Although the original fluid used with the traditional cast iron component hydraulic systems was water, it was soon found to have some major flaws. For one, as soon as the temperature dropped, it would freeze. If the climate conditions were too hot, it would then evaporate. Although water is still used in certain situations and applications, it will usually be emulsified with oil.
These days, the most typical hydraulic fluids are those made from refining mineral oil. In some cases, it’s necessary to make them fire resistant and in these cases they are likely to be manufactured from a variety of different materials blended together.
The advantage of using mineral oil is that is can generally handle extreme temperatures. However these fluids can also suffer from having a low flash point, sometimes between only 150° to 250°C. When there is a fire risk present, fire resistant fluids are typically used.
Although water is a suitable addition where there is a risk of fire, it has some quite obvious issues. By adding 10% emulsified oil to water, it’s possible to gain the required lubrication. Mixing 40% water with oil and special agents will produce a fluid that is fire resistant. A flash point as great as 600°C is possible from using synthetic fire resistant fluids. However, these types of fluids can be very expensive.
To find out more about industrial liquid lubricants and their categorisation you can check the standards of ISO3448 and BS4231.
Hydraulic Fluid is broken down into the following main categories:
Mineral oils – mineral oils are created as a result of refining crude oil and then improving their quality by adding certain substances. They may be labelled as HH which means that it’s a refined mineral oil that is non inhibited. HL has additives to make it anti corrosion and anti-rust. HM type has additives for anti-wear in addition to the additives of HL type.
Fire resistant fluids – there are 4 main types. HFAE is actually an oil in water emulsion. Type HFAB is a 40% water in oil emulsion. Type HRAS is a chemical solution in water and HFC is a water polymer solution containing water glycol. When a synthetic fluid is made from phosphate ester it’s known as type HFDR. HFDS is a synthetic oil that is made of chlorinated hydrocarbons.
Water / oil emulsions – this is when the predominant substance (around 60%) is the oil. Chemicals are used to enable the water to mix into the oil (also known as emulsify). When the fluid touches a hot surface, the water will turn to vapour and prevent a fire from occurring. This mixture also offers good lubrication properties.
Water glycol – known as HFC it comprises of 40% water mixed with 60% glycol. The result is a solution. This mix has the benefit of being able to work at a lower temperature than an emulsion whilst being able to produce an improved temperature viscosity trait.
Phosphate Esters - also known as HFDR these fluids are resistant to fire and will not ignite unless they reach above the temperature of 550°C. The main downside with them is their tendency to be chemically active which leads to them stripping paint and destroying rubber. This means that it’s necessary to use certain types of hoses, seals, etc that are able to withstand the chemical action. They can also melt the external insulation on electrical cables if they leak onto them. They are also known for being quite expensive.
When using hydraulic fluids, it’s critical that they are taken care of. Contamination accounts for up to 70% of faults in hydraulic system. It’s vital to avoid water, air and any solid matter from going into the fluid. This means that strict cleanliness is required when assembling units. Ideally it would take place in a dust free room that is designed to prevent contamination. After performing any work, a cleaning procedure should follow including the flushing of particles from pipes. Filtering systems should be used that can remove particles of between 3 microns to 10 microns (.001 mm = 1 micron).
Finally, due to the high expense of oil, it’s imperative to maintain it to provide a maximum life. Its condition should be checked regularly with records taken for each machine. Contamination should be avoided and filters used.