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ASME B18.2.4.4M-1982 (1999) — Metric Hex Flange Nuts
🔩 ASME B18.2.4.4M-1982 (1999) provides complete general and dimensional data for metric hex flange nuts. By adding an integral flange to the underside of a regular hex nut, the flange nut enlarges the bearing area, lowers unit bearing pressure, and offers a degree of self-retention, making it a favorite for automotive, agricultural, and construction machinery joints subject to vibration. Originally approved in 1982 and reaffirmed in 1999, it is an important member of the ASME B18 metric fastener series.
💡 The flange nut’s value lies in “two functions in one”: the flange replaces a separate washer, enlarging the bearing area, protecting the mating surface, improving preload distribution, and reducing vibration-induced loosening.
1. Scope 📌
This Standard covers the complete general and dimensional data for metric hex flange nuts recognized as an American National Standard. Key points:
- The dimensional data do not imply that all sizes are stock items; purchasers should consult manufacturers regarding lists of stock production hex flange nuts.
- Hex flange nuts purchased for U.S. Government use must additionally conform to Appendix I (government standard items and part-numbering system).
- All dimensions are in millimeters unless otherwise stated.
⚠️ Although the flange provides a self-retention character, a flange nut is not a true locking nut. Under severe vibration or cyclic loading, additional locking measures (nylon insert, thread adhesive) should still be applied per design.
2. Coordination With ISO Standards (ISO 4161) 🌐
The hex flange nuts covered by this Standard are essentially identical with ISO 4161. The dimensional differences between the ANSI standard and the ISO standard are very few and relatively minor, and none affect the functional interchangeability of nuts manufactured to the requirements of either. Letter symbols follow ISO usage, with capitals used instead of lowercase for data-processing convenience.
| Feature | Requirement | Remark |
|---|---|---|
| Width across flats | Standard hex series (mm) | Measured perpendicular to axis |
| Top of nut | Flat with chamfered corners | Chamfer = max across flats minus 15% |
| Thickness | Top to plane of bearing circle diameter | Excludes raised markings |
| Hex height | Junction with flange to top, at a corner | Measured at a hex corner |
| Wrenching height | Effective hex height for wrench engagement | Governs torque capability |
| Flange | Integral with hex, enlarged bearing area | Reduces bearing pressure |
| Threads | Metric coarse/fine | Per ASME B1.13M |
3. Geometry and Dimensional Requirements 🔧
The Standard defines each portion of the flange nut as follows:
- Width across flats: measured between two opposite wrenching flats, perpendicular to the nut axis.
- Top of nut: flat with chamfered corners; chamfer-circle diameter equal to maximum width across flats within minus 15%.
- Thickness: overall distance measured parallel to the axis from the top of the nut to the plane of the bearing circle diameter, excluding raised identification markings.
- Hex height: measured at a corner of the hex, from the junction of the hex portion with the flange to the top of the nut.
- Wrenching height: the effective hex height available for wrench engagement; it controls the torque that can be transmitted.
- Flange: integral with the hex; its bearing diameter is substantially larger than the circumscribed circle of the hex.
🚨 Flatness and surface quality of the flange bearing face govern joint integrity: a warped or burred flange concentrates preload on a small area, crushing the mating surface and losing preload. Inspect the flange face before assembly.
4. Materials, Threads, and Marking 📏
Material and mechanical-property requirements follow the metric hex nut system:
- Steel: hardness and proof load per metric property classes (e.g., 8, 10); carbon and alloy steel nuts carry the manufacturer’s mark.
- Corrosion-resistant steels and nonferrous metals: property classes and marking per the corresponding material standards (e.g., ASTM F 836M, F 467M).
- Threads: metric coarse or fine series, typically tolerance class 6H, manufactured per ASME B1.13M and accepted per the thread-gaging systems.
- Finish: zinc plating, chromate passivation, Dacromet, and others as agreed in the purchase order.
5. Engineering Practice Essentials 🛠️
- 🔍 Selection workflow: choose the flange nut size by bolt specification and joint load → check width across flats against wrench size → confirm material and property class → specify finish as needed.
- 🔧 Assembly: the flange replaces the separate flat washer in most cases; ensure the flange face seats fully on the mating surface when tightening.
- 📊 Anti-loosening: suitable for vibration-prone environments as a basic measure; for critical safety joints combine with nylon locking elements or thread-locking adhesive.
- 📋 Acceptance: verify dimensions per Table 1, gage threads, and run proof-load tests per batch; government items are ordered via the Appendix I numbering system.
6. Frequently Asked Questions ❓
❓ Q: Can a flange nut fully replace a “nut + flat washer” combination?
A: In most cases yes — the flange gives a larger bearing area and better load distribution; however, where oversized holes or special washers (e.g., beveled washers) are required, a separate washer is still needed.
A: In most cases yes — the flange gives a larger bearing area and better load distribution; however, where oversized holes or special washers (e.g., beveled washers) are required, a separate washer is still needed.
❓ Q: What is the difference between hex height and wrenching height?
A: Hex height is measured from the root of the hex (junction with the flange) to the top; wrenching height is the effective hex engagement available to transmit torque. Both must be met in production.
A: Hex height is measured from the root of the hex (junction with the flange) to the top; wrenching height is the effective hex engagement available to transmit torque. Both must be met in production.
❓ Q: Can nuts to this Standard be mixed with ISO 4161 nuts?
A: Yes. The Standard states that dimensional differences from ISO 4161 are minor and do not affect functional interchangeability, but the mating bolt’s thread and strength grade must still be matched.
A: Yes. The Standard states that dimensional differences from ISO 4161 are minor and do not affect functional interchangeability, but the mating bolt’s thread and strength grade must still be matched.
🔍 Q: Do flange nuts require witness/lock marks?
A: Not mandated by the Standard, but for critical joints or where assembly quality must be verified, painting a witness line makes loosening easy to detect during inspection tours.
A: Not mandated by the Standard, but for critical joints or where assembly quality must be verified, painting a witness line makes loosening easy to detect during inspection tours.
