another principle to be noticed in connection with hammers and forging processes is that of the inertia of the piece operated upon—a matter of no little importance in the heavier kinds of work.
when a piece is placed on an anvil, and struck on the top side with a certain force, the bottom or anvil side of the piece does not receive an equal force. a share of the blow is absorbed by the inertia of the piece struck, and the effect on the bottom side is, theoretically, as the force of the blow, less the cushioning effect and the inertia of the pieces acted upon.
in practice this difference of effect on the top and bottom, or between the anvil and hammer sides of a piece, is much greater than would be supposed. the yielding of the soft metal on the top cushions the blow and protects the under side from the force. the effect produced by a blow struck upon hot iron cannot be estimated by the force of the blow; it requires, to use a technical term, a certain amount of force to "start" the iron, and anything less than this force has but little effect in moving the particles and changing the form of a piece.
from this it may be seen that there must occur a great loss of power in operating on large pieces, for whatever force is absorbed by inertia has no effect on the underside. by watching a smith using a hand hammer it will be seen that whenever a piece operated upon is heavier than the hammer employed, but little if any effect is produced on the anvil or bottom surface, nor is this loss of effect the only one. the expense of heating, which generally exceeds that of shaping forgings, is directly as the amount of shaping that may be done at each heat; and consequently, if the two sides of a piece, instead of one, can be equally acted upon, one-half the heating will be saved.
another object gained by equal action on both sides of large pieces is the quality of the forgings produced, which is generally improved by the rapidity of the shaping processes, and injured by too frequent heating.
the loss of effect by the inertia of the pieces acted upon increases with the weight of the work; not only the loss of power, but also the expense of heating increases with the size of the pieces. there is, however, such a difference in the mechanical conditions between light and heavy forging that for any but a heavy class of work there would be more lost than gained in attempting to operate on both sides of pieces at the same time.
to attain a double effect, and avoid the loss pointed out, mr ramsbottom designed what may be called compound hammers, consisting of two independent heads or rams moving in opposite directions, and acting simultaneously upon pieces held between them.
it would be inferred that the arrangement of these double acting hammers must necessarily be complicated and expensive, but the contrary is the fact. the rams are simply two masses of iron mounted on wheels that run on ways, like a truck, and the impact of the hammers, so far as not absorbed in the work, is [114] neutralised by each other. no shock or jar is communicated to framing or foundations as in the case of single acting hammers that have fixed anvils. the same rule applies in the back stroke of the hammers as the links which move them are connected together at the centre, where the power is applied at right angles to the line of the hammer movement. the links connecting the two hammers constitute, in effect, a toggle joint, the steam piston being attached where they meet in the centre.
the steam cylinder which moves the hammers is set in the earth at some depth below the plane upon which they move, and even when the heaviest work is done there is no perceptible jar when one is standing near the hammers, as there always is with those which have a vertical movement and are single acting.
(1.) why is the effect produced different on the top and bottom of a piece when struck by a hammer?—(2.) why does not a compound hammer create jar and concussion?—(3.) what would be a mechanical difficulty in presenting the material to such hammers?—(4.) which is most important, speed or weight, in the effect produced on the under side of pieces, when struck by single acting hammers?
