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What Is a Bolted Joint?

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  • Written By: M. McGee
  • Edited By: Lauren Fritsky
  • Last Modified Date: 30 October 2014
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A bolted joint is a type of threaded fastener that connects separate pieces. These joints often look like a standard screw hole, although the hole will extend through two or more pieces. When fully tightened, the bolted joint can withstand huge amounts of pressure and torque without damage. This is a result of the forces at work between the bolt and connected surfaces.

The primary purpose of a bolted joint is connecting objects. This is typically accomplished by using a threaded hole that sinks through one object and into the next. A bolt then screws down into the hole and connects the pieces together. This connection is exceptionally strong, much more so than other common forms of connection.

The strength of this connection primarily stems from the threads in the hole and on the bolt. These threads vastly increase the surface area between the bolt and connected object. Since the connected surface area is very high, it increases the amount of friction between the two objects. This means it requires more power to overcome the friction and make the bolt move.

Since the threads are along the shaft of the bolt and the hole, they stack on top of one another. This allows for a huge increase in surface area with a very minor use of space. Since so little space is used for such a large gain, it is possible to spread bolted joints out over a larger distance.

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The biggest problem common in bolted joints comes from vibration. When a bolted joint vibrates, the two materials and the bolt will often vibrate at slightly different frequencies. This will cause the bolt to move very slightly with every vibration. This movement is so slight that it is generally invisible to the naked eye and is only noticeable over time. As the vibration continues, the bolt will slowly loosen until the connection becomes unstable.

The most common method of dealing with this problem is using a washer underneath the head of the bolt and any nuts on the fastener. Washers will help absorb a small amount of the vibrational movement and keep the fastener tight longer. In addition, a washer will provide a solid surface for the bolt or nut to sit on, allowing a tight connection in contoured or soft surfaces.

A bolted joint works best when it is designed with the forces placed upon it in mind. Certain bolt types and styles work best in certain circumstances. If the fastener has less torque on it than it was designed for, the bolt will loosen over time. If it has more than it was made for, the additional torque may cause a shearing force and break the bolt.

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