EASA/GCAA Part-66 · Module 7 — 7.x Torque Loading · B1/B2

Torque, Friction & Bolt Preload

Objective: relate applied torque to bolt clamp load with F ≈ T/(K·d), explain why friction consumes most of the torque, and recognise over-torque.

Bolted joint — torque to clamp load

Applied torque 40 N·m

Bolt diameter 10 mm

Friction factor K 0.2

Applied torque

0N·m

Clamp force (preload)

0kN

Bolt stress

0MPa

Status

Preload F ≈ T / (K · d) · most torque is spent on friction, not stretch

Tightening a bolt to a torque value is really a way of setting its clamp force (preload). The relationship is F ≈ T / (K·d), where K is the nut factor that lumps together thread and head friction — typically about 0.2 for dry steel. Because friction eats most of the effort, only a small fraction of your torque becomes stretch. That's why lubrication, surface finish and clean threads matter, and why over-torque yields or snaps the bolt.

Try this: Raise the torque slowly. Watch preload climb — until the bolt yields. What torque did that take?
Try this: Drop the friction factor K (e.g. lubricated threads). What happens to preload for the same torque?
Try this: Change the bolt diameter at fixed torque. Why does a bigger bolt get less preload from the same torque?