One of our members wrote me about a case study he read in which the life of a mineral hydraulic oil in a mobile hydraulic machine was extended from the 4,000 hours recommended by the machine’s manufacturer to 17,000 hours. The question raised was: “is this really possible?”
The first thing to keep in mind is that hydraulic oil should only be changed when either the base oil has become degraded or its additives have become depleted. Which means it should NOT be changed on an arbitrary number of hours in service. So in this respect, the 4,000 hour change interval recommended by the manufacturer doesn’t mean a great deal.
Given additive depletion or base oil degradation are the triggers for an oil change, it follows that if these things can be slowed, oil life can be extended. Base oil is degraded by chemical reactions in the presence of water (hydrolysis) and air (oxidation). And these chemical reactions are assisted and accelerated by HEAT. Some wear metals can act as catalysts too. So to give the base oil the longest life possible, it must be kept clean, dry and cool.
Similarly, if additives attach themselves to particles in the oil, they are lost when these particles are filtered out. Hydrolysis can also deplete additives, and elevated operating temperatures accelerate the consumption of anti-oxidation additives. So to give the additive package the longest life possible, the oil must be kept clean, dry and cool.
So to answer the question directly, if the oil is kept clean, dry and cool–especially COOL, I wouldn’t be at all surprised by the sort of life extension described above. According to hydraulic component manufacturer Danfoss, if a hydraulic system operates at 85ºC (185ºF), oil life is just 12% of what it would be if the system operated at 60ºC (140ºF). And if the system operates at 102ºC (216ºF), oil life is only 3% of what it would be if the system operated at 60ºC (140ºF).
The other thing to keep in mind, and this applies to most hydraulic machines, is there is always a certain amount of’ ‘bleed and feed’. In other words, used oil is lost and replenished with new oil when components are changed-out, hoses burst etc. Depending on the type of machine, the size of its reservoir and how it’s maintained, it is possible that a significant amount of a hydraulic system’s oil volume is replaced during the change interval. And while this is a negative in terms of oil spill and make-up oil cost, it has a positive effect on the oil’s documented ‘achieved life’.
To summarize, not keeping the hydraulic oil clean, dry and cool is costly for a number of reasons. Significant reduction in oil life is just one of them. And to discover 6 other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.