Gearbox Efficiency, Inefficiency and Thermal Rating 

As most technicians and engineers learn at school, a gearbox is by its nature an efficient asset. A  parallel shaft gear unit typically experiences losses of just 1–2 percent per stage. 

Example: a single-stage gearbox could have a nominal efficiency of 98–99 percent. 

The losses are of different types/sources:

? Gear losses 

? Lubrication losses 

? Seal (when present) losses 

? Windage losses (high-speed gears) 

? Bearing losses 

In 2003published a new bearing friction model in. This model is based on four sources of friction:

(1) M = Mrr + Msl + Mseal + Mdrag 


M is total frictional moment, N-mm.  

Mrr is rolling frictional moment, N-mm.  

Msl is sliding frictional moment, N-mm.


This new approach identifies the sources of friction in every contact occurring in the bearing and  combines them. In addition, the seal contribution and additional external sources can be added as  required to predict the overall into every single contact (raceways and flanges), changes of design  and surface improvements can readily be taken into consideration, rendering the model better able  to reflect improvement in bearing designs. 

This four-source model allows the designer to understand in detail the conditions under which the  bearing functions internally. The four sources of loss are plotted as a function of speed; it can be  checked where the bearing losses are driven by rolling or sliding sources. 

Using this new model will allow gear user and designer to make better predictions and provide a  better understanding of the bearing losses in a gear unit over various loading conditions. Thus, it  allows improved optimization of the sys- tem by providing a more accurate comparison between  different bearing types and designs.

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