The teeth on helical gears are trim at an angle to the face of 
the apparatus. When two tooth on a helical gear system engage, the get in touch with starts at one end of the tooth and steadily spreads as the gears rotate, until the two teeth are completely engagement.
This gradual engagement makes helical gears operate much more smoothly and quietly than spur gears. For this reason, helical gears are used in Super Power Lock almost all car transmissions.
Because of the angle of one's teeth on helical gears, they create a thrust load on the gear when they mesh. Gadgets that make use of 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 may be mounted about perpendicular shafts, adjusting the rotation position by 90 degrees.