasons for the faults are often not found on the hydraulic control components, which makes the repairing work in a dilemma. In the end, this kind of fault is often found in the executive components, that is, the serious wear of the cylinder piston sealing components. This kind of phenomenon is especially common in O-shaped sealing rings.

Example 1: A CB3463-1 program-controlled semi-automatic turret lathe with hexagonal turret appeared in the process of using, the turret tool rack feed speed can not be adjusted, the stop valve 2 still crawls and rushes forward after closing, and can not stop until the bottom of the cylinder, so the machine tool can not be used normally.

Inspection and maintenance: All hydraulic control components, especially speed control components, were inspected without any problems. Some speed regulating valves have been replaced, and the fault has not been eliminated. After disassembling and inspecting the cylinder, it was found that the "O" seal ring of the piston was seriously worn, resulting in the channeling of hydraulic oil between the two chambers of the cylinder. The reason is that the gap between rod chamber and rod-free chamber increases after wear of O-ring seal. When speed regulating valve 3 or globe valve 2 is closed, the return oil circuit with rod chamber is cut off. In theory, when the pressure oil enters the cylinder without rod chamber, a thrust F-free is formed on the piston, which causes the piston to move towards the rod chamber. Because the cylinder has rod chamber return oil shut off, the oil in the chamber quickly forms back pressure, which makes the force at both ends of the piston balanced and stationary. At this time, F-free, but no cavity oil hydraulic force is due to the piston. The area under stress is different, that is, P has > P has not. In practice, the pressure oil in the cylinder chamber tends to flow from the high pressure area to the low pressure area because of the gap between the two chambers. Once the flow occurs, the pressure P in the chamber decreases and P does not increase, and the thrust F at both ends of the piston has no more than F, so the piston loses its balance and moves to the rod chamber to find a new equilibrium point. So the piston moves back and forth until it is blocked by external force. The above phenomena have a great impact on the maintenance of machine tools. After replacing the "O" seal ring of cylinder piston, the machine tool failure is eliminated and the operation is restored to normal.

Example 2: In a CE7120 hydraulic copying lathe, the punch occurs when the copying tool holder leads to downward turning and longitudinal feeding cutting, which results in frequent knife drilling and scrap, and the machine can not work properly.

Inspection and maintenance: Inspection of all hydraulic control components are fault-free, maintenance work temporarily into the blind area. After removing and inspecting the longitudinal cylinder of the tool holder, it was found that the outer circle of the O-shaped seal ring of the piston oil seal had been ground. At the same time, it was found that the size of the outer circle of the piston was not up to standard, and the diameter direction was less than 0.8mm. The gap between the piston and the cylinder body was large, which led to oil channeling between the two chambers of the longitudinal cylinder. When repairing, after replacing the qualified piston and "O" seal ring, the machine tool Punch troubleshooting. The reason is that the tool holder is subjected to an overall impulse F at the moment when the profiling tool holder is in place; after the force is decomposed, there is a horizontal thrust F; 1 acts on the piston rod of the longitudinal cylinder, while the solenoid valve 34E1-25B works, the longitudinal cylinder has a rod chamber to connect the pressure oil, the rodless chamber to connect the return oil circuit of the speed regulating valve, and the piston has a rod. Cavity force F is accomplished by two parts, one is F; 1, the other is F1 = A1P1, where A1 is the force area without rod cavity, then F = F; 1 + F1. Normally, the piston moves to the rodless chamber under the action of force F. Because of the incompressibility of hydraulic oil, the hydraulic oil pressure in the rodless chamber rises sharply to form back pressure for the piston, and the force balance of the piston moves steadily according to the speed regulating valve. Because of the effect of% F; 1, the pressure P2 in the longitudinal cylinder chamber will be greater than P1. In fact, because of the gap between the two chambers of the cylinder, at the moment of P2 > P1, the hydraulic oil tends to flow from the rod-free chamber in the high pressure area to the rod-free chamber in the low pressure area. Once the flow occurs, P2 decreases, P1 increases, F1 increases, the force of the piston loses balance, and the piston drives the tool holder to move rapidly towards the rod-free chamber. After the instant, the horizontal thrust F acting on the piston of the longitudinal cylinder disappears due to the downward end of the profiling tool holder; 1. The piston moves quickly, the tool holder moves smoothly, and the punching phenomenon ends.