a gearwheel in which the teeth are sections of a helix described on the wheel face as opposed to being parallel with the wheel axis; such teeth are thus set at an angle to the axis.
We cut the reference cylinder across a point at a distance equal to half the value of the pitch or module of the first helixes drawn and we consider the two cylinders concentric with the reference cylinders known as the tip cylinder and the root cylinder: we are defining an helical gear.