Causes of non-rotational loosening
One of the key features of nuts and bolts is their ability to create and disassemble complex assemblies as needed. However, under certain conditions, a nut or bolt can become loose, leading to undesirable consequences. Self-loosening occurs when the nut or bolt rotates due to applied forces, potentially leading to complete disassembly and catastrophic failure of the product. Various locking devices aim to prevent or reduce this rotation of fasteners.
Introduction
While loosening is typically defined as a reduction in the preload of a fastener (the tensile load in the bolt) from its initial tightened state, another type of loosening can occur: non-rotational loosening. Over the next few newsletters, we will explore non-rotational loosening, which happens when no relative movement occurs between the internal and external threads—meaning no rotation takes place, but preload is still lost. A partial loss of preload can occur in all bolted joints, though some types are more prone to significant loss than others.
How does non-rotational loosening occur?
Non-rotational loosening can result from deformation of the fastener or the joint after assembly. The clamp load on a joint is due to the preload created from the axial extension of the fastener. Typically, both the fastener extension and joint compression are elastic. After assembly, changes in fastener extension and joint compression can lead to a loss of preload. These changes may be reversible due to differential thermal expansion of the bolt relative to the joint or permanent due to plastic deformation, creep, or stress relaxation.
Main causes of non-rotational loosening
Several factors contribute to non-rotational loosening, all involving either additional elongation of the bolt or a loss of compression in the joint post-installation. Terms such as relaxation and settlement are sometimes used to encompass these types of non-rotational loosening. The primary causes are:
Embedding: Localized plastic deformation occurring under the nut face, within the joint faces, and in the threads due to plastic flattening of surface roughness.
Stress relaxation: A form of creep where high stress in a bolt is gradually relieved over time, often at elevated temperatures, resulting in a reduction in bolt preload.
Creep: The tendency of a material to permanently deform under continuous stress. Creep can affect the joint material, any gaskets within the joint, and any paint on the plates of the joint.
Excessive bearing stress: When surface stress caused by the clamp load is too high, it leads to surface collapse, resulting in reduced bolt stretch and preload loss.
Differential thermal expansion: Bolts are usually tightened at ambient temperatures, but they may operate at elevated or cryogenic temperatures. Differences in the expansion/contraction characteristics of the joint and bolt materials can lead to increased or decreased bolt preload.
Yielding after tightening: If a bolt experiences stress beyond its yield point due to differential thermal expansion or a high external load on the joint, it can lead to a reduction in preload when the external load or temperature changes.
How to prevent and control non-rotational loosening ?
TRAXX recommends monitoring the behavior of an assembly and its fasteners over time. How? By adopting the method of measuring and controlling bolt tension. TRAXX designs and markets bolt load measurement devices based on ultrasonic technology. These devices allow precise monitoring of bolt preload at assembly, and also during follow-up checks after one day, one week, one month, or one year as part of quality verification processes. More rigorous and accurate than other methods aimed at preventing loosening (such as washers, locknuts, adhesives, or residual torque), this technique relies on measured data to ensure the long-term integrity of an assembly. If relaxation is observed, retightening is performed to restore the recommended preload.
How does preload control work?
Over time, the TRAXX-M2 device records and stores tightening data for each bolt: the bolt’s identifier, its characteristics, the date and time of tightening, and the applied tension during the initial assembly. This data is used to track bolt performance over time, enabling precise monitoring of the work done, operation tracing, result analysis, and scheduling of maintenance. This method is widely used in tightening maintenance and quality monitoring services.
For more information on this method, please contact us via the form on the contact page.
