How to select the correct hydraulic fluid

Hydraulic systems used to transmit power efficiently over relatively short distances are complex and precise machines. Using the right hydraulic fluid is essential to achieving the optimum performance and lifetime from the system. But how do you know which is the correct fluid to use?

With hydraulics, there are two primary considerations – the viscosity grade and the hydraulic oil type. These specifications are typically determined by the type of hydraulic pump employed in the system, operating temperature and the system’s operating pressure. But it doesn’t stop there.

Other items for consideration are: base oil type, overall lubricant quality and performance properties. A system’s requirements for these items can vary dramatically based on the operating environment, the type of machine for which the unit is employed and many other variables.

Selecting the best product for your system requires that you collect and utilize all available information.

The first selection criterion is the type of pump. Vane pumps provide a steady fluid flow but can suffer high rates of wear between the vanes and the cam ring if a suitable antiwear oil is not used. Piston pumps are more durable and can produce much higher operating pressures but are relatively complex and expensive. Gear pumps are less expensive, easier to lubricate, and can cope with higher amounts of contamination, but are the least efficient and can only provide relatively low pressures.

Hydraulic fluid has many roles in the smooth operation of a well-balanced and designed system. These roles range from a heat transfer medium, power transfer medium and a lubrication medium.

The chemical makeup of a hydraulic fluid can take many forms when selecting it for specific applications. It can range from full synthetic (to handle drastic temperature and pressure swings and reduce the rate of oxidation) to water-based fluids used in applications where there is a risk of fire and are desired for their high water content.

A full synthetic fluid is a man-made chain of molecules that are precisely arranged to provide excellent fluid stability, lubricity and other performance-enhancing characteristics. These fluids are great choices where high or low temperatures are present and/or high pressures are required. There are some disadvantages to these fluids, including: high cost, toxicity and potential incompatibility with certain seal materials.

A petroleum fluid is a more common fluid, and is made by refining crude to a desired level to achieve better lubricant performance with the addition of additives, which range from anti-wear (AW), rust and oxidation inhibitors (RO), and viscosity index improvers. These fluids offer a lower cost alternative to synthetics and can be very comparable in performance when certain additive packages are included.

Water-based fluids are the least common of the fluid types. These fluids are typically needed where there is a high probability of fire. They are more expensive than petroleum but less expensive than synthetics. While they offer good fire protection, they do lack wear-protection abilities.

Application should be the most critical attribute when selecting a hydraulic fluid in order to ensure the system’s ability to function properly and attain long life. When selecting a hydraulic fluid, it is very critical to determine the system’s needs: viscosity, additives, operation, etc.

When choosing a hydraulic fluid, ask the supplier if it is approved or recommended by the equipment manufacturer. Also ask for performance information against the international or national specifications. The fluid should be delivered clean and water-free so it will perform at its best. It’s very important to conduct used oil analysis as recommended by the supplier to determine when the oil may need maintenance or replacing.

For more information on choosing the right hydraulic fluid, contact our technical team on hos@hos.co.uk  or visit our dedicated SERVICES page, we’re here to help.