Maintenance and upkeep of the hydraulic system of the PY180 motor grader

The PY180 motor grader produced by China Foreign Construction Development Co., Ltd. adopts hydraulic transmission. It has won a wide market with its advantages such as smooth and uniform motion transmission, easy acquisition of large force and torque, light weight per unit power, small size, compact structure, sensitive response and long component life. However, due to its harsh usage environment and poor working conditions, it often malfunctions. According to the data, 75% of the faults in the hydraulic system of motor graders are caused by contamination of the hydraulic oil, and 20% are due to improper selection of the hydraulic oil. Therefore, the rational selection and correct maintenance of hydraulic oil are the prerequisites for giving full play to the working performance and economic performance of the motor grader and extending the service life of the hydraulic system and its components. Our company purchased two PY180 motor graders in 1998. Now, we would like to share with you the experience we have accumulated in maintaining hydraulic systems over the years.

1. Requirements for the performance of hydraulic oil

In hydraulic transmission, hydraulic oil serves as both the medium for transmitting power and a lubricant. In some components, it also functions as a seal. The heat in the system is dissipated through the oil, thus playing a role in heat dissipation. Therefore, to ensure the reliable, effective and economical operation of the hydraulic system, the hydraulic oil must meet the following requirements:

1) Appropriate viscosity. Viscosity indicates the magnitude of the frictional resistance between molecules when an oil flows. When the viscosity is too high, the resistance force of the oil flow is large, the energy loss is significant, and the system efficiency decreases. In addition, the no-load loss of the main unit increases, the temperature rises rapidly and the working temperature is high, and the "cavitation" phenomenon is prone to occur at the suction end of the main pump. When the viscosity is too low, it cannot ensure good lubrication conditions for hydraulic components, which will accelerate the wear of components, increase leakage, and also reduce the efficiency of the hydraulic system.

2) Good viscosity-temperature characteristics. The viscosity-temperature characteristic refers to the degree to which the viscosity of an oil changes with temperature, which is usually indicated by a viscosity index table. The larger the viscosity index, the less the viscosity of the oil in the hydraulic system decreases as the temperature rises during operation, thus preventing excessive internal leakage in the hydraulic system. The viscosity index generally should not be lower than 90.

3) Good anti-wear and lubricating properties. The purpose is to reduce mechanical friction and ensure sufficient oil film strength under different pressure, speed and temperature conditions.

4) It has high chemical reaction stability and is not prone to oxidation or deterioration. Practical experience has shown that for every 10℃ increase in oil temperature, the rate of chemical reactions approximately doubles. Hydraulic oil with good oxidation stability is less likely to oxidize and deteriorate over long-term use, which can ensure the normal circulation of the hydraulic oil.

5) The quality should be pure, and the content of mechanical impurities, moisture and dust should be minimized as much as possible.

6) The impact on the sealing parts should be small.

7) It should have good anti-emulsification properties and be less likely to cause foaming. Anti-emulsification property refers to the ability of an oil mixture mixed with water and not to form an emulsion and separate water from it after stirring. Anti-foaming property refers to the ability of the oil not to form an emulsion or separate bubbles from the oil after air is mixed in and stirred. When water or air is mixed in, the volumetric modulus of the hydraulic oil is reduced, its compressibility increases, the hydraulic components move slowly, and it is prone to shock and vibration.

8) Good anti-rust performance. Hydraulic oil covers the surface of the parts to prevent them from being oxidized and rusted.

9) Good shear stability. To improve the viscosity index of the oil, high-molecular polymers such as polymethyl propylene ester and polyisobutylene are often added to the oil. These substances have long molecular chains. When the oil flows through the slits of hydraulic components, it is subjected to significant shearing effects, which often cause molecular chain breakage and a decline in the viscosity-temperature characteristics of the oil.

10) The flash point and ignition point should meet the ambient temperature requirements, and the volatility should be low to ensure the safe use of hydraulic oil.

In addition, the selection of hydraulic oil for motor graders should also take into account the differences in their usage environments and regions. The oil used in winter and summer is also different. It is necessary to strictly follow the brand and model of hydraulic oil recommended by the manufacturer.

2. Causes and Hazards of hydraulic oil contamination

2.1 Sources of Pollutants

Contaminants in the new oil. Although hydraulic oil is refined and processed under relatively clean environmental conditions, during transportation and storage, it may be contaminated by pipelines, oil drums and storage tanks, and some dust, sand, rust, moisture and other liquids may mix into the oil.

2) Residual contaminants in components and systems. During the processing, assembly and cleaning of hydraulic components and systems, some contaminants may remain due to incomplete cleaning work.

3) External invading pollutants. During the operation of hydraulic components and machinery, due to incomplete sealing of the oil tank, damage to the sealing and protective devices of components, and other reasons, some contaminants such as dust, sand, and moisture may invade from the outside of the system.

4) Contaminants generated inside the hydraulic system. During operation, the hydraulic system itself generates some solid particulate contaminants, including metal particles or rubber powder produced by the wear and corrosion of hydraulic components, as well as contaminants generated by the oxidation of oil, etc.

2.2 Hazards of Hydraulic Oil Contamination

1) Contaminants often cause the throttle valve and pressure damping hole to be blocked or opened intermittently, and even jam the valve core, causing the working pressure and speed of the hydraulic system to change from time to time and affecting its normal operation.

2) Accelerate the wear of the hydraulic pump, motor and valve group, causing an increase in internal leakage.

3) Water mixed into the hydraulic oil corrodes metals and accelerates the aging and deterioration of the hydraulic oil.

4) Air mixed into the hydraulic oil can cause noise, vibration, crawling, cavitation and shock, thereby deteriorating the working performance of the hydraulic system.

Maintenance of hydraulic oil

3.1 Prevent oil contamination

Among the various pump and valve components used in motor motors, the fit clearances and working surfaces between the relatively moving parts are relatively small. There are also many damping holes and slit-type control valve ports in the hydraulic components. If dirt is mixed into the oil, blockage will occur, and even the mating surfaces will be scratched, increasing leakage, and even the valve core will be stuck, causing the components to malfunction. Therefore, keeping the oil clean is the key to the maintenance of hydraulic systems.

1) Hydraulic oil must undergo strict filtration. When filling the hydraulic oil tank, it should pass through a filter with a mesh size of 120 or more.

2) Regularly check the cleanliness of the oil and replace it as needed based on the working conditions. When replacing, try to drain as much of the approximately 40 liters of oil in the hydraulic system as possible. Among them, the method of using the system's external loop is relatively operable. The method is to first drain the waste oil from the fuel tank and radiator, and then add new oil. Remove the return oil pipe that has entered the fuel tank, start the engine, and let the waste oil completely flow out of the return oil pipe. Then it's done. It is particularly emphasized that the changes in the oil level in the fuel tank should be observed in a timely manner, and the safety level of the oil should be ensured. When changing to new oil, the filter element of the filter should be replaced simultaneously.

3) Hydraulic components should not be disassembled easily. If disassembly is necessary, the parts should be cleaned with kerosene or diesel and placed in a clean area to prevent impurities from mixing in during reassembly.

3.2 Prevent air from entering

Air dissolved in the oil will escape from the oil when the pressure is low, generating bubbles and creating cavitation.

3.3 Prevent moisture from entering

Measures such as filtration and oil-water separation should be taken to prevent water from mixing into the oil and causing emulsification.

3.4 Prevent the oil temperature from being too high

The working temperature of the hydraulic oil for motor graders is generally better within the range of 60 to 80℃. If the oil temperature is too high, it will cause the viscosity of the oil to decrease, the volumetric efficiency of the oil pump to drop, the lubricating film to become thinner, mechanical wear to increase, and the sealing parts to age and deteriorate, losing their sealing performance.

3.5 Regularly inspect the hydraulic system

It is necessary to regularly check the pressure at each detection point of the hydraulic system to prevent it from working under abnormal pressure for a long time. This can also reduce the wear of the hydraulic system and ensure its normal operation.