Abstract: Hydraulic systems are widely used in various industrial equipment. Besides system design, component manufacturing, and maintenance, the cleanliness of the hydraulic oil is a crucial factor in the normal operation of a hydraulic system. Oil contamination will affect the normal operation of the system, cause excessive wear on components, and even lead to equipment failure. Hydraulic oil is to hydraulic equipment what blood is to life; clean hydraulic oil circulating within the machinery is essential for ensuring normal operation and lubrication. Data shows that 70%-80% of unstable operation and malfunctions in hydraulic systems are related to hydraulic oil contamination. Hydraulic oil contamination refers to the presence of impurities such as moisture, air, small solid particles, and colloidal substances in the hydraulic oil. Hydraulic oil contamination is often a major cause of system failure. Therefore, controlling hydraulic oil contamination is extremely important. Keywords: Hydraulic system, hydraulic oil contamination, contamination control 1. Causes of Hydraulic Oil Contamination The causes of hydraulic oil contamination are complex, but generally fall into the following categories: 1.1 Residual Contamination: This mainly refers to contamination caused by residues such as sand, iron filings, abrasives, welding slag, rust, oil sludge, cotton yarn, and dust introduced during the manufacturing, storage, transportation, installation, and maintenance of hydraulic components, pipelines, and oil tanks. Even after cleaning, these residues may not be completely removed, leading to hydraulic oil contamination. 1.2 Intrusive Contamination: This mainly refers to contaminants from the surrounding environment, such as air, dust, and water droplets, entering the system through all possible entry points, such as exposed reciprocating piston rods, vent holes in the oil tank, and oil filling holes. Other causes include neglecting cleaning during maintenance, introducing environmental contaminants, using coarse filters instead of fine ones, not using filters for years, not cleaning filter screens frequently, not filtering oil during oil changes or replenishment, and using dirty oil containers without thorough cleaning, all of which introduce contaminants. 1.3 Pollution from byproducts: This mainly refers to hydraulic oil contamination caused by metal particles, wear particles from sealing materials, paint peeling flakes, moisture, air bubbles, and colloidal substances resulting from oil deterioration generated during the operation of the hydraulic transmission system. These particulate contaminants are similar to abrasives used in metalworking, spreading throughout the hydraulic system with the flow of hydraulic oil. When passing through hydraulic components such as pumps, cylinders, and valves, they exacerbate the wear of friction pairs, generating new contaminant particles, creating a vicious cycle, significantly reducing the service life of components, and seriously threatening the normal operation of the hydraulic system. Oil decomposition residues and surface-active media can corrode machine parts and disperse surface contaminants into the oil, making them difficult to remove. They also reduce the ability of filters to adhere to contaminants, often clogging throttling orifices. This can lead to hydraulic component failure and accidents. 1.4 Other oils mixed in with the hydraulic oil. Different types and grades of hydraulic oil have different chemical compositions. When other oils are mixed into the hydraulic oil, their chemical composition changes, thus altering their properties. 1.5 Contamination Level: The contamination of the working medium is expressed using a contamination level, which refers to the content of solid particulate contaminants per unit volume of the working medium, i.e., the concentration of solid particles in the working medium. To quantitatively describe and assess the contamination level of the working medium, the International Organization for Standardization (ISO) standard ISO 4406 provides a contamination level standard (Table 1.4). The contamination level uses two sets of numbers to represent the degree of solid particle contamination in the working medium. The first set of numbers represents the number of particles with a size of not less than 5 μm per mL of working medium, and the second set of numbers represents the number of particles with a size of not less than 15 μm per mL of working medium. The two sets of numbers are separated by a slash. For example, hydraulic oil with a contamination level of 18/15 indicates that it contains between 1300 and 2500 particles with a size of not less than 5 μm per milliliter, and between 160 and 320 particles with a size of not less than 15 μm per milliliter. 2. Hazards of Hydraulic Oil Contamination The main hazards of hydraulic oil contamination to the hydraulic transmission system are: 2.1 Solid particles and colloidal products clog filters, causing poor oil suction and difficulty in operation of the hydraulic pump, resulting in noise; they also clog the small holes or gaps of valve components, causing them to malfunction; 2.2 Tiny solid particles accelerate the wear of surfaces with relatively sliding parts, preventing hydraulic components from functioning properly; they can also scratch seals, increasing leakage; 2.3 The intrusion of moisture and air reduces the lubrication capacity of the hydraulic oil and accelerates its oxidation and deterioration; it causes cavitation, accelerating the damage of hydraulic components; and it causes vibration and creep phenomena in the hydraulic transmission system. [b]3. Control of Working Medium Contamination[/b] Because the causes of hydraulic oil contamination are complex, and the hydraulic oil continuously generates contaminants during the operation of the hydraulic transmission system, it is very difficult to completely prevent contamination. To extend the service life of hydraulic components and ensure the normal operation of the hydraulic transmission system, the degree of hydraulic oil contamination should be controlled within a certain range. The following measures are generally taken to control pollution: 3.1 Eliminate residual pollution: Before and after assembling the hydraulic system, parts must be thoroughly cleaned. 3.2 Reduce external pollution: To reduce pollution sources in the hydraulic system, improve the operating environment of the equipment, strengthen dust control, and reduce dust at the work site. An air filter should be installed at the oil tank vent to the atmosphere. Oil should be added to the oil tank through a filter. Maintenance and disassembly of components should be carried out in a dust-free area. 3.3 Filter out impurities generated by the system: Depending on the different requirements of the system and components, filters should be installed at the pump suction port, pressure line, pump suction line, return line, and the inlet of the servo valve or speed control valve, according to the required filtration accuracy. The dirt-holding capacity should also be considered when selecting filters. Under the same accuracy conditions, filters with a larger filtration area should be selected as much as possible. When necessary, an external circulation filtration system can be added (in which case a larger βn can be selected) to improve the system's contaminant control level. The filter screen should be checked regularly for cracks; if cracks are found, it should be replaced promptly. Deteriorated oil and oil with excessive cleanliness are prohibited from use. The inner wall of the oil tank should generally not be painted to avoid the formation of sediment in the oil. To prevent air from entering the system, the return oil pipe should be below the oil level in the tank, and the hydraulic pump and suction pipe should be strictly sealed. Filters of appropriate precision should be installed in relevant parts of the system as needed, and the filter elements should be checked, cleaned, or replaced regularly. 3.4 Controlling the operating temperature of hydraulic oil: In the absence of specific requirements, a volumetric speed control circuit can be considered as a priority, as it has low temperature rise and high efficiency. Increasing the oil tank capacity and using natural ventilation cooling can alleviate oil temperature rise. Alternatively, a dual-tank structure can be used to adjust oil temperature under different temperature rise conditions. When the system has significant power loss and high heat generation, and the structure does not allow for a large oil tank capacity, a cooler can be used for forced cooling. 3.5 Strengthening the maintenance and management of the hydraulic system 3.5.1 Selecting suitable hydraulic oil. Based on the characteristics of the hydraulic system and the operating environment, suitable hydraulic oil should be selected. Firstly, it should have suitable viscosity and a suitable solid particle contamination level. Secondly, the oil's oxidation resistance, demulsibility, and the presence of wear-resistant additives should be considered. Furthermore, the compatibility of the hydraulic working medium with the component metal materials and their sealing materials must be considered. 3.5.2 Strengthening oil management. To ensure the quality of oil products leaving the warehouse, regular sampling and testing of stored oil is necessary. New oil should be tested upon arrival at the warehouse; substandard oil should not be allowed to enter the warehouse. Oil should be properly stored; a "oil usage card" should be established for hydraulic equipment; oil should be filtered during oil transfer drums or injection; and attention should be paid to the cleanliness of containers such as oil drums, oil inlets, and funnel openings. 3.5.3 Regularly clean the filter elements, oil tanks, pipes, and internal components to remove dirt, and replace filter elements regularly. Establish a first-level maintenance system for the hydraulic system; 3.5.4 Determine whether the oil needs to be changed by checking the oil quality. Because different hydraulic oils have different service lives, the service life of the same hydraulic oil varies greatly under different equipment, environments, and maintenance conditions. Commonly used methods for detecting hydraulic oil contamination include: ferrography (this method cannot detect non-magnetic contaminants), spectroscopic analysis (this method cannot analyze particles larger than 5µm), gravimetric analysis (this method cannot verify the size and distribution of contaminant particles), and automatic particle counting (which can directly read the size and distribution of particles). These methods are limited in certain situations, such as in the field and production sites; in these situations, portable contamination measuring instruments can be used for detection. Examples of suitable instruments include DCA digital display contamination alarms, CM20 testers, KLOTZ contamination detectors, PFC200 particle counters, and PCM100 contamination detectors. If these instruments are unavailable, visual inspection and colorimetric methods can be used. Visual inspection involves estimating the degree of hydraulic oil contamination by observing its color, smelling its odor, and feeling its smoothness. Alternatively, two clean, transparent glass bottles can be used; one containing the hydraulic oil to be tested, and the other containing new hydraulic oil. Holding both bottles up to the sun allows for estimation of the contamination level. Colorimetric methods involve filtering contaminants from a specific volume of oil sample using filter paper, and then judging the degree of contamination based on the color of the filter paper. Specifically, take a small amount of the same quantity of used oil and a small amount of the same grade of pure oil, and drop them separately onto filter paper. After a certain period, compare the colors of the two filter papers to determine the degree of oil contamination and whether an oil change is necessary. When changing the oil, the oil tank should be cleaned, and the system piping components should be flushed. In order to effectively control the contamination of hydraulic systems and ensure the reliability of hydraulic system operation and the service life of hydraulic components, the cleanliness levels of typical hydraulic components and hydraulic systems formulated by the state are shown in the table. [b]References:[/b] (1) Liang Jie, Wang Huijun, et al. Hydraulic and Pneumatic Control. [J] Beijing: People's Transportation Press, 1999, 11(2): (2) Song Yonggang, Long Shuigen. Engineering Machinery English [J]. 20063): 56-59 (3) Wang Chunxing, Hydraulic Control System, Beijing: Machinery Industry Press, 2000 Editor: Chen Dong