The teeth on helical gears are cut at an angle to the face of the gear as shown in the figure below. 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 until the two teeth are in full engagement.

In spur gears the line of contact is parallel to the axis of rotation; in helical gears, the line is diagonal across the face of the tooth.

The gradual engagement makes helical gears operate much more smoothly and quietly than spur gears. Because of 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 can support this thrust load.