Understanding Locking Fasteners
Threaded fasteners (bolts, screws, etc.) can loosen when exposed to shock, vibration and other dynamic forces. In these situations, a "locking" fastener may be required. A locking fastener is designed to resist loosening and can be externally threaded, such as a screw, or internally threaded, like a nut.
In this section we'll look at some of the more common non-permanent locking schemes (an example of permanent locking uses a chemical reaction to form an adhesive that bonds with the mating threads). Basically, there are two types of locking fasteners: free-running and prevailing torque. Regardless of the type, the objective is to increase the "break-loose" torque, which is the amount of torque required to loosen the fastener once it is installed.
Free-running types often rely upon an additional component, such as a lock washer or jam nut. Once initially loosened, however, they offer no further resistance to loosening, which is their main disadvantage. Still, in some applications, that may not be a problem. Prevailing torque types, however, are designed to create friction. Even after slight loosening, friction continues to resist further loosening. However, that generally means there is substantial friction during assembly.
There are a variety of different free-running styles to choose from and, often, the application will dictate which is the most appropriate.
Said to be one of the first locking schemes, a thin "jam" nut is first installed and tightened, then the normal thick nut is installed and tightened against the jam nut. Although effective, extra bolt length is needed to accommodate the additional nut, and installation time is increased because assembly involves two nuts that must be individually tightened.
Slotted & Castle Nuts
Slotted nuts are used with bolts or studs that have a hole drilled in their threaded section. Once the nut is installed, a cotter pin or safety wire is inserted through opposing slots in the nut and the bolt's hole to prevent the nut from loosening. Unless the cotter pin or safety wire shears (breaks), the nut remains in position. This system does require that the hole be drilled in the correct place. And because the nut slots are spaced at 60-degree intervals, it may not be possible to tighten the nut precisely—it may have to be over- or under-tightened for the hole and a slot to align. Additional time is also required for assembly. Slotted nuts have a flat top; castle nuts have a higher profile because of their slotted cylindrical top.
Lock washers are a very popular locking choice especially with smaller-sized fasteners. Essentially, there are two types: spring action and tooth.
Spring action washers include split, conical (Belleville) and wave. These washers, placed under the nut or screw head, compress as the fastener is tightened and the spring-back tension deters loosening.
A tooth lock washer—internal, external, internal-external and countersunk external—creates a ratchet action by biting into the nut or screw head and the surface it contacts.
Despite their widespread usage, tooth lock washers can scratch mating surfaces. Conical washers do not damage surfaces like tooth lock washers, but must be oriented correctly when installed otherwise they will be ineffective. And because spring types flatten when tightened, their locking action doesn't come into play until the fastener has slightly loosened.
A final option is the lock set washer pair. Also called vibration-resistant lock and Nord-Lock washer assemblies, the set consists of two pieces: a top and a bottom washer. These reusable washers have wedges on one side, which should be installed interlocked, and radial teeth on the opposite side. When the screw or nut is tightened, the teeth bite into the screw head or nut and the mating material to prevent slipage, while the wedges increase tension to prevent loosening.
Aside from the additional time required, it can be difficult to install lock washers in tight places or when the bolt or screw is pointing down.
Sems are screws that have captive lock washers beneath their heads that can't be removed without damage. This one-piece approach saves time during assembly, but if the screw or lock washer becomes damaged, you have to replace both.
Also known as K-Lock nuts, Keps are similar to Sems in that they are nuts with pre-assembled lock washers (usually external tooth but sometimes conical). It can save time and make the job easier handling a single piece. But if either the nut or washer is damaged, both must be replaced. (See the discussion about Lock Washers above.)
Serrated fasteners, available as screws and as nuts, have serrated teeth that bite into the mating surface as they are tightened. In that respect, they act like tooth lock washers because the ratchet-action they create resists loosening. But, also like tooth washers, they damage the mated surface.
Prevailing torque locking fasteners use some form of interference to create friction with the mating threads. So even though there is friction to prevent loosening, there is also friction during assembly. Considering the two options—nuts and screws—nuts are more common. A single nut can be used with many different bolt and screw lengths and head styles, thus reducing inventory. Prevailing torque screws, however, must be used in situations like tapped holes.
There are two ways to make prevailing torque fasteners: using all metal, or by adding a non-metallic element. Flex top locknuts, for example, are all metal. As for non-metallic elements, nylon insert locknuts have a captive undersized nylon washer, and screws use a nylon patch or pellet 2 to 3 threads from the end. Not surprisingly, there are advantages and disadvantages to both types of locking fasteners.
Because nuts are the most popular, let's take a closer look at them: All metal nuts have fewer temperature and chemical restrictions, but they are less resilient, are prone to thread galling, and can be difficult to thread onto long bolts. Non-metallic designs are temperature limited and may not be used with certain chemicals, but can be threaded onto long bolts without thread damage. Also, because the non-metallic element tightly conforms to the mating threads, it provides an effective seal.
Be aware that flex top and nylon insert locknuts are basically one-way fasteners—because the locking feature is at one end, they can only be installed one way (locking feature last). Since most screws and bolts are tapered at their threaded end—and the first few threads may be damaged—be sure that the nut is threaded on far enough for its locking mechanism to engage full-size, undamaged threads. And even though most prevailing torque locknuts can be reused, torque may decline with subsequent reuses which will reduce effectiveness.
Finally, when using fasteners that rely upon deformed threads as the locking mechanism, do not mix grades. For example, if a heat-treated prevailing torque deformed threads nut is used with a standard low carbon steel bolt, the threads of either—or both—may be damaged.
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These reference charts provide additional details about locking fasteners…
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