Zero & Non-Zero Helix Angle Gears
As refresher, let’s start with the definition. Helix angle; the angle between the helix of the tooth flank in reference centerline (axis of rotation) of the gear (not to mix it up with the lead angle which is the complement helix angle measured between the perpendicular to the axis an the tooth flan helix)
Zero-helix-angle gears are also called straight or spur gears, non-zero-helix angle gears are called mostly “helical gears”.