Hydraulic fluids are meant to transmit motion from one piston to another. They are exposed to compression and expansion forces depending on the motion of the pistons. If they are compressed or expanded in the process, some energy will be lost to expansion and contraction processes respectively. Therefore, the fluids are generally incompressible to ensure maximum transmission of power within the systems. As the fluids resist expansion and contraction, they may be cooled or heated due to change in behavior of the molecules within the fluid. Therefore, the fluids are generally meant to operate at either extremely high or extremely low temperatures. They also contain additives that are meant to reduce the rate at which they heat or cool during contraction and expansion processes.
The metallic hydraulic systems have to be lubricated at all times to prevent formation of rust and exposure to friction. Therefore, hydraulic fluids are equipped with a lubricating feature. In fact, most of them are products of oil and other lubricating fluids. However, the lubrication done by the fluids is not sufficient to prevent corrosion and friction. Additional hydraulic oils have to be used to make the systems exceptionally efficient. With proper lubrication, an efficiency of up to 99% can be achieved.
Thermal stability is an equally important aspect when it comes to hydraulic systems. This is because the systems are exposed to heating and cooling processes as discussed above. Therefore, various additives are included in the hydraulic fluids to make them thermally stable. The additives are also used to ensure longer oil life, minimum corrosion, and general pump protection. The metallic hydraulic systems are generally reactive to most fluids especially those that are water based. The fluids used to run the systems are stable in nature and hardly react with metal even when they are exposed to heat.