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What is a Worm Shaft?

By Lori Kilchermann
Updated May 23, 2024
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A worm shaft, or a worm gear as it is often referred to, is a long cylindrical type of gear that provides a surface for ball bearings to ride on. Resembling a candy cane's stripes, this shaft has a raised ridge that wraps around the shaft and runs from top to bottom. This type of gear is most often found within the inner workings of a steering box on an automobile.

When used in an electric motor application, the worm shaft is commonly used to decrease the speed of a driven gear. As the teeth on a gear engage the worm shaft gear, the gear is rotated one gear tooth for every full turn of the worm gear. The drive speed is reduced by the number of teeth on the driven gear. For example, a five-tooth gear would have its speed reduced by a fifth and a 10-tooth gear would be reduced by a tenth.

Unlike a conventional gear set, the worm shaft cannot be reversed to increase a driven gear's speed. Such assemblies can be found in many electrical tools and devices such as electric drill motors and circular saws. They can also be found in sporting equipment such as fishing reels. Many fishing reels use a worm shaft to operate the line level wind on a bait-casting reel, as well as to operate the line winder on a spinning reel or a spin-casting reel.

By using the worm shaft on a fishing reel, the operator can crank the reel's handle at a comfortable speed while the worm gear slows the its action down to a manageable speed. This reduction in speed also increases the amount of power the reel has to do battle with a big fish. The mechanical advantage of this type of simple machine allows the use of small fishing reels to pursue larger fish.

While it is a typical custom to use grease to lubricate gears, the worm shaft operates at peak performance levels with a very heavy oil used as a lubricant. The fluidity of the heavy oil allows the gears to roll smoothly against each other, while grease tends to get sticky and cause erratic motion. In some machines, the mixture of heavy oil and light grease are combined to lubricate the gears. In any configuration, a clean and properly maintained worm gear will produce the best results.

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