The benefit to most threaded fasteners such as bolts and screws is that they can be installed and uninstalled multiple times to enable the assembly and reassembly of the products in which they are used. The downside is that they can unintentionally loosen and cause products to lose functionality, become damaged, or even fail.
The answer to that unintentional loosening issue is the use of self-locking fasteners. The self-locking fastener was developed to retain the advantage of reusability while preventing the problems of accidental disassembly when pre-load is lost. Also known as self-clinching fasteners, self-locking fasteners, once threaded into a softer, ductile metal, are held in place by means of a serrated clinching ring, knurl, ribs, or hex head which prevents the fastener from turning in the host material, effectively locking it so that it can’t be removed. Properly installed, self-locking fasteners become a permanent part of the assembly. An added benefit of self-locking fasteners is their decreased tendency to fatigue by reducing any vibration transferred to the fastener.
Self-locking fasteners are used primarily in situations where good pullout and torque loads are required, especially in thin sheet metal that can’t be securely fastened by other means. In many cases, self-locking fasteners – because of their locking power, low profile, and compact design – can enable the use of even thinner sheet metal for a lighter and smaller assembly form factor.
There are two basic types of self-locking fasteners:
Chemical additive, which involves two methods of application, pre-applied and applied-in-place:
Pre-applied chemical additive fasteners are manufactured with the adhesive coating pre-applied using a micro-encapsulation process. The adhesives are typically a two-part epoxy, with the material applied in a band around the threads and dry to the touch. When the fastener is rotated against its mating part the tiny capsules of adhesive burst, releasing the material in and around the threads, which then cures and forms the locking bond. Because the adhesive coating is pre-applied, it is generally more consistent and the assembly time is significantly less than for applied-in-place coatings. In some cases, pre-applied additive fasteners can be reused, depending on how much adhesive remains on the components after disassembly.
Applied-in-place chemical additive fasteners provide just one-time use and additional assembly time is required to coat the fastener with the additive. Application errors and contamination of other assembly parts are inherent risks with in-place application of chemical additive fasteners.
Prevailing torque fasteners, which are the only fasteners that retain their locking torque independent of an axial load. When pre-load is lost, the prevailing torque fastener can prevent loosening from shock or vibration. Prevailing torque fasteners come in two flavors: deflected or distorted thread (which, as the term suggests, lock via thread irregularities) and material-additive (which use a wedge to lock the fastener in place). Both require additional installation torque to assemble the fastener before engaging the bearing surface.
Self-locking fasteners provide a number of advantages over other kinds of fasteners, including greater reliability and holding power. They’re the ideal fastening solution for applications in which the attaching hardware can’t be easily reached after a cabinet, chassis, or other component is assembled. Self-locking fasteners can be installed during fabrication to simplify and speed up assembly operations and component mounting, especially for equipment installations and replacements performed in the field.
Manufacturers should consider self-locking fasteners for products in which simplified assembly and fewer parts can reduce production costs and speed up time-to-market in highly competitive marketplaces. Self-locking fasteners can reduce labor time, inventory, and overall product weight, as well as provide greater product design freedom. All of these characteristics make self-locking fasteners especially attractive for medical, computer, communications, automotive and aerospace products.