Hydraulic fluid is the energy transfer medium in all hydraulic systems. However, the job of hydraulic fluid goes beyond simple transmission of power. Although transmitting hydraulic energy is the core purpose of hydraulic fluid, it is useful in four secondary functions—heat transfer, contamination removal, sealing, and lubrication.
Hydraulic machines produce a lot of excess heat in normal operation, often caused by inefficiencies of the components themselves, like pumps and motors. Without a way to carry heat away from these components, they could easily overheat with resulting damage of seals and internal components, especially as a result of low local viscosity. As oil returns to the reservoir, it often passes through a cooler to help maintain optimal temperature range before it is pumped back out to the system. Conversely, hydraulic fluid can carry heat into a system during cold starts, when needed.
If closed-loop hydraulic systems didn’t bleed off fluid at a controlled rate, contamination would quickly accumulate to critical and damaging levels. Heat can be considered a form of contamination, but hydraulic fluid also carries particles and water away from sensitive components through filters or other conditioning devices, where it is cleaned up and returned to the circuit. Oil without impetus would remain within sensitive components, allowing essentially trapped contamination to slowly destroy its surroundings.
Although most believe hydraulic oil is what pieces of hardware—such as O-rings or U-cups—seal against, hydraulic fluid (especially oil) actually provides sealing within the internal components of pumps, valves and motors. A spool valve, for example, has a seal at each end to prevent oil from escaping the valve, but each notch on the spool is sealed from the neighboring cavities by only the tight metal-to-metal tolerances and the oil’s surface tension and resistance to shearing.
Lubrication is required in most hydraulic components to protect internal parts from wearing, or even downright melting, as a result of metal-to-metal friction. Oil provides full-film lubrication between moving parts, such as the slippers and lens plate of a piston pump. Without the lubricating properties of oil, hydraulic systems would be terribly inefficient and awfully unreliable.
These functions of hydraulic fluid are common to all types, except some water-based fluids, which require special design considerations during engineering. However, the majority of machines use refined or synthetic oil, which are formulated and manufactured to specific test standards for important properties like viscosity, pour point and viscosity index, to name a few.
Those three properties are often considered when choosing a fluid for a particular application, which is based on maintaining a specific viscosity throughout a particular set of ambient and machine operating conditions. For example, if ambient temperatures are low, you would choose an oil with lower rated viscosity and a low pour point, which is the temperature at which oil will still pour. If your machine sees varying temperature ranges, like an all-weather mobile machine, a high viscosity index is very important, which describes an oil’s ability to maintain its viscosity over a wide temperature range. It is important to consider the viscosity requirement of the components in your hydraulic system. A piston pump, for example, may require between 16 and 40 centistokes, which is a description of kinematic (i.e., measured while flowing) viscosity.
Hydraulic oil is engineered with other important properties, although these tend to be common regardless of brand, viscosity or application. Hydraulic oil has a package of chemical additives to improve the performance of both the oil and the components of the hydraulic system. These additives can improve the foaming resistance of the oil, its corrosion/rust resistance and the water-retention properties. The additive package of hydraulic fluid is what separates low and high quality fluids, and additives also improve the viscous properties of oil as well. When in doubt, always choose a premium fluid for your application at a viscosity appropriate for your operating conditions.