The teeth on helical gears are trim at an angle to the 
facial skin of the gear. When two tooth on a helical equipment system engage, the contact starts at one end of the tooth and steadily spreads as the gears rotate, until the 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 Motorbase almost 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 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 gear teeth are appropriate, they can be mounted on perpendicular shafts, adjusting the rotation position by 90 degrees.