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 starts at one end of the tooth and gradually 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. Because of this, helical gears are found in almost all car greenhouse motor transmissions.
Due to the angle of the teeth on helical gears, they create a thrust load on the gear when they mesh. Devices that use helical gears have bearings that may support this thrust load.
One interesting thing about helical gears is that if the angles of the apparatus teeth are appropriate, they can be mounted about perpendicular shafts, adjusting the rotation position by 90 degrees.