A helical gear such as shown in Figure 4.7 is a cylindrical gear in which the teeth flank are helicoid. The helix angle in reference cylinder is β, and the displacement of one rotation is the lead, pz.

The tooth profile of a helical gear is an involute curve from an axial view, or in the plane perpendicular to the axis. The helical gear has two kinds of tooth profiles – one is based on a normal system, the other is based on a transverse system.

Pitch measured perpendicular to teeth is called normal pitch, pn. And pn divided by π is then a normal module, mn.

The tooth profile of a helical gear with applied normal module, mn, and normal pressure angle αn belongs to a normal system.

In the axial view, the pitch on the reference is called the transverse pitch, pt. And pt divided by π is the transverse module, mt.

These transverse module mt and transverse pressure angle αt are the basic configuration of transverse system helical gear.

In the normal system, helical gears can be cut by the same gear hob if module mn and pressure angle at are constant, no matter what the value of helix angle β.

It is not that simple in the transverse system. The gear hob design must be altered in accordance with the changing of helix angle β, even when the module mt and the pressure angle at are the same.

Obviously, the manufacturing of helical gears is easier with the normal system than with the transverse system in the plane perpendicular to the axis.

When meshing helical gears, they must have the same helix angle but with opposite hands.

Fig.4.7 Fundamental relationship of a helical gear (Right hand)