Steam hammers are single acting when the hammer is raised by steam and falls by gravity alone; or are double acting when the hammer is raised and forced down by steam.

The smaller hammers have single frames and the larger hammers, as shown in Fig. 148, have double frames. This view shows a 2500-lb. hammer, i. e., one whose hammer and moving parts attached thereto weigh 2500 lbs.

The hammer H is raised and forced down by steam acting on a piston in the cylinder C. One of the levers at the side governs the steam supply to the hammer, and the other controls the rapidity and force of the blows. The hammer oscillations synchronize with the controlling lever movements, and an experienced man or boy can regulate the force of the Wow with great nicety. The dies DD are keyed to the hammer and to the anvil cap A.