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.http://www.yuchaobolt.com/bolt/hex-bolt/