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The function of a bolt is to provide a clamp load or preload (Fp) to sufficiently compress and stress the gasket and resist the parting forces exerted by the hydrostatic end force and other external loads. The hydrostatic end force is created by the pressure of the internal fluid across the internal area of flange. The internal area is generally the inside diameter of the sealing element.

All bolts behave like a heavy spring. As you turn down the nut against the flange, the bolt stretches and the flange and gasket compress. All bolt-tightening methods result in stretching the bolt. The torque-tightening method uses the thread helix of turning the nut against the reactive forces of the flange to stretch the bolt. The hydraulic bolt-tensioning method utilizes an annular piston threaded on the end of the bolt to provide an axial stretch.

There are many types of bolt with different shapes like hex bolt, carriage bolt, square head bolt, stud bolt, ball head bolt and so on.

The bolts always work together with a nut. A nut fastener is a mechanical fastener with a threaded hole. It is attached to a bolt, screw or stud to provide a clamping force and prevent axial movement. For example, a bolt is inserted through two parts, with aligned holes, and a nut is then screwed onto the bolt. The shaft of the bolt prevents radial movement of the parts, while the head of the bolt and the nut together prevent axial movement.

The most common type of nut is the hex nut. This has a hexagonal outer shape for the faces of tools to apply torque when fastening. To provide a secure fastening, this type of nut typically relies on torque being applied to generate an axial force. This axial force causes sufficient friction at the threads so as to prevent the nut from unscrewing. The force may act between the bolt head and the nut, causing compression of the parts being fastened. Alternatively, it may act between a pair of adjacent nuts, allowing parts to be fastened more loosely.

A nut and bolt clamping two parts together.

Thread-locking adhesive may also be applied to the threads of a nut to prevent it from working loose. Special locking nuts may also be used which work with devices such as pins, lockwire and nylon inserts to prevent loosening. Older nuts were often manufactured with a simpler square head and this design is still sometimes used to enable increased torque to be applied using a spanner. Also, the nut has different types as well to cooperate with bolts, such as hex nut, flang nut and square nut.