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 is often not found on the hydraulic control components (such as various pumps and valves), leading to difficulties in maintenance work. However, in the end, the cause of such faults is often found on the actuator components, which is caused by severe wear of the sealing components of the oil cylinder piston. This type of phenomenon is particularly common with O-ring seals.

 

 

 

Example 1: A certain CB3463-1 program-controlled hexagonal turret semi-automatic lathe encountered a malfunction during use where the feed speed of the turret tool holder could not be adjusted. After the shut-off valve 2 was closed, there was still crawling and forward punching, and it could not stop until the bottom of the oil cylinder, making the machine unable to function properly.

 

 

 

Inspection and maintenance: All hydraulic control components, especially the speed control components, were inspected and no problems were found. Some speed control valves have been replaced, but the fault remains unresolved. After disassembling and inspecting the oil cylinder, it was found that the "O" shaped sealing ring of the piston was severely worn, causing the hydraulic oil in the two chambers of the oil cylinder to cross flow. Analyzing the cause, due to the wear of the "O" shaped sealing ring, the gap between the rod and non rod cavities of the oil cylinder increases and communicates. When the speed control valve 3 or shut-off valve 2 is closed, it is equivalent to cutting off the return oil circuit of the rod cavity 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 a back pressure, causing the piston to balance and remain stationary. At this time, F has=F has no, and the pressure of the oil in the rodless chamber varies depending on the force area of the piston, that is, P has>P has no. In practice, due to the formation of a gap between the two chambers of the oil cylinder, the pressure oil in the cylinder chamber tends to flow from the high-pressure zone to the low-pressure zone. 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 does not exceed F, and the piston loses balance and moves towards the rod chamber to find a new balance point. In this reciprocating motion, the piston can only stop when there is an external force blocking it. The occurrence of the above phenomenon has had a significant 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 returned to normal.

 

 

 

Example 2: A certain CE7120 hydraulic profiling lathe experienced a punching of the profiling tool holder during the downward and longitudinal feed cutting process, resulting in frequent occurrence of tool breakage and scrap, and the machine tool was unable to operate normally.

 

 

 

Inspection and maintenance: Check that all hydraulic control components are free from faults, and the maintenance work has temporarily entered a blind spot. After disassembling and inspecting the longitudinal oil cylinder of the tool holder, it was found that the outer circle of the piston oil seal "O" shaped sealing ring had been ground flat. At the same time, it was found that the size of the outer circle of the piston did not meet the requirements (due to machine tool manufacturing reasons), and the diameter direction was less 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 cavities of the longitudinal oil cylinder. During maintenance, after replacing qualified pistons and O-ring seals, the machine tool punching tool fault was eliminated. The reason for this is that at the moment when the profiling tool holder descends into position, the tool holder is subjected to a force F 'as a whole. After the force is decomposed, a horizontal thrust F'1 acts on the piston rod of the longitudinal oil cylinder. At the same time, the solenoid valve 34E1-25B works. The rod chamber of the longitudinal oil cylinder is connected to pressure oil, and the rod 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, and 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 starts to move towards the rodless chamber under the action of force F. Due to the non compressibility of the hydraulic oil, the pressure of the hydraulic oil in the rodless chamber rises sharply to form a back pressure on the piston. The force balance of the piston moves steadily at the speed set by the speed control valve. At this moment, due to the effect of% F'1, the pressure P2 in the longitudinal oil cylinder cavity will be greater than P1. In practice, due to the clearance between the two chambers of the oil cylinder, at the moment P2>P1, the hydraulic oil tends to flow from the rodless chamber in the high-pressure zone to the rodless chamber in the low-pressure zone. Once the flow occurs, P2 decreases, P1 increases, F1 increases, and the piston loses balance due to force. The piston drives the tool holder to quickly move towards the rodless chamber. After an instant, due to the end of the profiling tool holder's downward movement, the horizontal thrust F'1 acting on the longitudinal oil cylinder piston disappears on its own, and the piston quickly moves to an end. The tool holder moves smoothly and the punching phenomenon ends.