The teeth of a helical gear are set at an angle (relative to axis of the gear) and take the form of a helix. This enables the teeth to mesh gradually, starting as point contact and developing into range get in touch with as engagement progresses. Probably the most noticeable benefits of helical gears over spur gears is definitely less noise, especially at medium- to high-speeds. Also, with helical gears, multiple tooth are generally in mesh, this means less load on each individual tooth. This results in a smoother transition of forces from one tooth to another, to ensure that vibrations, shock loads, and wear are reduced.
But the inclined angle of the teeth also causes sliding get in touch with between your teeth, which produces axial forces and heat, decreasing efficiency. These axial forces play a significant function in bearing selection for helical gears. Because the bearings have to endure both radial and axial 
forces, helical gears need thrust or roller bearings, which are usually larger (and more expensive) compared to the simple bearings used with spur gears. The axial forces vary in proportion to the magnitude of the tangent of the helix angle. Although bigger helix angles planetary gearbox provide higher rate and smoother motion, the helix angle is typically limited by 45 degrees because of the production of axial forces.