Factors influencing thread loosening

When we say that bolts come loose, this can mean several things. The nut that fixes the bolt in place could have rattled loose, or even possible came completely off. Or perhaps the bolt/ nut is still firmly in place, but after inspecting with a torque wrench, it had been identified that sufficient torque had been lost. At HARDLOCK when we refer to bolts coming loose, we refer to loss of preload or tension in a bolted joint.

How do bolts come loose? Unexpected External forces are usually at play. The joint can either rotate free or slacken free. Rotation usually occurs when the nut or bolt face slips when subjected to recurrent dynamic loads. Slackening or non-rotational loosening usually means that tension is lost even when there is no noticeable change to the nut bolt after installation. For example, one type of slackening is Embedment. Embedment can occur when the clamped components move laterally and clamped interfaces surfaces slip over each other causing flattening of the surface roughness and surface irregularities.

The tension in the bolt brings upon the opposing compression in the joint. This compression is called clamp force which is a vital force for keeping the assembly together and for preventing axial, bending and transverse movement. If the bolt comes loose, clamp force is lost, this can potentially result in loss or deterioration of the joints intended function wreaking havoc on the equipment/ device.

Bolt loosening modes:

When bolted joints come loose, problems can arise in many shapes and forms.

Joined parts separate.
Bolt or nut is found rattling loose or missing.
Seal failure because clamped interfaces Disengage. ( With gaskets etc )
Bolt fatigue failure ( Bolt breakage )

Depending on the intended application, bolt failure can lead to severe accidents and critical failure.

Optimal Preload (Bolt Tension)
= Resilient to upsetting forces

Loss of preload (Bolt loosening)

Nut comes loose, Bolt is directly subjected to dangerous loads

Bolt failure

Loosening Mechanisms

In order for a bolted joint to function properly, a bolt must be tensioned to the preload required for the application, then this preload must be sustained through usage. Loosening can occur when a bolted joint consists of too many interfaces ( intermediate parts ) or the bolted joint is subject to dynamic loading ( repeated exterior forces ).

Rotational loosening where the nut or bolt rotates loose is the easiest to identify whether bolt preload has been lost or not. Bolt preload can decline even when the bolt or nut doesn`t rotate loose. This type of loosening is called slackening or non-rotational loosening.

Strategies to prevent against Rotational and Non-rotational loosening

Rotational and Non-Rotational Loosening

1Non-rotational Loosening (Slackening)

Bolt preload is produced when the bolt or nut is tightened causing the bolt to stretch and the joint to be compressed. Even when the bolt has not rotated loose, there are cases where loss in bolt elongation and joint compression occur. This results in a loss of preload, letting our joints come loose. Loss in elongation of the bolt is caused by plastic elongation, bolt creep, differential thermal expansion, etc… Whereas loss in compression of the joint is caused by plastic flattening of the surface roughness between the joint interface, nut bearing face, bolt and nut thread flanks, etc…
There are three main types of rotational loosening; embedment, relaxation and creep. Creep becomes more severe when Differential thermal expansion occurs. Lastly, large external loads which exceed the initial preload, can cause the bolts to yield causing permanent elongation in the bolt.

You can do the following to help prevent non-rotational loosening:

Increase bolt length, and increase bolt diameter.
Improvise strategies to reduce abrasion and plastic flattening of the surface roughness.
Reduce preload of joint.
Use materials with the same thermal expansion coefficients for the bolt, nut, washers and the joint.

These prevention strategies are quite commonly utilized for joint design.

For upcoming projects and new designs these strategies will come into play. However, for existing designs, taking an effective prevention strategy for non-rotational loosening can be difficult and frustrating.

2Rotational Loosening (Spontaneous Loosening)

Rotational loosening is a form of loosening that occurs with relative movement of the nut to the bolt. The type of loading the joint is subject to will determines the degree of loosening that will take place. There are three main types of external loading; Cyclic Axial loading (bolt is pulled), Cyclic bending loading (bolt is bended) and Cyclic Transverse Loading (shear movement in the joint interface). Cyclic transverse loading has the biggest impact on loosening. Transverse movements causes side sliding of the nut relative to the bearing face. This relative motion translates to the thread, causing the nut to slip back in the loosening direction. Leading to preload loss.

Cyclic Transverse Loading will almost always lead to loosening and preload loss.
The following prevention strategies exist to prevent rotational loosening and side-sliding:

Increase initial preload to increase friction on bearing face.
Spread the burden by increasing the number of bolts.
Good Bolt joint design to improve resistance to external loads.
Increase bolt length, and increase bolt diameter.
Use a bolt locking device such as a locknut.

There are many Bolt locking devices available globally. There are locking devices for preload loss preventing , fall-off prevention and slackening prevention.