In sizing and specifying fasteners, most of our time is spent evaluating failure conditions of the fastener assembly when placed under physical load. We use engineering data to determine if a fastener will break, crack, wear, stretch, or decay during its use. All of the ways in which we examine for these types of material failures make the assumption that the fastener assembly itself is fully connected. In other words, we assume that a bolt and nut remain as tight as the day they were installed. Enter retention fasteners.
Well, we know from practical experience that at times nuts loosen, and bolts fall out. When a fastener joint in effect becomes loose and disassembles itself, this isn’t the result of stress or strain, but instead the result of vibration or oscillation. How do designers address keeping a fastener assembly intact under vibration? The answer is in adding a retention feature to the joint. In this article, we’ll take a look at common retention fasteners, otherwise known as ‘free-running’ and ‘prevailing torque’ fasteners.
Free-Run Style Retention
Loaded self-locking fasteners, also known as ‘free-running’ fasteners, are the first category of retention hardware that help combat vibration-induced loosening of a fastener joint. These solutions accompany or replace your normal fastener accessories, such as nuts and washers. They freely slip into place in the assembly and add a design element that deters the compression component (such as the nut) from rotating loose after installation. Here below are the most common free-run style choices:
|Locking Fastener Type||Description|
|Jam Nut||Similar to a regular hex nut and thinner in profile, a jam nut is placed on a bolt after the hex nut, such that the two nuts compress against each other, holding the primary hex nut in position, unable to spin loose.|
|Castle Nut||Similar to a regular hex nut with grooves cut into one side, a castle nut spins onto a bolt, aligning the grooves with a hole drilled through the bolt body. A cotter pin or wire is inserted through the groove and hole, locking the nut in place.|
|Lock Washer||A lock washer is placed below the assembly’s hex nut, either directly or with a flat washer against the material being bolted. Lock washers create additional friction and force intended to keep the nut from spinning loose, and come in multiple styles. Some lock washers deter loosening by adding compression force against the nut (such as with split helical lock washers), and others create interference force against the bolted material (such as with tooth lock washers).|
|Integrated Lock Washer Bolts / Sems||Sems bolts and screws are standard threaded fasteners that have a locking washer feature permanently attached to the bottom of their head. These function the same as separate lock washers, adding friction or compression forces to resist loosening.|
|Integrated Lock Washer Nuts / Keps||Keps nuts are standard nuts that have a locking washer feature permanently attached to one side of the nut. Same as Sems bolts, these function the same as separate lock washers, adding friction or compression forces to resist loosening.|
|Serrated Bolts and Nuts||Similar to standard bolts and nuts, serrated fasteners have one face cut with serrated teeth, which will engage the material being bolted for added friction.|
The benefit of free-run fasteners is that they’re relatively easy to apply to standard joints without any additional complication of the design. Some considerations when selecting a free-run style component might be that they:
- Only provide loosening protection when ‘under load’, IE when the bolted joint is fully tightened. They provide no benefit when the joint is not tightened or otherwise fully engaged
- May cause damage to the material being bolted, such as with serrated nuts and tooth lock washers
- Have relatively low holding force, used for mild to moderate vibration conditions
- May require longer bolts / screws, such as for the space needed to add jam nuts
- Are largely reusable without loss of performance, except for some styles of lock washers where the compression pattern will flatten after installation and / or over time
Most self-locking fasteners are readily available, and need limited technical data to specify. Still, you might want a second opinion or application review to make sure you’re specifying the ideal solution. E&T Fasteners offers expert opinions on self-locking hardware and more, helping you build out a complete project order in one stop. Contact us to discuss your application, or to see our standard fastener offerings, visit our product selection here.
Prevailing Torque Style Retention
Where free-running fasteners use surface compression or surface friction force to combat loosening of a bolted assembly, a different style known as Prevailing Torque fasteners take a more rigorous approach by adding a constant inter-thread mechanical interference feature to increase the torque needed to engage the assembly overall.
Said another way, a free-run feature is one that ‘pushes’ normal threads together to cause them to bind, but only works when the assembly is fully tightened. A prevailing torque feature is one that augments the threads themselves to increase the interference between threads, creating higher friction across the whole usable thread length in both directions (resisting tightening and loosening). In this way, a prevailing torque fastener set takes more force to assemble and disassemble and altogether provides more resistance and less risk of loosening than free-run hardware.
The most common example of a prevailing torque fastener is a nylon-insert lock nut (trade name: Nylok nut). This nut has a nylon ring permanently encased at one end of the nut, and the ring has a smaller diameter than the bolt threads such that it takes a wrench to spin the nut on or off of the bolt due to this positive interference. This additional or ‘prevailing’ torque required to install the nut also serves as the constant binding force that holds the nut in place once the joint is fully assembled.
Beyond lock nuts, the fastener world has many engineered, proprietary prevailing torque solutions available. Any possible means by which to increase the interference between threads is a potential prevailing torque solution. Common approaches and example manufacturers include:
- Poly-Lok Patch, by Long-Lok Fasteners Co, use a terephthalate compound material applied in a patch area to bolt threads, acting as a locking polymer that expands and fills up the thread space between bolt and nut
- All-Metal Torque Nuts, by TriState Locknuts Co, use a reduced-diameter metallic tail cap to grip bolt threads, in the same way that a nylon-ring lock nut functions but without the risks associated with the nylon material (temperature sensitivity, chemical compatibility, etc)
- Long-Lok Strip, by Long-Lok Fasteners Co, use a longitudinal slot cut into the bolt threads, where an engineered polymeric material strip is inserted to expand and fill up the thread space between bolt and nut
When evaluating prevailing torque fasteners, keep in mind that these styles may offer a limited number of reuses (disassembly and reassembly) as the torque-adding feature may wear or degrade over multiple uses. Also, because prevailing torque fasteners are specially engineered to offer a higher torque value than the break-out or loosening torque expected in your application, you’ll want to include calculated holding torque values in your specification.
E&T Fasteners is here to help you navigate fastener options to best meet your application and project needs. Price quotes, availability, lead times, delivery confirmations, unique application suggestions – whatever your need, we’re here to take your call or email. For support and advice from our fully trained staff, speak with an E&T sales representative today.
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