Locknuts
Locknut mechanisms, prevailing torque versus clamp force, and the limits of product and standard designations.
Tags: components, fasteners, threaded-joints
On this page
Bossard's BN 33024 is an A2 stainless prevailing-torque nut with a polyamide insert, listed for operation up to 70 °C. That limit belongs to this product. Prevailing torque is resistance to rotation on the unclamped mating thread. Use the specified torque and clamp-force method for the exact nut, finish and installation condition. Bossard, BN 33024.
What a locknut does
Locknuts use several mechanisms to resist unwanted rotation. Milsons, Locking Nuts, version 230309, p. 1.
A prevailing-torque nut resists rotation on an unclamped mating thread. Bolt Science, Quality Control in the Tightening of Bolts.
A turning-resistance measurement leaves joint clamp retention to verify separately.
Mechanism families
The examples below describe the cited supplier's constructions, not universal specifications for everything sold under a similar name.
| Family | Source-specific mechanism | Consequence of the distinction |
|---|---|---|
| Nonmetallic insert | Milsons Nyloc: nylon collar increases thread friction. | Insert identity is part of the product specification. A nylon example does not describe every nonmetallic insert. |
| All-metal distorted thread | Milsons Conelock: one-piece nut with distorted top threads. | The locking feature is in the metal thread region, rather than a polymer collar. |
| Serrated flange | Milsons serrated flange: serrations score the bearing surface. | This action requires bearing-face contact; it is not the same mechanism as thread drag before seating. |
| Half/jam arrangement | Milsons half nut: paired with another half or full nut. | The arrangement uses interaction between nuts, rather than an insert in one nut. |
Source for the constructions: Milsons, version 230309, p. 1. The consequences are explanatory synthesis. This guide does not establish a universal jam-nut order, torque split or strength equivalence. Likewise, the serrated example supplies no universal permission to mark a coated or finished bearing surface, or to substitute a washer beneath it.
Another construction in Milsons’ guide is the Fuji nut with a spring friction ring. Milsons, version 230309, p. 1.
Torque terms and clamp force
The following distinctions are explanatory terminology, not a replacement for a product's test procedure. Torque is expressed in N·m; clamp force or preload is a force, expressed in N or kN.
| Term | Meaning in this article | What it does not establish |
|---|---|---|
| Running torque | Torque while the nut is rotating along the thread; with an active locking feature and no bearing contact, this includes prevailing resistance. | Contact with the joint or achievement of clamp force. |
| Prevailing-on torque | Locking-feature resistance during installation under the specified measurement conditions. | The complete seated tightening torque. |
| Seating | The transition to bearing contact with the joint. A procedure may define a particular seating threshold. | A universal torque threshold or target preload. |
| Tightening torque | Applied torque during joint tightening, including frictional effects and the contribution associated with bolt extension. | A unique clamp force independent of friction and fastener design. |
| Removal torque | A direction and operation that need further definition: initial breakaway from a clamped joint and subsequent unseated running resistance are different measurements. | Equivalence to the original tightening torque. |
| Prevailing-off torque | Locking-feature resistance during removal under the specified test conditions, distinct from breaking a seated joint loose. | Proof of retained clamp force or permission to reuse. |
The table synthesizes the distinction between rundown and seating in Atlas Copco’s Prevailing torque compensation strategies, torque distribution in Bolt Science’s tightening article, and friction-dependent release torque in Bolt Science’s Untightening of Nuts & Bolts. It supplies no normative measurement procedure or test limits.
Synthesis: a measured torque cannot be converted into a reliable preload simply because the component is called a locknut. A blanket rule to add a catalog prevailing-torque maximum to an ordinary nut's torque is not established by these references. An assembly torque specification depends on the actual nut, mating fastener, friction condition and joint.
What standard designations establish
The public ISO catalog separates product geometry and classification from functional testing:
| Exact reference consulted | Publicly documented scope | Boundary of this evidence |
|---|---|---|
| ISO 7040:2025, edition 4, June 2025 | Regular hexagon prevailing-torque nuts with nonmetallic inserts; steel and stainless steel; metric coarse threads M3–M39; product grades A and B. | This is a product-standard scope, not a tightening instruction. ISO abstract and edition record. |
| ISO 7719:2025, edition 4, June 2025 | All-metal regular hexagon prevailing-torque nuts; steel and stainless steel; metric coarse threads M5–M39; product grades A and B. | It does not identify every proprietary all-metal locking design. ISO abstract and edition record. |
| ISO 2320:2015, edition 5, December 2015; confirmed in 2025 | Functional properties of prevailing-torque steel nuts, including combined prevailing-torque and torque/clamp-force testing at +10 °C to +35 °C. The abstract covers all-metal and nonmetallic-insert types, coarse M5–M39 and specified fine-thread combinations. | ISO explicitly identifies laboratory conditions and possible differences in practical prevailing torque. The test ambient range is not an operating-temperature rating. ISO abstract and confirmation record. |
Only these public abstracts and edition records were used; the full standards and their normative tables were not reviewed. No proof-load value, prevailing-on/off acceptance limit, test-cycle count or thread-fit tolerance is reproduced here. A product grade, material designation and mechanical property class describe different aspects of a nut. The ISO 7040 and ISO 7719 abstracts separately refer to product grades and selection of property classes or stainless steel grades. ISO 7040:2025, ISO 7719:2025.
Synthesis: outside dimensions and wrench size do not establish thread compatibility. Thread system, nominal diameter, pitch and fit; nut height and bearing geometry; material and mechanical property class; product grade; coating and lubrication; and locking construction are separate characteristics. A shared “M” diameter or similar hexagon shape is insufficient to establish an interchangeable assembly.
Product limits are specific
Bossard's BN 33024 listing describes a thin prevailing-torque hex locknut with a polyamide insert, A2 stainless steel and a blue zinc-plated surface. Its additional information gives an operating temperature up to 70 °C. The listing identifies DIN 985 as withdrawn and also displays an approximate UNI 7474 reference; no dated DIN or UNI edition is supplied. These are statements about that listing, not an equivalence certificate or a universal nylon-locknut temperature limit. Bossard, BN 33024.
This example illustrates why a broad family label or a standard's test context cannot replace an exact product limit. The sources reviewed do not establish universal reuse counts, service-temperature limits, corrosion life, vibration endurance or load ratings across locknut families. They also do not establish suitability for structural, aerospace or other qualified assemblies, or customer approval of a substitution. Reuse acceptability and assembly qualification remain product- and application-specific questions outside this draft's evidence.
Source provenance and coverage
Research independently reviewed October 30, 2026 (PDT, America/Los_Angeles), reusing the prior draft’s primary-source captures and checking the HTML publishers again. This is the business review date, not a source publication date.
- Milsons: supplier PDF, version 230309, p. 1 from the retained capture independently visually inspected. Fresh web retrieval timed out and direct HTTP returned 403; the retained document remains the evidence for the mechanism descriptions. The printed version code and URL directory do not establish a publication date. These are stocked-type descriptions, not manufacturer qualification evidence.
- ISO: original publisher's public HTML abstracts and edition/lifecycle records for the three exact editions above. ISO 2320's page reports confirmation in 2025. No full-standard access or normative-table verification is claimed. All three public pages were readable in fresh web retrieval. Retained ISO 7040 and ISO 7719 evidence came through that service after earlier direct HTTP downloads returned 403. Retrieval-service content is not proof of a direct origin download.
- Bossard: original supplier's English-language BN 33024 HTML product listing. No publication date or revision identifier was displayed in the consulted content; this is a retrieval snapshot, not a dated controlled drawing or lot certificate.
- Bolt Science and Atlas Copco: original technical publisher and tool manufacturer HTML articles cited above. No publication dates or revision identifiers were displayed in the consulted content. Numerical examples and controller-specific strategies are not adopted as general limits or installation instructions.
Coverage is limited to mechanism and terminology distinctions, public standard scopes, and one identified supplier temperature example. Detailed dimensions, certified material properties, installation procedures, reuse acceptance tests and customer-specific requirements remain outside the evidence reviewed.
Related reference
Hardware and component references provides broader assembly context.