Many cabinet shops use staples (wire bent into the shape of the letter "U") to fasten wood pieces together. Often this fastening is just temporary until the glue in a joint cures. At other times, the staple is the key fastener for a joint. Staples are also used for fastening upholstery fabric to the wood frame.
Different types of staples have been developed with various modifications in wire diameter, leg (or shank) length, leg coatings, point shape, and crown width. The diameter of light wire staples is measured in thousandths of an inch. Heavy wire staples are measured by gage; sizes range from 15 (0.072-inch diameter) to 18 gage (0.0475 inch). The length typically ranges from 1/4 to 2-1/2 inches. Both light and heavy wire staples are available in steel, coated steel (including nylon, cement, and epoxy), and stainless steel.
The points are often chisel shaped, which allows for easy penetration and encourages the staple legs to spread as it penetrates, increasing the withdrawal strength. Blunt points, rather than chisel points, will reduce the tendency for the staple to split the wood when it is first driven. The remaining classification factor is crown width, which is the distance between the legs of the staple. (For example, a staple for holding a frame together could be specified as cement-coated steel with chisel points, 2-inches long, 7/16 inch crown, and 16 gage.
The forces that try and pull a staple out of a joint in approximately the reverse direction of the way the staple was inserted are called withdrawal forces. The withdrawal strength of a staple is determined by several factors including:
Depth of penetration of the staple legs into the base or receiving member ; deeper penetration (longer staple legs), the higher the withdrawal strength.
Density of the wood in the base or receiving member . The denser the wood, the greater the withdrawal strength. Note that 10 percent denser can result in over 20 percent more withdrawal strength.
Coating on the staple legs, if any . The coating softens during insertion, with the heat, and then provides some bending between the wood and the staple legs.
Curvature of the legs as they enter and continue into the base or receiving member . Legs that penetrate straight into the base or receiving material and remain straight as they continue inward are not as strong as staples that have legs that diverge as the legs penetrate deeper.
Staple shank (or leg) diameter . The larger the diameter, especially with deeper penetration, the greater the strength.
The actual strength of a staple is similar to two nails of the same diameter and penetration. As a rough rule of thumb, if the diameter of a staple leg is 2/3 of a nail, the staple will provide about the same withdrawal resistance as the slightly larger diameter nail.
Dr. Carl Eckelman of Purdue University has suggested that the withdrawal strength of a staple is mathematically expressed as the product of three factors:[16.4 x leg diameter] x [(196 x depth of penetration) - 36] x [shear strength of wood/1130]
Using this formula, consider a staple that penetrates into the base material by 1/2 inch and is 16 gage (0.0625 inch). The wood has a shear strength of 1500 psi; data available from the Wood Handbook or other sources. Then the withdrawal strength for one staple is predicted to be 84 pounds. If the penetration is increased to 3/4 inch, then the withdrawal is 151 pounds. If the gage is now increased to 15 gage, then the withdrawal is 174 pounds. (It is assumed that the staple does'nt split the wood and the staple is put into the side grain and not the end grain. .)
Lateral (or shear) strength
Eckelman suggests that for staples loaded laterally (that is, from the side rather than from withdrawal), the strength can be estimated for 16 gage staples in side grain (not end grain) from the following equation
Using the initial example above with 1/2 inch penetration, this gives a shear strength of 159 pounds.
Of course, two staples, if they are not real close to each other, will double the lateral strength.
Sometimes when a stapled joint is pulled apart in withdrawal, the head of the staple is pulled through the top material. Pull through is dependent on the diameter and length of the staple's crown, and if the top is wood, its thickness and density. Having the two legs further apart (crown width), using a larger diameter (increasing the gage), using a thicker top panel, and using top wood with a higher density (often not possible) will increase pull-through resistance.
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