The teeth of a helical gear 
are set at an angle (in accordance with axis of the gear) and take the shape of a helix. This enables one’s teeth to mesh gear rack gradually, starting as point contact and developing into line 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 tooth are always in mesh, which means much less load on each individual tooth. This results in a smoother transition of forces in one tooth to another, to ensure that vibrations, shock loads, and wear are reduced.
However the inclined angle of one’s teeth also causes sliding get in touch with between your teeth, which produces axial forces and heat, decreasing effectiveness. These axial forces perform a significant function in bearing selection for helical gears. As the bearings have to withstand both radial and axial forces, helical gears need thrust or roller bearings, which are typically larger (and more costly) than the simple bearings used in combination with spur gears. The axial forces vary compared to the magnitude of the tangent of the helix angle. Although larger helix angles provide higher speed and smoother movement, the helix position is typically limited by 45 degrees because of the creation of axial forces.
