Helical Gears

Helical gear is similar to the spur gear except that the teeth are at an angle to the shaft, rather than parallel to its axis as in a spur gear. The resulting teeth are longer than the teeth on a spur gear of equivalent pitch diameter. The longer teeth cause helical gears to have the following differences from spur gears of the same size:

• Tooth strength is greater because the teeth are longer.
• Greater surface contact on the teeth allows a helical gear to carry more load than a spur gear.
• The longer surface of contact reduces the efficiency of a helical gear relative to a spur gear.

Helical gears may be used to mesh two shafts that are not parallel, although they are still primarily used in parallel shaft applications. A special application in which helical gears are used is a crossed gear mesh, in which the two shafts are perpendicular to each other. The basic geometry for a helical gear is shown in Figure 15.19.

Advantages: Helical gears can be used on non-parallel and even perpendicular shafts, and can carry higher loads than can spur gears.

Figure 15.18 Spur Gear

Figure 15.19 Helical Gear

Limitations: Helical gears are expensive and much more difficult to manufacture. They are also slightly less efficient than a spur gear of the same size.