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Hydraulic fluid function and basic requirements

Update:04-08-2016
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The oil pressure fluid is the working medium for transf […]

The oil pressure fluid is the working medium for transferring energy in the hydraulic system, and also has the functions of lubrication, sealing, cooling and rust prevention.
In the hydraulic system, since the pressure, speed and temperature vary within a wide range, in order to ensure the stability of the working state, the applied hydraulic oil is required to adapt to this change, and maintain stable performance without being changed due to external conditions. Causes great changes or damage, so the following basic requirements are imposed on hydraulic fluids:
a, has a suitable viscosity and good viscosity temperature characteristics. Viscosity requirements are in line with actual working conditions. If the viscosity is too large, the friction loss will increase. If the viscosity is too small, it will cause leakage. If the viscosity is too large or too small, the efficiency will decrease. Therefore, in order to enable the hydraulic pump to work stably, the viscosity of the hydraulic fluid changes little with temperature, that is, it has good viscosity-temperature characteristics.
b, has excellent lubricity. The hydraulic fluid lubricates the moving parts of the hydraulic system to reduce friction and reduce wear and ensure that the system can work normally for a long time. At present, the hydraulic system and components are developing toward high pressure and high speed, and the internal friction pair of the hydraulic component is in the boundary lubrication state. At this time, the hydraulic fluid should have good lubricity.
c, has good chemical stability. When the hydraulic fluid comes into contact with the air, it will produce colloidal deposits. These deposits adhere to the surface of the valve or the throttling gap will block the passages such as holes and gaps, affecting the movement of the components, thereby reducing system efficiency. Therefore, the hydraulic fluid should have good chemical stability.
d, good shear stability, hydraulic oil through the hydraulic components and narrow passages to withstand severe shear, causing some polymeric tackifier molecules to break, resulting in a permanent decrease in viscosity, which is particularly serious at high speeds and high pressures. In order to prolong the service life of hydraulic fluid, the shear safety of hydraulic fluid is better.
e, good demulsibility. Water may be mixed into hydraulic fluid from different sources. The hydraulic fluid contained in the water is easily emulsified under the vigorous stirring of the pump and other components, causing the hydraulic fluid to deteriorate or form sediment, hindering the heat conduction of the cooler and blocking the valves and pipes. Reduces lubricity and corrodes metals, so hydraulic fluids should have good demulsibility.
f, defoaming anti-foaming performance is good. In the atmosphere, mineral oil can usually dissolve 5%~10% of air. When air is mixed into hydraulic fluid, bubbles will be generated. Bubbles will circulate in the hydraulic system, which will not only reduce the rigidity of the system, but also deteriorate the dynamic characteristics and deteriorate the lubrication conditions. And it will also produce abnormal noise and vibration. In addition, the bubbles also increase the contact with the air to accelerate the oxidation, so the hydraulic fluid should have good defoaming and anti-foaming ability.
g, good rust resistance and low corrosiveness to metals. Metal parts that have been in contact with hydraulic fluid for a long time will cause rust under the action of moisture and air dissolved in the hydraulic fluid, which will deteriorate the accuracy and surface quality. Corrosion particles circulate in the system, which can also accelerate wear and system failure. Therefore, hydraulic fluids should have good rust resistance and non-corrosive properties.
h, compatibility with materials such as seals. The long-term coexistence of the sealing material in the hydraulic fluid will cause swelling and softening or shrinkage hardening, which will cause the seal to fail, cause leakage, and the system pressure will drop, resulting in abnormal work. Therefore, the hydraulic fluid should have good compatibility with the sealing material.

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