The teeth on helical gears are trim at an angle 
to the facial skin of the apparatus. When two teeth on a helical equipment system engage, the get in touch with starts at one end of the tooth and steadily spreads as the gears rotate, before two teeth are completely 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.
Due to the angle of the teeth on helical gears, they create a thrust load on the gear when they mesh. Helical Reducer Devices that use helical gears have bearings that can support this thrust load.
One interesting thing about helical gears is that if the angles of the apparatus teeth are right, they could be mounted about perpendicular shafts, adjusting the rotation position by 90 degrees.