Face Width Of Helical Gears

Machine Design

Gear Drives

Introduction:

To have more than one pair of teeth in contact, the tooth displacement (i.e. the advancement of one end of tooth over the other end) or overlap should be atleast equal to the axial pitch.

Face Width

Overlap = p_c = b tan α                           ...(i)

The normal tooth load (W_N) has two components ; one is tangential component (W_T) and the other axial component (W_A). The axial or end thrust is given by

W_A = W_N sin α = W_T tan α  

From equation (i), we see that as the helix angle increases, then the tooth overlap increases. But at the same time, the end thrust as given by equation (ii), also increases, which is undesirable. It is usually recommended that the overlap should be 15 percent of the circular pitch.

Overlap = b tan α = 1.15 p_c

where b = Minimum face width, and
m = Module.

Notes :                                                                                                                                                                                        Fig: Face width of helical gear.

1. The maximum face width may be taken as 12.5 m to 20 m, where m is the module. In terms of pinion diameter (D_P), the face width should be 1.5 D_P to 2 D_P, although 2.5 D_P may be used.

2. In case of double helical or herringbone gears, the minimum face width is given by

The maximum face width ranges from 20 m to 30 m.

3. In single helical gears, the helix angle ranges from 20° to 35°, while for double helical gears, it may be made upto 45°.