Hydraulic cylinder fault diagnosis and troubleshooting

Hydraulic cylinder fault diagnosis and troubleshooting

A complete hydraulic system is composed of a power part, a control part, an executive part and an auxiliary part, among which the hydraulic cylinder as the executive part is one of the important executive elements in the hydraulic system, which converts the hydraulic pressure output by the power element oil pump into mechanical energy to perform an action,
It is an important energy conversion device. The occurrence of its failure during use is usually related to the entire hydraulic system, and there are certain rules to be found. As long as its structural performance is mastered, troubleshooting is not difficult.

If you want to eliminate the failure of the hydraulic cylinder in a timely, accurate and effective manner, you must first understand how the failure occurred. Usually the main reason for hydraulic cylinder failure is improper operation and use, routine maintenance can not keep up, incomplete consideration in the design of the hydraulic system, and unreasonable installation process.

The failures that usually occur during the use of general hydraulic cylinders are mainly manifested in inappropriate or inaccurate movements, oil leakage and damage.
1. Hydraulic cylinder execution lag
1.1 The actual working pressure entering the hydraulic cylinder is not enough to cause the hydraulic cylinder to fail to perform a certain action

1. Under the normal operation of the hydraulic system, when the working oil enters the hydraulic cylinder, the piston still does not move. A pressure gauge is connected to the oil inlet of the hydraulic cylinder, and the pressure pointer does not swing, so the oil inlet pipeline can be directly removed. open,
Let the hydraulic pump continue to supply oil to the system, and observe whether there is working oil flowing out of the oil inlet pipe of the hydraulic cylinder. If there is no oil flow out of the oil inlet, it can be judged that the hydraulic cylinder itself is fine. At this time, other hydraulic components should be searched in turn according to the general principle of judging hydraulic system failures.

2. Although there is working liquid input in the cylinder, there is no pressure in the cylinder. It should be concluded that this phenomenon is not a problem with the hydraulic circuit, but is caused by excessive internal leakage of oil in the hydraulic cylinder. You can disassemble the oil return port joint of the hydraulic cylinder and check whether there is working fluid flowing back into the oil tank.

Usually, the cause of excessive internal leakage is that the gap between the piston and the piston rod near the end face seal is too large due to the loose thread or the loosening of the coupling key; the second case is that the radial The O-ring seal is damaged and fails to function; the third case is,
The sealing ring is squeezed and damaged when it is assembled on the piston, or the sealing ring is aging due to a long service time, resulting in sealing failure.

3. The actual working pressure of the hydraulic cylinder does not reach the specified pressure value. The cause can be concluded as a failure on the hydraulic circuit. The pressure-related valves in the hydraulic circuit include relief valve, pressure reducing valve and sequence valve. First check whether the relief valve reaches its set pressure, and then check whether the actual working pressure of the pressure reducing valve and sequence valve meets the working requirements of the circuit. .

The actual pressure values of these three pressure control valves will directly affect the working pressure of the hydraulic cylinder, causing the hydraulic cylinder to stop working due to insufficient pressure.

1.2 The actual working pressure of the hydraulic cylinder meets the specified requirements, but the hydraulic cylinder still does not work

This is to find the problem from the structure of the hydraulic cylinder. For example, when the piston moves to the limit position at both ends in the cylinder and the end caps at both ends of the hydraulic cylinder, the piston blocks the oil inlet and outlet, so that the oil cannot enter the working chamber of the hydraulic cylinder and the piston cannot move; Hydraulic cylinder piston burnt.

At this time, although the pressure in the cylinder reaches the specified pressure value, the piston in the cylinder still cannot move. The hydraulic cylinder pulls the cylinder and the piston cannot move because the relative movement between the piston and the cylinder produces scratches on the inner wall of the cylinder or the hydraulic cylinder is worn by unidirectional force due to the incorrect working position of the hydraulic cylinder.

The frictional resistance between the moving parts is too large, especially the V-shaped sealing ring, which is sealed by compression. If it is pressed too tightly, the frictional resistance will be very large, which will inevitably affect the output and movement speed of the hydraulic cylinder. In addition, pay attention to whether the back pressure exists and is too large.

1.3 The actual movement speed of the hydraulic cylinder piston does not reach the design given value

Excessive internal leakage is the main reason why the speed cannot meet the requirements; when the movement speed of the hydraulic cylinder decreases during the movement, the piston movement resistance increases due to the poor processing quality of the inner wall of the hydraulic cylinder.

When the hydraulic cylinder is running, the pressure on the circuit is the sum of the resistance pressure drop generated by the oil inlet line, the load pressure, and the resistance pressure drop of the oil return line. When designing the circuit, the resistance pressure drop of the inlet pipeline and the resistance pressure drop of the oil return pipeline should be reduced as much as possible. If the design is unreasonable, these two values are too large, even if the flow control valve: fully open,
It will also cause the pressure oil to return directly to the oil tank from the relief valve, so that the speed cannot meet the specified requirements. The thinner the pipeline, the more bends, the greater the pressure drop of the pipeline resistance.

In a fast motion circuit using an accumulator, if the movement speed of the cylinder does not meet the requirements, check whether the pressure of the accumulator is sufficient. If the hydraulic pump sucks air into the oil inlet during work, it will make the movement of the cylinder unstable and cause the speed to decrease. At this time, the hydraulic pump is noisy, so it is easy to judge.

1.4 Crawling occurs during hydraulic cylinder movement

The crawling phenomenon is the jumping motion state of the hydraulic cylinder when it moves and stops. This kind of failure is more common in the hydraulic system. The coaxiality between the piston and the piston rod and the cylinder body does not meet the requirements, the piston rod is bent, the piston rod is long and the rigidity is poor, and the gap between the moving parts in the cylinder body is too large.
The displacement of the installation position of the hydraulic cylinder will cause crawling; the sealing ring at the end cover of the hydraulic cylinder is too tight or too loose, and the hydraulic cylinder overcomes the resistance generated by the friction of the sealing ring during movement, which will also cause crawling.

Another main reason for the crawling phenomenon is the gas mixed in the cylinder. It acts as an accumulator under the action of oil pressure. If the oil supply does not meet the needs, the cylinder will wait for the pressure to rise at the stop position and appear intermittent pulse crawling motion; when the air is compressed to a certain limit When the energy is released,
Pushing the piston produces instantaneous acceleration, resulting in fast and slow crawling motion. These two crawling phenomena are extremely unfavorable to the strength of the cylinder and the movement of the load. Therefore, the air in the cylinder must be fully exhausted before the hydraulic cylinder works, so when designing the hydraulic cylinder, an exhaust device must be left.
At the same time, the exhaust port should be designed at the highest position of the oil cylinder or the gas accumulation part as much as possible.

For hydraulic pumps, the oil suction side is under negative pressure. In order to reduce pipeline resistance, large-diameter oil pipes are often used. At this time, special attention should be paid to the sealing quality of the joints. If the seal is not good, air will be sucked into the pump, which will also cause hydraulic cylinder crawling.

1.5 There is abnormal noise during the operation of the hydraulic cylinder

The abnormal noise produced by the hydraulic cylinder is mainly caused by the friction between the contact surface of the piston and the cylinder. This is because the oil film between the contact surfaces is destroyed or the contact pressure stress is too high, which produces frictional sound when sliding relative to each other. At this time, the car should be stopped immediately to find out the reason, otherwise, the sliding surface will be pulled and burned to death.

If it is the friction sound from the seal, it is caused by the lack of lubricating oil on the sliding surface and the excessive compression of the seal ring. Although the sealing ring with lip has the effect of oil scraping and sealing, if the pressure of oil scraping is too high, the lubricating oil film will be destroyed, and abnormal noise will also be produced. In this case, you can lightly sand the lips with sandpaper to make the lips thinner and softer.

2. Leakage of hydraulic cylinder

The leakage of hydraulic cylinders is generally divided into two types: internal leakage and external leakage. Internal leakage mainly affects the technical performance of the hydraulic cylinder, making it less than the designed working pressure, movement speed and working stability; external leakage not only pollutes the environment, but also easily causes fires, and causes great economic losses. Leakage is caused by poor sealing performance.

2.1 Leakage of fixed parts

2.1.1 The seal is damaged after installation

If the parameters such as the bottom diameter, width and compression of the sealing groove are not selected properly, the seal will be damaged. The seal is twisted in the groove, the seal groove has burrs, flashes and chamfers that do not meet the requirements, and the seal ring is damaged by pressing a sharp tool such as a screwdriver during assembly, which will cause leakage.

2.1.2 The seal is damaged due to extrusion

The matching gap of the sealing surface is too large. If the seal has low hardness and no sealing retaining ring is installed, it will be squeezed out of the sealing groove and damaged under the action of high pressure and impact force: if the rigidity of the cylinder is not large, then the seal will be damaged. The ring produces a certain elastic deformation under the action of instantaneous impact force. Since the deformation speed of the sealing ring is much slower than that of the cylinder,
At this time, the sealing ring is squeezed into the gap and loses its sealing effect. When the impact pressure stops, the deformation of the cylinder recovers quickly, but the recovery speed of the seal is much slower, so the seal is bitten in the gap again. The repeated action of this phenomenon not only causes peeling tear damage to the seal, but also causes serious leakage.

2.1.3 Leakage caused by rapid wear of seals and loss of sealing effect

The heat dissipation of rubber seals is poor. During high-speed reciprocating motion, the lubricating oil film is easily damaged, which increases the temperature and frictional resistance, and accelerates the wear of the seals; when the seal groove is too wide and the roughness of the groove bottom is too high, the Changes, the seal moves back and forth, and wear increases. In addition, improper selection of materials, long storage time will cause aging cracks,
are the cause of the leak.

2.1.4 Leakage due to poor welding

For welded hydraulic cylinders, welding cracks are one of the causes of leakage. Cracks are mainly caused by improper welding process. If the electrode material is improperly selected, the electrode is wet, the material with high carbon content is not properly preheated before welding, the heat preservation is not paid attention to after welding, and the cooling rate is too fast, all of which will cause stress cracks.

Slag inclusions, porosity and false welding during welding can also cause external leakage. Layered welding is adopted when the weld seam is large. If the welding slag of each layer is not completely removed, the welding slag will form slag inclusions between the two layers. Therefore, in the welding of each layer, the weld seam must be kept clean , can not be stained with oil and water; the preheating of the welding part is not enough, the welding current is not large enough,
It is the main reason for the false welding phenomenon of weak welding and incomplete welding.

2.2 Unilateral wear of the seal

The unilateral wear of the seal is particularly prominent for horizontally installed hydraulic cylinders. The reasons for unilateral wear are: first, the excessive fit gap between the moving parts or unilateral wear, resulting in uneven compression allowance of the sealing ring; second, when the live rod is fully extended, the bending moment is generated due to its own weight, causing the piston to Tilting occurs in the cylinder.

In view of this situation, the piston ring can be used as the piston seal to prevent excessive leakage, but the following points should be noted: first, strictly check the dimensional accuracy, roughness and geometric shape accuracy of the inner hole of the cylinder; second, the piston The gap with the cylinder wall is smaller than other sealing forms, and the width of the piston is larger. Third, the piston ring groove should not be too wide.
Otherwise, its position will be unstable, and the side clearance will increase leakage; fourth, the number of piston rings should be appropriate, and the sealing effect will not be great if it is too small.

In short, there are other factors for the failure of the hydraulic cylinder during use, and the troubleshooting methods after the failure are not the same. Whether it is a hydraulic cylinder or other components of the hydraulic system, only after a large number of practical applications can the fault be corrected. Judgment and quick resolution.