One’s teeth on helical gears are trim at an angle to the face of the gear. When two teeth on a helical gear system engage, the contact begins at one end of the tooth and gradually spreads as the gears rotate, until the two teeth are in full engagement.
This gradual engagement makes helical gears operate a lot more smoothly and quietly than spur gears. Because of this, helical gears are used in Gear virtually all car transmissions.
Due to the angle of one’s teeth on helical gears, they create a thrust load on the apparatus when they mesh. Gadgets that use helical gears possess bearings that can support this 
thrust load.
One interesting thing about helical gears is that if the angles of the apparatus teeth are appropriate, they could be mounted on perpendicular shafts, adjusting the rotation position by 90 degrees.