Explained: Why Bolts Loosen and How Preload Prevents It

Reasons for Loose Bolts

1. Insufficient tightening

The bolts that are not tightened or are not tightened properly are inherently under pre tightening force.

If they become loose again, the joint will not have enough clamping force to fix all parts together.

This may cause lateral sliding between two parts, and the bolt will be subjected to unnecessary shear stress, which may eventually lead to bolt fracture.

2. Vibration

The test of bolted connection under vibration shows that many small “transverse” movements cause the two parts of the connection to move with each other, and the bolt head or nut and the connected part will also move.

These repeated movements will counteract the friction between the bolt and the connected piece.

Finally, the vibration will cause the bolt thread to “rotate and loosen”, and the joint will lose the clamping force.

3. Embedding

The engineer who designs and develops the bolt tension is allowed to have a running-in period, which will cause a certain pre-tightening force loss.

During this period, the tightness of the bolt will be relaxed.

This relaxation is caused by the embedding between the bolt head and/or nut, thread and the joint surface of the connected part, and can occur in soft materials (such as composite materials) as well as hard polished metal.

If the joint is not properly designed, or if the bolt does not reach the specified tension at the beginning, the insertion of the joint may result in a loss of clamping force without reaching the required minimum clamping force.

There are micro unevenness between the joint surfaces.

After tightening, under the pre-tightening force of the bolt, the bump will collapse and permanent plastic deformation will occur, which will reduce the clamping length of the bolt and eventually lead to the reduction of the pre-tightening force of the bolt.

4. Gasket creep and thermal expansion

Many bolted joints include a thin and soft gasket between the bolt head and the joint surface to seal the joint and prevent gas or liquid leakage.

The washer itself also acts as a spring, springing back under the pressure of the bolt and the joint face.

Over time, especially when approaching high temperatures or corrosive chemicals, the gasket may “creep”, which means it loses elasticity, resulting in a loss of clamping force.

If the materials of bolts and joints are different, the temperature difference caused by rapid environmental change or industrial cycle process is too large, which will lead to rapid expansion or contraction of bolt materials, and may make bolts loose.

5. Impact

Dynamic or alternating loads from machinery, generators, wind turbines, etc. can cause mechanical impact – the impact force applied to the bolt or joint – to cause the bolt to slide relative to each other.

Like vibration, this sliding will eventually cause the bolt to loosen.

Even the impact often does not take such a large load into account when designing the joint connection.

What is preload?

Terms with multiple meanings in the project.

One is the tension (load) generated when the fastener is initially tightened.

When the bolt is extended, the part between the bolt and the nut is compressed, thus increasing the so-called clamping load until the tightening process is completed.

Hazards of loose bolts

1. Flange leakage

2. The fan rotor is separated from the engine room

3. Vibration connecting bolts of ship engine fall off

The vibration connecting bolts of the ship’s engine fell off and rolled with the ship, causing further damage to the equipment.

In bolted connections, tightening the nut actually causes the bolt to elongate, just like pulling a spring.

This tension, or tension, will produce opposite clamping forces, which will tightly fix the two parts of the connector together.

If the bolt is loose, the clamping force will be reduced.

Loose bolts are not just a headache. If the joint is not retightened quickly, it may start to leak liquid or gas, the bolt may break, the equipment may be damaged, or a catastrophic accident may occur.

Bolt on rail


“The best way to prevent looseness is to ensure that the pre-tightening force is sufficient, and the joint will not have problems such as sliding and opening.”

From the above analysis, it can be seen that there are three reasons for looseness due to insufficient or reduced preload.

Therefore, it is necessary to specially control the preload of bolts to control the risk of looseness.

As long as the pre-tightening force is sufficient to meet the requirements, and as long as the clamping length is not too short (such as 1k ≥ 3d), even if there is a certain vibration load, the bolt will generally not loosen itself.

A combination of good bolted connection design, appropriate clamping force development and appropriate bolt locking devices can reliably secure bolted connections to meet many of the loosening challenges presented here.

A good bolted connection will be designed with bolts and nuts of appropriate size and type, and the optimum tension will be specified to achieve the clamping force required to maintain the integrity of the connection.

In application, proper clamping force requires that the tension (pre-tension) in each bolt reaches the correct level and remains at that level throughout its service life.

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Founder of MachineMFG

As the founder of MachineMFG, I have dedicated over a decade of my career to the metalworking industry. My extensive experience has allowed me to become an expert in the fields of sheet metal fabrication, machining, mechanical engineering, and machine tools for metals. I am constantly thinking, reading, and writing about these subjects, constantly striving to stay at the forefront of my field. Let my knowledge and expertise be an asset to your business.

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