One’s teeth on helical gears are cut at an angle to the facial skin of the apparatus. When two teeth on a helical gear system engage, the contact starts at one end of the tooth and steadily spreads as the gears rotate, until the two teeth are in full engagement.
This gradual engagement makes helical gears operate much more smoothly and quietly than spur gears. Because of this, helical gears 
are used in almost all car transmissions.
Because of the angle of the teeth on helical gears, they create a thrust load on the reducers for greenhouse apparatus when they mesh. Products that make use of 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 right, they could be mounted in perpendicular shafts, adjusting the rotation angle by 90 degrees.