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 allows the teeth to mesh steadily, starting as point get in touch with and developing into series contact as engagement progresses. Probably the most noticeable advantages of helical gears over spur gears is less noise, especially at medium- to high-speeds. Also, with helical gears, multiple the teeth are constantly in mesh, this means much less load on each individual tooth. This outcomes 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 the teeth also causes sliding get in touch with between the teeth, which generates axial forces and heat, decreasing effectiveness. These axial forces play a significant role in bearing selection for helical gears. As the bearings have to withstand both radial and axial forces, helical gears require thrust or roller bearings, which are typically larger (and more costly) gear rack 
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 rate and smoother motion, the helix angle is typically limited to 45 degrees due to the production of axial forces.