Helical and Spiral Gears:

Helical gears are utilized to have smooth operation. The two mating helical gears are illustrated in Figure. They may be left-handed or right-handed, based on the direction in which the helix slopes away from the viewer. In given figure gear 1 is right-handed and gear 2 is left-handed. Both of the gears have same helix angle 'ψ' while shaft axes are parallel to each other however it is different for skew shafts. Some  terms involved in helical gears are as.

 Helical Gears