Why do bolts loosen and what can you do about it?
Knowing how to prevent bolt loosening is critical to ensuring a secure bolt joint. But first, you need to understand why it can happen. This guide will explain the reasons, along with how you can prevent bolts from loosening. We’ll cover:
What are the causes of loose bolts?
What is spontaneous loosening?
What is bolt slackening?
Devices to prevent loose bolts
How to stop bolts from vibrating loose
How to prevent screws from loosening
Vibration-resistant screws
What are the causes of loose bolts
Steel has some elasticity, so in bolted joints, the bolt stretches a little bit when the nut is tightened. This stretching creates axial tension, which generates an opposing clamp force. It’s this clamp force that clenches the joint’s two sections together.
Why is loosening prevention important for a bolted connection? 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. Post production, and your application’s performance is at risk or worse, safety becomes a serious threat.
The question is, why do bolts loosen in the first place? Why do screws keep coming loose? There are two primary reasons for bolt loosening.
● Spontaneous loosening
● Slackening
What is spontaneous loosening
Spontaneous loosening, also known as self-loosening or rotational self-loosening, refers to bolts that loosen over time. You'll need to take special care to prevent spontaneous bolt loosening. There are several reasons this can happen.
Shock
Shock is a sudden impact or jolt for a short period of time. For example, a machine subjected to a sudden stop can experience shock, causing instantaneous movement in joints, which can lead to loosening bolts due to loss of tension. The shock that causes bolts coming loose can create a dynamic loading condition that surpasses the static clamping force provided by the bolt.
Dynamic load
Dynamic loads cause fluctuating stress in bolts and can be created by any number of events, such as impact, vibration or motion.
Shock and dynamic load are related. While shock refers to an intense and very sudden event, dynamic load is a fluctuating load that over time, varies in magnitude or causes a change in direction with the result being a loosening bolt.
Vibration
Of all bolt-loosening reasons, vibration is perhaps the most common. Vibration causes bolted joints to make small, repeated “transverse” movements, meaning parts bolted move back and forth or side to side. These movements create friction between the bolt and joint threads that hold the joint together. The side sliding is actually what makes the bolt loose. The bolt separates from the nut’s mating threads, which in turn compromises the clamp force.
Just a minute rotation is enough for bolted joints to lose their preload. Preload is the required tightening load, when torque is applied to the bolt or nut with a wrench.
What is bolt slackening
Slackening also leads to the loss of preload, which causes bolt slipping. Slackening generally falls into three categories.
Settlement
This has to do with irregularities on the clamping surface of the materials that you’re bolting together. Preloading creates pressure on the surface, which causes the irregularities to flatten. Once the irregularities flatten, the joint loses its tightness, which leads to the preload diminishing. If the preload decreases enough, the bolt rotates loose.
Dynamic loads can also cause settlement. When the joint is subjected to high levels of stress from dynamic loads, the result is sometimes permanent deformation of the clamped material, versus existing elastic deformation, which is temporary.
Creep
Creep happens when the materials of a joint move slowly or deform permanently under constant high levels of mechanical stress below the material’s yield strength.The creep is more pronounced in high-temperature applications, where materials endure heat for longer periods and close to their recrystallization temperature. As the ambient temperature increases, so does creep, which can result in the bolt thread loosening due to reduced tension.
Creep, however, is normally not a problem until the application reaches operating temperatures greater than 250°C.
Relaxation
Creep and stress relaxation are similar and often used interchangeably. While creep refers to the slow and permanent deformation of a material under a constant load, relaxation, on the other hand, refers to the gradual loss of bolt tension over time, even if the load on the bolt remains constant.
While both creep and relaxation can cause bolts to loosen, they differ in the way they reduce the tension in the bolt. Creep causes permanent deformation of the material, which reduces the tension. Relaxation does not involve permanent deformation and may be reversible under certain conditions.
Relaxation can occur due to the settling or compression of the joint material around the bolt, which can reduce the clamping force and cause loose nuts and bolts.
How to stop bolts from loosening with devices
Consider the mechanical devices below to achieve optimal bolted joints. If you’re wondering how to stop nuts from loosening, the answer is often to try different nuts. Bolt locking methods include:
Washers
Some washers can stop bolts from loosening by providing additional friction or help distribute the load over a wider area, which can increase the clamping force between the bolt and the surface being fastened.
Spring washers: Also known as split or split lock washers. The purpose of spring washers is 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.
Toothed lock washers – external
Locks bolts, nuts and screws in place to prevent loosening. Also prevents loosening of PCB spacers caused by vibrations. Often known as an electrical contact washer and has tooth-like serrations on the external diameter that stop loosening through the use of friction.
You can learn more in our Complete guide to washers.
Nuts
How to stop a bolt from loosening involves the use of nuts. They do this by creating a threaded joint subjected to a compressive force that maintains the clamping force between the two surfaces being fastened together. The nut is threaded onto the bolt and tightened against the surface being fastened, creating tension and compression that helps keep the bolt in place.
Nyloc nuts: Provide extra friction to bolted connections by using a type of nut called a nyloc. As the nyloc nut is tightened, the nylon insert in the top part of the nut squeezes the bolt threads. A radial compressive force forms as it turns against the thread, creating the extra friction to keep bolts from loosening.
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.
Wide flange on one side acts as an integrated washer. The washer element helps to evenly distribute the pressure of the nut over the application being secured, thus mitigating the likelihood of damage or the chance of loosening, as a result of an uneven surface.
Special glues exist that can be applied to the threads of fasteners, but they become a problem when you need to disassemble and remove the bolt. Another issue: 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.
Learn more in our guide, What are the different types of nuts and bolts?
How to stop bolts from vibrating loose
A bolt loosening due to vibration is incredibly common. If you’re using plastic nuts, bolts and screws for smaller applications, such as electronics, make sure you use plastic or rubber washers. Electronic applications vibrate, and these materials are excellent at resisting vibration. For example:
Excellent anti-vibration loosening and shock absorbing properties. These durable washers also help to seal fluids and other liquid connections while offering superior insulation, particularly within electronic assemblies. Often used by manufacturers for spacing, buffing, sealing and for locking purposes.
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 screw keeps 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, such as:
Designed to lock in place after installation, creating a secure fastening assembly. Easily slides on to the screw, where the inside diameter grips the outside of the screw shank. Excellent sealing capabilities, chemical and corrosion resistance.
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.
Vibration-resistant screws
Sometimes, how to prevent loosening screws comes down to the screw itself. These designs are helpful for keeping screws tight:
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Used for making secure mechanical connections between mating components
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Socket enables driving for wrenches or sockets in limited spaces |
Available in slotted, Torx and crossed (Phillips) styles |
Head offers extra grip for tightening and loosening |
You can learn more in our guide, What are screw heads, threads and drives?
Tighten 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. In turn, that can lead to bolts breaking. Ideally, your quality control process will check the quality of the tightening process after your application has been assembled and any problems will be corrected.
Be aware of temperatures
How to keep a screw from loosening, 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.
Download free CADs and try before you buy
Free CADs are available for most solutions, which you can download. You can also request free samples to ensure the solutions you’ve chosen are exactly what you need. If you’re not quite sure which product will work best for your application, our experts are always happy to advise you.
You should also check out our full range of anti-vibration components.
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Questions?
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