Blacksmiths and metal-workers usually fasten one piece of metal around another by shrinking the first onto the second. This process, known as a shrink, is applied in attaching various kinds of bands, collars to shafts, in putting tires on locomotive wheels, and in shrinking steel gear rings to steel centers. The piece of metal to be shrunk on must be slightly smaller than the outside diameter of the part to which it is to be applied. It is heated and expanded until it can be slipped into place, so that when it cools it will cling to the part with sufficient strength to hold it in place during service.