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How hot is too hot?

Posted on Thursday Dec 26, 2019 at 10:01PM in Mechanical


Article By: Joe Hardwick

One of the most common concerns from a customer is one stating “My gearbox is running hot, what should I do”? 

Most of the time the concern is based on a personal feeling that the housing feels hot to the touch.  Generally, if a person can hold their hand on the housing for a few seconds, the temperature is not too high.


Temperature limits of industrial gearboxes are much higher than a person’s tolerance to heat.  Every person’s tolerance to heat is different, which makes this “test” of determining temperature purely subjective.  The limit of human touch to heat is generally 130 deg F-140 deg F.  To compare, water is said to cause 3rd degree burns to adult skin at 140 deg F in 5 seconds. 


General temperature limits of gearbox components are as follows:

Standard Rubber oil seal lips                 212 deg F

Mineral Oils                                                 212 deg F

Fluorinated Rubber Oil Seal Lips          300 deg F

Synthetic Oils                                              300 deg F


Roller bearings, bronze gears and steel gears have temperature limits that exceed 250-300 deg F and usually do not limit the thermal capacity of most general purpose industrial gearboxes.


Temperature is a result of load.  As load increases, temperature increases and as load decreases, temperature decreases.  If the load applied to a gearbox is cyclic in nature  the temperature should reach a medium and hold steady until the load cycle changes.


There are significant benefits to reducing the operating temperature of a gearbox.  Cooler temperatures can help extend the life of oil seals.  In fact, for nitrile lip seals, every 25 deg F change in temperature can either double the seal life or cut it in half.  For example, a gearbox may operate for 10,000 hours at 125 deg F, but another one may only operate 5000 hours at 150 deg F before seal failure occurs, in the same application.  Temperature has similar effects on Viton and other gearbox components.


If a gearbox is allowed to run at excessive temperatures other problems can arise such as burnt lubricant, galled bearings, damage to seals and gears which result in damage to other system components. 


The maximum housing limit on most gearboxes is from 180 deg F-200 deg F.  For quick, accurate temperature checks, use of infrared thermometers, thermocouples, or temperature sensitive crayons or marking devices such as those available from a Welding Supply, is suggested. 


If the temperature of the gearbox exceeds the manufacturers limit, there are a few options to consider.  If the physical space will allow a larger reducer should be considered or an auxiliary cooling device can be implemented.  Auxiliary cooling devices may include a shaft mounted fan, an electric fan or a radiant cooling system. 

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