The ultimate guide to hydraulic fluid

Hydraulic fluid is the lifeblood of any machinery with a hydraulic system, whether it’s simple or complex. Like engine oil or other fluids used in machines with working parts, we tend to forget that there are different types of hydraulic fluid for different purposes and applications. It isn’t all the same stuff. In this guide, we’ll take a close look at the different types of hydraulic fluid, what they do, and give you information about selecting the right fluid for your machinery.


What is hydraulic fluid?

The simplest answer is that hydraulic fluid is an energy transfer medium or a power transmission medium. There are many types of energy transfer mediums that have the role of relocating energy from one place to another, often converting it into a different form. Hydraulic fluid is a medium for mechanical energy transfer.

The power generated in a hydraulic system is mechanically similar to that in a pneumatic system, which uses air as the transfer medium. Air can be compressed. Hydraulic fluid cannot, which is why hydraulic systems deliver faster, more powerful mechanical performance.

View our range of Hydraulic rams and parts


What are the desirable properties in hydraulic fluid?

The core fluid features that are desirable in most, if not all, hydraulic systems are:

  • Non-compressible
  • Stable viscosity
  • Fire-resistant
  • Non-corrosive
  • Anti-wearing
  • Resistant to water contamination
  • Durable


The multi-purpose roles of hydraulic fluid

After energy transfer, hydraulic fluid has several secondary functions. This is why not all hydraulic fluids are the same. Manufacturers alter them to achieve different characteristics depending on the type of machinery and mechanical application. They do this with a base fluid altered with differing additives. These are the secondary functions of the fluid.


  1. It transfers heat
    The operational nature of hydraulic systems generate a lot of heat. The fluid transfers heat from the moving parts of the system when it returns to the main reservoir, often through a cooler. The fluid can also be used to transfer heat to the moving parts of the system when working in cold conditions.


  1. It removes contamination
    Contamination, in the form of dust, dirt, microscopic debris and water, gets into hydraulic systems. Any abrasion in the moving parts may introduce metal into the fluid. So hydraulic fluid also serves the purpose of carrying these contaminants out of the system when passing through the filter, which prevents damage to machinery over time. This is why regular filter checks and changes are so important.


  1. It acts as a sealant
    While a hydraulic system is technically fully sealed, there are microscopic spaces between seals, pistons and other moving parts. The fluid fills those spaces to create a complete seal. This complete seal has two crucial roles: to prevent contamination getting into the system and to keep pressure constant.


  1. It lubricates the system
    Like any system with metal moving parts, hydraulic systems need lubrication. Without it, parts wouldn’t slide or move easily, which would rapidly create wear and compromise the integrity of the entire system. Oil is the ideal liquid for lubrication, which is why it’s commonly used as the base for hydraulic fluid.


The different types of hydraulic fluids

Mineral-based hydraulic oils are made from refined crude oil with various additives and have very good properties for lubrication.

Synthetic-based hydraulic oils are chemically manufactured for superior performance at high temperatures but usually cost more than their mineral counterparts.

Fire-resistant oils were developed for applications like mining, where fire presents a very serious risk. While they offer this protection, they’re not as effective at preventing wear.

Biodegradable hydraulic oils have been developed for environmental protection where machinery operates in forests, water and the outdoors to mitigate the impact of spills.

Other oils include food-grade oils, specialised agricultural oils for machinery, and thermally stable oils for the aviation industry.

Take a look inside a Motus


The different additives in hydraulic fluid

There is no ideal product for every application, so different additives are blended with base oils to change the characteristics of the oil so they perform better in different applications. In most cases, there’s a trade-off for improving one characteristic of the fluid, meaning operators select products where the benefit far outweighs the performance reduction elsewhere. As above, fire-resistant oils aren’t as good at preventing wear but vastly reduce the risk of catastrophic accidents.

Examples include additives for:

  • Lubrication to form a thin film over moving parts, fill microscopic spaces and reduce friction.
  • Corrosion prevention with rust and oxidation inhibitors for ferrous metals and soft metals such as brass and bronze.
  • Reducing wear by creating a thin film over moving metal parts to reduce losing metal through abrasion.
  • Thermal stability for machines operating at high temperatures.
  • Foaming resistance, where air or water gets into the fluid and affects the performance of the pump.
  • Viscosity-temperature stability so the fluid remains the desired thickness at extreme temperatures, creates good seals and lubricates internal parts.
  • Durability to give the fluid a longer working life, so it takes longer to break down, reducing costs and repairs.
  • Safe disposability to avoid contamination or environmental hazards, which come with hefty penalties.
  • Detergency to keep systems clean and clear contaminants.
  • And others.


Cheap fluids versus expensive fluids

When evaluating what fluid to use in your machinery, cost is always a factor in business. However, cheaper products aren’t always a better option, even if they are kinder on the wallet. Expensive fluids usually have more additives, which means they will last longer, perform better and protect your machinery better. You’ll need to balance the up-front cost of cheaper products against regular fluid replacement, operating temperatures, maintenance and repairs.

If you need any advice, our experts are always available to help.

Contact us today or buy online.