The helix angle for single helical gears ranges from 

Explanation:

Helix angle:

• Helix angle is the angle between any helix and an axial line on its right, circular cylinder or cone.
• Helix angles are selected to obtain a minimum overlap ratio and to provide good load sharing.
• Helix angles vary from 5° to 45°.
• The helix angle for Single-helical gear falls between 15° and 35°, and for double-helical gears it falls between 20° and 45°.
• The thrust generated is also a function of the helix angle.
• An increase in the helix angle increases the thrust; thus, this increase is the main reason for the lower helix angles in single-helical gearing.