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To prevent spontaneous bolt loosening, the sliding between the connecting parts needs to be eliminated or at least reduced below the critical level. This can be achieved by increasing the axial tension, increasing friction between clamping components or reducing the cyclic load (e.g., shock, vibration or cyclic heat load).

Another common method is to increase the friction between bolt threads. There are many solutions to do this, and while some of them are effective, they also have their drawbacks. Glue or adhesives can be effective friction-based methods, but dry glue can be problematic when it comes to removing and removing bolts. In addition, increasing friction between threads reduces the preload that can be achieved at certain torque levels. Locking wire is a common method used in the aviation industry.

Fatigue is the sexual damage or deformation of bolts and clamping components. This is due to the loss of preload causing the joint to open. There are two basic mechanisms of preload loss - spontaneous loosening and relaxation.

Spontaneous or rotational loosening occurs when a bolt is spun loose by impact, vibration or dynamic load. Even a slight rotation is sufficient to cause the bolted connection to lose all its preload. This is a common cause of loose bolts.

Relaxation is caused by three mechanisms: subsidence, creep and relaxation. "Settling is critical when dynamic loads occur. When the joint is subjected to increased stress from dynamic working loads, the clamping material becomes deformed, "explains Harlen Seow, group technical manager at Nord-Lock. "Most bolted parts will return to shape after release if the stress in the part does not exceed yield strength. Certain materials in the contact surface, such as paint, are likely to deform, "he said.

If the material settles, even by a few microns, the bolt's stretch will be reduced and lead to a loss of preload.

Creep is a deformation that occurs as a result of prolonged exposure to high stress levels below the yield strength of the joint material. It is more serious in high-temperature applications.

Relaxation refers to the recombination of microstructure in the joint material, which converts the existing elastic deformation into plastic deformation over a period of time. Unlike settling or creep, fixture length does not change, making detection more difficult. "One way to measure preload losses is to measure bolt length after running for a period of time and compare it to bolt length immediately after tightening," added Seow. "However, this does not find relaxation, which makes it more problematic."

The key to avoiding fatigue is good design, which has become increasingly important in recent years due to the increased demand for many bolted connections and the increased use of lightweight materials. It is important to focus not only on the tensile capacity of the bolt, and therefore ignore other parameters, such as elasticity and stiffness, which are also important.

"The correct joint design is key to achieving a high-strength frictional gripper connection with a high preload level and therefore high slip resistance throughout the service life," Dinger said. "Until now, design engineers have focused on the failure of bolts to break. As performance improves and joint weight decreases, other failure mechanisms become increasingly important. In lightweight design, preload relaxation and self-loosening mechanisms are becoming more and more common. "