How to keep bolts from loosening
5 minutes | 24 Aug 2021
What is the reason why bolts and screws keep coming loose? The reasons can vary. Let’s talk about bolts first.
Steel has some elasticity, so in a bolted joint, the bolt stretches a little bit when the nut is tightened. This stretching creates tension, which generates an opposing clamp force. It’s this clamp force that clenches the joint’s two sections together. A loose bolt causes the clamp force to weaken, and that’s when serious consequences follow.
If it happens during production, then costly downtime is at stake. After production, and your application’s performance is at risk or worse, safety becomes a serious threat.
The question is, what are bolt-loosening reasons? Why do screws keep coming loose? Let’s take a closer look.
Vibration loosening of bolts is common and is a form of spontaneous loosening. Under vibration, bolted joints make small movements that cause the joint’s two sections to move repeatedly. These “traverse” movements, as they’re called, also move with the bolt or nut. This creates friction between the bolt and joint threads that hold the joint together. This side sliding is actually the cause of the bolt loosening from the nut’s mating threads and compromising the clamp force. In fact, all it can take for a bolted joint to lose its preload is a minute rotation. This is what happens when we think of a bolt loosening due to vibration.
How to stop bolts from vibrating loose
Consider the mechanical devices below. If you’re wondering how to keep nuts from loosening, the answer is often to try different nuts.
Bolt locking methods include:
1. Nyloc nuts: Provide extra friction to the bolt joint by using a type of nut called a nyloc. As the nyloc is tightened, the nylon insert in the top part of the nut squeezes the threads of the bolt. The radial compressive force as it turns against the thread creates the extra friction to prevent the bolt from loosening.
2. Jam nuts: A nut with a low profile and used as a type of locknut. Its name comes from the fact that it’s “jammed” against a standard nut, which locks the two nuts in place to prevent loosening.
3. Flanged nuts: These have a washer-like base to evenly distribute pressure. Its large head creates more friction between the head and part’s surface. This in turn lowers the surface pressure on bearing areas, reducing deformation and giving it resistance to vibration.
4. Plastic washers: If you’re using plastic nuts, bolts and screws for smaller applications, such as electronics, make sure you use plastic washers. Electronic applications vibrate, and plastic resists vibrations.
5. Spring washers: Also known as split or split lock washers. The spring shape is designed to help maintain the preload. The sharp edges where the split exists push into the mating surface. This creates more friction and reduces the risk of the bolt or nut loosening. Note, only use on small loads – large loads flatten the washer, rendering it useless. For grade 2 fasteners or metric property class 5.8 or lower.
Special glues exist that can be applied to the threads of fasteners, but they become a problem when you need to dissemble and remove the bolt. And then, by increasing the friction between threads with the use of glues, you also decrease the preload that can be achieved at a particular torque level.
How to prevent screws from loosening
In addition to the effects of vibration, screws can loosen due to the lack of adequate friction with its mating surface. How to keep screws from coming loose in wood is a common question, but it’s important to first understand why this happens. Screws wear down and lose their opposite thread pattern. Hence, there is no longer sufficient friction and the screws keep coming loose.
Using longer screws can help in these cases, as they have more threads than short screws. Or, use screws with a thicker diameter.
To stop screws from loosening due to vibration, you can and should use washers. Depending on the application, you can also consider screws designed to stand up to vibrations. If nylon is a good fit for your application, it’s an excellent material for resisting vibrations. The examples below are also lightweight, provide excellent insulation and corrosion resistance, and are good replacements for metal screws. Sometimes, how to keep a screw from loosening comes down to the screw itself.
Tighten loose bolts
It can happen that bolts were never tightened properly to begin with, which means the joint lacks clamp force. This can lead to sections side sliding, which puts unnecessary shear stress on the bolt, which can break it. Hopefully your quality control process will check the quality of tightening after your application has been assembled and any problems will be corrected.
Be aware of temperatures
How to keep a screw from coming loose, or a bolt, involves many factors, one of which is differential thermal expansion. Consider the material of the bolt and the joint. If they’re different, then significant temperature changes – whether rooted in the environment or industrial processes – can cause the fastener to quickly expand and shrink, which can loosen the screw or bolt.
Ensure that you consider temperatures that will affect the fastener – even if it’s temperatures experienced during transportation. Then choose a fastener material that can stand up to whatever comes.
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