Lubrication Checkup: Sludge Prevention

lubrication checkup 0809Symptom:  

Dear Dr. Lube: While changing out unreadable sight-gauges on some large gearbox reservoirs, we discovered a wall of sludge in the tank bottoms. Is it preventable?


Large gearboxes are almost always oil-lubricated, and typically employ a dual system wherein the reservoir is filled with oil to a determined level—usually designated on the sight gauge—to ensure partial lubricant coverage of lower mating gear teeth at all times. At speed, gears use surface tension on their teeth to “pick up” and transfer lubricant to other gears and bearings through “meshing” action and by “flinging and splashing” lubricant in all directions within the sealed reservoir. The “splash” method often involves a pressurized delivery system. An internal gear-driven pump picks up oil near the reservoir bottom and delivers it under pressure to bearings and gears that are difficult to service with traditional “splash” lubrication.

Sludge is a “telltale” sign of a neglected gearbox. Neglected oil decomposes in the presence of oxygen, water and heat. This oxidation process is caused by depletion of the lubricant’s additive package for various reasons. The end result is deposits of varnish, tar and contaminants that thicken much of the oil into that gooey soup we call “sludge.” It sits on reservoir bottoms and deposits on mechanical internals, leaving a liquid with little or no lubricating properties.


Reservoir sludge is most definitely preventable. Here’s how:

  • Mechanically remove as much sludge as possible and use a recommended solvent-based flushing fluid to clean the remainder.
  • Label the reservoir clearly to designate the correct lubricant manufacturer, product name and viscosity to be used, and indicate the same details on PM work orders.
  • If the reservoir exterior is regularly cleaned with water, ensure the fill cap and breathers (if applicable) are waterproof and always in place, or position a water-deflection shield over the reservoir. 
  • Ensure the correct lubricant is used for the application ambient temperatures. 
  • During change-outs, make sure the cap and breathers are reinstalled, and that the lubricant is transferred using dedicated clean equipment. 
  • Always fill to the correct level; do not overfill! 
  • Use regular oil analysis to determine when to change oil based on condition—and don’t forget to change it and the filter! MT

Dr. Lube, aka Ken Bannister, will present “Industrial Lubrication Fundamentals: Certification Preparatory Workshop,” a three-day, ICML-related Professional Development Course, at MARTS 2013. For details on this value-added lube-training opportunity, visit E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

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