Why bolts get loose?

Joints like racks and mudguards can remain maintenance-free even after running for ten thousand plus miles, yet there are certain bikes, which require frequent tightening of bolts. So, in this edition, we are going to discuss why do some rivets stay tight while other bolts get loosen so fast?

Well, if you think bit deeper then we can realize the concept behind this is quite simple. Frequent vibrations of the bike’s parts enable the nuts and bolts to loosen. (At the end of this post, we’ve attached a video which shows this in action).

The more the parts vibrate, the faster the bolts get detached. A major advantage of using a bolted joint other than riveted/welded joints is that it can be easily dismantled. However, this advantage can even cause trouble at times. Such accidental loosening, which is also known as vibrational loosening has been widely mistaken by the engineers. It is always important for the manufacturers to be alert about this mechanism.

Apart from the known factor that bolt detaching is caused due to vibration, we should also consider that the actual reason is side sliding of the fastener head relative to the joint, thereby resulting in relative motion in the threads. If this phenomenon does not occur then no matter how often the joint is subjected to severe vibration, loosening can be avoided.

Sometimes, burnout also results in self-detaching, thereby reducing the clamp force implied on the joint. Eventually, joint slips occur, which further leads the rivets to bending loads. The nuts will then rotate loose, as soon as the relative motion starts occurring between the male and the female threads. This motion then sooner or later breaks off the friction grip and instead, creates an off-torque. This off-torque makes the fastener get slowly detached.

The key to preventing self-loosening of bolts is by ensuring that

1. There is sufficient clamp force available on the joint interface, so as to reduce relative motion between the bolt head and the joint.
2. The joint should be designed as such to enable proper embedding and stress relaxation.
3. You’ve proper thread locking devices, such as flanged fasteners or torque prevailing fasteners.
4. The loosening of bolts is just one plain aspect of the bolted joint design, which the manufacturer should consider during the design process. And even if threads are firmly hold together by adhesive, this kind of situation cannot be avoided, if the bolt preload is inefficient in preventing the joint movement.

Here’s how to avoid this:

1. Do not use the same bolt to fasten several things.
2. The more ‘layers’ you have, the more motion occurs, thus the bolt gets detached quickly. This is common in the case of the front racks, which are attached to the cantilever brakes.
3. Always remember than even simple washers can increase the risk of fasteners getting disconnected. Don’t go for spacers either, as they are worse.
4. All the joints should be designed as such that rivets won’t be able to rotate. For instance, if the rack below is attached to the fork crown along-with straight tubes, which are alienated to the vibrations of the rack then in such a case, due to the curving of the tubes, each vibration would lead the bolt to loose gradually.

Applying state-of-the-art analytical method to stop vibrational loosening of threaded fasteners can be difficult. However, these days technology has much developed and programmers have come up with designs, which help engineers fix such issues.