Herringbone and double helical gears have two sets of teeth, of opposite hands, cut into one gear. It also eliminates the need for thrust bearings. (The difference between the two designs is that double helical gears have a groove in the middle, between the teeth, whereas herringbone gears do not.) This arrangement cancels out the axial forces on each set of teeth, so larger helix angles can be used. These arrangements have the appearance of two helical gears with opposite hands mounted back-to-back, although in reality they are machined from the same gear. The axial loads produced by helical gears can be countered by using double helical or herringbone gears. Although larger helix angles provide higher speed and smoother motion, the helix angle is typically limited to 45 degrees due to the production of axial forces. The axial forces vary in proportion to the magnitude of the tangent of the helix angle. Because the bearings have to withstand both radial and axial forces, helical gears require thrust or roller bearings, which are typically larger (and more expensive) than the simple bearings used with spur gears. These axial forces play a significant role in bearing selection for helical gears. This results in a smoother transition of forces from one tooth to the next, so that vibrations, shock loads, and wear are reduced.īut the inclined angle of the teeth also causes sliding contact between the teeth, which produces axial forces and heat, decreasing efficiency. Also, with helical gears, multiple teeth are always in mesh, which means less load on each individual tooth. One of the most noticeable benefits of helical gears over spur gears is less noise, especially at medium- to high-speeds. This allows the teeth to mesh gradually, starting as point contact and developing into line contact as engagement progresses. The teeth of a helical gear are set at an angle (relative to axis of the gear) and take the shape of a helix. Spur gears are simple and inexpensive to manufacture, but helical gears offer some important advantages over spur gears. Helical gears and spur gears are two of the most common gear types and can be used in many of the same applications.
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