Question 1

How is bolt torque calculated?

Accepted Answer

Torque follows T = K·D·F, where K is the nut factor (~0.2 dry, ~0.15 lubricated), D the nominal diameter and F the target preload. Preload is usually set to 65–90% of the bolt's proof load.

Question 2

Why does lubrication change the torque so much?

Accepted Answer

Most of the torque you apply fights friction under the head and in the threads, not stretch in the bolt. Lubricant lowers the nut factor K, so the same torque produces a higher preload — which is why you must never apply a dry-spec torque to a lubricated bolt.

Question 3

Is torque or preload what actually matters?

Accepted Answer

Preload. Torque is just an indirect, friction-dependent way to reach it. For critical joints, control preload directly (angle-of-turn, load-indicating washers or a tension tool) rather than torque alone.

Bolt Torque & Preload

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