A coupling nut is typically a metal fastener which is threaded on the inside. It is designed to secure two parts together by screwing the coupling nut onto the outside of another part, often a threaded rod. The outside of a coupling nut is usually hex shaped so that a wrench can tighten or loosen it.
Coupling nuts may be made out of any material, but the most common coupling nuts are metal due to the strength of the material and the variety of applications in which it may be used. Metal coupling nuts may be made out of stainless steel, carbon steel, brass or aluminum. In much rarer situations, they may be made out of PVC or similar materials.
This type of nut works through the use of threads, or grooves, in the metal. One continuous male groove runs along the inside of the nut. In addition, one continuous male groove runs along the outside of the part to be secured. When these male grooves meet, they intertwine, creating a secure connection which can be tightened by hand or with a tool. This mechanism of using grooves to secure parts is called threading.
There are many variations available for a standard coupling nut. First, the length of the nut can vary from under an inch (.0254 m) to 14 inches (.3556 m) or even longer in special applications. Second, the finish of the coupling nut can vary. They can be plain or galvanized, plated with another material, dyed or coated.
Finally, the shape and look of the nut can vary widely. Most coupling nuts are hex shaped on the outside so that a wrench can be used, and most are open in the center on the top and bottom to allow a rod to thread through the nut. A cap nut is different in that the top of the nut has a dome over the opening. This is used for the end of a coupling rod. An acorn nut is similar to a cap nut, but the dome is shaped more like a cone instead of a dome. In all these cases, the outside of the nut is hex shaped, but another standard coupling nut is square shaped on the outside.
An additional type of coupling nut contains something called a sight hole. This is a hole in the outside of the nut which travels all the way through the material. A sight hole on the nut allows a visual inspection of the threads on the inside, allowing verification that the threads are properly engaged.