Gear Oils: Part 3 – Gear Oil Change and Flushing Procedure

There are different procedures to follow depending on whether the gears are clean, contaminated or very contaminated.

  • Reasons for an oil change/flush include: Oxidation or varnish buildup, contamination, lifetime of the oil has been met or changing to a different oil type.
  • Guidelines on properly flushing a gearbox and changing the oil.

Depending on the situation, the procedures and guidelines to follow will vary when flushing a gearbox and changing the oil. There are several important principles to consider:

  1. Minimize interaction between old and new oil:
    If possible, the old gear oil should be drained while still warm, i.e., immediately after the gears have stopped. Draining should be followed by flushing to remove residues. The oil container and inside walls of the gearbox can also be cleaned by nonfraying cloth – not cleaning wool – or a rubber wiper.
  2. Remove deposits with a cleaning oil:
    Varnish and carbon buildup can be removed by means of cleaning oil and manual cleaning, as far as accessible. The cleaning oil is added at a concentration of 10% and drained after 24 to 48 hours of operation. Any remaining residues can be removed mechanically.
  3. Use great care when changing from mineral to synthetic:
    Simply draining the used mineral oil and filling with the new synthetic oil can cause problems. Older gears, in particular, can be assumed to contain oil residues in the casing and the oil lines, which might be dissolved by synthetic oils. If such residues are not removed, they may clog oil lines, filters, seals and pumps and damage teeth. To prevent such damage, the gears or lubricant circulation system should be flushed with the new synthetic oil after the old oil has been drained (ideally at operating temperature).


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