What is a Blind Bolt?

When you need a structural fastener that delivers more strength and adaptability than a typical rivet or weld, there is nothing like a blind bolt.

Blind bolts have been used for manufacturing and repair for decades. Because they come in all sizes and designs, it can be a challenge selecting the right fastener for the job. Choosing the right blind bolt is a matter of assessing the parameters of the application and selecting the right blind bolt to deliver maximum strength. You want to ensure that the blind bolt is easy to install, yet will be strong enough to create a lasting join.

Blind bolts were developed to deal with an age-old problem: forging a strong connection where conventional rivets or hex bolts are hard to fit or just can’t do the job. Blind bolt fasteners are incredibly versatile, largely because they can be installed from one side through pre-drilled holes without having to access the other side of your work to complete the lock, hence the name blind bolt. They are ideal for applications such as connecting box sections or even girder cavities, because they are quick and easy to install and provide superior strength.

The typical blind bolt consists of a steel pin, collar, and sleeve, and is installed using a blind bolt tool that functions in a similar fashion to a riveter. The blind bolt is inserted through the pre-drilled hole and the pressure on the collar creates a solid connection as the expander collapses to hold the blind side firm while the collar creates a seal on the side where the tool is applied. The beauty of the blind bolt is that it’s fast and easy to install, has more strength than a rivet, and is easy to inspect in order to ensure that the bolt has been installed properly.

What Is a Blind Bolt Used For?

A blind bolt is a fastener designed to deliver more strength and longer performance than a rivet or weld. They are called “blind” because they are installed from only one side of the material to be joined, which is very useful for repair and manufacturing applications. These bolts were also designed for installation in tight spaces where conventional rivets or hexagonal nuts would be hard to use.

A typical blind bolt is inserted through a pre-drilled hole. When tightened, it expands on the blind side to form a strong join. The blind bolt itself consists of a steel pin, a collar, and a sleeve. When it’s tightened, the collar creates pressure that collapses the expander on the blind side, and the sleeve collapses to create a seal on the visible side. They are easy and fast to install and to inspect, and they create a strong and lasting connection. That’s why they are popular for applications such as construction to hold steel girders together.

When you choose the right blind bolt, you must match the size and strength of the bolt to the job.

Vibration Resistance and Added Strength

Blind bolts are becoming increasingly popular in applications where fastener strength and resistance to vibration are essential. That’s why they are used for airplane construction, automotive manufacturing, shipbuilding, mining equipment, construction, materials handling—any application that requires a reliable bond and longevity, even with the added stress of continuous vibrations.

The strength of the blind bolt is determined by the length of the fastener. The grip length of the bolt should be the distance from the underhead-bearing surface to the first thread of the shaft. The grip itself is the total thickness of the material that is joined using the blind bolt. The grip length should be a few thousands of an inch less than the actual grip. To determine the right bolt size, you can use special grip gauges that are inserted into the hole. Note that every blind bolt grip gauge is different and they are not interchangeable, so be sure to use a grip gauge that matches the blind bolt manufacturer.

Another one of the appeals of using blind bolts is they are quite tamper-proof, which makes them ideal for applications such as mass transit, public buildings, or anywhere vandalism could be a problem. The hardness of the core bolt makes them difficult to remove, although fastener manufacturers do provide special removal kits. Using the proper removal kit makes it possible to extract the bolt without damaging the hole or the parent structure.

Choosing the Right Blind Bolt

When you are determining which blind bolt is best for the job, there are various factors to consider, such as:

1. Strength: The strength of the blind bolt relates to the length of the fastener. The grip is the total thickness of the materials being joined by the fastener. The grip length of the bolt should be the distance from the underhead surface to the first thread of the pin shaft and should be a fraction less than the actual grip. Grip gauges are inserted into the pre-drilled hole to determine the right bolt size, but gauges are unique to each manufacturer.
2. Tensile strength: Depending on the application, you want to be sure that the blind bolt you choose has sufficient tensile strength and shear resistance for the task. If you choose a bolt that is too narrow or made of the wrong material, it won’t last—even though the initial bond may be extremely strong.
3. Size: Blind bolts come in various thicknesses and lengths. You want to choose a bolt that is large enough to do the job and to deliver maximum strength.
4. Vibration resistance: One of the great strengths of the blind bolt is that it won’t loosen with continual vibration. That’s why they are so popular for heavy-duty applications, such as shipbuilding, bridge construction, creating mining and manufacturing equipment, or any situation in which continuous vibration is a factor. Be sure to use the correct locking bolts that will resist vibration.
5. Corrosion: Blind bolts are made of various materials. For example, many blind fasteners have an alloy steel stem, black oxide sleeve, and zinc-plated carbon steel collar. Other materials are available as well, such as titanium. It pays to choose the right materials for the application. In shipbuilding, for example, or other applications where the join is exposed to a corrosive substance such as sea water, choose materials that are less likely to corrode.

Two Great Blind Bolt Options

Bay Supply offers a variety of blind bolts for different applications. Here are some details on two of our most popular blind bolt products:

Huck: BOM

Bay Supply offers an extensive line of blind oversized mechanical bolts (BOM) from Huck. The Huck BOM comes in a variety of sizes, with steel shafts ranging from 3/16 inches to 5/8 inches in diameter. It is extremely vibration-resistant and has a unique push-and-pull installation design. Anyone can learn to install these fasteners in 10 minutes. The BOM can replace welding and is easy to inspect; if the pin position is correct on the visual side, then it has formed correctly on the blind side.

The BOM is extremely accurate and tough and is used for everything from military vehicles to amusement park rides. These are high tensile-strength fasteners designed for high-speed installation. The Hulk BOM also features a mechanical lock which creates a vibration-resistant joint and eliminates loose stems.

Avdel Avbolt

For heavy-duty fastening applications, the Avdel Avbolt is a structural blind fastener suitable for construction, railway, containers, mining, and even bridge-building. Avbolts offer high tensile and shear strength and are easy to install in tight spaces. They also have a locking feature that makes them highly resistant to vibration, and they are extremely tamper-resistant. Additionally, they have simple tooling, so they don’t require a lot of training to install.

The Avbolt features a stem made of alloy steel and black oxide, and its sleeve and collar are made of carbon steel that is zinc plated with a clear passivated finish. The Avdel Avbolts are also available in diameters range from 3/16 to 5/8 inches.

Working with a Blind Bolt Distributor 

When you work with a specialty engineered fastener distributor, you can receive recommendations from them on the best blind bolt based on your application and materials, along with proper installation tools, if you need them. 

When you work with Bay Supply in particular, you’ll unlock a number of helpful benefits, such as:

• Online account: You’ll be able to easily place new or recurring orders and manage your existing orders without having to pick up the phone.
• Industry discounts: If you are part of the green energy, rail and transport, trucking, or agriculture industries, you’ll receive product discounts exclusive to your industry.
• Quality support and repair center: Have questions about specific bolts or a tool that has broken down? Our experienced team can answer those questions or escalate your tool/part to our factory authorized repair center.
• Free domestic shipping: For any domestic order over $99, you’ll receive free ground shipping.

If you want to learn more about blind bolts and how they can outperform rivets and welds, especially in tough construction settings, give us a call. You can count on Bay Supply to offer a huge inventory of blind bolt products at a great price. Contact one of our fastener experts to discuss your blind bolt needs.

Article was originally published April 2018 and updated June 2020.