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ASME B18.16.2M-1979 — Prevailing-Torque Type Steel Metric Hex Locknuts — Dimensional Requirements
📐 ASME B18.16.2M-1979 is the document in the ASME B18.16 metric locknut family that defines dimensional requirements, complementing B18.16.1M (mechanical and performance requirements) and B18.16.3M (test and inspection requirements). It specifies the geometry, tolerances, and thread requirements for prevailing-torque type steel metric hex locknuts and hex flange locknuts — including width across flats, thickness, true position of the hexagon relative to the tapped hole, chamfers and countersinks, and flange (washer-face) dimensions — providing the common dimensional basis for design, manufacture, and interchangeability acceptance.
💡 This article is written from domain knowledge of ASME B18.16.2M. The available copy of the Standard is a scanned document without a text layer, so the content below is developed around the subject “dimensional requirements for metric prevailing-torque locknuts”; specific values are subject to the official text.
1. Scope and Dimensional Framework 📌
Compared with the ordinary hex nut dimension standard (e.g., ASME B18.2.4.1M for metric hex nuts), the B18.16.2M framework has two distinguishing features. First, the dimension tables cover both standard-height hex locknuts and hex flange locknuts. Second, because the prevailing-torque feature (deformed thread or insert) locally distorts the thread, the Standard makes special provision for thread dimensions and location (e.g., pitch diameter and the true position of the hexagon to the tapped hole) so that locking function and assembly interchangeability are both preserved.
⚠️ The deformed section of a prevailing-torque nut locally changes the thread, so its thread tolerances differ from those of free-running nuts. Do not apply ordinary hex-nut go/no-go gage logic to these products; follow this Standard’s thread provisions instead.
2. Principal Dimensions and Tolerances 📏
| Dimension Element | Function | Typical Provision |
|---|---|---|
| Width across flats | Determines wrench size and load section | Basic dimensions and tolerances by thread size |
| Thickness | Determines engaged thread length | Max/min thickness, standard or reduced height |
| Flange diameter and thickness | Larger bearing area, better load distribution | Limits for flange (washer-face) dimensions |
| True position, hexagon to tapped hole | Concentric assembly and torque transmission | Positional tolerance of hexagon center vs. hole axis |
| Chamfer and countersink | Guidance and thread start | Face chamfer angle and countersink depth |
| Thread | Fit accuracy and locking premise | Metric thread tolerance and deformation limits |
The dimension tables give nominal values and max/min limits for each element by thread size (for example, from M4 through the larger metric sizes covered). For hex flange locknuts, the flange face diameter and the flatness of the bearing surface directly affect the distribution of compressive stress and receive dedicated provisions; face chamfers and countersinks influence centering of the nut onto the bolt and coating uniformity.
⚠️ Dimensional tolerances and performance (B18.16.1M) are interdependent: too little deformation gives low prevailing torque; too much can push thread dimensions out of tolerance or make run-down resistance excessive. Manufacturing and inspection must combine dimensional control with performance testing.
3. Engineering Practice Essentials 🛠️
- 🔍 Design selection: choose the nut size and style (standard hex or hex flange) by bolt size and load; the flange style spreads compressive stress and suits members sensitive to bearing pressure.
- 🔧 Wrench matching: verify the width across flats against available sockets and torque tools to avoid slipping sockets and damaged corners.
- 📐 Inspection control: measure critical dimensions (flats, thickness, true position) on the Standard’s sampling plan; check the local thread deformation at the deformed section.
- 📋 Interchangeability: products from different suppliers of the same size must meet dimensional interchangeability; review dimensional and torque performance reports together at receiving inspection.
4. Frequently Asked Questions ❓
❓ Q: What advantage does a hex flange locknut have over a plain hex locknut?
A: The flange spreads the bearing area into a large face, lowering unit bearing pressure, improving stress distribution, reducing the need for washers, and limiting embedment of the mating surface — the flange also aids centering.
A: The flange spreads the bearing area into a large face, lowering unit bearing pressure, improving stress distribution, reducing the need for washers, and limiting embedment of the mating surface — the flange also aids centering.
❓ Q: Why does this Standard separately control the true position of the hexagon to the tapped hole?
A: Assembly torque is transmitted through the hexagon; if the hexagon is excessively eccentric relative to the hole, the socket applies load asymmetrically, destabilizing run-down and final clamp force. The positional tolerance keeps concentricity and supports automated assembly.
A: Assembly torque is transmitted through the hexagon; if the hexagon is excessively eccentric relative to the hole, the socket applies load asymmetrically, destabilizing run-down and final clamp force. The positional tolerance keeps concentricity and supports automated assembly.
🔍 Q: Does the deformed-thread process affect thread dimensions?
A: Yes. Deformation locally yields the thread — this is the mechanism that creates prevailing torque — but it can also reduce the pitch diameter at that location. The Standard limits such local deformation, and inspection should follow it rather than over-compressing the whole thread.
A: Yes. Deformation locally yields the thread — this is the mechanism that creates prevailing torque — but it can also reduce the pitch diameter at that location. The Standard limits such local deformation, and inspection should follow it rather than over-compressing the whole thread.
❓ Q: How does this Standard relate to B18.2.4.1M (metric hex nuts)?
A: B18.2.4.1M defines the dimensions of ordinary free-running metric hex nuts; B18.16.2M is the locking-function branch with a consistent external dimensional system for interchangeability, but with its own provisions for thread deformation, true position, and locking details.
A: B18.2.4.1M defines the dimensions of ordinary free-running metric hex nuts; B18.16.2M is the locking-function branch with a consistent external dimensional system for interchangeability, but with its own provisions for thread deformation, true position, and locking details.
