Lessons · Engineering · A bolted joint is held by preload, not by the bolt
A bolted joint is held by clamp, not by the bolt
A properly tightened bolt stretches and clamps the parts together. The external load is then mostly carried by friction between the clamped faces, and the bolt sees only a small share of it. A loose bolt has no clamp, so the load goes straight into the bolt in shear and it fails.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It reverses the intuition that a joint fails because the bolt was too small. Far more often the bolt was big enough and not tight enough, and the fix is a torque figure rather than a bigger fastener.
How to think about it
Ask what is holding the joint before sizing anything: friction from clamp, or the bolt itself in shear. If it is meant to be friction, the tightening specification is the design, and it needs a controlled method rather than a feel.
Worked example
Tightening stretches the bolt; the stretch is the clamp.A bolt is a stiff spring.
Clamp gives friction; friction carries the load.The bolt sees only a fraction of the external load.
Lose preload and the bolt takes the load directly.Which is how it fails.
Preload is lost to settling, vibration and heat.Which is why joints are re-checked.
Your turn
In a correctly tightened joint, what actually carries most of the external load?
The load is carried mostly by between the clamped faces
Solve one, graded on the server
The trap
Fixing a loosening joint with a bigger bolt. If preload is the problem, a bigger bolt loosens too; the answer is the right preload and something to keep it.