Originally Posted by Mothra

It's not so much the torque, but the linear force in shear at the teeth that'll break them off. Calculate cross-sectional area of teeth at the contact point and figure breaking force at yield-strength. Divide by number of teeth and divide torque through radius of gears and see which is greater. Power is just how fast you spin the gear, so it won't hurt them that much to spin it faster... until you get into 200,000rpms+ where inertial forces starts coming into play.