12 36
ratio centre distance pitch diameters (module 1 mm)
with 1 N·m in: out tooth load
Driver (pinion)Pitch circles, rolling on each otherLine of action, 20° pressure angleTwo gears behave like two circles rolling without slipping: the pitch circles. Pitch diameter is module × teeth, so the speed ratio is just the tooth ratio, and torque goes up by the same factor minus about 2% lost in the mesh. All the tooth force pushes along the line of action. Its tangential part does the work, and the rest (tan 20° ≈ 0.36 of it) tries to push the shafts apart. A smaller pinion makes more tooth force for the same torque.