Mechanical
Gears trade speed for torque and never give you both. Stack the ratios, then check that everything downstream can survive the torque you just created.
A 1750 rpm motor through two reductions. Ratios multiply, so 3:1 followed by 3:1 is 9:1 overall — and the torque rises by the same factor that the speed falls, minus efficiency.
Each mesh costs a couple of percent. Three stages at 97% each is not 97% — it is 0.97³, and that compounding is why long gear trains run hot.
All that multiplied torque has to react into a housing. Bolt torque converts a size and grade into the tightening figure that produces the clamp force — which is what actually stops a joint slipping.
Bearing life is brutally non-linear in load: roughly an inverse cube for ball bearings. Doubling the load does not halve the life, it cuts it to an eighth.
The ratio takes a minute. What it does to efficiency, joints and bearing life is the actual design work — and each of those tools will tell you when you have gone too far.