Prevailing torque nuts resist rotation through an intentional locking feature that creates torque before clamp load is developed. They include all-metal designs with locally deformed threads and nuts with a nonmetallic insert, commonly nylon. The added running torque can help keep a nut from backing off completely, but it is not the same as joint preload and does not repair a poorly designed bolted joint.
This guide focuses on selection, test language, reuse, temperature, installation, and the information a buyer should provide. TNHO supplies lock hex nuts for industrial fastening and other products in its nuts and washers range. Final approval should always be based on the application, governing standard, and samples of the actual nut-and-bolt combination.
Prevailing torque is the rotational resistance produced by the locking feature while the nut advances or retreats on the mating thread without bearing against the joint. Tightening torque is the total torque applied during installation. Once the bearing surface contacts the joint, tightening torque includes thread friction, under-nut friction, prevailing torque, and the portion that creates bolt tension. Confusing these quantities can lead to too little or too much preload.
A torque specification developed for a plain nut should not automatically be reused for a prevailing torque nut. The assembly method needs to account for the locking torque. Depending on the design standard, the installer may measure the prevailing torque and add or compensate for it, or use a validated torque-tension procedure for the complete joint. For safety-critical assemblies, engineering should establish the process with representative hardware and lubrication.
ISO 2320:2015 covers functional properties for all-metal and nonmetallic-insert prevailing torque steel nuts and was confirmed current in 2025. The standard itself warns that laboratory test torque may differ from application performance. This is why a catalog maximum or minimum should not be treated as a universal installation torque.
An all-metal prevailing torque nut uses controlled deformation in part of its thread or nut body. Common arrangements include top-lock, side-crimp, elliptical, and flexing segment designs. When the bolt enters the deformed zone, elastic interaction between the mating threads produces resistance. Because there is no polymer insert, an all-metal design is often considered for higher temperatures or environments that are incompatible with nylon. The exact limit still depends on nut material, coating, lubricant, and the applicable specification.
A nonmetallic-insert nut contains a polymer collar. The bolt thread forms a path through the insert and the interference creates running torque. These nuts are widely used because their locking behavior is easy to recognize and they can be economical for moderate-temperature service. Chemical exposure, heat, cold, aging, and repeated cycles can change the insert’s behavior.
ISO 2320 identifies application temperature ranges of approximately −50 °C to +150 °C for covered all-metal nuts and −50 °C to +120 °C for covered nonmetallic-insert nuts. Those ranges belong to that standard’s scope and should not be generalized to every proprietary nut. The drawing, material declaration, coating supplier, and application qualification may impose narrower limits.
NASA-STD-5020A treats prevailing-torque devices as a way to prevent complete separation or loss of hardware under vibration, not as a method for maintaining preload. That distinction is essential. A joint can lose clamp force because of embedding, gasket relaxation, thermal mismatch, plastic deformation, or insufficient initial preload even if the nut remains on the bolt.
The first line of defense against vibration loosening is a properly designed and preloaded joint that keeps the clamped parts from slipping. A lock nut adds retention. It does not eliminate the need to calculate grip length, bolt stiffness, joint stiffness, preload scatter, external load, temperature, and relaxation. Where complete hardware retention is critical, the applicable design authority may require a positive locking or secondary retention method.
| Selection factor | All-metal prevailing torque nut | Nonmetallic-insert nut | Buyer action |
|---|---|---|---|
| Locking mechanism | Controlled thread or body deformation | Polymer insert interference | Name the required design or standard |
| Temperature | Often selected for higher heat | Limited by insert material | State continuous and peak temperatures |
| Chemicals | Check metal and coating compatibility | Also check polymer compatibility | List fluids, cleaners, and exposure time |
| Reuse | Torque can change with cycling | Insert can wear or deform | Define cycle count and retest rule |
| Installation speed | Heat and galling risk may rise | Insert heating may change behavior | Validate tool speed and lubrication |
| Electrical path | Metallic path, subject to coating/contact design | Insert is not the intended current path | Test the complete assembly if conductive function matters |
A useful test identifies the nut standard and property class, mating bolt standard and coating, thread size and pitch, lubrication condition, test temperature, installation speed, seating condition, number of cycles, measurement points, and acceptance limits. Changing any of these can change the result. A zinc-flake-coated bolt, a bare test mandrel, and an oiled stainless bolt are not equivalent mating parts.
During a typical test, the nut is run onto a specified mandrel or bolt far enough for the locking feature to engage. Torque is measured while the nut continues to rotate without bearing on a joint. Removal torque is measured during reverse rotation at the prescribed location. Standards define exact sequences and limits; an improvised peak reading from a power tool is not a substitute.
The NASA Fastener Design Manual illustrates common deformed-thread and nylon-collar concepts and explains that repeated use can reduce prevailing torque. Current NASA mechanical-fastener torque guidance also calls for replacement when a prevailing-torque nut no longer meets its required value. For commercial products, use the applicable nut standard and customer requirement, not NASA limits by default.
Match diameter, pitch, thread series, tolerance class, direction, material, property class, and coating allowance. A nut that starts by hand for one turn can still be wrong. Cross-threading, damaged lead threads, oversized plating, or a mismatched pitch can create high torque that looks like locking action while damaging the joint.
Use a calibrated installation tool appropriate for the torque range. Control speed because rapid running can heat the insert or friction surfaces. Ensure enough bolt protrusion for full engagement of the locking feature, subject to the governing specification. Do not place the locking zone on incomplete runout threads. TNHO’s metric coarse versus fine thread guide and thread engagement guide help define the mating interface.
Stainless all-metal combinations may be vulnerable to galling. Coating, lubricant, material pairing, speed, and tightening method should be qualified together. Do not casually add lubricant after torque qualification: lower friction can increase preload at the same applied torque.
“Reusable” is not a permanent property. Each installation and removal cycle can burnish threads, redistribute coating or lubricant, wear a polymer insert, or relax a metal locking zone. Reuse is acceptable only when the applicable standard and application authority allow it and the nut continues to meet the defined prevailing-torque requirement.
For production, define whether nuts are single-use or the maximum qualified cycle count. For maintenance, provide a replacement rule that technicians can follow. Visual appearance alone is insufficient; a nut may look intact while its running torque has fallen. Discard hardware with cross-threading, cracks, severe corrosion, distorted bearing faces, damaged inserts, or values outside the approved range.
This educational episode from Würth demonstrates several locking-nut concepts and helps buyers visualize how their mechanisms differ. Product names in the video are examples; use the governing drawing and standard for procurement.
Provide the nut design, standard, size and pitch, property class, material, coating, lubricant, mating bolt or mandrel specification, temperature range, chemical exposure, required insertion and removal torque, test cycle, reuse rule, installation speed, assembly torque or tension requirement, lot traceability, certificates, packaging, and quantity. State whether the nut is expected to carry current or resist a specific vibration environment.
Ask the supplier to test representative finished nuts with the specified mating component. Check thread gauging, dimensions, coating, hardness or proof load where applicable, prevailing torque over the required cycles, bearing-face condition, and actual assembly performance. Compare the choice with the broader lock nut versus hex nut guide and the dedicated nyloc nut guide.
It resists nut rotation and can help prevent complete disengagement, but it does not guarantee retained preload. Joint design, initial tension, settlement, vibration, temperature, and external loading still determine clamp-force retention.
Only when the governing standard and application permit reuse and the nut still meets the required torque limits after the specified cycles. Replace damaged nuts and any nut that falls outside the acceptance range.
No. Prevailing torque is running resistance before the nut bears on the joint. Installation torque also includes bearing and thread friction and the torque that develops bolt tension.
Use the bolt or test mandrel defined by the applicable standard or purchase specification, including thread tolerance, coating, hardness, and lubrication. Changing the mating component can change the measured result.
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