Torque control infers preload through friction, which is exactly the thing that scatters by a quarter either way. Turn-of-nut measures geometry instead: past snug, every degree advances the nut a known fraction of the thread pitch, and that advance is shared between the bolt stretching and the joint squashing. No friction term appears in the arithmetic at all.
Because it needs one tool and no setup, and for most joints a quarter either way is survivable — the preload target already sits at 75% of proof, which leaves room above and enough clamp below. Torque is the right answer for ordinary work; it is the wrong answer when the joint is fatigue-critical or the bolt is expensive, which is exactly where angle control appears.
Because it measures a different thing. Torque reaches preload through friction under the head and in the threads, and that friction is what varies with finish, lubricant, temperature and reuse. Past snug, the nut advances pitch × angle/360 whatever the friction is, and the joint takes that advance as stretch and compression. The scatter falls from about ±25% to about ±15%, and the remaining error is mostly in finding snug.
Because preload is stored as stretch, and a longer bolt stretches further for the same force — it is a softer spring. Embedment removes a fixed few microns whatever the bolt, so it empties a large fraction of a short stiff bolt's stretch and almost none of a long one's. Drag the grip length above and watch the loss fall; it is the reason flange studs are longer than the joint strictly needs.
Surface asperities under the head, under the nut and at every mating face flattening out under load over the first hours and load cycles. You cannot avoid it — only make it matter less, with a more compliant bolt, or take it out, by re-torquing after a settling period. It is why critical joints are checked again after a few cycles rather than once.
A wrench measures twisting effort, not tension. T = K·D·F connects them through the nut factor K, which is why the dial shows a ±25% band.
The tension stretching the bolt and squeezing the joint: target about 70–80% of proof load, leaving margin for the service load on top.
A lumped friction coefficient for thread and under-head: K ≈ 0.10 lubricated to 0.21 dry or galvanised. The biggest source of uncertainty here.
The most a bolt takes and still returns to its original length: Fproof = As × σproof.
Roughly 90% of tightening torque is lost to friction and only about 10% stretches the bolt: about half under the head, about 40% in the threads. That is why lubrication changes preload so dramatically at the same torque. The shortcut: tightening torque ≈ 0.2 × preload × diameter for dry steel.
Modest: torque-to-tension scatter of ±25–30% is normal because K is hard to pin down (surface finish, plating, even how clean the threads are). For critical joints angle control or direct tension measurement is far more precise.
Not for two of them. ISO 898-1 gives class 8.8 two proof stresses, 580 N/mm² up to and including M16 and 600 above it, and SAE J429 gives grade 2 two: 55,000 psi to ¾″ and 33,000 above. Torque is directly proportional, so a single value would mis-specify most of the range: using 600 everywhere over-tightens every size up to M16 by 3.4%. The page resolves the figure for the size you pick and the grade selector shows where a grade steps.
It leaves margin. Proof load is roughly the bolt’s elastic limit; staying below it means the bolt holds tension without permanently stretching, with headroom for the service load.
Use the fastener or equipment manufacturer’s stamped spec. This tool is for estimation and understanding, and its results should be checked against an authoritative source.
Not for two of them. ISO 898-1 gives class 8.8 two proof stresses — 580 N/mm² up to and including M16, 600 above it — and SAE J429 gives grade 2 two, 55,000 psi to ¾" and 33,000 above. Torque is directly proportional to that figure, so a single value would mis-specify most of the range: using 600 everywhere over-tightens every size up to M16 by 3.4%. This page resolves the figure for the size you picked, and the grade selector shows both where a grade steps.
It leaves margin. Proof load is roughly the bolt's elastic limit; staying below it means the bolt holds tension without permanently stretching, with headroom for the service load on top.
Lubrication lowers friction (a lower K), so more of your twisting effort becomes useful tension instead of being lost to friction. Less torque is needed for the same clamp force.
Modest. Torque-to-tension scatter of ±25–30% is normal because K is hard to pin down. For critical joints, angle-control or direct tension measurement is far more precise.
No — use the fastener or equipment manufacturer's stamped spec. This tool is for estimation and understanding, and its results should be checked against an authoritative source.
Releases in which this page changed, newest last. Derived from the archived copy of every release, not from notes written afterwards — so it reflects what actually shipped. Site-wide passes are left out; they are in the full changelog.