Why Do Bolts Loosen?

Bolts are one of the most commonly used fasteners in different industries.

Despite their frequency of use though their design is not as perfect as one might think. While bolts are certainly capable of fastening, they also often suffer from self-loosening. The reasons why bolts loosen vary but the reality of their loosening is that any construction that they were a part of could become dismantled.

Bolt loosening is an important safety hazard.

Different applications can suffer different consequences as a result of bolts loosening the effects of bolts loosening can be so profound that it could lead to machinery breaking down, and bringing production plants to a standstill. With bolts loosening being an important safety hazard, it only makes sense for one to understand why bolts loosen and potentially also employ some solutions to reduce the chance of them loosening.

Bolt loosening could be caused by a variety of different reasons. Depending on your industry, you might find some of these causes of bolt loosening to be more prevalent or relevant. Regardless of frequency though they are incredibly important concerns for one to consider.

Vibration is one of the most common reasons that self-loosening take place.

Even small vibrations can at times be enough to slowly and over time cause a bolt to self-loosen. This is because small vibrations slowly cause the bolt to start rotating gradually in the direction of the torque. In time this will mean that the bolt connection is going to lose its effectiveness and even self-loosen. This is often referred to as spontaneous bolt loosening as it is caused by shock and vibration.

Slackening is another common reason for bolts loosening as it leads to loss of preload.

Slackening is caused by three mechanisms, namely settlement, creep, and relaxation. This non-rotational loosening of bolts results in a loss of tension that is usually not visually discernible by someone just looking at the bolt. What is important with Slackening is that it usually also entails the permanent deformation of the clamped material due to the increased stress that the joint experiences. In general, bolted joints will revert to their original shape once the stress is gone, but depending on the material that might not always be the case and instead, the bolts might be permanently deformed.

What is the settlement, creep, and relaxation? EMAIL: SALES@AYAFASTENERS.COM

 

Settlements have to do with the surface roughness and irregularities of the material you are using. The pressure applied on the clamped surfaces after the pre-loading could lead to deformation or irregularities being flattened. The result of that flattening is that the preload will decrease leaving the bolt looser than it had originally been.
Creep refers to the permanent deformation that occurs on the joint after long-term exposure to high-stress levels. The strength of the materials used for the joint could be important, especially in applications where the material will need to sustain high temperatures. To find more information about high-quality bolts, AYA FASTENERS is one of the leading suppliers. Ensuring the quality of the materials you are using can be the key to eliminating the chances of bolts loosening due to non-rotational loosening or slackening.
Finally, relaxation refers to a change in the microstructure of the materials of a joint from elastic deformation to plastic deformation. For this to occur there usually needs to be an extended period of time that has passed.
To measure the preload loss caused by this you can measure the original bolt length after the joint was originally tightened and then remeasure it over time. If you find that there is a change in length then you might be able to determine whether there is settlement or creep, however, this method will be unsuccessful in determining whether there is relaxation.

 

Preventing spontaneous bolt loosening means that the slip between the two parts will need to be minimized as much as possible. There are a few different ways in which this can be achieved.

Proper joint design can be the key to increasing the friction between the bolt and the nut that you are using. For the most part, design engineers focus their attention on reducing the chances of bolts breaking, however that is not the only important mechanism that they need to look at. In a lightweight design, the mechanism of preload relaxation also needs to be examined.
In terms of avoiding fatigue, in recent years the use of lightweight materials in joints has become more and more prevalent. However, just because having lightweight material has become more important, it does not mean that there has been a reduction in the need for elasticity and stiffness. As such, many different parameters can be important for ensuring the proper quality and design of the joints, which can be key for avoiding fatigue. Generally, a good bolted joint will require good bolt elasticity and long clamp length. The combination of these can be the key to stronger designs.
When dealing with an elastic joint you might also want to consider what length is available for your clamps and alter your design based on the availability of the bolts you need. Finding high-quality bolts, however, does not need to be a tough process. Instead, you can easily choose AYA FASTENERS for greater security and quality.