ISO purity classes of gear oils

Year of publication: 202


In the particle count, the newly filled gear oil shows an almost identical ISO cleanliness class after the change as the used oil used before. 

Shouldn't the new oil be much cleaner?

During an oil change, the often quite viscous used oil is drained through the oil drain plug located at the lowest point. Deposited wear metals and contaminants are also removed in this way with the oil at operating temperature. However, depending on the viscosity and temperature of the oil, 5 to 10 %, and often more in the case of worm and planetary gear units, of the old oil remains in the gear unit. Especially with cold oil, oil sludge and contaminants remain deposited in the oil pan.

When the gear is then filled with the new oil, the residues get into solution up to a size of about 40 μm. At the same time, they are stirred up after starting. An analysis carried out shortly after the oil change then still shows an impermissibly high cleanliness class. The oil change has thus actually failed its purpose.

OELCHECK advises:

  • Drain the oil within 15 minutes after the gear has stopped.
  • Briefly flush the gear preferably with a thinner oil of the same oil type.
  • Only fill the gear unit via filters with a pore size of 20 μm or finer.
  • Instead of changing the oil at regular intervals, clean the oil, e.g., with a mobile off-line filter during operation based on oil analyses.
  • Carry out oil analyses on a regular basis. This will help you determine whether an oil change is actually due.
  • Equip your gear unit with a filter system if the oil filling exceeds 200 l.