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Hydraulic Fluid - Preserving Quality ben lee

Tips for Preserving The Quality of Hydraulic Fluid

Preserving the quality of hydraulic fluid is something that will make a huge difference to the life span, lack of downtime and condition of your Hydraulic Systems and Machines. If it’s condition is allowed to degrade, you’ll be setting yourself up to have to deal with cavitation, machine damage and eventually the machine could even come to a halt. This is not going to look good in front of your boss nor your next employer.

That being the case, let’s explore what you need to know in order to preserve the quality of hydraulic fluid in your system.

After you’ve completed reading this post, you should understand what hydraulic fluid should be like, whether it needs any additives put into the fluid and how to get maximum life from your fluid. 


As we have covered, without your hydraulic fluid being in good condition, there is likely to be a negative effect on the running of your hydraulic machine. We always recommend to our customers that they continue to use the fluid that the manufacturer of their machine has suggested. It’s also wise to use filters in order to prevent the fluid from deterioration through contamination. The pump and reservoir unit should also be considered as these play a critical role in the health of your fluid.


Ideal hydraulic fluid properties

These are the properties that you want your hydraulic fluid to have in order for it to operate at it’s best.

Compressibility – it’s not very easy to squeeze liquids into a lesser volume. This is why precise motion control is one of the strong points of hydraulics. If air enters the system, then it takes it into being compressible and it won’t work as it should. You can test how compressible fluid is by forcing fluid into a rigid vessel with a screwed plunger and measuring the pressure.

Viscosity – this is a measurement of how easily the fluid will flow. Low viscosity fluid (for example water) will flow very easily, whereas high viscosity fluids will flow slowly and with some difficulty. This is what will result in loss of pressure. However, it’s only fluids with high viscosity that will lubricate well, so a balance needs to be sought.

Viscosity index – as a fluid gets hotter, its viscosity will usually decrease. This means that as it heats up it can become less effective at lubricating. Less change can be expected from a fluid with a low viscosity index. It might be possible to improve on this with the addition of chemical additives.

Air absorption – when liquids are under pressure they will absorb gas and then when the pressure is released, they will release it again. This can be seen with fizzy drinks. Air will not be absorbed easily by a good hydraulic fluid without causing foam and froth. It’s possible to add chemicals in order to improve this. Chemicals can be added to prevent foam from building up on the surface of the reservoir.

Oxidation – this can occur when oxygen is in the fluid and combining with elements. It can cause the oil to thicken to produce a varnish. This will stain the surface of the components and will reduce the life of the oil. Although there are additives that can help with this, it’s important to keep out air as much as possible. The main reason for air and fluid mixing is when there is foam and cascading in the reservoir.

Corrosion – corrosion of metals can be caused by hydraulic fluids. This can be helped by using materials that are compatible with it in addition to the addition of chemicals to the fluid.

Wear – when chemicals have been added which encourage the development of surface film where surfaces meet, such as in pumps and motors, it’s possible to slow down wear.

Pour point – this is the name given to the lowest temperature point at which the fluid will flow from a container when tipped up. If you’re working in cold climates, it’s possible to add chemicals to lower this temperature.

Flash point – the name given to the point when the vapour produced by a fluid will ignite when a naked flame makes contact with it. A Pensky Martins apparatus is used to measure this.

If you haven’t done so already, take time to become familiar with these technical engineering terms. They will prove useful for testing, explaining if you need advice from a third party and for keeping records of the condition of both the fluid and your hydraulic system.

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