Why does my nut and bolt keep working loose?

A properly tightened bolt is a stretched spring holding two things together. Most loosening is not the nut unscrewing at random but that spring going slack, after which vibration does the rest.

What you'll need

• nyloc nuts or threadlocker
• spanners or a socket set
• penny or plate washers (optional)

Step by step

1. Work out where it is loosening from.If the nut is backing off, it is vibration. If the nut is still tight against the material but the joint is loose, the material has crushed under the washer.
2. Tighten it properly in the first place.Under-tightened bolts loosen; that is the most common cause of all. The bolt needs enough tension to keep the faces clamped through the load cycle.
3. Use a bigger washer in soft material.In timber, plastic or thin sheet, a small washer embeds and the joint slackens. A penny washer or a plate washer spreads the pressure and stops the settling.
4. Retighten once after settling.New joints, especially in timber, need a second tighten after a few days or after the first heavy use. This alone solves many recurring problems.
5. Fit a nyloc nut for a removable joint.A nylon insert grips the thread and resists rotation. It is reusable a few times but loses grip, and it is not suitable for high temperatures.
6. Use threadlocker for a semi-permanent joint.An anaerobic adhesive that cures in the thread. Medium strength is removable with ordinary spanners; high strength usually requires heat, so choose deliberately.
7. Use two nuts where nothing else fits.A thin nut jammed against a full nut is the traditional lock. Tighten the first, then hold it and tighten the second against it.
8. Consider a serrated flange nut.The serrations bite into the surface, which works well on metal and badly on painted or soft material where they simply chew it up.
9. Look for the movement causing it.A bolt that repeatedly loosens is usually reporting a joint that is moving — an elongated hole, a missing brace, a flexing bracket. Locking the nut treats the symptom.

Tips

• Spring washers are far less effective than most people assume; a nyloc nut or threadlocker does the job much better.
• In timber, the answer is nearly always a bigger washer and a retighten rather than a cleverer nut.
• Mark the nut and bolt with a line of paint once tightened. At a glance you can then see whether it has moved.

Common mistakes

• Relying on a spring washer — Once the bolt is properly tightened the spring washer is flattened solid and contributes almost nothing. It is a habit rather than a solution.
• Just tightening it harder each time — In soft material this crushes more of it away, so the joint has less to bear against each time and loosens faster.

If it doesn't work

Questions people ask