_TRIP-HAMMERS._
Trip-hammers employed in forging bear a close analogy to, and were
no doubt first suggested by, hand-hammers. Being the oldest of
power-forging machines, and extensively employed, it will be proper
to notice trip-hammers before steam-hammers.
As remarked in the case of other machines treated of, there is no
use of describing the mechanism of trip-hammers; it is presumed
that every engineer apprentice has seen trip-hammers, or can do so;
and the plan here is to deal especially with what he cannot see,
and would not be likely to learn by casual observation.
One of the peculiarities of trip-hammers as machines is the
mechanical difficulties in connecting them with the driving power,
especially in cases where there are a number of hammers to be
driven from one shaft.
The sudden and varied resistance to line shafts tends to loosen
couplings, destroy gearing, and produce sudden strains that are
unknown in other cases; and shafting arranged with the usual
proportions for transmitting power will soon fail if applied to
driving trip-hammers. Rigid connections or metal attachments ace
impracticable, and a slipping belt arranged so as to have the
tension varied at will is the usual and almost the only successful
means of transmitting power to hammers. The motion of trip-hammers
is a curious problem; a head and die weighing, together with the
irons for attaching them, one hundred pounds, will, with a helve
eight feet long, strike from two to three hundred blows a minute.
This speed exceeds anything that could be attained by a direct
reciprocal motion given to the hammer-head by a crank, and far
exceeds any rate of speed that would be assumed from theoretical
inference. The hammer-helve being of wood, is elastic, and acts
like a vibrating spring, its vibrations keeping in unison with the
speed of the tripping points. The whole machine, in fact, must be
constructed upon a principle of elasticity throughout, and in this
regard stands as an exception to almost every other known machine.
The framing for supporting the trunnions, which one without
experience would suppose should be very rigid and solid, is found
to answer best when composed of timber, and still better when this
timber is laid up in a manner that allows the structure to spring
and yield. Starting at the dies, and following back through the
details of a trip-hammer to the driving power, the apprentice may
note how many parts contribute to this principle of elasticity:
First--the wooden helve, both in front of and behind the trunnion;
next--the trunnion bar, which is usually a flat section mounted
on pivot points; third--the elasticity of the framing called the
'husk,' and finally the frictional belt. This will convey an idea
of the elasticity required in connecting the hammer-head with the
driving power, a matter to be borne in mind, as it will be again
referred to.
Another peculiar feature in trip-hammers is the rapidity with which
crystallisation takes place in the attachments for holding the die
blocks to the helves, where no elastic medium can be interposed to
break the concussion of the dies. Bolts to pass through the helve,
although made from the most fibrous Swedish iron, will on some
kinds of work not last for more than ten days' use, and often break
in a single day. The safest mode of attaching die blocks, and the
one most common, is to forge them solid, with an eye or a band to
surround the end of the helve.
At the risk of laying down a proposition not warranted by science,
I will mention, in connection with this matter of crystallisation,
that metal when disposed in the form of a ring, for some strange
reason seems to evade the influences which produce crystalline
change. A hand-hammer, for example, may be worn away and remain
fibrous; the links of chains and the tires of waggon wheels do not
become crystallised; even the tires on locomotive wheels seem to
withstand this influence, although the conditions of their use are
such as to promote crystallisation.
Among exceptions to the ordinary plans of constructing
trip-hammers, may be mentioned those employed in the American
Armoury at Springfield, U.S., where small hammers with rigid frames
and helves, the latter thirty inches long, forged from Lowmoor
iron, are run at a speed of 'six hundred blows a minute.' As an
example, however, they prove the necessity for elasticity, because
the helves and other parts have to be often renewed, although the
duty performed is very light, such as making small screws.
(1.) What limits the speed at which the reciprocating parts of
machines may act?--(2.) What is the nature of reciprocal motion
produced by cranks?--(3.) Can reciprocating movement be uniform
in such machines as power-hammers, saws, or pumps?--(4.) What
effect as to the rate of movement is produced by the elastic
connections of a trip-hammer?