Hydraulic Fluids Overview

Hydraulic fluid is the medium that 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 large amount of excess heat in normal day-to-day operations. This heat is often caused by the inefficiencies of pumps and motors. The excess heat can be dangerous if there is no way to direct it away from the components in a 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 being pumped back 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 pump fluid at a controlled rate, it will contaminate the system 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 to the circuit. The absence of hydraulic fluid would trap the contamination and destroy the system.

Contrary to popular belief, hydraulic fluid, especially oil, is actually what seals 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 and the oil’s surface tension and resistance to shearing.

Most hydraulic components require lubrication to protect their internal parts from wearing. Lubrication also prevents melting, which may happen as a result of metal-to-metal friction. Oil lubricates 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 the hydraulic fluid appropriate for your application.

Hydraulic oil is engineered with additional important properties; 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 water retention properties. These additives are what separate low and high-quality fluids.

If possible, always choose premium fluid for your application, and choose World Wide Metric for premium-quality machine parts and equipment.