The 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 collection contact as engagement progresses. Probably the most noticeable benefits of helical gears over spur gears can be much less noise, especially at medium- to high-speeds. Also, with helical gears, multiple the teeth are usually in mesh, this means less load on every individual tooth. This results in a gear rack smoother transition of forces from one tooth to the next, so that vibrations, shock loads, and wear are reduced.
However the inclined angle of the teeth also causes sliding get in touch with between your teeth, which creates axial forces and heat, decreasing efficiency. These axial forces play a significant part in bearing selection for helical gears. As the bearings have to endure both radial and axial forces, helical gears require thrust or roller bearings, which are typically larger (and more expensive) 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 bigger helix angles provide higher quickness and smoother movement, the helix angle is typically limited by 45 degrees because of the creation of axial forces.