Abstract: Hydraulic system is a complex and high-precision mechanical, electro-hydraulic integrated system, and its faults have characteristics such as concealment, interleaving, randomness, and differences. Hydraulic system maintenance is an important condition for maintaining and restoring system operation, extending service life, and maintainability is a quality characteristic closely related to maintenance, reflecting whether the system is convenient for maintenance and an inherent characteristic given during system design. Studying the maintainability of hydraulic systems is of great significance in improving their maintainability and avoiding difficult maintenance situations.
Keywords: hydraulic failure, hydraulic component maintenance
1. Characteristics of hydraulic faults
Hydraulic failure refers to the loss of the intended function and the form of the problem of hydraulic components or systems. The loss of function includes the following three situations: first, complete loss of function, which refers to the occurrence of destructive faults, such as pump shaft twisting and electromagnetic burnout; Secondly, functional degradation refers to the occurrence of functional failures, such as a decrease in pump volumetric efficiency or a slowdown in hydraulic cylinder speed; The third is misoperation, which refers to faults caused by incorrect operation or assembly. And the problems mainly refer to the noise and vibration of the hydraulic system, such as high-frequency vibration sound when the relief valve releases pressure, vibration and noise caused by hydraulic impact, etc.
Compared with general mechanical or electrical faults, hydraulic faults have the following characteristics:
The first is concealment. Hydraulic system failures often occur within the system, and due to the inconvenient installation and disassembly, on-site inspection conditions are limited, making it difficult to directly observe various types of pumps, valves, hydraulic motors, hydraulic cylinders, etc. Due to the limited number of surface symptoms and the influence of random factors, fault analysis is also difficult.
The second is interleaving, where there are various overlaps and intersections between the symptoms and causes of hydraulic system failures. A symptom may be caused by multiple reasons, such as slow speed of the actuator. The causes include: excessive load, wear of the actuator itself, excessive guide rail error, leakage in the system, pressure regulation system failure, speed regulation system failure, etc. A symptom may also be formed by the superposition of multiple fault sources, such as when the pump, reversing valve, and hydraulic cylinder are all in a worn state, the efficiency of the system decreases significantly. When these components are replaced one by one, the system efficiency gradually improves.
The third one is randomness. During the operation of hydraulic systems, they are affected by various random factors, such as changes in grid voltage, environmental temperature, and equipment work tasks. The invasion of external pollutants is also random. Due to random factors such as these, the specific direction of change in the occurrence of faults is even more uncertain, which can cause difficulties in judgment and quantitative analysis. The fourth one is differentiation. Due to differences in design, processing materials, and application environments, the wear and degradation rate of hydraulic components varies greatly. General hydraulic component life standards are difficult to apply on site, and only specific wear evaluation standards for specific hydraulic equipment and components can be determined, which requires the accumulation of long-term operating data.
1) Content and requirements for minor repairs of equipment hydraulic systems
The minor repair of the hydraulic system of the equipment is mainly carried out by the operator, and under the guidance of the maintenance worker, the equipment is partially disassembled, inspected, cleaned, and the connecting parts are tightened. The content is to filter the oil in the fuel tank. If the oil is found to have deteriorated, it should be replaced; Check the filter and air filter, and replace them promptly if any problems are found; Clean the interior and exterior of the fuel tank; The sealing components should be replaced for the two joint surfaces that have been found to have leaks; Tighten the bolts and nuts on the pipe joints, covers, and flanges; Replace the faulty hydraulic oil pipe; Clear obvious external leaks in certain areas; Check if the electrical wiring of the solenoid valve electromagnet, pressure relay, and travel switch is in good condition.
2) Content and requirements for major repairs of equipment hydraulic systems
The major overhaul of the equipment hydraulic system is usually carried out by professional maintenance personnel, and there are several requirements during the overhaul:
One is to replace the hydraulic cylinder seal. If the hydraulic cylinder cannot be repaired, it should be replaced as a complete set. For parts such as piston rods, pistons, plungers, and cylinder barrels that can still be repaired, their working surfaces must not have cracks or scratches. The technical performance after repair should meet the requirements for use;
Secondly, all hydraulic valves should be cleaned and vulnerable parts such as seals and springs should be replaced. Components with severe wear and whose technical performance no longer meets the requirements for use should be replaced;
The third is to inspect and repair the hydraulic pump. After repair and testing, the main technical performance indicators of the pump must meet the requirements before it can continue to be used. Otherwise, a new pump should be replaced;
Figure 1 Inspection of Hydraulic Pump and Coupling
Fourthly, performance testing and calibration should be carried out on the pressure gauge. If it does not meet the quality indicators, a new gauge should be replaced. The new pressure gauge must be flexible, reliable, have clear and accurate readings, and the pressure gauge switch should achieve sensitive adjustment, safety and reliability;
Fifth, all kinds of oil pipes need to be cleaned thoroughly? Oil pipes with problems, such as pipes with obvious pits and striking points, and pipes with damaged high-pressure rubber hoses, should be arranged neatly and equipped with pipe clamps;
Sixth, the interior of the fuel tank should be cleaned thoroughly. Air filters and filters that cannot be used should be replaced in a timely manner. All accessories in the fuel tank should be equipped, and the oil level indicator should be clear and obvious (Figure 2);
Figure 2 Hydraulic Unit Oil Level and Oil Level Indicator
Seventh, all oil drain pipes should be inserted below the oil level to prevent foam and air inhalation;
Is the hydraulic system operating within the specified speed and pressure range? During movement, there should be no vibration, noise, or significant impact;
Ninth, there should be no foam in the oil tank when the system is working. The temperature inside the fuel tank should not exceed 55 ℃, and when the system is working continuously, the oil temperature should not exceed 65 ℃.
3) Precautions for hydraulic system maintenance
(1) It is prohibited to conduct maintenance on the system during operation, shutdown without pressure relief, or without cutting off the control power supply to prevent personal injury accidents.
(2) The maintenance site must be kept clean. Before dismantling components or loosening pipe fittings, the external surface dirt should be removed. During the maintenance process, clean protective equipment should be used in a timely manner? Seal all exposed channel openings to prevent contaminants from entering the system. Grinding, construction, and welding operations are not allowed at the maintenance site.
(3) When repairing or replacing components, they must be kept clean and free of sand, dirt, welding slag, etc. They can be rinsed first before installation.
(4) When replacing the seal, sharp tools are not allowed and care should be taken not to damage the seal or working surface.
(5) When disassembling or dismantling hydraulic components, attention should be paid to the direction and sequence of disassembly, and they should be properly stored. They should not be lost, and their precision machined surfaces should not be damaged. When assembling components, all parts must be cleaned thoroughly.
(6) When installing components, the tightening force should be uniform and appropriate to prevent deformation of the valve body, valve core jamming, or oil leakage at the joint.
(7) The replacement or replenishment of working fluid in the fuel tank must be done by filtering the new oil through a high-precision oil filter truck before injecting it into the tank. The grade of the working fluid must meet the requirements.
(8) Welding and machining on the accumulator housing are not allowed, as improper maintenance can cause serious accidents. If any problems are found, they should be promptly sent back to the manufacturer for repair.
(9) After the maintenance is completed, it is necessary to confirm the maintenance area. After confirming the accuracy, adjust according to the content of the hydraulic system debugging section and observe the maintenance parts. After confirming that they are normal, they can be put into operation.
2. Wear and leakage of main components in hydraulic system
2.1 Gear pump
The internal leakage of gear pumps ranks first among various hydraulic pumps, so their volumetric efficiency is low. At a certain speed and pressure, for gears without end face clearance compensation, the main cause of leakage is the gap between the gear end face and the bearing race or cover plate. The leakage amount here accounts for about 75% to 80% of the total leakage amount of the gear pump, and the leakage amount between the tooth tip and the cylindrical hole of the housing is about 15% to 20%. The leakage amount at the meshing point caused by tooth profile error only accounts for 4% to 5%. As for high-pressure gear pumps, methods such as installing axial clearance compensation devices or improving the manufacturing accuracy of gear teeth and housing are generally used to reduce leakage. The wear of gear pump components is the main reason for the increase in gear end face clearance and tooth tip clearance. It will reduce the main working indicator volume of the gear pump.
Figure 3 Gear Pump
2.2 Oil cylinder
Severe leakage inside the oil cylinder will result in insufficient thrust, decreased speed, and unstable operation during operation. Internal leakage is mainly caused by the aging and failure of the oil seal on the piston, severe wear of the sealing ring, reduced sealing ability, and increased clearance between the piston and cylinder bore. When high-pressure oil leaks into the low-pressure chamber, it will reduce the pressure difference on both sides of the piston and decrease the piston thrust. Internal leakage reduces the effective flow rate of the high-pressure chamber, resulting in a slower piston movement speed.
2.3 Directional valve
When the directional valve leaks a large amount of high-pressure oil into the low-pressure oil chamber due to wear between the slide valve and the valve body, it will also cause the actuator to act slowly and weakly. Under normal circumstances, there is less internal leakage between the high and low pressure oil chambers of the directional valve. When there are more abrasives in the oil, wear intensifies, and there are geometric errors in the mating surface, the internal leakage will increase sharply. According to hydraulic principles, the leakage flow rate is proportional to the cube of the single-sided gap. Therefore, as the gap increases with wear, the increase in internal leakage also changes rapidly. The internal leakage is also proportional to the square of the relative eccentricity e between the spool valve and the valve body hole, where eccentricity is the ratio of eccentricity to the average clearance. When the slide valve and valve body are not concentric due to wear, processing, or uneven clearance caused by geometric errors, the internal leakage will increase.
When the sharp edges of the slide valve and valve body cut off the oil circuit become dull, it will also cause the actuator to lag in operation due to the inability to quickly cut off the oil circuit. Improper pressure adjustment of certain valves or failure of other working components in the system can also cause leaks, such as safety valves with low pressure adjustment, valve cores stuck in the open position, and clogged filters in the suction path, all of which will result in insufficient oil pressure in the high-pressure chamber (at the same flow rate).
2.4 Damage to sealing components
The leakage of oil outside the hydraulic system due to damaged sealing components is called external leakage. Leakage not only affects the working performance of hydraulic systems, but also causes environmental pollution and energy waste, and in severe cases, can lead to production accidents. For example, leakage of the hydraulic cylinder in the cableway curved track lifting system can cause slow or impossible lifting of the curved track. Leakage can cause the oil level in the tank to drop, pump in air, affect the stability of the system operation, and even damage the components. The direct consequence of leakage is energy waste. The leakage rate is generally represented by the HFI (Hy hydraulic Fluid Index) value. The HFI value is the ratio of the volume of hydraulic oil supplied to the hydraulic system to the volume of the oil tank in a year. If HFI=3, it means that the amount of oil supplied to the hydraulic system in a year is three times the volume of the oil tank. There are many leakage points in a hydraulic system, one? There are about hundreds of leakage points in complex hydraulic systems. If oil leaks out of the system through the gap between the piston rod and the guide sleeve from the high-pressure chamber, the leakage amount accounts for 50% of the system's external leakage.
Under the action of high-pressure and high-frequency pulsating fluid, it is more prone to the phenomenon of sealing elements being squeezed out along the gap. The pressure fluid comes from one or two directions, and the damage to the seal varies. The sealing area of the relative moving parts should have a small friction coefficient to reduce the resistance and wear of the moving parts.
3. Repair of hydraulic components
1) Repair of directional valve
The function of a directional valve is to use the relative motion between the valve core and the valve body to change the direction of oil flow and open or close the oil circuit. The common faults of directional valves include: first, the valve core cannot move, second, the solenoid valve coil is burned, and third, noise occurs during operation. Considering the extremely important importance of safe and efficient operation of cableways, damaged directional valves are generally replaced locally (such as coils) or as a whole after identifying the cause; If there are no spare parts, the valve needs to be disassembled, inspected, cleaned, and repaired. If it can be repaired, it can be used urgently.
2) Repair of gear pump
The common faults of gear pumps include loud noise, insufficient displacement, or biting. Generally, new parts need to be replaced, and attention should be paid to injecting oil into the pump before trial operation to avoid dry grinding affecting its service life and even causing damage.
3) Maintenance of hydraulic cylinders
The repairable contents and methods of hydraulic cylinders are as follows:
(1) When the clearance between the cylinder and piston increases to 0.2mm or there are serious scratches on the cylinder wall and piston due to wear, the cylinder can be bored and honed to eliminate the scratches, while paying attention to eliminating the taper. The surface roughness after honing should reach Ra0.32-0.16 μ m, and the cylindricity error should not exceed 0.015mm? Then add larger sized pistons to restore the standard clearance.
(2) When there are scratches on the surface of the piston rod, causing oil leakage (one drop every 2-3 minutes), it can be repaired by brushing the surface of the piston rod with adhesive or silver welding. When there is severe corrosion on the surface of the piston rod or when the chrome plating layer on the surface of the piston rod falls off significantly within its working length, grinding can be performed first, followed by chrome plating repair. The failure of the dust seal on the piston rod can cause dust, chips, and sand to enter the hydraulic cylinder, so the seal should be replaced. When the bending deformation value of the piston rod is greater than the specified value by 20%, correction and repair are required.
(3) When the leakage inside the hydraulic cylinder exceeds three times the specified value of the product, it will cause a significant decrease in the working efficiency of the hydraulic cylinder, and the cause of the internal leakage should be checked. If the seal fails, it should be replaced; If the clearance is too large after the piston wears out, the piston should be re fitted and repaired. There is external leakage at both ends of the hydraulic cylinder cover. If the seal at the end cover is aged or damaged, it should be replaced; If the fastening screw is loose, it should be tightened; If the screw is too long to tighten the end cap, it should be checked and replaced.
(4) When the buffering effect of the buffering hydraulic cylinder is poor, the buffering device must be inspected. If a buffer device with a one-way throttle valve is used, the clearance between the buffer piston and the buffer sleeve should be checked. If there is any burrs, grooves, or excessive clearance, it should be redone according to the drawing. After assembly, the buffer performance should also be tested.
4. Conclusion
The use of hydraulic technology is one of the important technologies in modern industrial society, and it plays an increasingly important role in various industries. As a result, mechanical hydraulic equipment may experience some malfunctions in daily use due to various reasons, which requires maintenance personnel to carry out maintenance and timely detection of problems to improve the efficiency of equipment operation.
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