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Reducing and preventing oil contamination in hydraulic systems ben lee

Contaminated hydraulic oil is the biggest cause of system failure in hydraulic machinery and often it is entirely avoidable.  Mistakes happen, and there is always room for improvement in maintenance and routine replacement activities which can help reduce contaminants in the system.  Even when you have got everything right in that area, there are still extra tweaks you can make or things to avoid when refining your hydraulic machinery care process.


Using the correct weight and ISO rated hydraulic oil is essential operating practise with any type of hydraulic equipment.  Using liquid that is too thin or one designed specifically for a different type of motor, can cause serious damage to the internal parts through overheating or having an unsuitable level of intrinsic contamination.  However, it is possible to go one step further than simply using a dedicated oil; By checking the ISO rating of the standard oil and the rating that the machinery requires, and then using one with lower ratings, i.e. with a higher level of cleanliness it is possible to improve the lifespan of components operating at a higher than average pressure, speed or length of operation.  These factors affect the suitability of the standard hydraulic oil for any particular system and by taking into account any higher than average operational requirements, it is possible to avoid premature component failure caused by contamination levels in the fluid.


When looking at whether a different rated hydraulic fluid would be more suitable for your system and deciding to opt for a lower rated one, it is important that this decision is made with the most sensitive component in mind.  It may be a case of using the hydraulic fluid with that rating, or of installing added filtration systems before that part of the system, in order to clean the fluid as it passes through that part.  They say an army marches at the pace of the slowest person and it is similar concept to choosing hydraulic oil and filtration systems, when there are different levels of capability and tolerance between the component parts.


As a guide, the typical cleanliness required of hydraulic fluid for different types of components is as follows:



ISO Code

Servo control valves


Proportional valves


Vane and piston pumps


Direction and pressure control valves


Gear pumps and motors


Flow control valves and cylinders



An avoidable source of contamination in hydraulic fluid is paint flakes or rust in the system.  Sometimes a decision will be made to paint the inside of a hydraulic reservoir to prevent rusting, and on the surface.  This may seem like a sensible decision as tanks are not cheap to replace and when a piece of machinery is expected to last a long time, it is reasonable to take precautions against such problems.  Rust in hydraulic reservoirs can be caused by condensation and settled water in the space above the oil level, but a simpler solution is to keep the reservoir topped up and using a hygroscopic breather to reduce the potential for any water or vapour to form.  Painting the tank with a rust proof paint may not cause any problems, but the potential is definitely there and the risk is not worth taking.


Monitoring the cleanliness of hydraulic oil at all stages of its journey round the system is important for maintenance and replacement of filters and elements, but also for the daily operation of the machinery.  When it is possible to check that everything is operating as it should, then the focus can remain on the job at hand.  Monitoring also alerts users to a potential problem, as if the contamination level of the hydraulic fluid is too high at a particular point, an alarm or warning light can be deployed and the machinery switched off while the hydraulic fluid or filter element is replaced.  Being aware and alert to these issues and resolving them before they cause damage to the parts, is preferable to continuing blindly and then incurring hefty costs down the line.


The steps outlined above go a lot further than simple best practice – these are next-level preventative activities, that can save time and money for companies already acting in a contamination-aware manner.  There are always small improvements that can be made to the operation of hydraulic machinery and it is hard to implement them all, but at least the knowledge expansion can inform suitable changes to your operating practices.

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