**6.1 How does bolt and nut loosen?**

Threaded fasteners were designed to be removed anytime. The problem is that it loosens unintentionally. Thread is a rolled cone and unrolled thread is an inclined plane. As shown in **Figure 4**, "F sin a" of the induced load assists the loosening. When an object is in an equilibrium state between gravity and friction on an inclined surface, and then, if there is a sideway transverse impact or vibration applied to the

**Figure 4.** *Object on an inclined plane.*

object, the object easily slides down meaning loosening of the fastener also downhill is 30% less effort (easier to slide down) than uphill (going up) [1, 2].

#### **6.2 Reasons for failure**

#### *6.2.1 Relaxation*

In a flanged assembly, a cut gasket receives a high pressure from bolt tightening. In the beginning, cut gasket has a certain level of flexibility to push back the flange pressure to achieve a sealing. As the flange pressure continues to press the cut gasket, the cut gasket comes to a point that it no longer pushes back and loses a flexibility. A gradual decrease of cut gasket thickness over time is called "creep." As the cut gasket starts to reduce its thickness due to the flange pressure, the cut gasket becomes "compression set." Once the compression set was done, the bolt tension of the flange is reduced, and then, the flange surface pressure reduces leading to a leak, and the bolt and nut assembled become susceptible to an unintentional loosening against external vibration and impact. The same is applied when the flange substrate is soft enough to be deformed by the bolt and nut with a high tightening torque. The bolt tension is reduced, and the assembly is easy to loosen. Another one is "settling" of the substrate surface as its surface irregularity. Especially, peak is flattened by the flange pressure to settle down a bit. However, this settling is very minimal to cause the reduction of bolt tension (see **Figure 8**).

In a flanged assembly, initial settling of flange and creeping of cut gasket might be compensated by the bolt elasticity to provide a sealing and no bolt loosening, but as time goes by and the cut gasket thickness becomes thinner, then bolt elasticity can no longer compensate the loss of flange pressure. The bolt tension will be reduced, and bolt and nut will loosen eventually. Therefore, it is a routine work and guideline for mechanical assembly manufacturers that they do a "re-torqueing" of the bolt and nut assembly before shipping after they manufacture and store at the warehouse. Again, such above method will add additional cost to the assembly [1, 2].

#### *6.2.2 Loosening*

After the tightening procedure, the clamp load is maintained by the preload in the bolt. This occurs because the bolt has been stretched like a spring, and the tension acts to pull the flange or substrate. As shown in **Figure 5**, an object is in an equilibrium state (no move) in an inclined surface as the gravity and friction are applied equally. A test was done using a simulated object and inclined plane to see how the object moves when a transverse force is applied to the side of the object. When it was pushed sideway, it slides down and then stops. Then another sideway push, it slides down and stops again. Consequently, when there was a continuous sideway pushes, the object continued to slide down. As bolt and nut are tightened more and more, it means that one thread climbs up the mating thread. It also means that the height energy becomes higher and has a high tendency to slide down when there was a loosening parameter combining external ones (impact and vibration) and internal one (plastic deformation). This is an exact representation of a real threaded assembly as each thread is an inclined surface (see **Figure 5**) [1].

When there were internal or external parameters, such as impact and vibration, it generates a sliding force and consequently results in insufficient clamp load and loosening.

A special testing machine (transverse shock tester) was used to compare how fast the mechanical locking devices lose the initial bolt tension under shock and vibration. Bolt and nut are used together with locking devices such as spring washer, nylon insert, and

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*Liquid Thread Locking Solution for Machinery Assembly Industry*

serrated bolt. They are assembled with a pre-determined tightening torque in the testing machine. Then, air hammer is actuated by hitting both sides of the assembled part. The machines monitor the loss of initial bolt tension as time goes by. Those mechanical fasteners assembled lose their initial bolt tension within "20 s" (see **Figure 6**) [1, 2].

When substrates have a different thermal expansion coefficient, it causes a sliding movement when subject to high or low temperature. It results in loss in bolt tension, reduction in clamp force, and consequent assembly failure (see **Figure 8**).

When mechanical locking devices are used for assembly, metal-to-metal contact area is 15% between male and female threads only and between bolt head and substrate. The rest 85% is all empty which is susceptible to the ingression of moisture or other chemical environments resulting in the corrosion inside. Once it is corroded, it is quite difficult to disassemble by tools. Then they have to end up with cutting the whole fastener assembly by torch and procure new fasteners again. For bigger size

The flange assembly tightened with fasteners often has a flange sealing failure. It is difficult to achieve a uniform, consistent flange pressure with normal bolt and

bolt, quite often, the assembly failure occurs (see **Figures 7** and **8**) [1].

*DOI: http://dx.doi.org/10.5772/intechopen.85090*

*6.2.3 Differential thermal expansion*

*6.2.4 Corrosion*

**Figure 5.**

**Figure 6.**

*Transverse shock test.*

*Transverse loosening of a bolt and nut.*

*6.2.5 Tension scatter*

**Figure 5.** *Transverse loosening of a bolt and nut.*

*Adhesives and Adhesive Joints in Industry Applications*

**6.2 Reasons for failure**

*6.2.1 Relaxation*

*6.2.2 Loosening*

object, the object easily slides down meaning loosening of the fastener also downhill

In a flanged assembly, a cut gasket receives a high pressure from bolt tightening. In the beginning, cut gasket has a certain level of flexibility to push back the flange pressure to achieve a sealing. As the flange pressure continues to press the cut gasket, the cut gasket comes to a point that it no longer pushes back and loses a flexibility. A gradual decrease of cut gasket thickness over time is called "creep." As the cut gasket starts to reduce its thickness due to the flange pressure, the cut gasket becomes "compression set." Once the compression set was done, the bolt tension of the flange is reduced, and then, the flange surface pressure reduces leading to a leak, and the bolt and nut assembled become susceptible to an unintentional loosening against external vibration and impact. The same is applied when the flange substrate is soft enough to be deformed by the bolt and nut with a high tightening torque. The bolt tension is reduced, and the assembly is easy to loosen. Another one is "settling" of the substrate surface as its surface irregularity. Especially, peak is flattened by the flange pressure to settle down a bit. However, this settling is very

In a flanged assembly, initial settling of flange and creeping of cut gasket might be compensated by the bolt elasticity to provide a sealing and no bolt loosening, but as time goes by and the cut gasket thickness becomes thinner, then bolt elasticity can no longer compensate the loss of flange pressure. The bolt tension will be reduced, and bolt and nut will loosen eventually. Therefore, it is a routine work and guideline for mechanical assembly manufacturers that they do a "re-torqueing" of the bolt and nut assembly before shipping after they manufacture and store at the warehouse.

After the tightening procedure, the clamp load is maintained by the preload in the bolt. This occurs because the bolt has been stretched like a spring, and the tension acts to pull the flange or substrate. As shown in **Figure 5**, an object is in an equilibrium state (no move) in an inclined surface as the gravity and friction are applied equally. A test was done using a simulated object and inclined plane to see how the object moves when a transverse force is applied to the side of the object. When it was pushed sideway, it slides down and then stops. Then another sideway push, it slides down and stops again. Consequently, when there was a continuous sideway pushes, the object continued to slide down. As bolt and nut are tightened more and more, it means that one thread climbs up the mating thread. It also means that the height energy becomes higher and has a high tendency to slide down when there was a loosening parameter combining external ones (impact and vibration) and internal one (plastic deformation). This is an exact representation of a real threaded assembly as each thread is an inclined surface (see **Figure 5**) [1].

When there were internal or external parameters, such as impact and vibration,

A special testing machine (transverse shock tester) was used to compare how fast the mechanical locking devices lose the initial bolt tension under shock and vibration. Bolt and nut are used together with locking devices such as spring washer, nylon insert, and

it generates a sliding force and consequently results in insufficient clamp load

is 30% less effort (easier to slide down) than uphill (going up) [1, 2].

minimal to cause the reduction of bolt tension (see **Figure 8**).

Again, such above method will add additional cost to the assembly [1, 2].

**122**

and loosening.

**Figure 6.** *Transverse shock test.*

serrated bolt. They are assembled with a pre-determined tightening torque in the testing machine. Then, air hammer is actuated by hitting both sides of the assembled part. The machines monitor the loss of initial bolt tension as time goes by. Those mechanical fasteners assembled lose their initial bolt tension within "20 s" (see **Figure 6**) [1, 2].
