The teeth of a helical gear are set at an angle (relative to axis of the gear) and take the shape of a helix. This allows the teeth to mesh steadily, starting as point contact and developing into line contact as engagement progresses. One of the most noticeable benefits of helical gears over spur gears is much less noise, especially at medium- to high-speeds. Also, with helical gears, multiple the teeth are generally in mesh, which means less load on every individual tooth. This results in a smoother transition of forces from one tooth to another, so that vibrations, shock loads, and wear are reduced.

But the inclined angle of the teeth also causes sliding contact between your teeth, which creates axial forces and heat, decreasing efficiency. These axial forces perform a significant function in bearing selection for helical gears. As the bearings have to endure both radial and axial forces, helical gears require thrust or roller bearings, which are usually larger (and more expensive) than the simple bearings used with spur gears. The axial forces vary compared to the helical gear china magnitude of the tangent of the helix angle. Although larger helix angles offer higher acceleration and smoother movement, the helix position is typically limited by 45 degrees due to the production of axial forces.