Hydraulic Oils and Transmission Fluids

Hydraulic oils and transmission fluids are used to transmit power in hydraulic equipment and power transmission applications. There are several basic types of products. Hydraulic fluids transmit power or pressure generated by a pump through hydraulic lines to a cylinder or actuator. Transmission fluids lubricate gearbox assemblies where power is transmitted from an engine to a driving axle. Most industrial hydraulic oils and transmission fluids consist of petroleum or mineral oils, oil-water emulsions, synthetic lubricants, or water-glycol mixtures. High water content fluids (HWCF) and high water base fluids (HWBF) are used in applications where the leakage of a flammable fluid is likely to cause ignition. Wax, paraffin and stearate compounds are well-suited for anti-corrosive and anti-static applications. Some natural oils pose fewer risks to workers and have a reduced environmental impact. Hydraulic oils and transmission fluids that are silicone-based or thickened with sodium or calcium soaps are commonly available.

Hydraulic oils and transmission fluids vary in terms of kinematic viscosity, operating temperature, pour point, boiling point, and flash point. Kinematic viscosity is the time required for a fixed amount of fluid or oil to flow through a capillary tube under the force of gravity. Units of measure include stoke, centistoke (1/100 of stoke) and Saybolt universal seconds (SUS). Pour point, the lowest point at which a fluid flows, is generally 15° F to 20° F below the system’s lowest end-use temperature to prevent pump damage through cavitation. Flash point is the lowest temperature at which a liquid produces sufficient vapors to form an ignitable mixture in air near the surface of the liquid. Fire point and autogenous ignition are important considerations when selecting fire resistant fluids. Fire point is the lowest temperature at which a liquid can produce sufficient vapors to form a mixture in air that continuously supports combustion after ignition. Autogenous ignition (AIT) is the temperature at which ignition occurs spontaneously.  

Hydraulic oils and transmission fluids are used in a variety of applications and industries. Some products are designed for aerospace, automotive, marine, or military applications. Others are used with combustion engines, processing equipment, compressors, piston pumps, gears, and final drives. Passivators or deactivators are applied to internal or machined surfaces. Fluids with extra pressure (EP) additives form a film to prevent sticking or seizing under heavy loads. Industrial hydraulic oil and transmission fluids can inhibit oxidation, corrosion, or microbial growth; provide sealing or barrier functions; or dampen shock and vibration. Biodegradable products are designed to break down into harmless chemicals when released into the environment. Non-foaming characteristics are achieved through the use of additives that break out entrained air. Hydraulic oils and transmission fluids that are rated for food, beverage, and pharmaceutical applications are also available.

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