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What Is a Flange Bolt?

By K'Lee Banks
Updated May 17, 2024
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A flange bolt differs from the common bolt in that it has a wider surface area under the head to provide for an even distribution of clamping pressure. This wider surface acts like a washer, eliminating the need to combine one with this kind of bolt. Flange bolts are available in numerous sizes, strengths, and configurations. Like most other types of fasteners on the market, metric flange bolts are available for any applications that require them.

Specially designed flange bolts are equipped with anti-rotational “teeth” designed to “bite” or dig into the material one might need to fasten together. Similar to the locking rings one might see on a soda bottle or milk jug, these teeth on the flange bolt can be turned backward, but often require more force. This type of design is what makes a flange bolt uniquely and strategically different from a common bolt.

An essential addition to the flange bolt is the flange nut. While it is similar to an ordinary nut, one side of the flange nut is wider than the other to ensure even pressure distribution. The flange nut and flange bolt work well together, and in some cases may reduce costs by eliminating the need for purchasing separate washers. Proper torque on a flange bolt-nut combination will ensure that the fasteners never become loose by themselves. Only an outside force, such as a mechanic, can loosen these types of flange bolts.

The types of flanges one intends to bolt together do not necessarily require a special type of flange bolt, such as standard or metric. Instead, it is typically the physical size of a flange system, or the specific number of boltholes, that will indicate the required bolt size or grade one should use. The bolt grade indicates its strength, so the higher the grade a bolt has, the stronger it is. For example, high-pressure connections, such as in a steam line, may require grade eight bolts for the utmost reliability and safety.

A flange bolt is suitable with most any flange system. The sizes and grades, or strength ratings, can and will change depending on the application at hand. Sizes are available in standard or Society of Automotive Engineering (SAE) measurements, such as 1/4 inch and 1/2 inch, as well as metric measurements, such as 8mm and 10mm. Designed to hold connections tightly, without slippage or loosening, flange bolts are standard in most industrial applications where secure connections are crucial to production.

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