Terms used in Helical Gears

Machine Design

Gear Drives

Introduction:

A helical gear has teeth in form of helix around the gear. Two such gears may be used to connect two parallel shafts in place of spur gears. The helixes may be right handed on one gear and left handed on the other.

Terms

Helical gear

The pitch surfaces are cylindrical as in spur gearing, but the teeth instead of being parallel to the axis, wind around the cylinders helically like screw threads.The teeth of helical gears with parallel axis have line contact, as in spur gearing.This provides gradual engagement and continuous contact of the engaging teeth. Hence helical gears give smooth drive with a high efficiency of transmission.The helical gears may be of single helical type or double helical type.In case of single helical gears there is some axial thrust between the teeth, which is a disadvantage.In order to eliminate this axial thrust, double helical gears herringbone gears) are used.It is equivalent to two single helical gears, in which equal and opposite thrusts are provided on each gear and the resulting axial thrust is zero.

1. Helix angle. It is a constant angle made by the helices with the axis of rotation.

2. Axial pitch. It is the distance, parallel to the axis, between similar faces of adjacent teeth. It is the same as circular pitch and is therefore denoted by pc. The axial pitch may also be defined as the circular pitch in the plane of rotation or the diametral plane.

3. Normal pitch. It is the distance between similar faces of adjacent teeth along a helix on the pitch cylinders normal to the teeth. It is denoted by pN. The normal pitch may also be defined as the circular pitch in the normal plane which is a plane perpendicular to the teeth. Mathematically, normal pitch,

pN = pc cos α

Note : If the gears are cut by standard hobs, then the pitch (or module) and the pressure angle of the hob will apply in the normal plane. On the other hand, if the gears are cut by the Fellows gear-shaper method, the pitch and pressure angle of the cutter will apply to the plane of rotation. The relation between the normal pressure angle (φN) in the normal plane and the pressure angle (φ) in the diametral plane (or plane of rotation) is given by

tan φN = tan φ × cos α