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What is a Superbolt?

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  • Written By: Dorothy Distefano
  • Edited By: Michelle Arevalo
  • Last Modified Date: 25 November 2016
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A superbolt is a special type of mechanical fastener that is used to replace hex nuts, bolts, and other threaded fasteners. This type of bolt is normally used with larger nuts and bolts, typically greater than 1 inch (25 cm) in diameter, which require a high amount of force to properly tighten. By using a superbolt, the force required to tighten a large fastener over several smaller bolts is spread, so that simple hand tools may be used. This provides an alternative to using large tools, like hydraulic wrenches and sledgehammers, which are difficult to handle and can pose safety risks.

A superbolt is made up of three primary components. The first is a hardened washer that provides a precision flat surface for smaller bolts to push against. The second is the round-nut body, which is threaded onto the bolt and sits against the washer. The third is a series of smaller bolts, known as jackbolts, which are threaded through the nut body and are tightened. When the jackbolts are tightened, they push against the hardened washer surface and secure the mechanical joint by placing the existing nut or bolt in pure tension.

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Superbolts are normally used for large-scale equipment applications that require very high forces or pressures. A few examples include mechanical presses, where large diameter columns are used to distribute extremely high working loads, and steam turbines, which generate very high internal pressure that must be contained without leakage using metal-to-metal mechanical joints. The force necessary to properly tighten these fasteners becomes very difficult to achieve without large, cumbersome tools that can pose significant safety hazards.

There is a wide variety of superbolts available to meet many different applications. Guidelines are available through superbolt manufacturers to help customers select the correct size and type of superbolt for a specific set of operating conditions. These guidelines include the type of mechanical connection, operating temperature, working loads, and material consideration. Superbolts can also be custom-designed and fabricated to meet unique equipment applications.

Use of a superbolt can provide significant time-savings when compared to other mechanical fastening methods because only hand tools are used and worker safety is improved. Since a superbolt generates pure tension force when tightened, it also eliminates problems with seizing and thread galling that can occur with conventional fasteners. While effective, superbolts are more expensive than standard mechanical fasteners, and the cost-versus-benefit of their use should be evaluated for each specific application.

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