Loose bolts can be an irritating nuisance, but they can also lead to serious and even catastrophic results.
For example, in 2012, 432 barrels of synthetic drilling fluids were released into the Gulf of Mexico when 4-foot bolts on a drilling rig failed. After an investigation, the bolts’ manufacturer had to replace 10,000 bolts deployed, and the oil drilling company had to disrupt deep-water operations.
"The safety and reliability of bolted joints often determine the overall reliability and safety of mechanical and structural systems,” says Dr. Cheng Siong Phua, vice president of technology for STANLEY® Engineered Fastenings Asia Pacific & Global Electronic Division.
Bolt self-loosening is caused by any type of dynamic load, such as vibration or changes in temperature, insufficient clamp load and poorly fitting parts, allowing relative movements to increase the risk of self-loosening, according to a 2017 article in Engineer Live magazine. The sum of these very small movements ultimately results in the loosening of the threaded assembly.
By far the most frequent cause of loosening is side sliding of the nut or bolt head relative to the joint, resulting in relative motion occurring in the threads, according to Bolt Science, a U.K.-based consultancy and training company.
“A loosening problem is indicative of insufficient preload that results in joint movement,” says Bill Eccles, founder of Bolt Science and a consultant who specializes in bolting problems.
Relative motion occurring in the threads can be attributed to three common problems: bending of the parts; differential thermal effects; and external forces on a joint.
The safety and reliability of bolted joints often determine the overall reliability and safety of mechanical and structural systems.—Dr. Cheng Siong PhuaVice President of Technology
STANLEY Engineered Fastening, Asia Pacific & Global Electronic Division
One conclusion that experts agree on is that vibration has a far greater effect in a joint loaded in shear than in a joint loaded only in tension. For example, when the axis of a bolt is parallel to even severe vibration, it might result in a reduced preload of up to 40 percent over a long time, but will usually not result in total loss of the preload or in the loss of a fastener. On the other hand, severe transverse vibration, which is perpendicular to the axis of the bolt, can and often does cause catastrophic failure of all preload.
It’s even possible that a certain type of vibration will sometimes tighten, sometimes loosen, and sometimes do neither to a fastener when fasteners are subjected to neither pure transverse nor pure axial motion but to a combination.This creates arc slip.
As counterintuitive as it may sound, preload loss can occur with no fastener rotation and no relative movement between the external and internal threads whatsoever. This is called relaxation. One form of relaxation—embedding—happens when surface roughness flattens and the real area of contact becomes less than the apparent area of contact in the threads, joint faces and under the nut face.
The ongoing trend toward lighter, smaller products leads to an increase in the amount of vibration in a joint, according to a 2014 article in Assembly magazine. These two trends can sometimes work in tandem with each other. For example, lighter materials vibrate more, while smaller fasteners, in general, can handle less vibration. As a result, fasteners in small, lightweight products are susceptible to loosening, unless design options are considered to address this risk.