Helical gears are similar to spur gears except that their teeth are cut at an angle to the hole (axis) rather than straight and parallel to the hole like the teeth of a spur gear.
Helical gears are used to connect non-intersecting shafts. Boston standard helical gears with 45-degree helix angles (a term that will be discussed below) are used to connect parallel shafts or shafts at right (90º) angles. 
Helical gears are manufactured as both right and left-hand gears. The teeth of a left-hand helical gear lean to the left when the gear is placed on a flat surface. The teeth of a right-hand helical gear lean to the right when placed on a flat surface.
Opposite hand helical gears run on parallel shafts. Gears of the same hand operate with shafts of 90º.

Helical gears are similar to spur gears except that their teeth are cut at an angle to the hole (axis) rather than straight and parallel to the hole like the teeth of a spur gear.
Helical gears are used to connect non-intersecting shafts. Boston standard helical gears with 45-degree helix angles (a term that will be discussed below) are used to connect parallel shafts or shafts at right (90º) angles. 
Helical gears are manufactured as both right and left-hand gears. The teeth of a left-hand helical gear lean to the left when the gear is placed on a flat surface. The teeth of a right-hand helical gear lean to the right when placed on a flat surface.
Opposite hand helical gears run on parallel shafts. Gears of the same hand operate with shafts of 90º.