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Threaded Fasteners: More than Appearances

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May 2012

 

Fasteners keep our machines together. They’re simple, strong—and often neglected, overlooked and misunderstood. They’re also critical to our machines’ condition, and ultimately to our own longevity. It’s worth looking at them and knowing what we’re looking at.

Bolts hold things together, or keep things from shifting. Tension pulls on the bolt; shear forces try to bend it or cut it off. (The best example of a fastener designed for use in shear is a pin.)

A bolt is usually stronger in shear than in tension (and it is stronger yet in “double-shear,” where it is supported in two places, with a center load), but it is in shear that we see most failures—precisely because engineers know how to load bolts optimally, and engineers try to put fasteners into shear rather than tension, when there is a choice.

A bolt in a tension application will sometimes have a reduced-diameter shank to prevent other things from rubbing against it; a bolt in shear is usually snug around its circumference.

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