6 Reasons Why Bolts Become Loose (And 5 Prevention Methods)

Bolts are one of the most popular types of threaded fasteners, used to secure materials together, usually in combination with a Nut, to form a bolted joint.

Given their versatility in being easily installed, adjusted or even removed as required, they are suitable for a wide range of applications across multiple industries, including Automotive, Aerospace and Manufacturing.

As a non permanent fastening, Bolts are also susceptible to loss of preload and self loosening, particularly when subjected to harsh vibrations. In this article, we take a look at some of the main reasons why bolts and other threaded fasteners become loose and what you can do to prevent it.

Before we look at some of the best methods of prevention, including those backed by Junker tests and industry leaders, it is important to understand the cause(s) of the problem.

Causes Of Bolt Loosening

Bolts and other fasteners can become loose for a variety of reasons, including shock, vibration and dynamic loads. If a bolted application is poorly designed or not installed correctly, it is far more likely to become loose. 

1) Vibration

Fasteners subjected to vibration are loosened gradually due to the small relative movements and load changes leading to short-term frictionless situations where the nut or bolt unwinds.

2) Thermal Cycling

Metals expand when heated and shrink when they cool. This can lead to loosening of a bolted joint as the preload changes with the bolt length. This can cause a joint to loosen. This can be used to your advantage, in the case of heating up a stuck fastener to loosen it.

3) Over-tightening 

It is important to adhere to torque specifications during installation as over-tightening nuts and bolts can damage the threads, negatively affecting the performance of the fastener. 

4) Under-tightening 

The bolt must be sufficiently tightened in order to be effective. This is because the applied preload transfers through the fastener to act as a clamping force. If there is insufficient preload on the bolt, the fastener will rotate and work loose due to the lack of clamping force. Accu has a range of Washers which can be used to increase preloading, for example Spring Washers & Serrated Locking Washers.

In tension applications, a bolt that is not sufficiently tightened can cause one of the fastened surfaces to slip, which puts shear stress on the bolt, possibly causing it to break.

5) Damaged Components

If the fastener has been damaged, this will impair its performance. This could be from installation error, reusing old fasteners, or even a manufacturing defect. Any damage may cause a false torque reading, causing the fastener to be looser or tighter than desired, leading to the problems previously mentioned. 

6) Poor Installation Conditions

Dirt, grease or lubricants can affect the capability of a fastener to maintain preload in a joint. Lubrication in a bolted joint can lead to over torquing and elongation or failure of the fastener. Lubrication reduces the torque for a given clamping load, meaning much more stress can be put through the fastener for a given torque, increasing the risk of failure.

5 Tips To Prevent Your Fasteners From Working Loose

The combination of good design, careful installation and using proven preventative methods helps to make a bolt more resistant to loosening, especially in regards to the factors above. Accu has an extensive range of preventative products which are used by leading names in engineering. The following 6 tips will help you to protect your assemblies from loosening.

1) Use Locking Nuts As Part Of The Assembly

Locking Nuts are specifically designed to provide protection against self loosening. Nylon Locking Nuts and All Metal Locking Nuts are two of the most popular examples for low cost anti-vibration resistance, with a premium option available in Aerotight Nuts offering very effective resistance to loosening once installed. 

See our Locking Nuts article to learn more about the differences between various types of Locking Nuts. 

2) Thread Locking Patches Provide Friction And Protection

AccuLock is a specialist thread locking compound by Accu that can be added to the threads of Stainless Steel Screws and Bolts to help protect the fasteners from corrosion, galling and fatigue. 

AccuLock Thread Locking Patches are a popular method of maximising the efficiency of threaded assemblies. The threadlocker adhesive chemically bonds materials together to increase friction which helps to resist loosening and vibration. It is a common choice in the Aerospace industry to avert fastener fatigue and in the Automotive industry to help prevent self-loosening and corrosion. 

AccuLock patches can be added to virtually any fastener and can be used in conjunction with mechanical options such as Anti-Vibration Washers and Locking Nuts. 

3) Nord-Lock Washers Are Proven To Resist Vibration And Backed By Junker Tests

Specialist Anti-vibration devices such as Nord-Lock Washers are used to resist vibration and dynamic loads. These washers are used in pairs and feature interlocking cams that provide a unique wedge locking system which secures critical fasteners subjected to extreme vibration or dynamic loads.

Unlike traditional anti-vibration washers, which rely on friction to prevent bolts from loosening, Nord-Lock Washers secure the mating components together using tension.

This tension is achieved by the interlocking cams on the inside of the pair of washers attaching to one another, creating a cam inclination of a greater angle than that of the bolt’s thread pitch. As a result of this setup, the tension creates a wedge effect under the head of the bolt, which prevents the nut from rotating loose.

Nord-Lock Washers can be used with screws and bolts up to high tensile grades, such as our Grade 12.9 Cap Head Screws. 

To prove the effectiveness of this specialist wedge locking washer as a method of preventing bolts from working loose, Nord-Lock Group conducted the Junker test, according to DIN 65151, which can be seen in the video below.

4) Use Fasteners With Serrated Heads When Washers Are Unsuitable

Serrated Flanged Hexagon Bolts and Serrated Socket Cap Head Screws have underhead serrations which bite into the surface to increase friction. These fasteners help to provide added grip to the surface of the assembly and reduce the risk of loosening. Serrated Bolts and Screws are often used in applications where washers are unsuitable.

5) Use Mechanical Locking Methods

Mechanical fastener locking provides assurance against loosening, by placing a mechanical barrier in the way of a fastener becoming undone.

In the case of a drilled fastener, such as Capstan Screws, lock wire can be used to ensure the fastener is kept tight and cannot be removed unintentionally. This is common in motorsports applications.

In Automotive suspension applications, a Split Pin is often used in conjunction with a Castle Nut to ensure critical assemblies do not work loose under vibration. 

Another option is Tab Washers, which can be used alongside Belleville Washers, a type of conical spring washer designed to act as a spring for preload, to effectively stop Bolts from turning loose.

Causes Of Vibration & Loosening

For further information on why vibrational loosening is an essential consideration for structures and assemblies, please see our article on Fasteners and Vibration.