One’s teeth of a helical gear are set at an angle (relative to axis of the apparatus) and take the form of a helix. This enables the teeth to mesh gradually, starting as point contact and developing into line contact as engagement progresses. One of the most noticeable benefits of helical gears over spur gears can be much less noise, especially at moderate- to high-speeds. Also, with helical gears, multiple tooth are constantly in mesh, this means less load on every individual tooth. This results in a smoother changeover of forces from one tooth to the next, to ensure that vibrations, shock loads, and wear are reduced.
However the inclined angle of one’s teeth also causes planetary gearbox sliding contact between the teeth, which produces axial forces and heat, decreasing efficiency. These axial forces play a significant function in bearing selection for helical gears. As the bearings have to endure both radial and axial forces, helical gears need thrust or roller bearings, which are typically larger (and more costly) than the simple bearings used with spur gears. The axial forces vary in proportion to the 
magnitude of the tangent of the helix angle. Although larger helix angles provide higher speed and smoother movement, the helix angle is typically limited to 45 degrees due to the production of axial forces.