Maintenance methods and measures for common hydraulic systems of construction machinery

For mechanized construction enterprises, the technical condition of construction machinery is a direct factor determining whether the enterprise can produce normally. For construction machinery with hydraulic transmission, the normal operation of the hydraulic system is a major indicator of its good technical condition. Qualified hydraulic oil is the guarantee for the reliable operation of hydraulic systems, and correct maintenance is the fundamental for the reliable operation of hydraulic systems. For this reason, based on my work experience, I will make a rough discussion on the maintenance of the hydraulic system of construction machinery in a general working environment.

1. Select the appropriate hydraulic oil

Hydraulic oil plays a role in transmitting pressure, lubricating, cooling and sealing in hydraulic systems. Improper selection of hydraulic oil is the main cause of early failure and reduced durability of hydraulic systems. Hydraulic oil should be selected in accordance with the grade specified in the random "User Manual". In special circumstances where substitute oil is needed, its performance should be as close as possible to that of the original grade. Hydraulic oils of different grades must not be mixed to prevent chemical reactions and changes in their performance. Dark brown, milky white and odorous hydraulic oil is deteriorated oil and should not be used.

2. Prevent solid impurities from mixing into the hydraulic system

Clean hydraulic oil is the lifeblood of a hydraulic system. There are many precision pairs in the hydraulic system, some of which have damping holes and some have gaps, etc. If solid impurities invade, it will cause scratches on precision mating parts, jamming, blockage of oil passages, etc., threatening the safe operation of the hydraulic system. The common ways for solid impurities to invade a hydraulic system include: unclean hydraulic oil; The refueling tools are not clean. Improper refueling, maintenance and repair; Hydraulic component desquamation, etc. Solid impurities can be prevented from invading the system from the following aspects:

2.1 When refueling

Hydraulic oil must be filtered before refilling, and the refilling tools should be reliably clean. The filter at the fuel tank filling port should not be removed just to increase the refueling speed. Refueling personnel should wear clean gloves and work clothes to prevent solid impurities and fiber impurities from falling into the oil.

2.2 During maintenance

When disassembling the oil tank filling cap, filter cover, inspection hole, hydraulic oil pipe and other parts, avoid dust when the system oil passage is exposed. The disassembled parts must be thoroughly cleaned before opening. When removing the oil tank cap of the hydraulic oil tank, first remove the soil around the oil tank cap. After loosening the oil tank cap, clear the debris remaining at the joint (do not rinse with water to avoid water seeping into the oil tank). Only after confirming that it is clean can the oil tank cap be opened. When using wiping materials and hammers, one should choose wiping materials that do not remove fiber impurities and special hammers with rubber attached to the hitting surface. Hydraulic components and hydraulic hoses should be carefully cleaned and dried with high-pressure air before assembly. Choose genuine filter elements with intact packaging (if the inner packaging is damaged, even if the filter element is intact, it may still be unclean). When changing the oil, clean the filter at the same time. Before installing the filter element, carefully clean the dirt at the bottom of the filter housing with wiping materials.

2.3 Cleaning of the hydraulic system

The cleaning oil must be of the same grade as the hydraulic oil used in the system. The oil temperature should be between 45 and 80 degrees Celsius, and a large flow rate should be used to remove impurities in the system as much as possible. The hydraulic system should be cleaned more than three times. After each cleaning, all the oil should be discharged from the system while it is still hot. After cleaning, clean the filter, replace the new filter element and then add new oil.

3. Prevent air and water from invading the hydraulic system

3.1 Prevent air from invading the hydraulic system

Under normal pressure and temperature, the hydraulic oil contains air with a volume ratio of 6 to 8%. When the pressure drops, the air will be released from the oil, and the bubbles will burst, causing "cavitation" of the hydraulic components and generating noise. A large amount of air entering the oil will intensify the "cavitation" phenomenon, increase the compressibility of the hydraulic oil, make the operation unstable, reduce the work efficiency, and cause adverse consequences such as the "crawling" of the actuator. In addition, air can also oxidize the hydraulic oil, accelerating its deterioration. To prevent air invasion, the following points should be noted:

After maintenance and oil change, the air in the system must be removed as specified in the random "User Manual" before normal operation can be carried out.

2. The suction pipe opening of the hydraulic oil pump must not be exposed above the oil surface, and the suction pipe pipeline must be well sealed.

3. The seal of the oil pump drive shaft should be in good condition. When replacing the oil seal at this location, a genuine "double-lip" oil seal should be used instead of a "single-lip" one, as a "single-lip" oil seal can only seal oil in one direction and does not have the function of sealing air. After a major overhaul of a Liugong ZL50 loader in our company, the hydraulic oil pump experienced continuous "cavitation" noise and automatic rise in the oil level of the oil tank. After checking the repair process of the hydraulic oil pump, it was found that the problem was caused by the misuse of a "single-lip" oil seal on the drive shaft of the hydraulic oil pump.

3.2 Prevent water from invading the hydraulic system

Excessive water in the oil can cause rust on hydraulic components, emulsification and deterioration of the oil, reduction of the strength of the lubricating oil film, and accelerate mechanical wear.

In addition to preventing water from entering during maintenance and repair, it is also important to note that when the oil storage drum is not in use, the lid should be tightened and it is best to place it upside down. Oil with high water content needs to be filtered multiple times. Each time it is filtered, the dried filter paper should be replaced. When there is no dedicated instrument for detection, the oil can be dropped onto a heated iron plate. Only when no steam comes out and it burns immediately can it be refilled.

4. Precautions during homework

4.1 Mechanical operations should be gentle and smooth

Mechanical operations should avoid rough handling; otherwise, shock loads will inevitably occur, causing frequent mechanical failures and significantly shortening their service life. The impact load generated during operation, on the one hand, causes premature wear, fracture and breakage of mechanical structural components, and on the other hand, creates impact pressure in the hydraulic system. This impact pressure can damage hydraulic components, lead to premature failure of oil seals and the pressing points between high-pressure oil pipe joints and rubber hoses, resulting in oil leakage or pipe burst, and frequent action of relief valves, causing the oil temperature to rise. Our company once purchased a new UH171 front shovel excavator. During operation, the oil pipe at the doumen gate would leak or burst every 4 to 6 days. The oil pipe was a genuine imported product and was found to have no quality issues after testing. Through on-site observation, it was found that it was caused by the strong impact on the limit block and the door frame when the hopper door opened and closed. To effectively avoid generating shock loads: operating procedures must be strictly followed; The opening and closing of hydraulic valves should not be too sudden or too fast. Avoid causing the components of the working device to move to the limit position and generate strong impacts. Hydraulic equipment without impact function cannot use the working device (such as the bucket of an excavator) to forcefully impact the working object to achieve the purpose of crushing. There is another issue worth noting: The operator must remain stable. Because the free clearance of each device's control system varies to some extent, the wear degree of the connection parts is different, and thus the clearance is also different. The output of the engine and the hydraulic system is also not the same. These factors endow the equipment with its personality. Only when the operators of this equipment carefully explore and correct their control actions to adapt to the characteristics of the equipment, and after long-term operation, can they develop good operation habits that are in line with the characteristics of the equipment. One of the factors is that the general mechanical industry adheres to the principle of assigning specific personnel to specific mechanisms.

4.2 Attention should be paid to cavitation and overflow noise

During operation, always pay attention to the sounds of the hydraulic pump and the relief valve. If the hydraulic pump makes "cavitation" noise and the noise cannot be eliminated after venting, the cause should be identified and the fault eliminated before use. If an actuator operates slowly when there is no load and is accompanied by a sound of overflow from the relief valve, the machine should be stopped immediately for maintenance.

4.3 Strictly implement the shift handover system

When the shift handover driver parks the machinery, he must ensure the safety of the incoming driver during the inspection and that the accurate oil level is checked. Whether the system leaks, whether the connections are loose, whether the piston rods and hydraulic hoses are damaged, whether the connection of the low-pressure oil inlet pipe of the hydraulic pump is reliable, and whether the oil level in the oil tank is correct, etc., are the key points that the incoming driver checks for the hydraulic system. For the atmospheric pressure oil tank, it is also necessary to inspect and clean the vent hole of the oil tank to keep it unobstructed, so as to prevent the vent hole from being blocked and causing vacuum in the oil tank, which may make it difficult for the hydraulic oil pump to draw oil and lead to damage.

4.4 Maintain an appropriate oil temperature

The working temperature of the hydraulic system is generally controlled between 30 and 80℃ (the dangerous temperature should be ≥100℃). Excessively high oil temperature in the hydraulic system can lead to: reduced oil viscosity, which is prone to leakage and decreased efficiency; The strength of the lubricating oil film decreases, accelerating the wear of machinery. Generate carbides and silt; The oxidation of oil accelerates the deterioration of oil quality. Premature aging of oil seals and high-pressure rubber hoses, etc. To avoid excessive temperature: Do not overload for a long time; Pay attention to not contaminating the radiator fins with oil to prevent dust from adhering and affecting the heat dissipation effect. Maintain an adequate oil level to facilitate the circulation and heat dissipation of the oil. Don't work all day long in the hot summer. Avoid the high-temperature time at noon. When the oil temperature is too low, the viscosity of the oil is high, its fluidity is poor, the resistance is large, and the working efficiency is low. When the oil temperature is below 20℃, sharp turns can easily damage hydraulic motors, valves, pipes, etc. At this point, warm-up operation is required. Start the engine and let it idle without load for 3 to 5 minutes. Then, increase the engine speed by pressing the accelerator at medium speed. Operate the handle to bring any action of the working device (such as the bucket opening of the excavator) to the limit position and hold it for 3 to 5 minutes to allow the hydraulic oil to overflow and heat up. If the oil temperature is lower, the warm-up operation time needs to be appropriately increased.

4.5 Control of air pressure and oil volume in the hydraulic oil tank

During operation, the pressure of the pressure tank should be constantly monitored. The pressure must be maintained within the range specified in the random "User Manual". If the pressure is too low, the oil pump will not draw enough oil and is prone to damage. If the pressure is too high, it will cause oil leakage in the hydraulic system and easily lead to pipe burst in the low-pressure oil circuit. For equipment that has undergone maintenance and oil change, after all the air in the system has been drained, check the oil level status as specified in the random "User Manual". Park the machine on a flat surface, turn off the engine for 15 minutes, and then recheck the oil level. If necessary, replenish the oil.

4.6 Other Notes

During operation, it is necessary to prevent falling stones from hitting components such as hydraulic cylinders, piston rods, and hydraulic oil pipes. If there are small points on the piston rod that are damaged, the edges around the small points should be ground off in time with an oilstone to prevent damage to the sealing device of the piston rod. It can continue to be used when there is no oil leakage. For equipment that has been continuously shut down for more than 24 hours, oil should be injected into the hydraulic pump before starting it to prevent the pump from being damaged due to dry grinding.

5. Regular maintenance precautions

At present, some hydraulic systems of construction machinery are equipped with intelligent devices, which have a warning function for certain hidden dangers in the hydraulic system. However, the monitoring scope and degree of these devices have certain limitations. Therefore, the inspection and maintenance of the hydraulic system should combine the monitoring results of the intelligent devices with regular inspection and maintenance.

5.1250 hours of inspection and maintenance

Check the attachments on the filter screen of the filter. If there is too much metal powder, it often indicates that the oil pump is worn or the cylinder is pulled. In this case, a confirmed diagnosis must be made and corresponding measures taken before starting the machine. If the filter screen is found to be damaged or dirt accumulates, it should be replaced in time. If necessary, change the oil at the same time.

5. Inspection and maintenance after 2500 hours

Regardless of the condition of the filter element, it should be replaced, as it is difficult to detect any minor damage to the filter element with the naked eye. If it operates at high temperatures for a long time, the filter element should be replaced in advance appropriately.

5.31000 hours of inspection and maintenance

Clean the filter, replace the filter element, clean the hydraulic oil tank and replace the hydraulic oil. The oil change time for long-term high-temperature operation should be appropriately advanced. Of course, it is the most economical to guide oil change through oil quality testing and analysis. However, it should be noted that for oil that has been used for a longer period, it should be tested every 100 hours to detect and replace deteriorated oil in a timely manner.

5,47,000 hours and 10,000 hours of inspection and maintenance

The hydraulic system needs to be inspected by professionals, and necessary adjustments and maintenance should be carried out. Based on practice, imported hydraulic pumps and motors must undergo major overhauls after working for 10,000 hours; otherwise, they may be damaged due to disrepair, which would be a fatal blow to the hydraulic system.