_COMPOUND HAMMERS._
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 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?