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ASME B18.16.4-2008 (2013) — Serrated Hex Flange Locknuts (Inch Series, 90,000 psi)
🔒 ASME B18.16.4-2008 (2013) is an American National Standard covering the dimensions, materials, and performance of serrated hex flange locknuts (inch series), with a proof stress rating of 90,000 psi. The nut carries a pattern of serrations machined on the bearing face of the hex flange; during tightening, the serrations embed into the mating surface and generate a locking torque that resists loosening under vibration and impact — a “surface-mechanical locking” type of fastener. Approved by ANSI on August 25, 2008, issued October 7, 2008, and reaffirmed in 2013, the nut is widely used in automotive, agricultural equipment, and general industrial vibration-prone joints.
💡 Two selling points of a serrated flange locknut: the flange bearing face spreads clamp stress to protect the joint surface, and the serrations add a mechanical locking torque — together they work reliably without washers in low- to medium-load vibration service.
1. Scope and Document Structure 📌
The Standard contains 18 sections covering the full chain from terminology and reference standards through dimensions, serrations, threads, material and hardness, mechanical requirements, finish, marking, inspection, and workmanship:
- Dimensions (Sec. 5): regular and large series listed separately (Tables 1 and 2), giving width across flats, thickness, flange diameter and flange thickness.
- Serrations (Sec. 6): profile, spacing, and completeness of the serrations on the flange bearing face.
- Threads and true position (Secs. 8, 10): positional tolerance of the hexagon to the tapped hole, thread size and tolerance.
- Material and hardness (Sec. 11) and mechanical requirements (Sec. 12): steel grades, hardness after heat treatment, 90,000 psi proof load, and associated torque values (Table 3).
| Series | Style | Main Features | Typical Applications |
|---|---|---|---|
| Regular (Table 1) | Regular Serrated Flange Locknut | Standard flange diameter and thickness | General industrial assembly, automotive chassis |
| Large (Table 2) | Large Serrated Flange Locknut | Enlarged flange, larger bearing area | Soft or pressure-sensitive mating surfaces |
| Proof load & torque (Table 3) | Proof Load and Torque Values | Loads and recommended torques for the 90,000 psi proof stress | Preload and clamp-force calculations |
⚠️ “90,000 psi” is a proof-stress class, not a material designation. Base preload design on the proof loads and torque values in Table 3, not on experience torques for plain flange nuts — the serration embedment consumes part of the applied torque.
2. Technical Requirements 📏
The mechanical core is the 90,000 psi proof-load stress: the nut, when subjected to the corresponding axial proof load, must show no harmful deformation or thread damage. To deliver the locking effect, the Standard controls the serration profile and the flatness and completeness of the flange face; to protect assembly quality, it controls the true position of the hexagon relative to the tapped-hole axis, the countersink, and end chamfers. Threads conform to the inch Unified series with the specified tolerance classes, including defined thread start and runout requirements.
| Requirement | Section | Key Content |
|---|---|---|
| Dimensions | 5 | Flats, thickness, flange dimensions for regular/large series |
| Serrations | 6 | Profile, distribution, and completeness |
| Tops of nuts | 7 | Top-face geometry and parallelism |
| True position, hexagon to tapped hole | 8 | Concentricity tolerance for symmetric socket loading |
| Countersink | 9 | Thread-start guidance |
| Threads | 10 | Inch Unified thread tolerance classes |
| Material and hardness | 11 | Steel grades and heat-treated hardness range |
| Mechanical requirements | 12 | 90,000 psi proof load and torque values (Table 3) |
⚠️ Serration locking is most effective once the teeth embed — but that means ring-shaped indentations on the mating surface. For thin sheet, coated parts, or aluminum workpieces where surface integrity matters, evaluate whether serration embedment is acceptable, or use a plain flange locknut or a washer.
3. Marking, Inspection, and Engineering Practice 🛠️
- 🏷️ Marking (Sec. 14): nuts carry the manufacturer’s identification and grade designation on the top or side per the specified marking system, ensuring traceability.
- 📋 Inspection and quality assurance (Sec. 15): dimensional, hardness, and mechanical tests per the Standard’s sampling plan; defective parts and lots are disposed of per the applicable clauses.
- 🔧 Assembly control: apply torque per Table 3, accounting for serration embedment energy; after repeated removals, inspect the serrations and flange face and replace when worn.
- 🔍 Comparison to ISO standards (Sec. 2): the Standard dedicates a section to comparing with corresponding ISO flange-nut requirements — confirm alignment for export projects and import substitution.
4. Frequently Asked Questions ❓
❓ Q: How do serrated flange locknuts prevent loosening?
A: The serrations on the flange bearing face embed into the mating surface during tightening, creating a mechanical locking torque that resists counter-rotation under vibration; the distributed clamp pressure from the flange also raises friction, and the two together provide the locking action.
A: The serrations on the flange bearing face embed into the mating surface during tightening, creating a mechanical locking torque that resists counter-rotation under vibration; the distributed clamp pressure from the flange also raises friction, and the two together provide the locking action.
❓ Q: What does “90,000 psi” mean?
A: It is the proof-load stress of this nut grade — the nut must sustain the axial load corresponding to 90,000 lb/in² without harmful deformation or thread damage. It defines the performance level and the usable preload envelope.
A: It is the proof-load stress of this nut grade — the nut must sustain the axial load corresponding to 90,000 lb/in² without harmful deformation or thread damage. It defines the performance level and the usable preload envelope.
🔍 Q: Regular or large series — which one should I choose?
A: The regular series meets general load and locking needs; when the mating material is soft or a lower contact pressure distribution is required, choose the large series with a bigger flange diameter to spread the bearing load.
A: The regular series meets general load and locking needs; when the mating material is soft or a lower contact pressure distribution is required, choose the large series with a bigger flange diameter to spread the bearing load.
❓ Q: Do serrations leave marks on the mating surface?
A: Yes — serration embedment leaves a ring-shaped impression, which is the normal result of the locking mechanism. Evaluate acceptability where appearance or surface integrity matters, and note that for high-strength joints such impressions generally do not affect structural capacity.
A: Yes — serration embedment leaves a ring-shaped impression, which is the normal result of the locking mechanism. Evaluate acceptability where appearance or surface integrity matters, and note that for high-strength joints such impressions generally do not affect structural capacity.
