A Brief Discussion on the Maintenance of Hydraulic Systems in Construction Machinery

Abstract

The hazards caused by improper maintenance of the hydraulic systems of commonly used construction machinery were analyzed, and how to correctly maintain the hydraulic systems of construction machinery was discussed.

Key words

Correct maintenance of the technical condition of the hydraulic system of construction machinery

For mechanical construction enterprises, the technical condition of construction machinery is a direct factor for the normal production of the enterprise. Most of the current engineering machinery adopts the integration of mechanics, electricity and hydraulics. The normal operation of the hydraulic system is a major sign of its good technical condition. Qualified hydraulic oil is the guarantee for the reliable operation of the hydraulic system. Proper maintenance is the fundamental for the reliable operation of hydraulic systems. For this reason, based on work practice, the author makes a rough discussion on the maintenance of the hydraulic system of engineering machinery.

1. Select the appropriate hydraulic oil

Hydraulic oil plays a role in transmitting pressure, lubricating, cooling and sealing in the hydraulic oil system. Improper selection of hydraulic oil is the main cause of early failure and reduced durability of the hydraulic system. 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. 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 range and accuracy 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.

2.1 Inspection and maintenance after 250 hours

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 regard, the machine can only be turned on after a confirmed diagnosis and corresponding measures have been taken. If the filter screen is found to be damaged or dirt accumulates, it should be replaced in time. If necessary, the hydraulic oil should also be replaced simultaneously.

2.2 500-hour inspection and maintenance

After the construction machinery has been in operation for 500 hours, the filter element should be replaced regardless of its condition. This is because 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.

2.3 1000h inspection and maintenance

At this point, the filter should be cleaned, the hydraulic oil tank should be cleaned, the filter element and the hydraulic oil should be replaced. For long-term high-temperature operation, the oil change time should be appropriately advanced. It is the most economical to guide the oil change through oil quality testing and analysis. However, it is necessary to pay attention to the hydraulic oil that has been used for a long time. It should be tested every 100 hours to detect and replace the deteriorated hydraulic oil in time.

2.4 Inspection and maintenance at 7000h and 10000h

At this point, the hydraulic system of the construction machinery needs to be inspected by professionals, and necessary adjustments and maintenance should be carried out. According to practice, imported hydraulic pumps and hydraulic motors must undergo major overhauls after working for 10,000 hours; otherwise, they may be damaged due to lack of maintenance, which would be a fatal blow to the hydraulic system.

3. 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 are equipped with damping holes or 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 substances to invade 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:

(1) When refueling, the hydraulic oil must be filtered and the refueling tools should be reliably clean. The filter at the filling port of the hydraulic oil tank should not be removed just to increase the refueling speed. Refueling personnel should wear clean gloves and work clothes.

(2) When maintaining, remove the oil tank filling cap, filter cover, inspection hole, hydraulic oil pipe and other parts. When the oil passage of the hydraulic system is exposed, avoid raising dust. 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 it is necessary to use 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 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 remove the dirt inside the filter housing with cleaning materials.

(3) The cleaning oil for the hydraulic system must be of the same grade as that 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 from 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 again, replace the new filter element and add new oil.

4. Prevent air and water from invading the hydraulic system

4.1 Prevent air from invading the hydraulic system

Under normal pressure and temperature, the hydraulic oil contains 6% to 8% air by volume. When the pressure drops, the air will be released from the oil, causing the bubbles to burst and resulting in "cavitation" of the hydraulic components, 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 work efficiency, and cause adverse consequences such as "crawling" of the actuating elements. 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 should be removed in accordance with the provisions of the random "User Manual".

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. A "single-lip" oil seal cannot be used as a substitute because 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.

4.2 Prevent water from invading the hydraulic system

Excessive water in hydraulic 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 necessary to pay attention to tightening the lid of the oil storage drum when it is not in use, and it is best to place it upside down. Hydraulic oil with high water content needs to be filtered multiple times, and the dried filter paper should be replaced each time it is filtered. When there is no dedicated instrument for detection, hydraulic oil can be dropped onto a heated iron plate. Only when no steam comes out and it burns immediately can it be refilled.

5. Precautions in the homework

5.1 Construction machinery operations should be gentle and smooth

When operating construction machinery, rough handling should be avoided; otherwise, impact loads will inevitably occur, causing frequent malfunctions of the machinery and significantly shortening its service life. The impact load generated during operation, on the one hand, can cause premature wear, fracture and breakage of the structure of construction machinery, and on the other hand, it can also cause impact pressure in the hydraulic system. This impact pressure can damage hydraulic components, lead to premature failure of oil seals and the pressing points of high-pressure oil pipe joints and rubber hoses, resulting in oil leakage or pipe burst, and frequent actions of relief valves can cause the oil temperature to rise. Our company has newly purchased a UH171 front shovel excavator. During operation, the oil pipe at the doumen gate leaks or bursts every 4 to 6 days. The oil pipe is a genuine imported product randomly and has been inspected and tested without any quality issues. 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 extreme limit position and generate a strong impact. 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 also differs, and the output of the engine and hydraulic system is not the same. These factors endow the equipment with its individuality. Only when the operator using the equipment carefully explores and corrects their control actions to adapt to the individuality of the equipment, Only after long-term operation can one develop good operating habits that match the characteristics of the equipment. The construction machinery industry adheres to the principle of assigning specific personnel and mechanisms, which is also one of the factors.

5.2 Attention should be paid to cavitation and overflow noise

During the operation of construction machinery, always pay attention to the sounds of the hydraulic pump and the relief valve. If the hydraulic pump makes "cavitation" noise, 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.

5.3 Strictly implement the shift handover system

When the shift handover driver parks the construction machinery, he must ensure the safety and convenience of the inspection by the incoming driver. The inspection items include whether the hydraulic system is leaking, whether the connections are loose, whether the piston rods and hydraulic hoses are damaged, whether the low-pressure oil inlet pipe connection of the hydraulic pump is reliable, and whether the oil level in the hydraulic oil tank is correct, etc. In addition, the vent holes of the atmospheric pressure hydraulic oil tank should be inspected and cleaned to keep them unobstructed. This is to prevent the blockage of the vent holes from causing a certain degree of vacuum inside the hydraulic oil tank, which may make it difficult for the hydraulic oil pump to draw oil or even cause damage.

5.4 Maintain an appropriate temperature of the hydraulic oil

The working temperature of the hydraulic system is generally controlled between 30 and 80 degrees Celsius. Excessively high oil temperature in the hydraulic system can lead to: a decrease in the viscosity of the hydraulic oil, which is prone to leakage and a decline in efficiency; The strength of the lubricating oil film decreases, accelerating the wear of machinery. Generate carbides and silt; The oxidation of the oil accelerates and the oil quality deteriorates. Premature aging of oil seals and high-pressure rubber hoses, etc. To avoid excessively high temperatures; Do not overload for a long time; Pay attention to the radiator. The heat sink fins should not be contaminated by 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 hydraulic oil. In the scorching summer, do not work all day long. Avoid working during the midday when the temperature is high. When the temperature of hydraulic oil is too low, its viscosity is high, fluidity is poor, resistance is large, and working efficiency is low. When the oil temperature is below 20℃, sharp turns can easily damage hydraulic motors, valves, pipes, etc. At this point, a warm-up run is required. After starting the engine, 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 machine operation time needs to be appropriately increased.

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

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

Precautions

During the operation of construction machinery, 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 construction machinery that has been continuously shut down for more than 24 hours, oil should be injected into the hydraulic pump before starting to prevent the pump from being damaged due to dry grinding.