The hex bolt is among the most commonly used fasteners, as its basic shape offers a universal application. This bolt is comprised of a 6-sided head, threaded shaft, and is made from a variety of steel grades. Hex bolts are threaded, and need a matching nut size in order to complete operation. These are commonly used in construction and custom auto applications.
The carriage bolt is most commonly used in wood lumber construction for fastening large structural wood members. Carriage bolts are made with a smooth, rounded head, and small cubic section underneath that serves to lock the head in place when tightened. Like the hex bolt, these bolts also have threaded shafts that require the use of a matching threaded nut.
The lag bolt resembles the hex bolt in that it also has a hex head, but the lag bolt is made with a pointed wood thread shaft and is used for fastening wood without the use of a threaded nut. Before these bolts can be used, a pilot hole must be drilled in the wood to prevent splitting.
Eye bolts are unlike other bolts as they are made with circular rings instead of a wrench head; these circular heads are used for attaching rope or chain. The eye bolt is also made with pointed lag threads for use in wood without needing a threaded nut. These bolts are most commonly used for hanging heavy fixtures like lighted ceiling units or storage containers.
The U-bolt is in the shape of a "U" and is threaded on both ends for the attachment of a metal strap and nuts. The purpose of the U-bolt is for securely attaching pipes or other round objects to structural members, and is also available with a square bend instead for use with non-round objects.