Hydraulic Oil: Depletion, Degradation and Filtration

Hydraulic Oil



In part 1 of the hydraulic oil series we discussed the basics of viscosity and concluded the importance of selecting the correct oil viscosity based on the environmental and operational effects on the system.  In part 2 we discussed a few general conditions that affect oil life. Heading into part 3 we deepened the discussion on how temperature affects the life of the oil as well as the service life of the components in the system.  And finally, in part 4, we complete our blog series by explaining, depletion and degradation process as well as the need for filtration. 

    The animating element of a hydraulic system is the hydraulic oil.  For something so important you would think it would get more attention.  We buy it in bulk so we can get the best price but we give little attention to how we store it and transfer it into our equipment.  We allow it to overheat and get dirty and yet we expect it to perform everyday and keep our machines running. Fortunately, hydraulic oil is quite forgiving and even under abuse and neglect, it continues to perform.  But if we treat it properly, keep it clean and moderately cool, it will take care of our equipment and take care of our customers. 


Depletion and Degradation 

    As shown above, when oil is operated in its ideal temperature range and kept clean the life of hydraulic oil is not fixed. Oil in a hydraulic system should not be changed at an arbitrary time frame.  The life span of oil could range from 500 hours to 5000 hours depending on the operating conditions of the oil. Considering the disposal cost of used oil, the benefits of using a fluid analysis service to evaluate the condition of the oil becomes feasible.

    Depletion and degradation of oil additives are the markers for considering when the oil should be changed. Hydraulic oil operating at high temperatures undergoes a process called Oxidation.  Oxidation of hydraulic oil is a process where oxygen combines with hydrocarbon molecules resulting in the formation of organic acids, thus increasing the acidity level of the oil. A fluid analysis service can provide the TAN value for the oil.  TAN is the Total Acidity Number of the oil. An analysis will determine if the TAN value is out of the acceptable range.   This process is accelerated by heat and high concentrations of wear particles.  If you have sludge in the bottom of your hydraulic reservoir then you probably have highly oxidized oil. 



    Just because the hydraulic oil is dirty doesn’t mean that it should be discarded.  Maintaining proper filtration will extend the life of the hydraulic oil. In many cases, changing the filter will bring the oil back to the specification requirements for the hydraulic system.  Clean oil has fewer wear particles which contribute to the Oxidation process. So keeping the oil clean inhibits the Oxidation process even if the oil is operated at higher temperatures. Refer to my post on filtration for additional information on this topic.  


Written by:  Pat Kinnison & Chrystal Bates

All right reserved

Copyright 2020