In fig. 7 let Z be the fulcrum knife-edge, X the knife-edge on which
the load R is hung, and H the centre of gravity of the weights to the
right of Z, viz. the weight, W, of the steelyard acting at its centre
of gravity; G, the travelling poise; P, acting at M; and the weights,
Q, hung on the knife-edge at V. Then if Z be below the line joining
X and H, the steelyard will be "accelerating"; i.e. with the smallest
excess of moment on the left-hand side of the fulcrum, the end C of
the steelyard will rise with accelerating velocity till it is brought up
by a stop of some sort; and with the smallest excess of moment
on the right-hand side of the fulcrum, the end C of the steelyard will
drop, and will descend with accelerating velocity till it is brought
up by a similar stop. If Z be above the line XH, the steelyard is
"vibrating"; i.e. it will sway or vibrate up and down, ultimately
coming to rest in its position of equilibrium. Steelyards, again, are
frequently arranged as counter machines, having a scoop or pan
resting on a pair of knife-edges at the short end, which is prevented
Fig. 7.
from tipping over by a stay arrangement similar to that of other
counter machines.
Steelyards are largely used in machines for the automatic weighing out of granular substances. The principle is as follows: The weighing is effected by a steelyard with a sliding poise which is set to weigh a definite weight of the material, say 1 ℔. A pan is carried on the knife-edges at the short end, and is kept from tipping over by stays. A packet is placed on the pan to receive the material from the shoot of a hopper. A rod, connected at its lower end with the steelyard, carries at its upper end a horizontal dividing knife, which cuts off the flow from the shoot when the steelyard kicks. When the filled packet is removed, the steelyard resumes its original position, and the filling goes on automatically.
The automatic personal weighing machine found at most railway
stations operates by means of a steelyard carrying a fixed weight on
its long arm, the load on the platform being inferred from the position
of the steelyard. In fig. 8 the weight on the platform is transferred
Fig. 8.
by levers to the vertical
steel band, A, which is
wrapped round an arbor
on the axle of the disk-
wheel, B, to which is
rigidly attached the
toothed segment, C. The
weight, D, is rigidly
attached to the axle of the
wheel, B, and the counter-
balance, E, is hung from
the wheel, B, by means of
a cord wrapped round it.
When the pull of the band,
A, comes upon the wheel,
B, it revolves through a
certain angle in the direction
of the arrow until the
three forces, viz. the pull of
A, the weight, D, and the
counterbalance, E, are in
equilibrium. The toothed
segment, C, actuates the
pinion, F, which carries
the finger, G, and this
finger remains fixed in
position so long as the
person is standing on the
platform. If now a small
weight, as a penny, be
passed through the slot, H,
it falls into the small box, I,
and causes the lever, J, to
turn; the lever, J, which turns in friction wheels at K, and is
counterbalanced at O, carries a toothed segment, L, which
actuates a small pinion on the same axle as F, and is free to
turn on that axle by a sleeve. This small pinion carries a finger,
M, which is arranged to catch against the finger, G, when moved up
to it. Consequently as the lever, J, turns, the finger, M, revolves,
and is stopped when it reaches G. The sleeve of the pinion which
carries M also carries the dial finger, and if the dial is properly
graduated its finger will indicate the weight. The box, I, has a
hinged bottom with a projecting click finger which, as the box descends,
plays idly over the staves of a ladder arc. When the weight
is removed from the platform, the counterbalance, E, causes the
finger, G, to run back to its zero position, carrying with it the finger
M, and causing the click finger of the box, I, to trip open the bottom
of the box and let the penny fall out. The lever, J, regains its zero
position, and all is ready for another weighing. Since so small a
weight as a penny has to move the lever, J, together with the dial
finger, &c., it is evident that the workmanship must be good and the
friction kept very low by means of friction wheels.
Some of the largest and most accurate steelyards are those made for testing machines for tearing and crushing samples of metals and other materials. They are sometimes made with a sliding poise weighing 1 ton, which has a run of 200 in., and the steelyard can exert a pull of 100 tons.
Balances are frequently used as counting machines, when the
articles to be counted are allot the same weight or nearly so, and
this method is both quick and accurate. They are also used as trade
computing machines, as in the case of the machine made by the
Computing Scale Company, Dayton, Ohio, U.S.A. In this machine
the goods to be priced are placed on the platform of a small platform
machine whose steelyard is adjusted to balance exactly the weight
of the platform, levers and connexions. The rod which transmits
the pull of the long body lever of the platform machine to the knife-edge
at the end of the short arm of the steelyard is continued upwards,
and by a simple mechanical arrangement transmits to an
upper steelyard any additional pull of the long body lever due to the
weight of goods placed on the platform. This upper steelyard is
arranged as in fig 9, where A is the point where the pull of the long
body lever due to the weight of the goods on the platform comes upon
the steelyard; C is the fulcrum of the steelyard, which with the
steelyard can be slid to-and-fro on the frame of the machine; and Q
Fig. 9.
is a poise which can be slid along the upper bar of the steelyard.
The steelyard is exactly in balance when there is no weight on the
platform and Q is at the zero end of its run, at O. Suppose that
the weight of the goods on the platform is (p) ℔, and that 1nth
of this weight is transmitted by the long body lever to the point
A, so that pn ℔ is the pull at A. Let the lower bar of the steelyard
be graduated in equal divisions of length, d, each of which
represents one penny, so that the distance CA=q×d represents
q pence. Then the number p×q represents the total value of
the goods on the platform. If Q ℔ be the weight of the poise Q,
the position of Q when the steelyard is exactly in balance is
given by the equation pn×q·d=Q×OQ, or OQ=p×q×dnQ. If
therefore the upper bar be graduated in divisions, each of which
is dnQ the indication of the poise Q, viz. p×q graduations, gives
correctly the value of the goods. Thus to ascertain the value of
goods on the platform of unknown weight at a given price per ℔,
it is only necessary to slide the steelyard till the weight acts at
the division which represents the price per ℔, and then to move
the poise Q till the steelyard is in balance; the number of the
division which defines the position of the poise Q will indicate the
sum to be paid for the goods. When the load on the platform is
large, so that the value of the goods may be considerable, it is
convenient to measure the larger part of the value by loose weights
which, when hung at the end of the steelyard, represent each a
certain money value, and the balance of the value is determined by
the sliding poise Q.
In the machines commonly used to weigh loads exceeding 2 cwt. the power is applied at the end of the long arm of the steelyard and multiplied by levers from 100 to 500 times, so that the weights used are small and handy. The load is received upon four knife-edges, so that on the average each knife-edge receives only one-fourth of the load, and, as will be seen, it is immaterial whether the load is received equally by the four knife-edges or not, which is essential to the useful application of these machines.
In fig. 10 AB is the steelyard. The platform and the load upon it are carried on four knife-edges, two of which, x1 and x2, are shown, and the load is transferred to the steelyard by the two levers shown, the upper one CD being known as the "long body," and the lower one EF as the "short body." If z1x1=z2x2, and z1t=z2y2. then the leverage of any portion of the load applied at x2, will be the same as the leverage of any part of the load applied at x2, and the pressure produced at y1 will be the same for equal portions of the load, whether they were originally applied at x1 or x2. Platform machines, like steelyards, may be arranged either on the "accelerating" principle or on the "vibrating" principle. If in fig. 10 g1 be the centre of
