nut, in technology, fastening device consisting of a square or hexagonal block, usually of metal, with a hole in the centre having internal, or female, threads that fit on the male threads of an associated bolt or screw. A bolt or screw with a nut is widely used for fastening machine and structural components.
In addition to the standard square and hexagonal nuts, there are many special types. Several are illustrated in the , including the slotted or castellated nut; when this nut is tightened on the bolt, the slots are aligned with a hole in the bolt and locked in place by a cotter pin or wire lacing to prevent loosening or unscrewing. Locking can also be accomplished by tightening a thin nut called a jam nut against a standard nut. Another locknut contains a fibre or plastic insert near the top of the nut; locking occurs when this insert interferes with the bolt threads as the nut is tightened. The wing nut is used in applications in which frequent adjustment is necessary and hand tightening is sufficient.
Self-retained nuts provide a strong, permanent fastener for many types of thin materials; they are threaded blocks held in special enclosures attached to the part by welding, rivetting, screwing, or snap-on attachments. Single-thread nuts are formed by stamping a thread-engaging impression in a piece of flat metal.