An anchor nut is a hardware fastener with a threaded hole through it, which is secured to a work surface, usually by rivets or screws. Anchor nuts are an alternative to tapping threads into a work surface, especially in spots that are difficult to access for such a purpose, or that aren't thick enough. In addition, using an anchor nut is a reliable way of ensuring the exact placement of nuts or screws when fastening additional components onto the workpiece.
In the aerospace industry, significant problems occurred when nuts were welded onto work surfaces. The heat associated with the welding would sometimes deform and weaken the work surface, and the welds themselves would sometimes break under the high stresses associated with aerospace applications. Work surfaces themselves were often too thin to be tapped to accept screws or bolts, even when they were strong enough to support the load those nuts were intended to secure. Anchor nuts were developed to address these problems, and subsequently have also found great popularity in the automotive industry. Anchor nuts are also used in other applications, including decorative fasteners on cabinetry.
Made of steel or some other hard metal suitable for hardware, an anchor nut consists of the nut itself with one or more protrusions, or lugs, with holes for the rivets or screws to secure it to the workpiece. When an anchor nut has a single protruding lug, the lug will have two holes. Two-lug anchor nuts have a single hole in each lug, which can protrude from opposite sides of the nut or at right angles to each other, depending on the configuration of the work surface.
Some anchor nuts have no lugs, but are designed to be built right into the workpiece. Some, called street plate nuts, are discus-shaped, with tapered edges and the threaded hole cut right through the center. Others, often hex-shaped, are forced into an appropriately-shaped hole in the work surface.
Rivets are the fastener of choice for anchor nuts because they're relatively easy to use and won't loosen or detach in most industrial and machine environments. An anchor nut can also be welded onto the work surface, although this isn't appropriate for some high-stress applications like aerospace. Securing the anchor nut with rivets, screws, or welds distributes the stress across two points instead of concentrating it on a single location, which is one of the drawbacks of tapping threads into the work surface itself, if it were thick enough to be tapped.
Anchor nuts are sometimes called plate nuts when they're stamped from a single piece of sheet metal, flat or dimpled, thick enough to drill a hole through and tap threads. Alternately, and especially when the plate nut is dimpled, a long tube with internal threads may be attached to the plate to accept a screw or bolt. When there's the possibility of misalignment of manufactured parts, it's sometimes advisable to use floating anchor nuts, which have a degree of controlled movement.