What Are Hydraulic Fluids and Their Common Types?

Posted on August 9, 2021 Jerold Perkins Aviation

Within the realm of hydraulics, hydraulic fluids allow for power to be efficiently transferred throughout a system for the means of actuating and carrying out processes. Hydraulic fluids can come in various forms, all of which feature zero compressibility for the means of providing ample power transfer capabilities. With their ability to transfer power, a hydraulic fluid can provide the energy needed to drive various systems including hydraulic brakes, aircraft flight control systems, lifts, excavators, garbage trucks, and much more. To better understand how hydraulic and hydraulic fluids are used for various common operations, we will discuss their types and applications in more detail.

According to Pascal’s principle of fluid mechanics, a pressure change in a confined and incompressible liquid will result in an equal transmission of that pressure change throughout the rest of the liquid. With an incompressible hydraulic fluid or hydraulic oil that is enclosed within a system, this principle holds true as force exerted to the liquids will allow for power to be transferred with ease. Beyond such roles, hydraulic fluid can also serve other purposes, commonly being used to transfer heat, lubricate parts, enhance pump efficiency, and more.

Different Types of Hydraulic Fluids

In the time of ancient Egypt, water served as the primary hydraulic fluid for all applications. By the 1920s, however, mineral oil began to overtake water in use due to its lubricative abilities and resistance to higher heats. Due to these inherent properties, many modern hydraulic fluid mixtures take advantage of mineral oil as their base stock. Natural oils can come in a variety of forms, rapeseed or canola oil being common due to its reliability and biodegradability. If a fire resistant hydraulic fluid or temperature resistant fluid is required, other base stocks may be used such as phosphate ester, glycol ethers, and silicone oils.

Beyond base stock fluids, there are also other types that one may take advantage of such as biodegradable hydraulic fluids and anti-wear hydraulic fluids. Biodegradable hydraulic fluids are best suited for environmentally sensitive applications such as marine dredging or the operation of farm tractors. Such oils may be specified as ISO 32, ISO 46, or ISO 68 oils, and classification may follow ASTM standards. Anti-wear hydraulic fluids, on the other hand, are those that are made from a petroleum base fluid. To enhance their anti-wear capabilities, such liquids have an anti-wear additive such as Zinc dialkyldithiophosphate (ZDDP) added to their mixture. Anti-wear hydraulic fluids can range in viscosity, and they are commonly used for pumps, excavators, dump trucks, and other applications.

In some instances, such as within aviation applications, hydraulic fluids must be rated to certain performance standards and specifications to be considered fit for use. These fluids typically follow military standards, featuring prefixes and ratings that dictate their quality. For example, hydraulic fluid Mil-P-RF83282 is a type that features fire resistance, taking advantage of a synthetic hydrocarbon base to achieve a higher flashpoint, self-extinguishing properties, and backward compatibility with other fluids. Hydraulic fluid Mil-H-5606 is another example, coming in the form of a mineral base fluid that is flammable, features a low flashpoint, and is usable in environments ranging from -65 degrees Fahrenheit to 275 degrees Fahrenheit. As such, it is very useful to choose hydraulic fluids based on their specifications and standardization to find an optimal fit for a particular need.

As hydraulic fluids are often subject to extreme pressures and temperatures, it is crucial that such systems are well maintained so that failure does not occur. Safety procedures must also be followed during maintenance for the protection of all personnel. If fire hazards are a concern, one should always take advantage of fire resistant fluids for increased protection.

If you find yourself in need of hydraulic fluids or aircraft components such as O-rings and pumps, Aerospace Exchange has you covered with our unrivaled inventory of items. Aerospace Exchange is a leading supplier of hydraulic fluids, providing competitive pricing and rapid lead-times on items that have been sourced from top global manufacturers. Take the time to explore our offerings as you see fit, and our team is always on standby to assist customers through the purchasing process as needed. Get started today with a competitive quote for your comparisons which you may receive through the submission of an Instant RFQ form as provided on our website.

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