The 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 series contact as engagement progresses. One of the most noticeable advantages of helical gears over spur gears is 
usually less noise, especially at moderate- to high-speeds. Also, with helical gears, multiple tooth are generally in mesh, this means much less load on every individual tooth. This gear rack results in a smoother transition of forces from one tooth to the next, so that vibrations, shock loads, and wear are reduced.
But the inclined angle of one’s teeth also causes sliding get in touch with between your teeth, which generates axial forces and heat, decreasing efficiency. These axial forces play a significant role 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 usually larger (and more costly) compared to the simple bearings used in combination with spur gears. The axial forces vary in proportion to the magnitude of the tangent of the helix angle. Although larger helix angles offer higher rate and smoother motion, the helix position is typically limited to 45 degrees due to the creation of axial forces.