Why Do Bolts Loosen and How to Keep Them Tight

Bolts, the humble yet vital parts of our machinery, play a crucial role in binding together various mechanical systems. Despite their significance, the discussion around bolts often falls in the background until the moment they become loose. Understanding why bolts loosen over time is essential, as is knowing how to prevent this unwanted eventuality and ensuring the longevity of our machinery and equipment.

Why Bolts Become Loose Over Time

Bolts can become loose due to several factors, each of which can compromise the stability and safety. Understanding these causes can help prevent bolt failure and ensure long-term reliability.

1. Vibration

One of the most common reasons bolts loosen is vibration. Machinery, vehicles, and other mechanical systems experience constant movement, which creates vibrations that gradually work bolts loose over time.

In high-vibration environments, such as engines, heavy machinery, and construction equipment, bolts must withstand continuous shaking forces that can weaken their grip.

If bolts are not adequately secured using locknuts, washers, or thread-locking compounds, vibration will eventually cause them to come undone.

2. Thermal Expansion and Contraction

Temperature fluctuations cause materials to expand when heated and contract when cooled. Since bolts and the materials they fasten may have different expansion rates, the resulting stress can lead to bolts gradually loosening.

This is especially relevant in industries where equipment operates in extreme temperatures, such as aerospace, automotive, and industrial manufacturing.

3. Improper Torque Application

Applying too much or too little torque when installing bolts is another significant cause of loosening. Over-tightening can strip threads, distort bolt heads, or weaken the material being fastened, ultimately leading to failure.

Under-tightening fails to generate enough clamping force, allowing bolts to shift and loosen over time. Using a torque wrench ensures that the correct amount of force is applied to secure the bolt without damaging it.

4. Joint Settling and Material Deformation

Over time, the materials clamped together by a bolt can compress or deform, reducing the preload force that keeps the bolt tight. This is especially common in wooden structures, plastic fastenings, and soft metal applications, where gradual settling can create gaps, causing bolts to loosen.

5. External Forces and Impact Loads

Machinery that experiences high-impact loads, such as construction equipment, vehicles, or industrial presses, can generate forces that displace bolts. External shocks, sudden forces, and repeated stress cycles can push bolts out of position if they are not adequately secured.

6. Corrosion and Wear

Corrosion weakens bolts and the surfaces they fasten, reducing their holding strength. Rust buildup can also prevent bolts from properly engaging with their threads, making them more likely to come loose over time. Regular maintenance and the use of corrosion-resistant coatings can help prevent this issue.

How To Keep Bolts Tight

The good news? There are several effective strategies to keep bolts from loosening. By implementing these methods, you can maintain equipment stability and prevent costly failures.

1. Use Washers to Distribute Load

Washers are essential for spreading the load over a larger surface area, reducing pressure points that could lead to loosening. Flat washers help minimize stress on the bolted joint, while lock washers create tension that helps bolts stay secure. Using spring washers, split washers, or Nord-Lock washers can enhance resistance against loosening due to vibration.

2. Secure Bolts with Locknuts

Locknuts have a special design that helps resist loosening caused by vibrations or torque forces. Common types include nylon-insert locknuts, which use a plastic ring to grip the bolt, and metal locknuts, which create additional friction to prevent movement. Using locknuts is a simple but highly effective way to maintain bolt tightness.

3. Apply Thread-Locking Fluid (Threadlocker)

Threadlocker is an adhesive specifically designed to prevent bolts from loosening. When applied to the threads before installation, the liquid hardens and bonds the bolt to the material, securing it in place. Thread-locking fluid is available in different strengths: 

• Low strength (removable by hand tools) – Ideal for small screws.
• Medium strength (requires some force to remove) – Suitable for most bolts.
• High strength (permanent locking) – Used for critical applications.

4. Ensure Proper Torque Application

Correct torque application is critical in keeping bolts tight without damaging them. Using a torque wrench allows for precise tightening, ensuring the bolt has sufficient tension without being over-tightened. Always follow manufacturer torque specifications to avoid overloading or under-securing the bolts.

5. Use Double Nuts for Extra Security

Double nutting is a method where a secondary nut is installed on top of the primary nut to prevent loosening. The second nut is tightened against the first, creating a locking effect that holds both in place. This is especially useful for high-vibration applications like machinery and construction.

6. Consider Safety Wire or Cotter Pins

In high-risk applications, safety wire or cotter pins can be used to physically lock bolts in place. These are commonly used in aerospace, automotive racing, and heavy machinery where absolute security is required. The wire or pin prevents the bolt from rotating, ensuring it stays in place even under extreme conditions.

7. Regular Maintenance and Inspections

Regularly inspect bolts for signs of wear, corrosion, or loosening. Check high-vibration areas frequently and re-tighten bolts as necessary. Preventative maintenance helps identify potential failures before they lead to costly downtime or accidents.

Browse Our Bolts and Fasteners

Bolts are widely preferred in numerous industries due to their convenient dismantling feature. However, this very quality also exposes them to the risk of self-loosening and loss of preload. 

Loose bolts can damage expensive equipment and there are also safety concerns with bolts that keep coming loose. If you’re in need of quality bolts and caps to fasten and secure anything from wood to metal to plastic, ASMC Industrial has everything you need.

At ASMC Industrial offers, we offer a wide variety of wholesale bulk bolts and screws , a huge inventory of wholesale nuts and bolts, and different sizes and styles of wholesale washers . We have over 100,000 products in our inventory and our online store is stocked with every type of bolt or screw you’ll need. 

Our extensive range of products eliminates the hassle of finding those hard-to-locate sizes and makes your work more efficient. When you buy wholesale bulk screws and bolts from ASMC Industrial, you not only get a high-quality product at an affordable price but also experience outstanding customer service.