The teeth on China Vacuum Pumps helical gears are cut at an angle to the facial skin of the gear. When two tooth 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 much more smoothly and quietly than spur gears. Because of this, helical gears are found in virtually all car transmissions.
Because of 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 have bearings that may support this thrust load.
One interesting thing about helical gears is that if the angles of the gear teeth are appropriate, they could be mounted on perpendicular shafts, adjusting the rotation position by 90 degrees.
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