Helical gears are similar to spur gears except that the gears teeth are at and angle with the axis of the gears. A helical gear is termed right handed or left hand as determined by the direction the teeth slopes away from the viewer looking at the top gear surface along the axis of the gear. ( Alternatively if a gear rest on its face the hand is the direction of slope of the teeth) . Meshing helical gears must be of opposite hand. Meshed helical gears can be at and angle to each other (up to 90o ). The helical gear provides a smoother mesh and can be operated at greater speeds than a straight spur gear. In operatation helical gears generate axial shaft forces in addition to the radial shaft force generated by normal spur gears.

In operation the initial tooth contact of a helical gear is a point which develops into a full line contact as the gear rotates. This is a smoother cycle than a spur which has an initial line contact. Spur gears are generally not run at peripheral speed of more than 10m/s. Helical gears can be run at speed exceeding 50m/s when accurately machined and balanced

A helical gear train with parallel axes is very similar to a spur gear with the same tooth profile and proportions. The primary difference is that the teeth are machined at am angle to the gear axis.

Designing helical gears is normally done in accordance with standards the two most popular series are listed under standards above: The notes below relate to approximate methods for estimating gear strengths. The methods are really only useful for first approximations and/or selection of stock gears (ref links below). — Detailed design of spur and helical gears should best be completed using the standards. Books are available providing the necessary guidance. Software is also available making the process very easy.

Crossed Helical Gears
When two helical gears are used to transmit power between non parallel, non-interesecting shafts, they are generally called crossed helical gears. These are simply normal helical gears with non-parallel shafts. For crossed helical gears to operate successfully they must have the same pressure angle and the same normal pitch. They need not have the same helix angle and they do not need to be opposite hand. The contact is nor a good line contact as for parallel helical gears and is often little more than a point contact. Running in crossed helical gears tend to marginally improve to area of contact

Designing crossed helical gears is normally done in accordance with standards the two most popular series are listed under standards above: The notes below relate to approximate methods for estimating gear strengths. The methods are really only useful for first approximations and/or selection of stock gears (ref links below). — Detailed design of spur and helical gears should best be completed using the standards. Books are available providing the necessary guidance. Software is also available making the process very easy.