By Julie Pereyra, Sales Engineer, Nord-Lock Group
Depending on the application, bolt loosening can have profound consequences. One loose bolt can bring a whole production plant to a standstill and cost a company thousands. In other applications, loose bolts can pose a significant safety hazard. So, why do bolts loosen?
Broadly speaking, there are two main causes:
- Spontaneous loosening – shock, vibration, dynamic load
- Slackening – settlement, creep, relaxation
“The main causes and the consequences of failure depend on the purpose of the bolted joints, on the environment and usually on the industry,” said Georg Dinger, Siegenia-Aubi KG, who has studied the causes and effects of self-loosening of bolts extensively.
“For example, the petrochemical industry is primarily concerned with corrosion problems, while fatigue and vibration loosening are usually of minor concern. On the other hand, the automotive industry would probably name self-loosening and corrosion as the two main problems. The primary concerns for the structural steel industry are joint slip and corrosion, but self-loosening and leakage are less common. The aerospace industry would probably list fatigue first.”
Repeated relative displacements between the contact surfaces, which are under the influence of the shank torque and result from the thread pitch torque, can lead to a gradual rotation of the bolt or the nut.
“This causes a bolt preload loss and consequently a loss of function in the bolt connection. The effect is well known, but prevention is usually performed experimentally only after the occurrence of self-loosening events,” Dinger said.
An ounce of prevention
To prevent spontaneous bolt loosening, the slip between the joined parts needs to be eliminated or at least reduced to below critical levels. This can be achieved by increasing the axial tension or the friction between the clamped parts or decreasing the cyclic loading (for example, shock, vibration or cyclic thermal loading).
Another common method is to increase the friction between the bolt threads. Although there are several effective ways to do this, some have disadvantages worth considering.
For example, glue or adhesives can be an effective friction-based method, but dried glue can be problematic when dissembling and removing the bolt. Furthermore, increasing the friction between threads would decrease the achievable preload at a specific torque level. Locking wire is a common method in the aviation industry.
Fatigue is permanent damage or deformation in the bolt and clamped parts. It’s caused by loss of preload resulting in an opening in the joint. Spontaneous loosening and slackening are the two basic mechanisms for loss of preload.
Essentially, spontaneous loosening or rotational self-loosening occurs when a bolt rotates loose because of shock, vibration or dynamic loads. Even a slight rotation can be enough for a bolted joint to lose all of its preload. This is the most typical cause of bolt loosening.
Slackening is caused by three mechanisms: settlement, creep or relaxation.
“Settlement is critical when it happens due to dynamic loads. It is the permanent deformation of the clamped material when the joint is subjected to the increase of stress from dynamic working loads,” explained Harlen Seow, technical manager with the Nord-Lock Group. “Most parts of a bolted joint will revert to shape after being released if the stress in the parts has not gone beyond their yield strengths.”
This means some materials in the contact surface, such as paint, will most likely deform permanently.
Creep is a permanent deformation that occurs due to long-term exposure to high levels of stress below the yield strength of the materials in the joint. It’s more severe in high-temperature applications.
Relaxation is when the microstructure in the materials of a joint restructure, converting existing elastic deformation to plastic deformation over a period of time. Unlike settlement or creep, the clamp length does not change, which makes it more difficult to detect.
“One way to measure preload loss is to measure bolt length after a period in operation and compare to the bolt length immediately after tightening,” Seow said. “However, this will not detect relaxation, which makes it more problematic.”
The key to avoiding fatigue is good design, which has grown in importance in recent years because of the increased demands on many bolted joints and increased use of lightweight materials. It’s important to avoid solely focusing on the tensile capacity of bolts while overlooking other parameters — such as elasticity and stiffness, which can also be important.
“Correct joint design is the key to achieving a high strength friction grip connection with a high preload level, and thus a high slip resistance over the entire lifetime,” Dinger said. “Up until now the focus for design engineers has been on the failure with bolts breaking. Other failure mechanisms have become more and more important as performance is increased and the weight of joints is decreased. The mechanisms of preload relaxation and self-loosening are more and more common in lightweight designs.”
Depending on the bolt, the application and the cause of preload loss, there are generally multiple options for designing more optimal bolted joints.
“In cases where there’s thermal loading, the joint can be optimized by choosing materials with equal thermal expansion coefficient for the clamped parts,” Dinger said. “To help minimize settlement and maintain a high preload during operation, you can reduce the roughness between contact surfaces. Measures such as fine hole diameters or toothed surfaces can help minimize relative displacement.”
Overall, achieving the optimal bolted joint involves factoring in multiple variables and design options.
“In general, a good bolted joint is made up of very elastic bolts and very stiff clamp parts, and there are different ways of achieving this,” Seow said. “One way of improving bolt elastically is to have long clamp length. But if you have a flange, where the clamp length can’t be too long, you can change the design by using more but smaller bolts. So instead of using five bolts, you can use ten smaller bolts, which will create a more elastic joint.”