WEIGHING MACHINE. The common turnpike weighing machine is a compound steelyard (see BALANCE), and is used for ascertaining the weight of loaded carts. The apparatus, of which a ground-plan is here given (fig. 1), is arranged in a box A B C D, which is about 12 inches deep, and is sufficiently sunk in the ground to place the highest part of the completed machine on a level with the line of road. A fixed wooden bridge, by which the horse may pass across the machine, is laid over the middle part of the box. At each side of this bridge is a metal wheel-tract. On these the cart is rested by its two wheels and a prop-staff. The tracks are fixed to and borne up by a platform (or skeleton frame) placed below the wooden bridge, but above the works. The platform has four short feet, the lower ends of which are formed into small hemispherical cavities. These cavities rest at E F G H, on pointed studs about an inch long. The two studs at E and F are fixed on the crooked lever A K B, with their points upward, and the two at G and H are similarly fixed on the companion lever D I C. Sharpened studs, also about an inch long, project downward from the four extremities of the crooked levers. The points of these studs rest in small hemispherical cavities, formed at the upper ends of blocks which are fixed as bearers for them in each corner of the box. The short parts of the crooked levers parallel to O I, and near the middle of the box, have their under edges sharpened, and rest on the angle of a sustaining fulcrum I K, which passes through, and is fixed to the lever O M. This lever is also crossed at L by the fulcrum which forms the common centre of movement for the whole machine. The angle of the fulcrum points downward, and rests, as in the ordinary balance, on concave arcs, situated one on each side of the lever O M. These arcs are formed on the upper end of a block, which rises from and is fixed to the bottom of the box. The lever is again crossed at M, by a fulcrum having its angle pointing upward; and from this angle a receiving-scale for the weights is hung by a shears or double ring. In cases where it is wished that the machine when unloaded should preserve equilibrium, the lever O M is prolonged to the left, and loaded at that end, to the extent requisite to balance the weight of the levers.
A side view of B K, one of the limbs of the crooked lever A K B is here given, in the direction of the diagonal B D. The lever, as shown in fig. 2, is kneed to an extent sufficient to admit of the angular points of the fulcrum being placed in the same horizontal line. The other limb, A K, is of the same shape. The form of the entire lever, A K B, is thus easily understood. Its companion lever, D I C, is of the same construction. The limbs of the levers are therefore so shaped that the fourteen sharp bearing-parts of all the fulcrum are situated, during equilibrium, in one horizontal plane. These bearing-parts, and the parts they bear against, are all formed of polished steel.
If now E A be supposed a tenth part of the length of E K, then 100 lb. placed on the platform, or on the wheel-tracks, and pressing, therefore, on the studs E F G H, will
be sustained by 10 lb. of an upward pressure at I K. And if, again, O L be a tenth part of L M, the 10 lb. of upward pressure will be given by 1 lb. placed in the receiving scale at M. These proportions are assumed, for easy arithmetical illustration; but in the practical construction of the machine, the distance between the resting-points at E A, and at O L, is made to bear a greater ratio to the distance between the resting-points at E K, and at L M, than has here been supposed. Instead of using a receiving-scale, the lever O M may be prolonged to the right, and graduated as a steelyard. And in machines where an upward instead of a downward pressure is given at M, the lever O M is prolonged to the left, and the fulcrum L and its resting-block removed to N.
Machines consisting of variously modified systems of levers have been used for a considerable period. The one above described is that generally employed. The arrangement of the levers is well conceived, and suited for the purpose to which the machine is chiefly applied. Its first contrivance and employment is said to have been for weighing the riders of race-horses.
H. D. Schmidt of Vienna exhibited a model of a weighing-table, in which the mode of suspending the platform on a quadruple lever, and connecting this lever with the steelyard, was the same as usual. But there was a good arrangement for relieving the steel fulcrum of the levers when the machine is not in use from the weight of the platform, and of carriages passing over it. Under the points where the platform rests the four levers are furnished with bosses projecting downward, and nearly touching the upper surfaces of four short pillars of iron fixed in the masonry at the bottom of the pit. When the machine is not required for weighing, the long end of the lever is lowered, together with the steelyard, counterpoise, and framework, by means of a winch and screw, until the bosses of the levers rest on
the pillars beneath, and these support the platform and any carriage moving over it. On raising the frame with the steelyard and the end of the long lever, the weight of the platform is thrown on its fulcrum, and the machine is ready for use.
H. Pooty and Son exhibited a locomotive weighing-machine, consisting of a system of three tables, placed so that a locomotive running over them may have each pair of its wheels on a separate table. The three counterpoises may then be adjusted until each one correctly balances the proportion of the weight bearing on its table. A bar with a light lever, and a handle projecting from it over each steelyard, enables the attendant standing opposite the centre one to check the vibrations of all three, and inspect with accuracy the state of adjustment. By the use of this multiplied weighing-machine, the proportion of weight borne by each pair of wheels may be tested, and the inquirer is thus enabled to alter the adjustment of the springs, until he has secured such a distribution of the load as shall be most likely to lead to good working.
The principles on which weighing machines in general rest are briefly stated under MECHANICS, § 148 (D). See also BALANCE for a notice of Steelyards, the Hydrostatic Balance, the Balance of Torsion, &c.; while some other kinds of balance, employed for special purposes, will be found under different heads. (C. T.)