Engineering Guide · 7 min read
Flange Bolts and Nuts: Why the Flange Matters
Understand why fasteners have a flange, where flange bolts and flange nuts spread load, and how bearing faces, washers, locking features and standards differ.
The short answer: a flange gives a bolt head or nut a wider, built-in bearing face. Its purpose is to spread clamping force into the part beneath it and, in a suitable joint, reduce the number of loose washers needed during assembly. A flange bolt provides that face at the head; a flange nut provides it at the nut. The shared shape solves a contact problem at either end of the joint—it does not make every flange fastener a locking fastener.[1][2]
Use this Guide to understand the flange as a design feature. For head identification, inch and metric series, and DIN 6921 replacement history, continue with Hex Flange Bolts: ASME B18.2.1, ISO 4162 and DIN 6921. The Bolt and Screw Head Types Guide compares the wider head family.
Why add a flange to a fastener?
A conventional hexagon gives a wrench somewhere to grip. The face beneath it has a different job: transmitting the fastener's clamping force into the joint. A compact wrenching shape may leave too little bearing area for the part being clamped. Adding an integral flange lets the bearing face extend beyond the hexagon without enlarging the wrench size to the same extent.
That explains the design's purpose: combine a wrenching body and a wider seat in one component. A separate flat washer can provide the wider seat too, but it adds another part to feed, position and retain during assembly. Where the drawing permits the flange to take over that load-spreading role, the bolt or nut arrives with its bearing face already attached. Fastenal describes this combination for flange bolts and the larger bearing area of flange nuts.[1][2]
The force still has to enter the clamped parts at both ends. The illustration shows why a flange on the bolt does not also enlarge the bearing face under the nut:
What a wider bearing face changes
At a given clamping force, more effective bearing area means lower average contact pressure. This can help avoid local indentation where a small head or nut presses into sheet steel or a softer material. The useful area is the portion actually supported by the mating part. An oversized hole, slot or nearby edge can remove much of the support suggested by the flange's outside diameter.
Here is a calculation that isolates the area effect:
A calculated example—not standard dimensions or a load rating. Assume a fully supported, flat annular contact face around a 9 mm hole and a clamping force of 10,000 N. The nominal contact area is A = π(D² − d²)/4, and mean pressure is p = F/A. Increasing the assumed bearing diameter from 13 mm to 18 mm changes the area from 69.1 mm² to 190.9 mm², and the mean pressure from 144.7 MPa to 52.4 MPa. These are illustrative inputs, not dimensions assigned to an ASME, ISO or DIN fastener. Real contact pressure is uneven; flange flexure, incomplete support and the mating material require separate checks.
Why does indentation matter after tightening? Permanent compression beneath a head or nut reduces the joint's thickness and can release some of the bolt's initial tension, or preload. Nord-Lock describes preload loss from settlement of surface irregularities and from creep, with softer materials more susceptible to creep.[3] A wider supported seat addresses local pressure; it does not remove the need to check the material, temperature and long-term deformation.
Flange bolt, flange nut or both?
Choose the flange location by looking at the two outer faces of the joint. The bolt head bears on one face; the nut bears on the other. Each face may need a different solution.
| Fastener arrangement | Where the wider seat acts | When it is useful |
|---|---|---|
| Flange bolt into a tapped part | Under the bolt head; there is no separate nut. | A bracket or guard needs a wider seat and the flange has sufficient flat support and tool clearance. |
| Ordinary bolt with a flange nut | Under the nut on the opposite outer face. | The nut-side surface needs more support or an integral seat simplifies handling there. Check the head-side seat independently. |
| Flange bolt with a flange nut | At both outer faces. | Both sides benefit from a wider seat and the full assembly has been checked for fit, preload and surface compatibility. |
| Bolt or nut with a separate washer | Where the specified washer is installed. | A larger diameter, particular hardness, replaceable contact surface or another washer function is required. |
A flange nut can therefore solve a problem that a flange bolt alone leaves untouched. For example, a steel bracket on one side and a softer housing on the other may require different bearing areas or washer arrangements. Specify the complete stack rather than choosing both ends from the word flange.
A flange and a locking feature do different jobs
The flange describes the bearing geometry. Locking may come from another feature: teeth on the bearing face, a nylon insert or a designed distortion in the nut's thread. Fastenal distinguishes nylon-insert and top-lock mechanisms from the load-bearing function of the flange.[2] A smooth flange by itself is not a specification for vibration resistance.
Serrations also change the surface interaction. NASA's Fastener Design Manual describes serrated faces engaging the mating surface, with restrictions where damage or corrosion matters.[4] Check a selected tooth design against painted, coated or sensitive parts, and retain any specified locking device. The washer and locking-method comparison covers the separate vibration question.
Changing from a loose washer to a flange also changes the tightening interface. Under-head or under-nut friction, finish and washer presence affect the torque–tension relationship.[5] Carry over a torque value only when the assembly specification covers the new arrangement.
Which standards belong to bolts and which to nuts?
A family resemblance is not a shared product specification. Inch bolts and nuts have separate ASME documents; metric flange-bolt and flange-nut standards likewise identify different products.
| Product | Standard route | What to specify |
|---|---|---|
| Inch hex flange screw | ASME B18.2.1-2012 (R2021), which includes several inch head forms.[6] | The hex flange screw form, thread, size and required edition. |
| Inch hex flange nut | ASME B18.2.2-2022 includes hex flange and large hex flange nuts.[7] | The exact nut form and size. A particular locking nut may require its own product standard. |
| Metric hexagon bolt with flange | ISO 15071:2011 and ISO 4162:2012 cover specified small-series products.[8][9] | The stated standard and series; compare the complete head dimensions. |
| Metric hexagon nut with flange | ISO 4161:2012 covers style 2, metric coarse threads M5–M20.[10] | The nut standard, thread, property class, finish and bearing-face requirement. |
DIN 6921 is a legacy flange-bolt reference; DIN 6923 is a legacy flange-nut reference. For nuts, DIN Media records DIN EN 1661:1998-02 as replacing DIN 6923:1983-06.[11] Confirm the drawing's permitted replacement and its dimensions rather than treating a DIN, EN and ISO number as interchangeable labels.
A flange bolt also differs from a bolt used to join pipe flanges. The former names the fastener's head geometry; the latter describes the equipment being joined. Pipe-flange assemblies may use headless studs and separate nuts, as explained in Stud Bolts vs Threaded Rod.
Connect the joint requirements to the product
Start with the function needed at each end, then check the actual drawing and product specification. The catalog has separate UNC hex flange bolt and UNF hex flange bolt variants. For the nut side, compare the nylon-insert hex flange nut with the hex flange nut listed to ASME B18.16.4. Their locking features and specifications need separate review; the shared flange shape does not make them equivalent.
Before requesting a quotation, provide:
- Both bearing surfaces: mating materials, thicknesses, hole or slot geometry, available flat area and surface finish.
- The fastener pair: bolt and nut forms, thread compatibility, material/property requirements and the exact standards and editions.
- The complete assembly: washer and locking requirements, tightening instructions, access limits and the mating-detail drawing.
The washer selection Guide helps decide which washer functions still matter. Use How to Specify a Bolt to turn the chosen arrangement into an inquiry.
References
- Fastenal. Fastener type descriptions: flange bolt and flange screw. Integral bearing face, load distribution and assembly handling.
- Fastenal. Nut type descriptions. Flange bearing area, nylon-insert and top-lock mechanisms.
- Nord-Lock Group. Safe Bolted Joint Design, printed page 6, “Settlement and creep.”
- NASA. Fastener Design Manual, NASA RP-1228 (1990), printed page 8, serrated-face heads and nuts.
- Fastenal. Bolted Joint Design, revision March 4, 2009, PDF pages 9–10, torque-controlled tightening.
- ASME. ASME B18.2.1-2012 (R2021), public scope.
- ASME. ASME B18.2.2-2022, table of contents. Table 1.1.1-13 identifies hex flange and large hex flange nuts.
- ISO. ISO 15071:2011, public scope for small-series flange bolts.
- ISO. ISO 4162:2012, public scope for small-series flange bolts.
- ISO. ISO 4161:2012, public scope for style 2 flange nuts with coarse thread.
- DIN Media. DIN EN 1661:1998-02, replacement record for DIN 6923:1983-06; consult the publisher's accompanying warning notice with this edition.