The Mechanics of Bolted Joints: Torque vs Preload
A threaded fastener is essentially a stiff tension spring. Tightening a bolt is not merely about turning the nut; the true engineering objective is to achieve an accurate elastic clamp preload (F) that holds the joint faces clamped securely without exceeding the bolt's yield strength under external dynamic loads.
The Short-Form Torque Equation (T = K × D × F)
The relationship between applied wrench torque (T) and the resulting clamping tension (F) is defined by:
In standard dry fasteners, approximately 90% of applied torque is lost strictly to friction (~50% overcoming under-head washer friction and ~40% sliding over the thread helix). Only 10% of torque actually stretches the bolt into elastic tension!
Why Lubrication Drastically Changes Bolt Tension
Applying anti-seize paste or molybdenum grease slashes the nut friction factor (K) from 0.20 down to 0.12. If you tighten an anti-seized bolt to a dry torque specification, the bolt experiences nearly 67% higher clamping tension, snapping the shank or stripping the threads!