Hydraulic Oil
Hydraulic oil is the "blood" of a hydraulic system. It not only transmits power but also performs a variety of critical functions, including lubrication, sealing, cooling, and rust prevention.
Hydraulic oil used in hydraulic systems performs multiple functions to achieve system performance:
Energy Transfer: Hydraulic oil's most fundamental role. In a closed system, it transfers pressure energy generated by the pump to the actuator, thereby driving the equipment. Power is transmitted through the pressure acting on the moving parts.
Lubrication: Lubricates contacting surfaces and relative motion between hydraulic cylinder components and seals, as well as other system components such as pumps and valves, reducing friction and wear and extending component life.
Corrosion Protection: Prevents component corrosion.
Sealing: Forms an oil film in gaps, helping to improve sealing effectiveness.
Cooling: Removes heat from high-load areas, moving or turbulent areas, and then distributes it throughout the system, including the oil tank and cooling equipment, thereby cooling the system.
Cleanliness: Maintains system cleanliness by transporting contaminants and wear particles to filters or settling areas. (However, the oil itself can also be a carrier of contaminants.)
The oil used in hydraulic systems comes in a variety of chemical compositions and viscosity grades to suit specific application conditions.
Viscosity: The most important performance indicator.
Viscosity (thickness) measures the internal resistance of an oil to flow. Excessively high viscosity can lead to poor flowability, difficulty starting, high power loss, and increased heat generation. Excessively low viscosity can increase internal leakage, reduce volumetric efficiency, impair lubricity, and exacerbate wear, thus impacting seal performance.
The viscosity of most typical hydraulic oils decreases with increasing temperature and increases with increasing pressure. The viscosity index (VI) can be used to determine the degree to which viscosity changes with temperature. A higher VI value indicates that the oil's viscosity is less affected by temperature, resulting in more stable performance and suitability for environments with large temperature fluctuations.
Hydraulic Fluid Classification.
The most commonly used hydraulic fluid: This type of hydraulic fluid is based on mineral oil and contains various additives. It offers a high cost-effectiveness and meets the needs of most industrial applications. In China, it is commonly categorized as: HL (general-purpose - low pressure, anti-oxidation and anti-rust), HM (anti-wear, high pressure, most commonly used), HV (high viscosity index, high pressure, low temperature), HS (ultra-low temperature), HG (hydraulic guide oil), and HR (HL with improved viscosity-temperature properties).
The viscosity grade of the working fluid is specified based on the average kinematic viscosity at 40°C (mm/s). The density of mineral-based hydraulic fluids is generally between 850 and 960 kg/m².
Among sealing ring materials, EPDM and silicone rubber are not suitable for this type of hydraulic fluid. Other materials, such as polyurethane, nitrile rubber, and fluororubber, perform well with mineral-based oils. When using, pay attention to the material's temperature resistance.
Special Operating Conditions:
Biodegradable hydraulic fluids that can reduce environmental impact in the event of an accidental leak include synthetic esters (HEES), natural vegetable oils (HETG), and polyalphaolefins (PAO).
Hydraulic systems in confined spaces or near fire: Use flame-retardant water-based or synthetic ester hydraulic fluids.
In HFC water-glycol fluids, avoid using polyurethanes and silicone rubbers that are not resistant to hydrolysis.
In HFD phosphate ester fluids, avoid using any polyurethanes, nitrile rubber, or silicone rubber. Fluororubber and EPDM rubber are recommended.
In biodegradable hydraulic fluids, avoid using EPDM and silicone rubber.
Polytetrafluoroethylene (PTFE) has no restrictions on use in the above hydraulic fluids due to its excellent chemical properties and temperature resistance.
For more information on seal selection for sealing systems with special hydraulic fluids, please contact us.
Possible consequences of chemical incompatibility between hydraulic fluid and seal material:
Absorption and reaction of the seal material with the incompatible fluid can reduce seal life and performance.
Material volume changes – "swelling" or "shrinking" – affect contact stress and friction.
Common hardening and embrittlement of seal materials.
Softening, loss of strength, or degradation of seal materials.
Degradation of polymer chains or cross-links, leading to material fatigue or loss of elasticity.
In addition to specific hydraulic fluids, factors that damage seal materials include exposure to fluids in other machine components (such as grease, fuel, and coatings), environmental factors (such as humidity or radiation), degradation, and other chemical substances produced by reactions with system fluids, additives, and contaminants.
Contamination is the leading cause of hydraulic system failure (accounting for approximately 70%-80%). Keep the system clean and regularly inspect and replace filters.
Avoid prolonged high oil temperatures (usually 30-60°C is recommended), as this accelerates oxidation. Prevent leaks: Also prevent the ingress of external contaminants through leaks. Regular monitoring: Perform oil analysis to monitor viscosity, acidity, moisture content, and particulate contamination.
Regular oil changes: Change oil according to oil analysis results or the hours specified in the equipment maintenance manual. Additionally, for contaminated hydraulic fluids, selecting the appropriate seal type can minimize the impact of contaminants on the hydraulic sealing system. For more information, please contact us.
