Bolts Are Springs: Why Preload Matters More Than Torque

A bolt holds a joint together because it is stretched, like a spring. Not the threads, not the friction: the stretch. This video shows the bolt lengthening as the nut turns, the joint as two springs, and why an external load barely reaches a well-preloaded bolt.

It also shows why torque is a poor stand-in for preload, where ninety percent of the torque actually goes, and what a drop of oil on the threads does.

In this video

• Preload: the force a stretched bolt stores
• The joint diagram in one picture
• Why preloaded bolts don't fatigue
• Torque, friction scatter, angle tightening and ultrasound