An ordinary form of steam hammer is shown in Fig. 154. Its essential parts are an inverted steam cylinder, to whose piston rod the hammer head is attached, and the frame for carrying the whole. The hammer is raised by admitting steam beneath the piston. The blow is dealt by exhausting the steam from beneath the piston and admitting above. The head is thus accelerated by gravity and by the pressure of steam above the piston. The valve gear is so arranged that the intensity of the blow may be varied by changing the amount of steam admitted to the piston on its downward stroke. The steam admitted below on the same stroke forms a cushion for the absorption of the momentum of the head. In this way the lightest of taps and the heaviest of blows can be delivered by the same hammer. These hammers are also made in a great variety of sizes. Steam hammers are rated by the weight of the falling parts, i.e., the piston rod, ram or head, and hammer die. A hammer in which these parts weigh 400 pounds would be called a 400-pound hammer. Steam hammers are made in two distinct parts: the frame, carrying the hammer or ram, and the anvil, on which the hammer strikes. The frame is carried on a heavy foundation, and the heavy anvil, which is generally made of cast iron and fitted with a die block of tool steel, rests upon a heavier foundation of timber or masonry capped with a timber. The object of these separate foundations is to allow the anvil to give slightly under a blow without disturbing the frame. On very light power hammers the anvil and frame are sometimes made together. Hammer Dies. The dies, as most commonly used with a steam hammer, have flat faces. The beat ones are made of tool steel. These dies may be made of tool steel and left unhardened, then when the dies become battered out of shape from use, they may be trued up and refaced without going to the trouble of annealing and hardening. Dies of gray cast iron and cast iron with a chilled face are also quite commonly used. Ordinary gray cast iron is used, particularly when special shaped dies are employed for welding and light bending.
Fig. 155. Tongs for Heavy Work.
Tongs for steam-hammer work should always be fitted carefully, and should grip the stock firmly on at least three sides. A quite common shape for tongs for heavy work is shown in Fig. 155. To hold the tongs securely on the work and to make it easier to handle them, a link is sometimes used of the shape shown. This is driven firmly over the handles of the tongs and the projecting ends are used as handles for turning the work.