cylinder, which is filled with oil or other liquid, is fitted with a piston having a piston-rod passing downwards and terminating in an attachment for the goods to be weighed. As the goods are lifted by the crane the whole of their weight is taken by the liquid in the cylinder, and the pressure on the liquid, as indicated by a pressure gauge, gives the weight. The gauge has a plain dial, marked off to indications given by the application of standard tons and cwts.; it could probably be read to about 12% of the load weighed.
Fig. 12.
Spring Balances.
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 springs in general are very accurate and uniform in their extension, and are very permanent when fairly well used; but their indications are apt to vary from fatigue of the springs if they are kept extended by a weight for a long time. Their indications also vary with the temperature, so that for good work it is advisable that spring balances should be frequently checked with standard weights. For the sake of compactness and convenience of reading the extension of the springs, and consequently the load, is frequently indicated on a dial, by means of a small rack and pinion, which give motion to a finger on the dial-plate, but the regularity and correctness of the indications of the finger will depend upon the condition of the rack work and upon the friction, and these will vary with the wear of the machine. For the above reasons spring balances are not in general so accurate as knife-edge machines. It is found that when a spiral spring is extended by a weight it has a tendency to turn a little round its axis. Therefore an index pointer attached to the bottom of the spring, and moving past a scale would rub slightly against the case. To correct this tendency the spring is usually made half with right-hand spiral and half with left-hand spiral.
The extension of a spiral spring is given by the formula:—
Extension＝W4nR3/Er4,
in which W＝weight causing extension,
in ℔s; n＝number of coils; R＝radius of spring, from centre of coil
to centre of wire, in inches; r＝radius of wire of which the spring is
made, in inches; E＝coefficient of elasticity of wire, in ℔s per square
inch. The value of E depends upon the tempering of the wire and
will vary accordingly: for the springs of trade balances E will
usually be about 10,500,000. For the application of the above formula
it is necessary to measure (R) and (r) very accurately, by reason of
the high powers involved, but when this has been carefully done the
formula may be relied upon. Thus in the case of a spring for which
the values of the quantities were W＝7 ℔, n＝51, R＝·30 in., r＝·038 in.,
E＝10,500,000, the formula gives extension—1·764 in., while
direct experiment gave extension—1·75 in. And with a very long
and weak spring for which the values of the quantities were W＝12 oz.,
n＝233, R＝·35 in., r＝·0085 in., E＝10,500,000, the formula gives
extension＝22·78 in., while direct experiment with the spring gave
23·5 in.
Automatic Weighing Machines.
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 ℔. 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. It is to meet such cases that the greater number of automatic weighing machines have been invented. Some examples of these machines will now be given.
Automatic Computing Spring Weighing Machine for Retail Purposes (fig. 13).—A light and carefully balanced drum with its axis horizontal is enclosed within a cylindrical casing, and rotates freely in bearings formed in the ends of the casing. The casing is fixed in supports on the top of a strong frame, which also carries a small platform machine of ordinary construction on which the goods to be weighed are placed. The pull of the load is transmitted to a hook which hangs freely from the middle of a horizontal bar below the drum casing. At each end of the drum casing is attached a vertical spiral spring, and by the extension of these springs the weighing of the goods is effected.
| From the Notice issued by the Standards Department of the Board of Trade, by permission of the Controller of H.M. Stationery Office. |
| Fig. 13.—Price-computing Spring Weighing Machine. |
There are also two vertical racks, one at each end of the casing, in connexion with the two springs, and these actuate pinions on the axle of the drum and cause it to revolve as the springs extend. The horizontal bar which receives the pull of the load is connected at its ends with the two spiral springs and pull of vertically upon them. Above the horizontal bar, and parallel with it, is a rod which is connected at its ends with the lower ends of the vertical racks, and at its middle with the horizontal bar. The connexion with the horizontal bar is through the medium of an adjustable cam. This cam can be turned by hand in a vertical plane by means of a worm and wheel movement, and by turning the worm the vertical distance between the bar which is attached to the springs and the rod which is attached to the racks can be increased or diminished, and thus the racks can be moved relatively to the springs. By this means the zero of the scale on the drum can be adjusted to the fixed index on the casing when there are no goods on the platform. There is also a compensation arrangement for effecting automatically the same adjustment for changes of temperature. To deaden the vibration of the springs after a load has been placed on the platform, and thus to enable the weights arid values of the goods to be read rapidly, the piston of a glycerin cylinder is attached to the end of the lever which pulls upon the hook of the horizontal bar and is worked by it in the glycerin.
