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6.8 Helical 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)
   Details of contact ratio of helical gearing are given later in a general coverage of the subject; see SECTION 11.1.

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

   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.

(6-10)

  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.
   If center distance, ax is given, the normal coefficient of profile shift xn1 and xn2 can be calculated from Table 6-2. These are the inverse equations from items 4 to 10 of Table 6-1.


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