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Gearing Systems : Spur, Helical, and Herringbone Gears

Spur gear, helical gears, herringbone are very similar except for how the teeth are cut.

Spur Gear

A spur gear is one of the simplest, most common, and least expensive gear made. It essentially is cylinder that has teeth cut parallel to the axis of the gear. Refer to the image below. For a pinion and gear to mesh, the axis of both gears must be parallel.

spur gear

Spur gears are typically very noisy gears due to the fact that the faces of the gear teeth come in contact all at once. However, despite this, spur gears are very efficient gears with an efficiency of 98 to 99%.

Helical Gears

A helical gears teeth are cut at a certain angle with respect to the axis of the gear. By cutting the gear like this, the gear and pinion can mesh parallel to their axis or they can be meshed at an offset angle dependent on the angle of the teeth on the gears. Refer to the image below.

 helical gear

These gears are more expensive to make then the common spur gear, but they run much quieter then the spur gear. This is because there is a smoother and more gradual contact between the angle surface of the teeth when they mesh. The surface of the face of a spur gear tooth on the other hand all comes in contact at once causing more noise. Thrust bearings however should be used with helical gears to conteract the additional thrust load caused by added friction between the gear teeth.

Due to the additional friction between the gear teeth there is a loss of efficiency for helical gears in comparison to spur gears. A parallel helical gear set has an efficiency of 96-98%, while helical gears that are crossed (set at an angle) have an efficiency of 50 to 90% depending on the angle.

Herringbone Gears

Herringbone gears are essentialy two helical gears of identical pitch and diameter that are joined together. This provides an advantage over a helical gear because the thrust load on the gear will be canceled, which means thrust bearings will not be needed in the design. These gears are very expensive to make however and are normally only used for large, high-power applications. Refer to the image below.

undercutting


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