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Originally Posted by thunder1
I work in a machine shop and rebuild gearboxes also, when we rebuild them with new bearings and set preload ,we do what is call a breakaway reading at each set of gears and bearings with a torque wrench, could be inch pounds,
no matter what is hook to the gearbox,"electric motor hand crank "the amount of torque should be the same .So way the percentage?
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Your breakaway reading measures the residual friction in the bearings and seals. It is a good way of measuring the 'stiffness' of a new box and I guess you are right that it will remain fairly constant.
In addition to this, each pair of gears generates friction proportional to the loading (torque) applied so here is the 'percentage bit'. Any pair of gears (and especially helical type) involves pairs of teeth 'rubbing' together and the friction of this uses energy to produce heat. The harder you load the gears the more they rub...