Hydraulic fluid is the medium which transfers energy in all hydraulic systems. Although this seems like a simple enough concept, the job of hydraulic fluid is much more complex than just the transmission of power. Hydraulic fluid is additionally useful for four secondary functions: heat transfer, contamination removal, sealing and lubrication.
Hydraulic machines tend to emit a good amount of excess heat in their normal day-to-day operations. This heat is often caused by inefficiencies of pumps and motors. The excess heat has the potential to be dangerous if there is not a way to direct it away from the components in your system. Seals and internal components can easily be damaged, especially as a result of low local viscosity. When oil returns to the reservoir, it should pass through a cooler to help maintain an ideal temperature range before it is pumped back out into the system. Hydraulic fluid can also carry heat into a system during cold starts.
If a closed loop hydraulic system is not able to bleed off fluid at a controlled rate, the system will become contaminated to the point where it may be dangerous. Heat is considered a form of contamination along with particles and water. Hydraulic fluid can carry these things away from sensitive components through filters or a conditioning device where it is removed, cleaned and returned into the circuit. Without hydraulic fluid, the contamination would become trapped and destroy the system.
Contrary to popular believe, hydraulic fluid (especially oil) is actually what provides sealing within the internal components of your system’s pumps, valves and motors. For example, a spool valve has a seal at each end in order to prevent oil from escaping the valve but the notch on the spool is sealed from neighboring cavities by the metal-to-metal tolerances, the oil’s surface tension and resistance to shearing.
For most hydraulic components to protect their internal parts from wearing, lubrication is required. Lubrication also prevents melting which could happen as a result of metal-to-metal friction. Oil lubricates in between moving parts and without it, hydraulic systems would be inefficient and completely unreliable.
These four secondary functions are common to all types of hydraulic fluid, with the exclusion of some water-based fluids. The majority of hydraulic systems use refined or synthetic oil which are fluids that are created according to specific test standards for important properties such as viscosity, pour point and viscosity index. These three properties are significant when choosing your hydraulic fluid appropriate for your application.
Hydraulic oil is engineered with additional important properties; however these will be common regardless of the brand, viscosity or application. Hydraulic oil normally features chemical additives that improve the performance of the oil and the components of your hydraulic system. Features include improvement with the foaming resistance of the oil, its corrosion, rust resistance and its water retention properties. These additives are what separates low and high quality fluids. If possible, always choose and premium fluid for your application.
“Hydraulic Fluids” Design World. June 2014: 84. Print