5 Reasons Why Bolts Are Loose
Insufficient tightening
Undertightened or falsely tightened bolts are inherently under-tightened, and if loose, the joint does not have enough clamping force to hold the parts together. This can lead to lateral sliding between the two parts, and the bolt is subjected to unnecessary shear stress, which can eventually cause the bolt to break.
02
vibration
Tests of bolted connections under vibration have shown that many small "lateral" movements cause the two parts of the connection to move relative to each other, as well as the bolt head or nut and the connected parts.
These repeated movements cancel out the friction between the bolt and the parts being connected. Eventually, the vibration will cause the bolt to "spin loose" on the threads and the joint will lose its grip.
03
embed
Engineers who design and develop bolt tension allow for a break-in period, a loss of preload, during which the bolt loosens in tightness.
This loosening is due to entrapment between the bolt head and/or nut, threads and mating surfaces of the connected parts and can occur in both soft materials such as composites as well as hard polished metals.
If the joint is not designed properly, or if the bolt is not at the specified tension at the beginning, the embedding of the joint may cause a loss of clamping force and the required minimum clamping force is not achieved.
There are microscopic unevenness between the joint surfaces, and the bumps will be crushed and permanently plastically deformed under the action of the bolt pre-tightening force after tightening, so that the clamping length of the bolt will be reduced, and finally the pre-tightening force of the bolt will decrease.
04
Gasket Creep and Thermal Expansion
Many bolted joints include a thin, soft gasket between the bolt head and the joint surface to seal the joint against gas or liquid leakage. The washer itself also acts as a spring, rebounding under the pressure of the bolt and mating surfaces.
Over time, especially near high temperatures or corrosive chemicals, a gasket can "creep," which means it loses its elasticity, resulting in a loss of clamping force.
If the bolts and joints are made of different materials, the rapid expansion or contraction of the bolt material due to rapid environmental changes or excessive temperature differences due to industrial cycling processes may cause the bolts to loosen.
05
shock
Shock - Larger shock loads exceed the frictional force when the bolt is preloaded, resulting in slippage.
Dynamic or alternating loads from machinery, generators, wind turbines, etc. can cause mechanical shocks - impact forces applied to a bolt or joint - causing the bolts to slide relative to each other.
Like vibration, this slippage can eventually cause the bolt to loosen. Even shock is often not considered such a large load when designing joint connections.
What is preloading?
A term that has many meanings in engineering. One is the tension (load) developed by the fastener when it is initially tightened. When the bolt is stretched, the components between the bolt and the nut compress, increasing the so-called clamping load until the tightening process is complete.
Dangers of loose bolts
01
Flange Leakage
02
The fan rotor is separated from the nacelle
03
Ship engine vibration connection bolts 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 a bolted joint, tightening the nut actually lengthens the bolt, like pulling a spring. This pulling force, or tension, creates an opposing clamping force, holding the two parts of the connected piece tightly together.
If the bolt is loose, the clamping force will be weakened.
Loose bolts are more than just a headache. If joints are not retightened quickly, fluid or gas may begin to leak, bolts may break, equipment may be damaged, or a catastrophic accident may occur.
fully fueled launch vehicle
bolted rails
"The best anti-loosening is to ensure that the pre-tightening force is sufficient to prevent problems such as slippage and opening of the joint"
From the above analysis, it can be seen that there are three reasons for loosening due to insufficient or reduced pre-tightening force. Therefore, special control of the pre-tightening force of the bolts is required to control the risk of loosening.
As long as the pre-tightening force is sufficient to meet the requirements, as long as the clamping length is not too short (such as lk≥3d), even if there is a certain vibration load, the bolt will generally not loosen itself.
The combination of good bolted joint design, proper clamping force development, and proper bolt lockouts can reliably secure bolted joints to meet the many loosening challenges presented here.
A good bolted connection will be designed with the proper size and type of bolts and nuts, and specify the optimum tension to achieve the clamping force necessary to maintain the integrity of the connection.
Proper clamping force in an application requires the correct level of tension (preload) in each bolt to be maintained at that level throughout its service life.
Therefore, it is very important to maintain proper tension on the bolts. Ultrasonic waves can be used to measure the axial force of the bolts during design to ensure that the pretightening force of the bolts meets the design requirements.
5 Reasons Why Bolts Are Loose
Insufficient tightening
Undertightened or falsely tightened bolts are inherently under-tightened, and if loose, the joint does not have enough clamping force to hold the parts together. This can lead to lateral sliding between the two parts, and the bolt is subjected to unnecessary shear stress, which can eventually cause the bolt to break.
02
vibration
Tests of bolted connections under vibration have shown that many small "lateral" movements cause the two parts of the connection to move relative to each other, as well as the bolt head or nut and the connected parts.
These repeated movements cancel out the friction between the bolt and the parts being connected. Eventually, the vibration will cause the bolt to "spin loose" on the threads and the joint will lose its grip.
03
embed
Engineers who design and develop bolt tension allow for a break-in period, a loss of preload, during which the bolt loosens in tightness.
This loosening is due to entrapment between the bolt head and/or nut, threads and mating surfaces of the connected parts and can occur in both soft materials such as composites as well as hard polished metals.
If the joint is not designed properly, or if the bolt is not at the specified tension at the beginning, the embedding of the joint may cause a loss of clamping force and the required minimum clamping force is not achieved.
There are microscopic unevenness between the joint surfaces, and the bumps will be crushed and permanently plastically deformed under the action of the bolt pre-tightening force after tightening, so that the clamping length of the bolt will be reduced, and finally the pre-tightening force of the bolt will decrease.
04
Gasket Creep and Thermal Expansion
Many bolted joints include a thin, soft gasket between the bolt head and the joint surface to seal the joint against gas or liquid leakage. The washer itself also acts as a spring, rebounding under the pressure of the bolt and mating surfaces.
Over time, especially near high temperatures or corrosive chemicals, a gasket can "creep," which means it loses its elasticity, resulting in a loss of clamping force.
If the bolts and joints are made of different materials, the rapid expansion or contraction of the bolt material due to rapid environmental changes or excessive temperature differences due to industrial cycling processes may cause the bolts to loosen.
05
shock
Shock - Larger shock loads exceed the frictional force when the bolt is preloaded, resulting in slippage.
Dynamic or alternating loads from machinery, generators, wind turbines, etc. can cause mechanical shocks - impact forces applied to a bolt or joint - causing the bolts to slide relative to each other.
Like vibration, this slippage can eventually cause the bolt to loosen. Even shock is often not considered such a large load when designing joint connections.
What is preloading?
A term that has many meanings in engineering. One is the tension (load) developed by the fastener when it is initially tightened. When the bolt is stretched, the components between the bolt and the nut compress, increasing the so-called clamping load until the tightening process is complete.
Dangers of loose bolts
01
Flange Leakage
02
The fan rotor is separated from the nacelle
03
Ship engine vibration connection bolts 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 a bolted joint, tightening the nut actually lengthens the bolt, like pulling a spring. This pulling force, or tension, creates an opposing clamping force, holding the two parts of the connected piece tightly together.
If the bolt is loose, the clamping force will be weakened.
Loose bolts are more than just a headache. If joints are not retightened quickly, fluid or gas may begin to leak, bolts may break, equipment may be damaged, or a catastrophic accident may occur.
fully fueled launch vehicle
bolted rails
"The best anti-loosening is to ensure that the pre-tightening force is sufficient to prevent problems such as slippage and opening of the joint"
From the above analysis, it can be seen that there are three reasons for loosening due to insufficient or reduced pre-tightening force. Therefore, special control of the pre-tightening force of the bolts is required to control the risk of loosening.
As long as the pre-tightening force is sufficient to meet the requirements, as long as the clamping length is not too short (such as lk≥3d), even if there is a certain vibration load, the bolt will generally not loosen itself.
The combination of good bolted joint design, proper clamping force development, and proper bolt lockouts can reliably secure bolted joints to meet the many loosening challenges presented here.
A good bolted connection will be designed with the proper size and type of bolts and nuts, and specify the optimum tension to achieve the clamping force necessary to maintain the integrity of the connection.
Proper clamping force in an application requires the correct level of tension (preload) in each bolt to be maintained at that level throughout its service life.
Therefore, it is very important to maintain proper tension on the bolts. Ultrasonic waves can be used to measure the axial force of the bolts during design to ensure that the pretightening force of the bolts meets the design requirements.
In threaded fasteners, preload is the internal tension (stretching force) created when a bolt is tightened. A bolt acts like a stiff spring: as you torque it, you stretch the bolt along its length, which clamps the jointed parts tightly together.
This clamping force creates friction between the mating surfaces, holding the joint rigid and keeping the threads engaged.
[ BOLT HEAD ] │ │ ▲ │ │ │ Applied Preload (Tension/Stretch) ▼ ▼ │ ═══════════════════ ◄── Clamping Force Joint Material ═══════════════════ ◄── Clamping Force ▲ ▲ │ │ │ │ Reaction Force │ │ ▼ [ NUT / THREAD ]
Why Bolts Come Loose
A bolt loses its preload and backing out occurs through two main mechanisms: rotational self-loosening and non-rotational relaxation.
1. Rotational Self-Loosening (Vibration & Dynamic Load)
When a bolted joint experiences transverse (side-to-side) vibration or dynamic shearing forces, the relative motion between the male and female threads momentarily eliminates thread friction.
Under repeated micro-slips, the elastic energy stored in the stretched bolt causes it to rotate backward along its helix angle.
Once micro-rotation begins, the bolt rapidly spins loose, shedding all clamping force.
2. Non-Rotational Relaxation (Loss of Preload Without Turning)
A bolt can lose its tension even if the threads never turn:
Settling & Surface Roughness: Under load, microscopic high spots (asperities) on the joint faces, washers, and thread flanks flatten out over time, reducing joint thickness and relaxing the bolt's stretch.
Gasket Creep: Soft materials like rubber or composite gaskets compress continuously under load, causing tension loss.
Thermal Expansion Mismatch: If the bolt and joint materials expand at different rates under heat, the joint can loosen when temperatures drop.
Yielding (Over-Tightening): If a bolt is torqued beyond its yield strength, it stretches permanently (plastic deformation) and can no longer maintain tension.
Key Factors for Joint Security
| Mechanism | Cause | Common Solutions |
| Side-to-Side Shear | Transverse vibration causing thread slip | Wedge-locking washers (Nord-Lock), threadlocking fluid (Loctite) |
| Joint Settling | Surface compression and smoothing | High-stiffness Belleville spring washers, smoother joint surfaces |
| Inaccurate Torque | Under-torquing leads to insufficient preload | Angle-controlled tightening, tension indicators, calibrated torque wrenches |
Maintaining Proper Preload
Preventing bolt loosening starts with achieving the correct initial preload-typically targetted at 60% to 70% of the bolt's yield strength. Proper preload ensures the clamping force exceeds any external shear forces acting on the joint, stopping the micro-movements that trigger self-loosening in the first place.
