The reason why the two Hex Nuts system is effective in […]
The reason why the two Hex Nuts system is effective in resisting self loosening is due to the way the threads are jammed together (hence the term jam nut being frequently used for the thin nut). Since the bolt thread is in contact with the top flank of the small nut and the bottom flank of the top nut, relative thread movement is not possible. For self-loosening to occur, relative movement between the bolt and nut threads must occur. It is this jamming action that is the secret of the two-nut method.
Many types of old machinery have two nuts on the bolts. A thin nut is frequently used in these applications. Sometimes the thin nut can be observed below the standard thickness nut and on other installations, it’s on top. Although it may seem counter-intuitive, the thin nut should go next to the joint and not be put on last. In other applications, for example on column attachments, two standard thickness nuts are frequently used.
Some washers have a special design that attempts to help keep the nut and/or bolt from coming loose. Known as a lock washer these may be a split ring type, star type, wavy or any one of a number of other designs. A lock washer will be used with the part of the assembly that most likely could turn such as the nut. It could be used under the bolt head in instances where the bolt screws into threads in one part of the assembly.
Some assemblies are designed to have the parts move with respect to one another when the bolt is actually tight. The travel of the nut may be restricted by a shouldered design of the bolt or a sleeve can be installed around the bolt in the hole. In these cases a washer is often used to keep the moving part from wearing the bolt head or nut. The washer may be made of a material designed to reduce the friction between itself and the moving part. This can also help to keep a turning part from trying to loosen the nut through friction.