When you tighten a bolt, what actually holds the joint together is tension (clamp force) stretching the bolt — but a wrench measures torque, the twisting effort. The short-form relationship T = K · D · F connects them through a friction term called the nut factor (K). The catch: K varies with lubrication, plating, and surface condition, so the same torque can produce noticeably different tension. That's why the dial above shows a ±25% band, not a single exact value.
An M10 class 8.8 bolt, dry (K ≈ 0.20), targeting 75% of proof load: the tensile stress area is about 58 mm², giving a proof load near 34,800 N.
Target preload is 75% of that, and T = K · D · F works out to roughly 52 N·m. Lubricate the same bolt (K ≈ 0.13) and the same clamp force needs only about 34 N·m — which is exactly why applying a dry torque spec to a lubricated bolt can over-tension it.
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.