One's teeth on helical gears are cut at an angle to the face of the apparatus. When two teeth on a helical gear system engage, the contact starts at 
one end of the tooth and gradually 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. Gearbox Because of this, helical gears are found in 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. Devices 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 gear teeth are correct, they can be mounted about perpendicular shafts, adjusting the rotation position by 90 degrees.