The teeth on helical gears are trim at an angle to the facial skin of the gear. When two teeth on a helical gear system engage, the contact starts at one end of the tooth and steadily spreads as the gears rotate, before two teeth are in full engagement.
This gradual engagement makes helical gears operate a lot more smoothly and quietly than spur gears. For this reason, helical gears are used in virtually all car transmissions.
Because of the angle of the teeth on helical gears, they V Belt Pulley create a thrust load on the apparatus when they mesh. Gadgets that use helical gears possess bearings that may support this thrust load.
One interesting thing about helical gears is that if the angles of the gear teeth are right, they could be mounted on 
perpendicular shafts, adjusting the rotation position by 90 degrees.