Articles 

What’s a Locknut and Why Do You Use Them?

2011-02-23

A threaded bolt/nut joint is held together by forces whose total strength in compression is greater than the forces that tend to pull them apart. If not, the joint will loosen, and vibration would cause it to fall apart. Sounds simple enough, but it is not. Firstly, the forces that the joint will see are not always known, or not known exactly. Secondly, external forces may exceed engineering calculations. For example, the best engineering does not take into account a tire hitting a pothole or curb at 60 miles per hour. For these reasons, the employment of a locking element on the fasteners is a common practice. Among its usages are; as a fail/safe device, to forestall any mis-installation by the factory, repair shop, or other handler, the locking fastener will prevent complete separation rapidly of the joint. Thereby, allowing time for the failing joint to be noticed and acted upon. Further, the perceived safety of the joint may require a cosmetic appearing fastener, even if it never performs any function (the customer wants to see some visual assurance that the parts are secure). The largest usage type fastener that employs locking elements is the internal threaded part, hereafter called the nut. While there is some use of locking elements on externally threaded fasteners, the use of a retention feature is usually limited to specific cases. These are: externally threaded parts that are threaded into tapped holes; where the nut member can not be turned (as in welded or fixed position parts); where the design intent is such that the bolt member is to be fixed at a position other than full installation (as in a stop bolt or where the part is used as a limiting device).