<Dd> The imaginary surface in which contact occurs between two engaging tooth surfaces . It is the summation of the paths of action in all sections of the engaging teeth . </Dd> <Dt> Plane of action </Dt> <Dd> The surface of action for involute, parallel axis gears with either spur or helical teeth . It is tangent to the base cylinders . </Dd> <Dt> Zone of action (contact zone) </Dt>

Advantages and disadvantages of different types of gears