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What is a Square Nut?

By Lori Kilchermann
Updated May 17, 2024
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A square nut is a steel fastening device with four sides that threads onto a steel bolt. Made prior to the hexagonal nut, the square nut uses a purpose-built wrench to tighten and loosen it from its bolt. The square nut can be used from small fastener applications to very large ones, as the success of the design has proved worthy for a myriad of uses. One reason the square nut is so successful is the ease of removal and installation as the square design has proved durable and practically immune to being rounded off by a wrench.

One of the earliest fasteners, the square nut was prized by many as an easy-to-use fastener capable of securing even the most difficult components. The first versions of the square nut were carved out of wood and used on threaded wooden pegs. The square design made it possible to easily fashion a properly-sized wrench or tool to tighten the nut into place. As technology began to replace the wooden fasteners with more durable and longer-lasting steel versions, the square nut was soon forged out of steel bar stock and adapted to use on steel bolts and fasteners.

The sharp corners of the nuts were often destructive to the wood or metal surrounding the fastener. Large flat steel washers were soon used behind every nut to provide some protective material around the corners. The washers proved to not only protect the components from damage from the corners of the nuts, but they also succeeded in spreading the load of the forces pressing against the nuts across a larger area, giving the fastener more strength. The nuts themselves were often forged with a flat side and a convex side. The flat side was created to allow the nuts to fit flat against a flat washer, while the convex side served to provide a minor bit of relief when the nut was used under a hubcap or cover.

Modern construction continues to use the square nut in the manufacture of many furniture items. Often used as a blind nut, the square nuts are frequently placed inside of metal channel to prevent them from turning as the bolt side of the fastener is tightened into place. The square nuts can be slid into position inside the channel and make a perfect foundation for a hidden fastener. The nuts are also used in modern railroad track construction. The square nut is placed inside of the rail's channel, where it makes contact with the top and bottom of the rail, preventing it from turning when pressure is applied.

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