It's also the elastic modulus of the gears. Every solid thing has some stiffness or measure of how much it compresses/stretches when it's loaded. There will be

It's also the elastic modulus of the gears. Every solid thing has some stiffness or measure of how much it compresses/stretches when it's loaded. There will be finite (but extremely small) compressive/tensile forces that are absorbed in the stretch of the material that the gears and driveline are made out of. Like stretching a rubber band to double its length takes a force you can feel but stretching it 1/100 of that (or one one millionth or lower) takes effort you don't even notice.