SEED Guides Gears Number of teeth

To llustrate the choice of teeth numbers consider a 30:1 target reduction. Alternative solutions are:
(72/12)(60/12) (1) overall reduction, R = 30
(72/13)(72/13) (1) overall reduction, R = 30.67 - (2.2% error, closest to equal sharing)
(72/13)(70/13) (1) overall reduction, R = 29.82 - (0.6% error,very near to equal sharing)

It should be noted that although in this example the target ratio was attainable exactly, this will not always be so. Also it may be observed that whilst a common module size is required for pinion and wheel gears in each meshing gear pair, they need not be the same at each reduction stage. The changing speeds and torques at each stage may mean that a different module size is desirable from strength and gear size considerations.