Anchor Nuts

Nov. 15, 2002
Plate or anchor nuts have one or more lugs projecting from the base of the threaded body.

Plate or anchor nuts have one or more lugs projecting from the base of the threaded body. The nuts are attached by riveting or welding the lugs to the work surface.

Plate nuts provide permanent attachment for threaded fasteners at inaccessible or blind locations. They assure positive positioning of the mating bolt and are self-wrenching. Plate nuts are preferred for stressed-skin applications because they do not introduce additional stresses around the bolt hole.

Riveting is most commonly used to attach plate nuts. Projection and spot welding are specified when it is desirable, for stress purposes, to minimize the number of drilled holes in the workpiece. The most common anchor nut is the two-lug plate nut, where the lugs protrude on either side of the nut body. A corner anchor nut has two lugs at a 90° angle.

A number of nuts can be assembled on a single retainer, generally referred to as a "gang channel." Gang channels are produced in straight or curved lengths, with a variety of nut spacings.

Nonrotating plate nuts: These are used where little or no misalignment is tolerated. Bolts may be specified with regular-height nuts to use full bolt tensile strength. Low-height, lightweight designs may be selected for shear applications. Miniature designs are available for close-clearance installations. For safety, appearance, and sealing, capped nuts which cover the bolt ends are used.

For flush mounting, countersunk nuts are used with dimpled sheet metal to accommodate the screw head. Variable-depth counterbore nuts permit use of constant bolt lengths to fasten panels of various thicknesses.

Floating plate nuts: These nuts are used where bolt hole misalignment can occur in assemblies. Tolerances between holes can also affect alignment. Units with 0.015, 0.020, and 0.030-in. radial floats are provided for such misalignments. Oversize retainer holes facilitate assembly.

Swivel, or self-aligning, plate nuts are three-piece units consisting of a nut, concave or convex base ring, and retainer. These nuts simplify assembly of tapered or nonparallel components. The self-aligning design accommodates angular misalignment of mounting surfaces as well as some bolt-hole radial misalignment.

Floating, replaceable-element plate nuts: These multipiece units allow the nut element to be removed and replaced without disturbing the permanently attached retainer. They are usually high-performance nuts and are often specified for repeated use.

Clip nuts are sometimes used as plate nuts to eliminate riveting. They are self-retaining and easily installed and removed.

Plate nuts use the following locking techniques: Nylon inserts are held like a washer in a cylindrical counterbore at one end of the nut. Walls of the nut are crimped or rolled over the washer to hold it firmly in place.

Elliptical offsets produce locking torque by deforming the reduced portion of the nut body. Usually, this portion is a turretlike section at the top of the nut.

Beam offsets contain multiple segments in the top portion of the nut which are bent inward, providing elastic interference with the bolt. A lubricant must be used with this type.

Plate nuts are not usually used in primary tensile applications. Most are specified as 125 or 140-ksi nuts. Some anchor, swivel, variable-counterbore and channel designs are rated at 160 ksi, and specials to 180 ksi.

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