Key points
- What it is. Galling is cold-welding between two metal surfaces under load. On a clamp thread it ends as a seizure, and it is a drawing-stage decision, not user error.
- Worst pairing. Stainless bolt in a stainless body is the most common cause. Zinc-plated grade 8.8 steel in an aluminium body is the more durable pairing.
- Cheapest fix. A rolled thread costs about 8–12% more per bolt and leaves a harder, smoother flank than a cut thread — the difference between three seasons and one.
- What to ask. Thread rolled or cut, bolt coating, proof-load sample rate, and thread engagement length in mm on the drawing.
Every rental company has a box of dead clamps. They are not broken — the jaw is fine, the casting is fine. The T-bolt will not turn. Somebody put it away wet after a festival, it sat in a road case for three months, and by the next load-in the thread had welded itself to the nut.
The industry calls it galling. It is the single most common failure mode in stage hardware, and it is almost always a design decision rather than a user error.
What galling actually is
When two metal surfaces slide against each other under pressure, the oxide layer that protects them can tear. Fresh metal contacts fresh metal, and at the microscopic level the two surfaces cold-weld together. On a coarse thread under load, that weld becomes a seizure.
Three things make it worse: dissimilar metals (stainless bolt in an aluminium body is the classic pair), cut threads (which leave torn flanks that give the weld somewhere to start), and no lubricant or coating between them.
A clamp that seizes in year one did not fail in year one. It failed the day the thread was cut.
The three choices that prevent it
1. Roll the thread, do not cut it
A cut thread removes material and leaves a torn surface. A rolled thread is formed by displacing the metal, which leaves the grain structure running along the flank instead of across it. The flank is smoother, harder at the surface, and far less likely to initiate a weld. It costs roughly 8–12% more per bolt and it is the difference between a clamp that lasts three seasons and one that lasts one.
2. Pair the materials deliberately
Stainless-on-stainless is the worst combination available and it is also the most common, because stainless looks premium. In a clamp body, the better pairing is a zinc-plated grade 8.8 steel bolt running in an aluminium body, with a zinc-plated nut. The zinc acts as a sacrificial layer and a dry lubricant. If the customer insists on stainless for corrosion reasons, specify an anti-seize coating on the thread — and say so on the drawing, or the factory will skip it.
3. Give the thread somewhere to go
A T-bolt that bottoms out under load has nowhere to release. Slightly coarser pitch and a longer engagement length spread the load over more flank area, which lowers the contact pressure below the threshold where galling starts. It also means the clamp still works when the thread is half-full of sand, which is the actual operating condition.
What to ask before you buy
| Question | Good answer | Walk away |
|---|---|---|
| Is the thread rolled or cut? | Rolled | "Same as everyone" |
| Bolt material and coating? | 8.8 steel, zinc plated | Stainless, uncoated |
| Proof load per batch? | 2× SWL, 3 pcs per 1,000 | "We test samples" |
| Thread engagement length? | Stated in mm on the drawing | Not known |
The part nobody puts in the catalogue
None of this shows up in a product photo, and none of it shows up in a quotation. It shows up in year two, in a road case, at 2 a.m., when someone is trying to hang a light and the wing nut will not move.
That is the whole argument for buying hardware from the people who cast it. A trader can source you a clamp. Only the factory can tell you why the last one seized — and change the thread.
