The teeth of a helical gear are set at an angle (in accordance with axis of the apparatus) and take the shape of a helix. This enables one's teeth to mesh gradually, starting as point contact and developing into series contact as engagement progresses. Probably the most noticeable advantages of helical gears over spur gears is much less noise, especially at moderate- to high-speeds. Also, with helical gears, multiple teeth are generally in mesh, which means much less load on every individual tooth. This results in a smoother changeover of forces from one tooth to another, so that vibrations, shock loads, and wear are reduced.
But the inclined angle of one's teeth also causes sliding contact between your teeth, which generates axial forces and heat, decreasing effectiveness. These axial forces perform a significant function in bearing selection for helical gears. Because the bearings have to endure both radial and axial forces, helical gears require thrust or roller bearings, which are usually planetary gearbox larger (and more expensive) compared to the simple bearings used with spur gears. The axial 
forces vary compared to the magnitude of the tangent of the helix angle. Although larger helix angles offer higher acceleration and smoother movement, the helix position is typically limited to 45 degrees due to the production of axial forces.