Weighing machine

“ For many purposes spring balances are the most convenient of all weighing machines. They are rapid in action, the indication is in general clear, and there is no need of loose weights except for testing the machine occasionally. Their action depends upon the extension of one or more spiral springs, and as the extension is proportionate to the weight which causes it the graduation is very simple. The accuracy of spring machines depends upon the accuracy of the springs and the workmanship of the machines. ”

“ The tension is thus directly measured by a system of levers, exactly resembling those used in a common weighing machine, and this is done so accurately that even when a chain is being subjected to a strain of many tons, an additional pull, such as one can give to the shackle-link with one hand, at once shows itself in the weighing-room. The person who has charge of this part of the machine places on the end of the lever a weight of as many pounds as the number of tons strain to which the chain to be tested has to be submitted. ”

“ The weighing-machines, of which there are sixteen, are actuated by a small atmospheric engine in one corner of the room, the only manual assistance required being to keep them supplied with coins. It is said that sixty thousand sovereigns and half-sovereigns can be weighed here in a single day. The weighing-machine in question is the invention of Mr Cotton, a former governor of the Bank, and among scientific men is regarded as one of the most striking achievements of practical mechanics. ”

“ The same principle is employed in his weighing machines. [Pg 399]
By one of these hydraulic testing machines the tensile strength of forged links has been ascertained by the exertion of a power amounting to over 700,000 pounds before breaking a link, the chain breaking with a loud report.
The most delicate materials are tested by the same machine—the tensile strength of a horsehair, some of which are found to stand the strain of one and two pounds. Eggs and nuts are cracked without being crushed, and the power exerted and the strain endured automatically recorded. ”

“ A crane machine of peculiar construction, well adapted for weighing heavy loads, and extremely simple and compact, which does not properly come under any of the heads under which the machines have been classified, is the hydrostatic weighing machine. ”

“ Sometimes the object is to weigh many loads in succession, the loads being of varying weight, and to present the total weight at the end of a day’s work; this is the case with machines for weighing coal and other minerals. Of course the introduction of automatic mechanism introduces friction and other complications, and it is difficult to construct automatic machines that shall be as accurate in their weighing as the simpler weighing machines, but in many weighing operations a moderate degree of accuracy will suffice, and speed is of great importance. ”

“ Similar principle applied by compound levers with unequal arms. The table is supported on four points on the arms of levers loosely jointed together in the centre; one lever is extended and coupled by a rod to a graduated lever with sliding weight. Knife edges are used for bearings for all weighing machines by leverage. See No. 958. This construction is the basis of most of the ordinary weighing machines in use.
1728. Duckham’s patent hydraulic weighing machine. The article to be weighed is suspended from the hook, and exerts a pressure on the ram. ”

“ During the last few years great efforts have been made to expedite the operation of weighing machines by the introduction of machinery, more or less complicated, which renders the machines to a great extent self-acting. The object aimed at varies very much with different machines. Sometimes the object is to weigh out parcels of goods in great numbers of the same definite weight. Sometimes the object is to weigh out parcels of goods, of unknown weight, as in ordinary retail dealing, and to give the exact value of each parcel at different rates per ℔. ”

“ The machines, while necessarily of the finest adjustment, are ponderous and heavy, weighing in some cases as much as four hundred tons, this being the weight of the machine itself, exclusive of its foundations. ”

“ One may make himself a proximate idea of the enormous power of pressure of such a machine, if he can imagine what a strength is needed to bend an iron plate of 250 meters thickness, in cold condition; being also 1.5 meters in width, and 5.00 meters in length, and weighing about 14,555 kilogrammes, or 14,555 tons. ”

“ If the weight of the bar has been correctly stated in the memorandum, the scale which holds it should be exactly half an ounce in excess. This or any less excess of weight over the three hundred and sixty ounces in the opposite scale is instantly registered by the machine, a pointer travelling round a dial until it indicates the proper amount. The function of the machine, however, is limited to weighing half an ounce only. ”

“ There are many reliable kinds of weighing machines, but the ordinary shop scales and weights still remain the most popular, and the price of a set of weights and scales, with weights sufficient to weigh from 1⁄4 oz. ”

“ It may hence be proved, that in the elevation of the weight a quantity of power is expended equal to that which would be necessary to elevate the weight if the power were immediately applied to it, without the intervention of any machine. ”

“ The larger will, as a rule, sustain a greater weight in relative proportion to its area than the smaller one, and do the work with less relative horsepower. As a general thing well-constructed machines will average a supporting capacity of one pound for every one-half square foot of surface area. Accepting this as a working rule we find that to sustain a weight of 1,200 pounds—machine and two passengers—we should have 600 square feet of surface. ”

“ Of these only 100 foot-pounds are usefully employed, and thus 50 foot-pounds, one-third of the whole, have been expended on friction. Here we see that notwithstanding a small force overcomes a large one, there is an actual loss of energy in the machine. The real advantage of course is that by the pulley-block I can raise a greater weight than I could move without assistance, but I do not create energy ”

“ No power less than a ton can, in the ordinary acceptation of the word, support the weight of a ton. It will, however, be asked how it happens that a machine appears to do this? how it happens that by holding a silken thread, which an ounce weight would snap, many hundred weight may be sustained? To explain this it will only be necessary to consider the effect of a machine, when the power and weight are in equilibrium. ”

“ But the sustaining surface is only four times as great. As compared with the smaller machine, its ratio of effectiveness is reduced to one-half. It may carry two or three buttons, but will not carry over four, because the total weight, machine plus buttons, can only be quadrupled, and if he more than quadruples the weight of the machine, he must less than quadruple that of the load. How many such enlargements must he make before his machine will cease to sustain itself, before it will fall as an inert mass when we seek to make it fly through the air? ”

“ But since it may occasionally happen that we have but a small force, and want to move a great weight without dividing it into pieces, then we must have recourse to a machine by means of which we shall remove the given weight, with the given force, through the required space. ”

“ Of this kind is the weight of any piece in the mechanism whose centre of gravity alternately rises and falls; for during the rise of the centre of gravity that weight acts as a resistance, and energy is employed in lifting it to an amount expressed by the product of the weight into the vertical height of its rise; and during the fall of the centre of gravity the weight acts as an effort, and exerts in assisting to perform the work of the machine an amount of energy exactly equal to that which had previously been employed in lifting it. ”

“ Furthermore, if we had a machine so perfect as to enable us to apply all the heat thus produced to the raising of a weight, we should be able, by it, to lift the mass of iron to the precise point from which it fell. ”

“ To raise 30 lbs. one foot, a force of 17·4 lbs. must therefore be exerted through 3·38 feet. The number of units of work expended is thus 17·4 × 3·38 = 58·8. Of this 30 units, equivalent to 51 per cent., are utilized. The remaining 28·8 units, or 49 per cent., are absorbed by friction.
276. We have pointed out in Art. 222 that a machine in which less than half the energy is lost by friction will permit the load to run down when free: this is the case in the present instance; hence the weight will run down the plane unless specially restrained. That it should do so agrees with Art. ”

“ We infer from this result that the mechanical efficiency of this machine is 56 ÷ 2·6 = 21·5. I add a second 56 lb. weight to the load, and I find that a power of 5·0 lbs. raises the load of 112 lbs. The mechanical efficiency in this case is 112 ÷ 5·5 = 22·5. We adopt the mean value 22. Hence the mechanical efficiency is reduced by friction from 31 to 22.
308. We may compute from this result the number of units of energy which are utilized out of every 100 units applied. Let us suppose a load of 100 lbs. is to be raised one foot; a force of 100 ÷ 22 = 4·6 lbs. will suffice to raise this load. ”

“ Here is a fine Atwood's machine, which has often been used in this Institution: you see two weights, one heavier than the other, connected by a string over a pulley. The heavier weight falls, pulling the other up; but the fall is comparatively slow, and can be made as slow as we please by making the weights nearly equal. ”

“ Now, we know, by the laws of mechanics, that, whatever be the construction of a machine, the space passed over by the weight, is to that passed over by the moving power, in the reciprocal ratio of the latter to the former. It is known also, that a man can act with an effort equal only to about 30 pounds for eight or ten hours, without intermission, and with a velocity of about 10,000 feet per hour. ”

“ There is a great degree of taste and finish employed in its design; indeed the builder may flatter himself that he has one of the best machines built in this country.
This machine, according to its size, weight, and cost, is divided into Nos. 4, 5, and 6, capable of planeing metal from four feet long, by eighteen inches wide and high, to six feet six inches long, by twenty-four inches wide and high—and weighs respectively 1000, 2,600, and 3,500 pounds. ”

“ High Arm Construction.—Sewing machines, as now constructed, exhibit a rather short and very high arm, a form of framework that has been found to contribute in no small degree to the light running capabilities of fast speed machines. While it reduces the length of the various parts concerned in the transference of the motive power, it adds to their rigidity and diminishes their weight, maintaining at the same time the capacity of the machine to accommodate the largest garments beneath the arm. ”

“ If this Machine is well made and proportioned, I think it is one of the best that can be used, to do much work by a given power: It gives no shock to the water; which, when once in motion, continues to rise, and escapes when arrived at it’s proper height: and, being spread over a large surface, no part of it is raised higher than enough. The perfection of the Machine depends on a due relation between the centrifugal force, and the sine of the angle, which the threads of the screw make with the horizon ”

“ In a machine of which each piece is to move with a uniform velocity, if the effort and the resistance be constant, the weight of each piece must be balanced on its axis, so that it may produce lateral pressure only, and not act as a reciprocating force. ”

“ I have thought it necessary to do, because, if I mistake not, I see almost all mechanics deceiving themselves in the belief that, by the help of a machine, they can raise a greater weight than they are able to lift by the exertion of the same force without it.—Now if we take any determinate weight, and any force, and any distance whatever, it is beyond doubt that we can move the weight to that distance by means of that force ”

“ If the diameter of the wheel be considerably enlarged, the machine will become unwieldy, and the power will work through an unmanageable space. If, on the other hand, the power of the machine be increased by reducing the thickness of the axle, the strength of the axle will become insufficient for the support of that weight, the magnitude of which had rendered the increase of the power of the machine necessary. To combine the requisite strength with moderate dimensions and great mechanical power is, therefore, impracticable in the ordinary form of the wheel and axle. ”

“ But that plate is supposed screwed down to a horizontal bench, at or near the height of a man’s breast; the slides or cases are fastened to it, and the man is supposed to work the Machine nearly as he would a die-stock in tapping a screw. This however is not indispensable; the Machine might be placed vertically, and these motions given by any proper mover; or a weight may be suspended to the arm F, so as to add continuity to pressure. ”

“ In order to lift 100 lbs. the expression 0·0143 R shows that 1·43 lbs. would be necessary: hence the mechanical efficiency of the screw is 100 ÷ 1·43 = 70. Thus this screw is vastly more powerful than any of the pulley systems which we have discussed. A machine so capable, so compact, and so strong as the screw, is invaluable for innumerable purposes in the Arts, as well as in multitudes of appliances in daily use. ”