Bolts come loose in two ways. Vibration or sideways movement turns the nut back (self-loosening), or the joint settles and the bolt loses tension without turning (slackening).
What keeps a bolt tight in the first place
A tightened bolt behaves like a stiff spring. Tightening stretches it a tiny amount, and that stretch pulls the parts together. This clamping force is called preload.
While the preload is high enough, friction in the thread and under the bolt head holds everything still. Loosening starts when something takes that preload away, or lets the parts slip despite it.
Cause 1: vibration turns the nut back
The most common cause is sideways movement in the joint. When the clamped parts shift even slightly across each other, the thread and the bolt head slip for a moment. During that slip, friction almost disappears.
A thread is a slope. With no friction holding it, the load on the bolt pushes the nut a tiny way back down that slope. One slip does almost nothing. Thousands of slips, on a conveyor, crusher, vehicle or pump, walk the nut off.
Engineers have known this since the 1960s, when Gerhard Junker showed that sideways (transverse) movement is far more damaging than vibration along the bolt. The vibration test named after him is still the standard way to compare locking methods.
Cause 2: the joint settles and the bolt slackens
Sometimes the nut never turns, but the bolt still loses its grip. This is called slackening. Common reasons:
- Embedding: the high spots on contact surfaces flatten in the first hours after tightening.
- Soft materials: aluminium, copper and composites slowly creep under load.
- Gaskets and coatings: they compress or squeeze out over time.
- Temperature: parts that expand at different rates change the clamp force as they heat and cool.
Short bolts suffer most. If a bolt only stretches a little when tightened, a small amount of settling removes a large share of its preload.
Cause 3: not enough preload to start with
A bolt that was never tightened enough lets the joint slip from day one. Uneven tightening, unknown friction from oil or coatings, and worn threads all leave bolts with less preload than the drawing asks for.
How to stop bolts coming loose
- Tighten to the right preload with a controlled method. Our guide on torque or tensioning explains the options.
- Lock the bolt so it cannot turn. Nord-Lock Original washers use wedge action, so a bolt that tries to turn loose is pulled tighter instead.
- Where joints settle, use Nord-Lock X-series washers, which also keep tension up as the joint relaxes.
- For special joints, use the matching product: wheel nuts for heavy vehicles and steel construction washers for HV and HR structural bolts.
- Design for a longer clamp length where you can. A longer bolt stretches more and tolerates settling better.
What does not work well
Methods that only add friction, such as spring washers, give little protection once a joint is properly tightened and starts to vibrate. We compare them in Nord-Lock vs spring washer.