Gear Contact Ratio
The contact ratio of helical gears is enhanced by the axial overlap of
the teeth. Thus, the contact ratio is the sum of the transverse contact
ratio, calculated in the same manner as for spur gears, and a term
involving the axial pitch.
6.9 Design Considerations
6.9.1 Involute Interference
Helical gears cut with standard normal pressure angles can have considerably higher pressure angles in the plane of rotation - see Equation (6-6) - depending on the helix angle. Therefore, the minimum number of teeth without undercutting can be significantly reduced, and helical gears having very low numbers of teeth without undercutting are feasible.
6.9.2 Normal vs. Radial Module (Pitch)
It is not that simple in the radial system. The gear hob design must be
altered in accordance with the changing of helix angle b
even when the module m, and the pressure angle at, are the same.
Obviously, the manufacturing of helical gears is easier with the normal system than with the radial system in the plane perpendicular to the axis.
6.10 Helical Gear Calculations
6.10.1 Normal System Helical Gear
In the normal system, the calculation of a profile shifted helical gear, the working pitch diameter dw and working pressure angle awt in the axial system is done per Equations (6-10). That is because meshing of the helical gears in the axial direction is just like spur gears and the calculation is similar.
Table 6-1 shows the calculation of profile shifted helical gears in the
normal system. If normal coefficients of profile shift xn1 xn2 are zero,
they become standard gears.