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Design and Application of the Worm Gear

William P. Crosher
William P. Crosher
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ASME Press
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Few engineering subjects are more misunderstood than worm gear efficiencies. Most articles, referencing worm gearing, draw attention to the commonly held supposition that all worm gearing transmits power with low efficiencies. This lack of understanding is the prime cause of many of the difficulties that arise when making the proper selection.

A typical example of the problem can be taken from the recognized book Machine Design. (Irving J. Levinson Reston Publishing, 1978) In discussing worm gear drives, the following totally incorrect statements are made, “…the efficiency of a worm drive is closer to 50 percent than 100 percent” and, it continues to unintentionally mislead “…to prevent seizure of worm to gear, the worm is usually a hardened steel and the gear bronze…,” this would imply that using any other combination will cause a seizing of the mesh, or the prevention of seizing, is the prime reason for material selection.

It is frequently suggested that spur, helical, double helical, herringbone or bevel gearing are more efficient modes of power transmission. In fact, when the requirements of the selection are reviewed the overall efficiencies are usually very close and, for most applications, any difference is insignificant. The lower efficiencies are to be expected with small horsepowers, low-input speeds and designs requiring minimum backlash. These conditions do not normally prevent the worm gear from being the optimum economic choice for the drive. When horsepowers are low any additional power loss is insignificant and, if speeds and powers are high then the efficiencies approach those of any alternative gearing selection.

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