A helical gear has teeth that are cut at an angle—that is, they are helical to the gear axis of rotation. This allows multiple teeth to be in mesh at the same time. Helical gears permit the tooth load to be evenly distributed and are used in applications requiring high torque transfer loads. In addition, helical gears perform more quietly than spur gears because they create a wiping action as they come into and out of mesh with mating gears. The main disadvantage of helical gears is the axial thrust they create in operation. As torque transfers from one gear to its mating gear, fore or aft thrust is created on each shaft. This fore or aft force is known as axial thrust force; it has to be absorbed by thrust washers, taper roller bearings, other transmission gears, shafts, and, ultimately, the transmission housing.
Helical gears can have either right- or left-handed geometry, depending on the direction of the spiral when viewed face-on. It is possible to determine whether a helical gear is left- or right-handed using the hand-rule: Hold the gear alternately in each hand. The hand in which the helical teeth align with the thumb determines whether it is left or right cut. When designed to mesh, helical gears must be mounted on parallel shafts, with one gear using right-handed geometry and its mate using left-handed geometry. Two helical gears manufactured with helices in the samedirection spiral cannot mesh in a parallel-mounted arrangement. Both helical and spur gears are commonly used in heavy-duty truck transmission gearing.