Wing Nuts, Disc Nuts and Washers: Selection and Tightening
> In brief > - The two designs fail in different ways. > - Torque and thread condition decide whether the joint holds. > - Reuse is acceptable only if the thread is still in specification.
The nut is the cheapest component in the tie assembly and the one most often reused past its service life. Its geometry decides whether the joint holds through vibration or slowly walks loose.
Our own production and test data are available on request — see how our factory works and contact the engineering team for the documents behind any figure quoted here.
Wing Nut is a nut with two large wings for hand tightening, suitable for wall formwork and tolerant of site conditions. Disc Nut refers to a plate-like nut with a large bearing face, used to spread load and resist embedding into soft materials. Plum or Corrugated Washer means a washer that spreads load, provides friction to resist back-off, and protects the panel or waler face from the nut's rotation. Thread Fit is a check for nut reuse, ensuring the nut can be threaded by hand without needing a wrench, indicating no damage or coating issues.
Nut types and where they fit.
WING NUT (butterfly): two large wings for hand tightening, cast or forged, available in class 4.8 to 8.8. Fast to assemble, tolerant of site conditions, and the standard for wall formwork. Hand torque is limited, which is the point: the tie carries tension, not the nut.
DISC NUT: a plate-like nut with a large bearing face. The wide face spreads load into the waler or panel and resists embedding into timber. Preferred where the nut bears directly on soft material.
DOUBLE-EAR DISC NUT: two ears give more grip and a larger bearing face, used on heavy walers and where a single wing nut would deform the waler flange.
HEX NUT: standard hardware, requires a wrench, and provides the highest controlled torque. Common on anchors and prestressed bars where a torque value is specified, less common on reusable wall formwork because of assembly speed.
Standard basis: ISO 3501
The washer is not optional.
A plum or corrugated washer under the nut does three things: it spreads the load, it provides friction that resists back-off, and it protects the panel or waler face from the nut's rotation. Omitting it is the most common cause of a nut that loosens during vibration and a panel that shifts at the joint.
How tight is right.
Formwork ties work in tension, so the nut does not need high torque — it needs full face contact and no play. Practical guidance: tighten by hand until the nut seats, then a further quarter turn with the tool of the system (many systems provide a short wrench for exactly this).
Over-tightening does harm: it crushes timber beams, embeds the nut into the waler, stretches the thread locally, and pre-loads the rod so that the working tension adds to an already high stress. On hot-dip galvanized assemblies over-tightening also galling the thread, which makes removal after the pour difficult.
Standard basis: EN 13670
Reuse checks.
Thread fit by hand on a sample per batch — a nut that needs a wrench to run down is already damaged or coated out of tolerance.
Wing or ear deformation, which reduces grip and indicates the nut has been loaded in shear rather than tension.
Bearing face wear on disc nuts, which reduces the contact area and lets the nut embed.
Do not mix nuts from different systems on the same rod even when the thread matches: class, bearing face and wing geometry differ, and the joint capacity is set by the weakest element.
| Nut Type | Torque and Thread Condition | Consequence for Joint Integrity |
|---|---|---|
| Wing Nut | Limited hand torque, thread tolerance | Resists over-tightening, may walk loose |
| Disc Nut | High bearing face, moderate torque | Spreads load, resists embedding in soft material |
| Double-Ear Disc Nut | Larger bearing face, higher grip | Better for heavy walers, resists deformation |
| Hex Nut | Requires wrench, highest controlled torque | Ensures high torque, less common due to speed |
| Plum Washer | Spreads load, adds friction, protects surface | Prevents loosening, avoids panel shift at joint |
| Reuse Check | Thread fit by hand, inspect for wear | Ensures proper fit, prevents joint failure |
References and standards.
- EN 14399-1 — High-strength structural bolting assemblies
- ISO 16047 — Hexagon head bolts and hex nuts
- High-strength bolting guidance
- EN 1090 — Execution of steel structures
- ISO 2859-1 — Sampling procedures for inspection by attributes
- ISO 9001:2015 — Quality management systems
About us: This article covers the selection and tightening of wing nuts, disc nuts, and washers in formwork systems. Our production and test data are available on request. Our contact page provides access to the engineering team for detailed inquiries, and the about page documents our factory processes and capabilities. This article was reviewed by the Qianheng Formwork Engineering team.