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15

2023

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08

Maintenance methods and measures for hydraulic systems of commonly used construction machinery

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For mechanized construction enterprises, the good technical condition of construction machinery is a direct factor in whether the enterprise can produce normally. For hydraulic transmission construction machinery, the normal operation of the hydraulic system is a main indicator of its good technical condition. Qualified hydraulic oil is the guarantee for the reliable operation of the hydraulic system, and correct maintenance is the foundation for the reliable operation of the hydraulic system. Therefore, based on my work experience, I will make a rough discussion on the maintenance of hydraulic systems in construction machinery in general working environments.
1. Choose the appropriate hydraulic oil
Hydraulic oil plays a role in transmitting pressure, lubrication, cooling, and sealing in hydraulic systems. Improper selection of hydraulic oil is the main reason for early failures and decreased durability of hydraulic systems. Hydraulic oil should be selected according to the brand specified in the random "User Manual". In special cases where alternative oil is required, its performance should be as good as the original brand. Different grades of hydraulic oil cannot be mixed to prevent chemical reactions and changes in performance of the hydraulic oil. Dark brown, milky white, and odorous hydraulic oil are spoiled oils and cannot be used.
2. Prevent solid impurities from entering the hydraulic system
Clean hydraulic oil is the life of the hydraulic system. There are many precision components in the hydraulic system, some with damping holes and some with gaps. If solid impurities invade, it will cause precise components to be pulled, jammed, and oil passages to be blocked, endangering the safe operation of the hydraulic system. The general ways for solid impurities to invade hydraulic systems include: unclean hydraulic oil; Refueling tools are not clean; Careless refueling, repair, and maintenance; Hydraulic components peeling, etc. Solid impurities can be prevented from invading the system from the following aspects:
2. When refueling
Hydraulic oil must be filtered and filled, and the refueling tool should be reliable and clean. Do not remove the filter at the fuel tank filler in order to increase refueling speed. Refueling personnel should use clean gloves and work clothes to prevent solid and fiber impurities from falling into the oil.
2. During maintenance
Remove the hydraulic oil tank filler cap, filter cover, inspection hole, hydraulic oil pipe, and other parts to avoid dust when exposing the system oil channel. The disassembled parts must be thoroughly cleaned before opening. When disassembling the hydraulic oil tank filler cap, first remove the soil around the tank cap. After loosening the tank cap, remove any remaining debris from the joint area (do not rinse with water to prevent water from seeping into the tank). Only after confirming cleanliness can the tank cap be opened. When using wiping materials and hammers, choose wiping materials that do not remove fiber impurities and specialized hammers with rubber attached to the striking surface. Hydraulic components and hydraulic hoses should be carefully cleaned, dried with high-pressure air, and then assembled. Choose genuine filter cartridges with intact packaging (damaged inner packaging, although the filter cartridge is intact, it may still be unclean). When changing the oil, clean the filter at the same time. Before installing the filter element, use wiping materials to carefully clean the dirt at the bottom of the filter housing.
2. 3. Cleaning of hydraulic system
The cleaning oil must use hydraulic oil of the same brand as the system used, with an oil temperature between 45 and 80 ℃. Use a large flow rate to remove impurities from the system as much as possible. The hydraulic system needs to be cleaned repeatedly three times or more. After each cleaning, fully drain the system while the oil is hot. After cleaning, clean the filter and replace it with a new filter element before adding new oil.
3. Prevent air and water intrusion into the hydraulic system
3.1 Preventing Air Intrusion into the Hydraulic System
Under normal pressure and room temperature, hydraulic oil contains air with a volume ratio of 6-8%. When the pressure decreases, the air will dissociate from the oil, causing bubbles to rupture and causing "cavitation" of the hydraulic components, resulting in noise. A large amount of air entering the oil will exacerbate the phenomenon of "cavitation", increase the compressibility of hydraulic oil, unstable operation, reduce work efficiency, and cause adverse consequences such as "crawling" of the actuator. In addition, air can also oxidize hydraulic oil, accelerating its deterioration. The following points should be noted to prevent air intrusion:
1. After maintenance and oil change, the air in the system must be removed according to the accompanying "User Manual" in order to operate normally.
2. The suction pipe opening of the hydraulic oil pump must not expose the oil surface, and the suction pipeline must be well sealed.
3. The sealing of the oil pump drive shaft should be good. When replacing the oil seal at this location, it should be noted to use a "double lip" genuine oil seal, and cannot be replaced by a "single lip" oil seal because the "single lip" oil seal can only seal oil in one direction and does not have the function of sealing gas. After a major overhaul of a Liugong ZL50 loader in our unit, the hydraulic oil pump experienced continuous "cavitation" noise and automatic oil level increase in the oil tank. After checking the repair process of the hydraulic oil pump, it was found that it was caused by the misuse of a "single lip" oil seal on the drive shaft of the hydraulic oil pump.
3. 2. Prevent water intrusion into the hydraulic system
Excessive moisture in the oil can cause hydraulic components to rust, oil emulsion to deteriorate, lubricating oil film strength to decrease, and accelerate mechanical wear.
In addition to preventing water intrusion during maintenance, it is also important to note that when the oil storage tank is not in use, the lid should be tightened and it is best to place it upside down; Oil with a high water content needs to be filtered multiple times, and the dried filter paper should be replaced every time it is filtered. When there is no dedicated instrument to detect it, the oil can be dropped onto a hot iron plate, and no steam can be emitted and immediately burned before filling.
4. Precautions during homework
4.1 Mechanical operations should be gentle and smooth
Mechanical operations should avoid being rough, otherwise they will inevitably generate shock loads, causing frequent mechanical failures and greatly shortening their service life. The impact load generated during operation not only causes early wear, fracture, and fragmentation of mechanical structural components, but also generates impact pressure in the hydraulic system. The impact pressure can also cause damage to hydraulic components, premature failure of oil seals, high-pressure oil pipe joints, and pressure joints with rubber hoses, oil leakage, or frequent action of overflow valves, and oil temperature rise. Our company once purchased a new UH171 front shovel excavator. During operation, the oil pipe of the bucket door leaks or bursts every 4-6 days. The oil pipe is a randomly imported pure genuine product, and after testing, there are no quality issues. Through on-site observation, it was found that it was caused by a strong impact on the limit block and door frame when the door was opened and closed. To effectively avoid generating shock loads, it is necessary to strictly follow operating procedures; The opening and closing of hydraulic valves should not be too strong or too fast; Avoid strong impacts caused by the movement of working device components to their extreme positions; Hydraulic equipment without impact function cannot use working devices (such as excavators' buckets) to violently impact the working object to achieve the purpose of crushing. Another noteworthy issue is that the operator should maintain stability. Because there are certain differences in the free clearance of each equipment's control system, the degree of wear on the connecting parts, and therefore the clearance, as well as the output of the engine and hydraulic system, these factors give the equipment its personality. Only by carefully exploring and correcting their operating actions to adapt to the equipment's personality, can the operators who use the equipment develop good operating habits that conform to the equipment's personality after long-term operation. The general machinery industry adheres to the system of fixed personnel and machines, which is also one of the factors.
4.2 Pay attention to cavitation and overflow noise
During operation, always pay attention to the sound of the hydraulic pump and overflow valve. If the hydraulic pump experiences "cavitation" noise that cannot be eliminated after exhaust, the cause should be identified and the fault should be eliminated before use. If a certain actuator operates slowly without load and is accompanied by an overflow valve overflow sound, it should be immediately shut down for maintenance.
4.3 Strictly implement the handover system
When the handover driver parks the machinery, it is necessary to ensure the safety and accurate oil level of the handover driver during inspection. Whether the system leaks, whether the connections are loose, whether the piston rod and hydraulic hose are damaged, whether the low-pressure oil inlet pipe connection of the hydraulic pump is reliable, and whether the oil level of the oil tank is correct are the key points for the driver to inspect the hydraulic system. For atmospheric pressure oil tanks, it is also necessary to check and clean the vent holes of the tank to keep them unobstructed, in order to prevent the blockage of the vent holes from causing vacuum in the tank, which may cause difficulty in oil suction and damage to the hydraulic oil pump.
4.4 Maintain appropriate oil temperature
The working temperature of the hydraulic system is generally controlled between 30 to 80 ℃ (dangerous temperature ≥ 100 ℃). Excessive oil temperature in the hydraulic system can lead to a decrease in oil viscosity, which can easily cause leakage and decrease efficiency; The strength of the lubricating oil film decreases, accelerating mechanical wear; Generation of carbides and sludge; Oil oxidation accelerates the deterioration of oil quality; Premature aging of oil seals and high-pressure hoses. To avoid excessive temperature: do not overload for a long time; Be careful not to contaminate the radiator fins with oil to prevent dust from adhering and affecting the heat dissipation effect; Maintain sufficient oil volume to facilitate oil circulation and heat dissipation; Don't work all day in the hot summer, avoid the hot noon time. When the oil temperature is too low, the viscosity of the oil is high, the fluidity is poor, the resistance is high, and the work efficiency is low; When the oil temperature is below 20 ℃, rapid turning can easily damage hydraulic motors, valves, pipelines, etc. At this point, it is necessary to warm up the engine and start it. After idling at no load for 3 to 5 minutes, increase the engine speed at a medium speed throttle. Use the joystick to bring any action of the working device (such as excavator bucket opening) to the limit position, and hold it for 3 to 5 minutes to allow the hydraulic oil to warm up through overflow. If the oil temperature is lower, it is necessary to increase the warm-up running time appropriately.
4.5 Control of air pressure and oil volume in hydraulic oil tank
Pressure type oil tanks should always pay attention to the air pressure during operation, and their pressure must be maintained within the range specified in the accompanying "User Manual". If the pressure is too low and the oil pump suction is insufficient, it is easy to be damaged. If the pressure is too high, it can cause oil leakage in the hydraulic system and easily cause low-pressure oil pipeline explosion. After the maintenance and oil change of the equipment, after exhausting the air in the system, the oil level status should be checked according to the instructions in the accompanying user manual. The machine should be parked in a flat place, and the oil level should be rechecked after the engine is turned off for 15 minutes. If necessary, it should be replenished.
4.6 Other precautions
During operation, it is necessary to prevent flying stones from hitting hydraulic cylinders, piston rods, hydraulic oil pipes, and other components. If there are small scratches on the piston rod, the edges around the small points should be promptly polished off with an oilstone to prevent damage to the sealing device of the piston rod. It can continue to be used without oil leakage. For equipment that has been continuously stopped for more than 24 hours, oil should be injected into the hydraulic pump before startup to prevent damage caused by dry wear of the hydraulic pump.
5. Precautions for regular maintenance
At present, some engineering machinery hydraulic systems are equipped with intelligent devices, which have warning functions for certain hidden dangers in the hydraulic system. However, their monitoring range and degree have certain limitations. Therefore, the inspection and maintenance of hydraulic systems should combine the monitoring results of intelligent devices with regular inspection and maintenance.
5.1 250h inspection and maintenance
Check the attachments on the filter screen, such as excessive metal powder, which often indicates oil pump wear or cylinder pulling. For this, it must be diagnosed and corresponding measures must be taken before starting the machine. If damage or accumulation of dirt is found on the filter screen, it should be replaced in a timely manner, and if necessary, the oil should be changed at the same time.
5.2 500h inspection and maintenance
Regardless of the condition of the filter element, it should be replaced because it is difficult to detect minor damage to the filter element with the naked eye. If the filter element is operated at high temperatures for a long time, it should be replaced appropriately in advance.
5.3 1000h 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 operations should be appropriately advanced. Of course, it is most economical to guide oil change through oil quality testing and analysis, but it is important to pay attention to extending the use of the oil and testing it every 100 hours to detect and replace spoiled oil in a timely manner.
5.4 7000h and 10000h inspection and maintenance
The hydraulic system needs to be inspected by professional personnel for necessary adjustments and repairs. According to practice, imported hydraulic pumps and motors must be overhauled after 10000 hours of operation, otherwise the hydraulic pumps and motors may be damaged due to disrepair, which is fatal to the hydraulic system.

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