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ASME B18.16.1M-1979 — Prevailing-Torque Type Steel Metric Hex Locknuts — Mechanical and Performance Requirements
🔒 ASME B18.16.1M-1979 is a member of the ASME B18 fastener family developed by Subcommittee 16 (Locknuts) for metric-series locknuts. The family addresses prevailing-torque type steel hex locknuts and hex flange locknuts. “Prevailing torque” means the nut generates a continuous friction-resistant torque during run-down — via an all-metal deformed thread or a nylon insert — rather than relying on a separate elastic element for locking. This Standard specifies the mechanical and performance requirements, and is used together with B18.16.2M (dimensions) and B18.16.3M (inspection and test requirements).
💡 This article is written from domain knowledge of ASME B18.16.1M. The available copy of the Standard is a scanned document without a text layer, so the content below is developed around the subject “mechanical and performance requirements for metric prevailing-torque locknuts”; specific values are subject to the official text.
1. Scope and Positioning 📌
The B18.16 series is the complete technical set issued by the B18 Committee for the metric locknut market. B18.16.1M focuses on material, hardness, heat treatment, proof load, torque-tension relationship, and locking performance. Its core objective is to ensure the locknut does not undergo unintended loosening within the specified service load and temperature envelope, while preventing thread failure from over-tightening during assembly. It corresponds in principle to ISO requirements for prevailing-torque nuts (e.g., performance classes in the ISO 2320 / ISO 7042 family) but retains U.S. industry experience and acceptance practice.
⚠️ The prevailing-torque nut relies on friction in the thread to resist loosening, and the locking torque naturally decreases with the number of run-downs. The Standard limits both the first-run torque and the residual torque after multiple removals; never treat a single-assembly torque value as a permanent performance figure.
2. Mechanical and Performance Requirements 📏
The Standard typically addresses performance through the following layers:
- Material and hardness: defines applicable steel grades and hardness ranges so the nut has both load-carrying capacity and the elastic-plastic behavior needed to generate a prevailing torque; hardness testing is carried out at specified locations and frequencies.
- Proof load: an axial proof load is applied without producing harmful deformation or thread damage, verifying the nut’s capacity under maximum assembly preload.
- Torque-tension behavior: bounds the relationship between assembly torque and resulting axial clamp force, preventing insufficient clamp or overload from friction variation.
- Prevailing torque: sets limits on first-run-on torque, first breakaway torque, and fifth breakaway torque (after repeated cycles), which is the key measure of locking effectiveness.
| Performance Item | Purpose | Typical Method |
|---|---|---|
| Hardness | Load capacity and process consistency | Rockwell/Vickers at specified locations |
| Proof load | Thread integrity at max preload | Axial tension test, no harmful deformation |
| First-run-on torque | Assembly behavior and locking onset | Torque measured during standard run-down |
| Breakaway torque (multiple) | Loosening durability | Residual torque after repeated cycles |
| Axial load vs. torque | Preload control | Load cell plus torque wrench calibration |
⚠️ Prevailing-torque testing must be performed at the specified thread size, lubrication condition, and run-down speed. Lubricants (e.g., assembly oil) significantly change torque values, so test and assembly conditions should match, or acceptance data become non-comparable.
3. Engineering Practice Essentials 🛠️
- 🔍 Selection principle: choose the appropriate property class by load level; prefer all-metal deformed-thread types for vibration and impact, and nylon-insert types where repeated assembly and protection of mating surfaces matter.
- 🔧 Assembly control: with torque control, account for the prevailing torque contribution and leave enough “clamping torque”; with angle control, define the starting position carefully.
- 📊 Inspection management: sample and accept per the Standard’s plan, with emphasis on fifth breakaway torque not falling below the lower limit to keep “single-use locking” products out.
- 📋 Traceability: obtain material certificates and hardness/torque test reports at purchase, and define lot identity for traceability.
4. Frequently Asked Questions ❓
❓ Q: What is a “prevailing-torque” locknut?
A: It is a nut whose locking action comes from a continuous friction resistance generated in the thread itself — typically by an all-metal deformed thread or a nylon insert. The locking torque exists throughout run-down, unlike locking methods that depend on separate elements such as disc springs or toothed washers.
A: It is a nut whose locking action comes from a continuous friction resistance generated in the thread itself — typically by an all-metal deformed thread or a nylon insert. The locking torque exists throughout run-down, unlike locking methods that depend on separate elements such as disc springs or toothed washers.
❓ Q: Why does the Standard specify fifth breakaway torque?
A: Prevailing torque decays with the number of run-downs; the fifth breakaway torque reflects residual locking ability after repeated servicing, and is the key acceptance check that the product remains reliable in maintenance environments and is not merely “one-time locking.”
A: Prevailing torque decays with the number of run-downs; the fifth breakaway torque reflects residual locking ability after repeated servicing, and is the key acceptance check that the product remains reliable in maintenance environments and is not merely “one-time locking.”
🔍 Q: Does the proof load test destroy the nut?
A: No. The proof load is a verification loading below the failure load; a conforming nut shows no harmful permanent deformation or thread damage after the load is removed and may continue in service, unlike a destructive tensile test.
A: No. The proof load is a verification loading below the failure load; a conforming nut shows no harmful permanent deformation or thread damage after the load is removed and may continue in service, unlike a destructive tensile test.
❓ Q: How does this relate to ISO locknut requirements?
A: The two align in principle on material, property classes, and prevailing-torque testing, but differ in specific limits, test detail, and acceptance practice. Export and domestic projects should verify the referenced edition to avoid mixing acceptance standards.
A: The two align in principle on material, property classes, and prevailing-torque testing, but differ in specific limits, test detail, and acceptance practice. Export and domestic projects should verify the referenced edition to avoid mixing acceptance standards.
