How to Classify Hydraulic Oil Leaks

Slow leaksOne of our members wrote me with the following question:

“We are trying to give one of our customers a short definition of what constitutes a hydraulic oil leak. Is there a standard or a general definition of what constitutes a hydraulic leak?”

It’s not rocket science. In the case of a hydraulic system, any hydraulic fluid that appears on external surfaces can and should be classified as a leak. With one important exception, which I’ll get to in a minute.

By way of external arbitration, in their maintenance manuals the U.S. Army defines three classes of leaks:

–Class I: Leakage indicated by wetness or discoloration, but not great enough to form drops.
–Class II: Leakage great enough to form drops, but not enough to cause drops to drip from the item being checked/inspected.
–Class III: Leakage great enough to form drops that fall from the item being checked/inspected.

According to Army guidelines, Class I and II leaks are considered minor leaks (except fuel leaks) and machine operation can continue under these conditions. When operating machinery with these types of leaks, fluid levels must be checked regularly. Class III leaks must be reported to maintenance for corrective action. Furthermore, if a Class III leak is discovered during operation, the task can be completed as long as the leak is drops only and not a steady stream of fluid, and the fluid level is maintained within its allowed operating range.

The stark reality is most hydraulic machines in the civilian world operate continuously with Class I and II leaks. And far too many hydraulic machines are operated for extended periods with Class III leaks!

But at least the Army is crystal clear about what constitutes a leak: wetness or discoloration is sufficient. So a hydraulic coupling or connector should not even get damp with oil otherwise it constitutes a leak. HOWEVER, for a cylinder rod seal, it can be different story.

When the rod extends, the rod seal leaves a film of oil on the rod. This oil film is a few to several microns thick. And it’s desirable and necessary for lubricating the interface between the rod and its seals. Rod wiper design is a juggling act between allowing this oil film to ‘pump back’ into the cylinder, and preventing dirt invasion-which is after all the wiper’s primary function. As a result, it’s not unreasonable for the area around the rod wiper’s lip to appear damp with oil. In other words, in the case of cylinder rod seals, a Class I situation does not constitute a leak.

Furthermore, a Class II situation can be the result of an over aggressive wiper seal-rather than a defective rod seal. If the design of the wiper is such that its lip is too sharp and/or it exerts too much force on the rod’s surface, the surface oil film is ‘cut’ by the wiper when the rod retracts. In other words, an aggressive wiper does not allow the normal film of oil that coats the rod’s surface to pump back into the cylinder. So with the above in mind, as a general rule I would not declare a cylinder rod seal to be leaking until the Class III criteria is met: sufficient oil to form drops that fall or dribble from the rod. Changing out a hydraulic cylinder before that point is reached 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.

2 thoughts on “How to Classify Hydraulic Oil Leaks

  1. SAE J1176 “External leakage classifications for hydraulic systems” is what we use to classify leakages in validation testing and field-audits. It is a classification scale of 0 to 5 for both “dusty” and “dust-free” environments, and gives more resolution and definition than the military specification quoted in this article (which is often required, particularly in validation testing and analysis). SAE Purchasing guidelines J1754 for hydraulic hose and hose assemblies also reference this standard in conjunction with other relevant performance standards for hydraulic hose and hose assemblies (J517, J343, etc.)

    ISO Technical Report ISO/TR 11340 “Rubber and Rubber Products – Hydraulic Hose Assemblies – External leakage classifications for hydraulic systems” is basically the same as the SAE J1176 “Dust-Free” classification scale/definitions, and is also an excellent classification of hydraulic leak severity as it also has a pictorial representation of the leakage classification.

    • Thanks for this Paul. For our readers, here are the classifications the above mentioned scale numbers represent:

      0 = no indication of moisture (dry)
      1 = non-recurring fluid (weep)
      2 = recurring fluid that does not result in the formation of a droplet (seep)
      3 = recurring fluid that results in the formation of a non-falling droplet (droplet)
      4 = recurring fluid where a droplet forms and falls (drip)
      5 = recurring fluid where the frequency of droplets makes a measurable stream (flow)




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