Lubrication Checkup: Gearbox Overheating

lubrication checkup 0809Symptom:  

“We’ve recently noticed that one of our critical gearboxes has started to run so hot that it can be barely touched by hand. It’s normally just warm to the touch. How detrimental is this situation, and what can we do about it?” 


Judging by your description of the gearbox to the touch of a hand, I estimate its temperature to be around 150 F (or 65 C)—the approximate temperature when the hand has to be lifted within three seconds of touch. That’s too hot, especially since the normal “warm to the touch” temperature would be approximately 104 F (or 40 C). 

An oil-temperature rise reacts according to the Arrhenius rule—a temperature- change-dependent failure-rate rule—that states for every 18 degrees F (10 degrees C) increase in temperature of the oil, the lubricant life cycle is halved. Your higher gearbox temperature means the unit is in danger of a premature failure.


1. Check the current oil level and for evidence of leakage. Is the drain plug tight? Low oil levels in both splash and pressured systems can cause overheating.

2. When did you last change the oil? Old, oxidized oil can cause sludge build-up in the bottom of the reservoir and increased viscosity. If the oil was recently changed, was it replaced with incompatible gear oil or incorrect viscosity? Both scenarios can cause internal friction, leading to overheating. 

3. If you have a pressurized system, check for a plugged suction.

4. Is the gearbox full of debris or dirt? Are the oil-fill cap and reservoir breather in place? Internal and external debris can create a thermal blanket that raises the temperature of the gearbox and oil.

5. Is a new external heat source causing the temperature to change? Did the operating parameters change and surpass the equipment’s design criteria? Use an infrared thermometer or camera to check for heat diffusion on the gearbox and local “hot spots.”

Often, one or more of the above situations can be responsible for a hot-running gearbox—pinpointing the culprit(s) is difficult without seeing the equipment. If my recommendations don’t resolve your issue, considering contacting a professional lubrication-management consultant for assistance. Good Luck! MT

Ken Bannister of Engtech Industries, Inc., is a Lubrication-Management Specialist and author of Lubrication for Industry (Industrial Press) and the 28th Edition Machinery’s Handbook Lubrication section (Industrial Press). For in-house ICML lubrication certification training, Ken can be reached at 519-469 9173 or by email at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

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