Staple (fastener)

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A staple is a type of fastener, usually made of wire or thin flat metal, which has the general form of an upside-down letter "U": there are two "legs" that drive into the material being fastened, and a piece between them that prevents holds the entire staple to the surface of the material. For different applications, there are a great number of staple shapes, materials, thickness of materials, and tools for inserting them.

Perhaps the most common staple in daily use is used to fasten sheets of paper together. The tool for this purpose is called a plier stapler. The staples themselves are made of thin wire, with the ends of the leg beveled for sharpness. When using the tool, it separates a staple from the strip in which they are produced, pushes the legs through the material, and then bends the legs toward one another to anchor the stable. Paper staples differ from most industrial staplers in bending the legs; most industrial types leave the legs straight and rely on friction to hold them in place.

A common type is used for fastening thin material, such as a backing of particleboard or the paper edge of foil-backed insulation, to a wooden frame. These may be placed by hand and hammered in, or, more commonly, inserted by a tool called a staple gun. The staple gun may be muscle-powered but with strength-multiplying levers, or may have electric, compressed air, or even explosive power for the driving operation. The staples used in this common application differ in the length of the legs; the bridge between them is a constant width. A shallow staple might have 1/4" legs, while a large one could be 9/16" or longer. The longer the leg, the more likely it is that power assistance will be needed to drive it. Staple guns often have an adjustment that allows the driving force to be increased or decreased to optimize the power for the material being fastened; too much power and the staple can fracture the work, while too little power can cause the staple not to penetrate.

To use a staple gun, one either squeezes a handle, or pushes a button to actuate the power source, after the staple exit has been carefully positioned over the work. A variant, often used for repetitive jobs such as anchoring sheet insulation, is the hammer tacker. The latter is loaded with staples, but is swung at the work, with a motion much like that of hammering; the force of the swing pushes on a lever at the bottom of the tacker, and transfers mechanical energy to the staple driver at the top of the tool.

These common staples have their bridge at right angles to the legs, since the goal is using the bridge to hold the sheet to the base. A variant is called a crown staple. In this type, the bridge curves upward from the legs and does not press against the work. Crown staples are used to secure wire and cable to surfaces; the wire is held between the curve of the staple and the surface. To use these effectively, it is essential that the crown put minimal pressure on the insulation of the wire, and, above all, not cut through the insulation into the conductor.