One’s teeth of a helical gear are set at an angle (relative to axis of the apparatus) and take the shape of a helix. This allows the teeth to mesh steadily, starting as point contact and developing into line get in touch with as engagement progresses. One of the most noticeable benefits of helical gears over spur gears is certainly much less noise, especially at moderate- to high-speeds. Also, with helical gears, multiple tooth are at all times in mesh, which means much less load on every individual tooth. This outcomes in a smoother changeover of forces in one tooth to another, so that vibrations, shock loads, and wear are reduced.
However the inclined angle of one’s teeth also causes sliding get in touch with between the teeth, which produces axial forces and heat, decreasing effectiveness. These axial forces enjoy a significant part in bearing selection for helical gears. As the planetary gearbox bearings have to withstand both radial and axial forces, helical gears need thrust or roller bearings, which are usually larger (and more expensive) than the simple bearings used with spur gears. The axial forces vary compared to the magnitude of the tangent of the helix angle. 
Although bigger helix angles offer higher rate and smoother movement, the helix position is typically limited to 45 degrees because of the creation of axial forces.
