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Machine Screw Nuts

Machine screw nuts are hex and square nuts available in much smaller sizes than full-size hex nuts and are usually used with machine screws. Hex is short for hexagon, which means they have six sides. Square nuts have four sides.

Threads are standard right-hand and Unified inch coarse series (UNC, Unified National Coarse) or Unified inch fine (UNF, Unified National Fine).

Machine screw nut size refers to its nominal thread diameter. Typically, sizes range from #0 to about 3/8" for hex and #2 to about 1/2" for square. Sizes less than 1/4" are listed as a number size (the larger the number the larger the size) while sizes that are 1/4" and larger are specified in inches, usually fractional rather than decimal. Not all types are available in all sizes.

Width across flats, which is wrench size, ranges from 1.67 to 2.60 times the nominal machine screw nut size and thickness is 0.621 to 0.864 times the size. Although the across flats dimension for 5/16" and 3/8" hex machine screw nuts is larger than full-size hex nuts, hex machine screw nuts are thinner than same sized hex nuts. Square nuts offer a large surface area on their flats for improved wrenching. Hex nuts have flat tops with chamfered (beveled) corners; bottoms are flat and corners may or may not be chamfered (chamfered bottom corners are common). Square nuts have flat tops and bottoms (no chamfer).

Common materials for hex machine screw nuts include steel, stainless steel, brass, silicon bronze, and nylon. For square nuts, steel is typical. Steel remains the least costly material followed by stainless steel; copper alloys (such as brass and silicon bronze) are the most expensive.

A common finish for steel machine screw nuts is zinc plating. Zinc, the most popular and least expensive commercial plating, offers moderate corrosion resistance. Stainless steel, though, is a better choice when corrosion is of concern. When submerged in salt water without free oxygen, silicon bronze is a preferred material because stainless steels can suffer from severe pitting corrosion (stainless steel needs oxygen to create its self-healing, corrosion-resisting chromium oxide film).

Refer to ASME B18.6.3 for specifications relating to hex and square machine screw nuts.

The different types of machine screw nuts are summarized below…

Table 1. Machine Screw Nuts. Machine screw nuts are available in hex and square types. Hex is more common, but square has advantages in some applications.

Machine Screw Nuts
Hex Machine Screw Nuts
Hex Machine Screw Nuts range in size from #0 to #14, and 1/4", 5/16" and 3/8". Hex is the most common style and is available in the largest variety of materials.
Square Machine Screw Nuts
Square Machine Screw Nuts range in size from #2 to #12, and 1/4", 5/16", 3/8" and 1/2". The large surface area on the flats of these nuts reduces the chance of wrench slippage.

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