12.4 Helix angle

ψ{β} is the standard or generating helix angle. The helix angle of a gear varies with the diameter at which it is specified. The standard helix angle is measured on the generating pitch cylinder.

For hobbed gears, the helix angle may be freely chosen because the hobbing machine can be adjusted to cut any helix angle. For pinion-shaped gears, the helix angle must correspond to the helical guides that are available for the gear-shaping machine.

ψ (deg.) Application

0 spur
10-20 single helical
20-40 double helical

Low helix angle: provides low thrust loads but results in fewer teeth in contact (smaller face contact ratio, mF and higher noise generation. For the full benefit of helical action, mF {εβ} should be at least 2.0. If mF < 1.0 the gear is a low contact ratio (LACR) helical gear and is rated as a spur gear. Maximum bending strength is obtained with approximately 15 degree helix angles.

High helix angle: provides smooth-running, quiet gearsets but results in higher thrust loads unless double helical gears are used to cancel internally generated thrust loads.

The plus button PIC at the side of the field can be used to define the ’hand’ (left or right) of the helix.


Figure 12.2: AGMA 2001/2101 - Helix angle