There are generally two conditions which necessitate an oil change. One of these is oxidative degradation. And the only way to know if the oil's life has expired based on this condition is to do regular oil analysis.
A hydraulic oil's oxidative condition is determined by an absolute measure of its total acid concentration. When oxygen combines with hydrocarbon molecules a chain reaction occurs, which results in the formation of organic acids. These substances darken the oil, increase viscosity, reduce foaming resistance and air release, and form varnish and sludge. In other words, the oil becomes unserviceable.
The total acid number (AN) test result is expressed by the volume of the alkaline, potassium hydroxide (KOH) in milligrams (mg), required to neutralize the acidic components contained in one gram (gm) of used oil.
Due to their additive composition, new zinc-based, mineral hydraulic oils can have a rather high initial AN of 1 to 1.5 mg KOH/gm. This number initially decreases as additives deplete. But as the oil starts to age and oxidize, the formation of acidic by-products reverses this trend and causes AN to rise.
For mineral hydraulic oils, AN of 2.0 mg KOH/gm is the typical trigger value for an oil change. But for synthetic esters and some triglycerides (vegetable based oils) AN can be as high as 5.0 mg KOH/gm before an oil change is required.
With the above in mind here's your mission, should you chose to accept it:
- If you don't know the AN value which should trigger an oil change for each of the hydraulic oils you are using - contact your oil supplier to find out.
- If you don't know the current AN of the hydraulic oils you have in service, isn't it time you found out? This is one reason why failing to regular oil analysis can be a costly mistake. And to discover six 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.