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Choosing the right hydraulic oil

Hydraulic oil is a non-compressible fluid that plays a vital role in the smooth operation of hydraulic systems. In addition to its primary role in transferring power and heat within hydraulic systems, it has a few secondary functions — contamination removal, sealing, and lubrication.  

Hydraulic oil has a wide variety of applications. It powers forklifts and log splitters, allows pilots to control vital systems within aircraft, and ensures the proper functioning of hydraulic brakes on tractors and other farm equipment.

Hydraulic fluids can either be mineral (extracted from naturally occurring crude oil deposits that have undergone a refining process) or synthetic (engineered by chemical synthesis). 

The importance of choosing the right hydraulic oil

Using the right type of oil can improve efficiency and reduce wear and tear, which means using less fuel / energy and extending the life of your equipment.

There are three essential properties to keep in mind when choosing an oil:

  • Temperature range (monograde or multi-grade [higher viscosity index])
  • ISO VG / DIN or SAE requirement (which viscosity grade classification)
  • Zinc-based or zinc-free, anti-wear technology
  • Detergent or no detergent
  • OEM (Original Equipment Manufacturer) requirements
  • Special requirements of area of use or application (food grade / biodegradable / fire resistant technologies, extended drain)

Temperature range (monograde or multi-grade [higher viscosity index])

The grade of an oil describes how temperature affects its viscosity, or thickness. A multi-grade (higher viscosity index) oil will maintain viscosity under a greater range of temperatures, so it’s ideal for equipment that has to deal with extreme changes in climate and for starting equipment at low temperatures. Monograde oils will suffice for equipment that runs continuously.

In extreme cold, hydraulic oil is too viscous and this can lead to a considerable drop in efficiency, which can slow machinery down or even cause it to cease functioning. On the other hand, extremely high temperatures can make the fluid too thin, which can make the system less responsive and more susceptible to wear. Multi-grade oils have different viscosities at low and high temperatures, and as a result, they are effective throughout the year. 

ISO VG / DIN or SAE requirement (Which viscosity grade classification?)

It is always advised to stick to the OEM recommendation as to which type of oil to use. However, how does an SAE type oil differ from ISO / DIN? SAE types can have detergency benefits, meaning how it will act to emulsify water rather than allow it to be separated easily. This can be beneficial for small quantities of water. But, it can affect filterability and have other detrimental effects on equipment when water is higher.  

Detergent or no detergent?

Some hydraulic fluids also contain additives known as detergents, which help remove contaminants from hydraulic systems. This can be especially useful for mobile hydraulic applications, such as construction vehicles, where sludge and varnish build-up is common. 

Zinc-based or zinc-free, anti-wear technology

The source of zinc in traditional hydraulic oils is related to the anti-wear additive system of the hydraulic oil. Zinc-free hydraulic oils used specially developed zinc-free additive systems. These hydraulic oils offer increased performance in systems with known water contamination because zinc-based hydraulic oils are  affected more by the formations of deposits and sludges, due to negative interactions with water and other contaminants. This in turn could lead to filter blockages. Zinc-free hydraulic oils are well suited for systems that have sensitive filterability requirements and/or known accidental water contamination. Furthermore, at higher temperatures zinc and other ash containing oils may form deposits leading to fouling of equipment.

OEM (Original Equipment Manufacturer) requirements

Always make sure to use oils that meet the requirements of the OEM. Some specific things to be on the lookout for are for example the viscosity: is it ISO VG / DIN  [46, 68] or SAE [10W]. Are there any specific requirements in terms of performance [Caterpillar TO-4].

Special requirements of area of use or application

Avoid mixing different hydraulic oils

Different grades and additives are finely balanced by the manufacturer of any particular oil for a given application, so mixing different fluids in one hydraulic system is something that should be avoided. Mixing fluids with different additives may result in unanticipated and unwanted chemical reactions, which can lead to harmful deposits inside sensitive machinery.

For more clarity, view the diagram below: