The performance and evaluation methods of hydraulic oils are crucial to ensure the optimal functioning of hydraulic systems, which are widely used in industrial, construction, and automotive machinery. Hydraulic oil must possess several key properties to meet the demanding requirements of modern hydraulic systems, which are becoming more compact, high-pressure, and automated. This article outlines the essential properties of hydraulic oil, the methods used for their evaluation, and the importance of each property for the effective operation of hydraulic systems.
1. Good Shear Stability
Shear stability is a vital property for hydraulic oils, especially those containing viscosity index improvers (VII), which are high molecular weight polymers designed to enhance the oil’s viscosity-temperature characteristics. However, under shear conditions, the polymer chains of these VII additives can break, leading to a reduction in the oil’s viscosity and thus deteriorating its performance. Therefore, hydraulic oils must exhibit good shear stability to maintain their viscosity even after prolonged shear forces.
The shear stability of hydraulic oils can be evaluated using several standard methods, including the ultrasound shear test (SH/T 0505-92), diesel nozzle shear test, Vickers pump shear test, and FZG gear machine shear test. These tests involve measuring the percentage decrease in kinematic viscosity at a specified temperature after applying shear forces to the oil.
2. Excellent Extreme Pressure and Anti-Wear Properties
The trend towards smaller and higher-pressure hydraulic pumps has made it essential for hydraulic oils to possess excellent extreme pressure (EP) and anti-wear properties. Extreme pressure refers to the ability of hydraulic oil to maintain a lubricating film between moving parts under high loads, preventing metal-to-metal contact and wear. Anti-wear properties are crucial for protecting hydraulic components and ensuring smooth and reliable operation under stress.
To evaluate the anti-wear properties of hydraulic oils, the four-ball wear test is commonly used (SH/T 0189-92). This test involves subjecting oil to a load of 150N or 400N at 75°C and 1200 RPM for 60 minutes, then measuring the wear scar diameter on the balls to assess wear resistance. Additionally, gear test rigs (SH/T 0306-92) and the vane pump test (SH/T 0307-92) can be used to evaluate the wear characteristics of hydraulic oils in practical applications.
3. Good Fluidity
The fluidity of hydraulic oil is critical for its ability to transfer energy efficiently. Fluidity is directly related to viscosity, pour point, and viscosity-temperature behavior. Hydraulic oils must remain fluid at low temperatures to ensure the smooth operation of hydraulic systems. Poor fluidity at low temperatures can result in high friction losses and slow pump speeds, while excessive thinning at high temperatures can reduce the oil’s lubricating ability.
Hydraulic oils are typically formulated with additives that improve their viscosity-temperature behavior. These oils should maintain an optimal balance between low-temperature fluidity and high-temperature stability. The pour point and low-temperature viscosity are key parameters in determining the suitability of hydraulic oil for specific operating environments.
4. Incompressibility and Anti-Foam Characteristics
The incompressibility of hydraulic oil is essential for the accurate and reliable transmission of energy within the hydraulic system. If air is entrained in the oil, it can significantly affect its incompressibility. This can lead to erratic system behavior, decreased performance, and potential damage to the hydraulic components.
To maintain hydraulic oil’s incompressibility, it is crucial to prevent air from entering the system. Additionally, anti-foam agents are added to hydraulic oils to prevent the formation of foam, which can further compromise system performance. The air release value of hydraulic oils is measured to assess their ability to release dissolved air under specific conditions, using the SH/T 0308-92 method.
Foam stability is evaluated using the foam test (GB/T 12579-90), which measures the tendency of the oil to generate foam and the stability of any foam that forms under specified conditions.
5. Excellent Oxidation Stability
Oxidation stability refers to the ability of hydraulic oil to resist chemical degradation when exposed to oxygen at elevated temperatures. Oxidation leads to the formation of sludge and acidic compounds that can damage hydraulic components, such as seals and pumps, and impair the oil’s lubricating properties.
To evaluate the oxidation stability of hydraulic oils, the oil is subjected to accelerated oxidation tests, such as the rotating bomb oxidation test (SH/T 0193-92). This test measures the time required for the oil to reach a specified acid number, indicating the onset of significant oxidation. The test conditions typically involve exposing the oil to oxygen at high pressure and temperature in the presence of a catalyst.
6. Sealing Compatibility
Sealing compatibility is another crucial property for hydraulic oils, as hydraulic systems often experience internal leakage or external leakage due to improper sealing materials or oil incompatibility. These leaks can lead to system inefficiencies, environmental contamination, and damage to components. Hydraulic oils must be compatible with the sealing materials used in the system to prevent swelling, degradation, or leakage.
The sealing compatibility of hydraulic oils is evaluated by measuring the volume expansion of a rubber ring submerged in the oil at 100°C for 24 hours. The change in the inner diameter of the ring is used to calculate the sealing compatibility index (SH/T 0305-92).
7. Anti-Rust Properties
Anti-rust properties are essential for preventing corrosion of the metal parts in hydraulic systems, especially in the presence of water or moisture. Rust can significantly damage hydraulic components and compromise the performance of the system.
To evaluate the anti-rust properties of hydraulic oils, the oil is tested using the GB/T 11143-89 method, which involves immersing a steel rod in a mixture of oil and distilled water or synthetic seawater at 60°C. After the prescribed time, the rod is examined for signs of rust, and the degree of corrosion is assessed.
8. Good Filtration Performance
As hydraulic systems become smaller, more precise, and more sensitive to contaminants, it is crucial for hydraulic oils to have good filtration properties. The presence of even small particles can cause wear, clog filters, and lead to system malfunctions.
The filtration performance of hydraulic oils is evaluated using the SH/T 0210-92 method, which measures the time required for a specific volume of oil to pass through a 1.2 μm filter membrane under a vacuum of 86.7 kPa. Oils that are contaminated with water are also tested to assess how the presence of water affects the filtration performance.
9. Demulsibility and Hydrolytic Stability
Demulsibility refers to the oil’s ability to separate from water when mixed, while hydrolytic stability measures its ability to resist degradation when exposed to water. These properties are particularly important for hydraulic oils used in environments where water contamination is likely, such as marine or offshore applications.
The demulsibility of hydraulic oils is evaluated using the GB/T 7305-87 method, which measures how long it takes for an oil-water emulsion to separate into distinct layers after agitation. Hydrolytic stability is evaluated by sealing the oil, water, and a copper strip in a pressurized glass bottle and exposing it to elevated temperatures for a specified time (SH/T 0301-92). The oil’s ability to resist degradation and separation is then assessed.
Conclusion
Hydraulic oils play a critical role in the performance, efficiency, and longevity of hydraulic systems. The evaluation of their properties—such as shear stability, extreme pressure and anti-wear characteristics, fluidity, incompressibility, oxidation stability, sealing compatibility, anti-rust performance, filtration ability, and demulsibility—is essential for selecting the right oil for specific applications. By using the appropriate testing methods, manufacturers and engineers can ensure that hydraulic oils meet the demanding requirements of modern hydraulic machinery and equipment, thereby optimizing performance and reducing the risk of failure.
