Machinery Magazine - May 10, 1917
If is often necessary, in the shop or engineering office, to weigh small quantities of materials with a rather high degree of precision, and yet the amount of work to be done may not be great enough to warrant the purchase of an analytical or similar balance. The price of an accurate, sensitive, chemical balance is rather high, and the theoretical and technical knowledge required for its manufacture precludes its being made successfully by anyone not expert in that line. The simple and inexpensive balance here described, however, can be made easily and at small cost, is easily adjusted, and will give satisfactory precision for such work as the weighing of small samples, moisture determinations, and a number of other uses for which an accurate balance of small capacity is required.
The beam A should
be made of steel or invar; the latter, which is a comparatively inexpensive
nickel-steel alloy, is preferable on account of its negligible coefficient
of thermal expansion. If machinery steel is used, it should be carefully lacquered
or enamelled to prevent rusting. In the case shown, the beam is 1 1/2 inch
wide; its short arm is 13/32 inch thick throughout, but its long arm tapers
from 3/8 to 3/16 inch at its outer end. Its fulcrum B may be either two cone
points spaced about an inch apart along a line perpendicular to the plane
of the beam or a knife-edge an inch long. If the latter is used, which is
the preferable form, it should be absolutely straight, and finished, by smooth
grinding, to a clean, sharp edge having an included angle of approximately
90 degrees. The load pan is suspended from a cone pivot C at the end of the
beam. Both the knife-edge B, if one is used st the fulcrum, and the cone pivot
C should be made of tool steel, carefully hardened and ground; the cone pivot
should be perfectly sharp and have an apex angle of approximately 90 degrees.
The knife-edge, the cone, and their bearings must be glass-hard, and the wearing
surfaces of the bearings must be polished smooth. The V-bearing D of the knife-edge
as an approximate angle of 150 degrees and the cup bearing surface E of the
cone pivot C has a radius of 5/16 inch. The utmost care must be taken in finishing
the knife-edge, cone pivot, and bearings that co-act with them, the sensitiveness
and accuracy to be obtained in the use of the balance depends more directly
on these factors than on any other parts or the instrument. Both the knife-edge
B and the cone pivot C should be set snugly and firmly into the beam A. For
this purpose dowel pins or short screws may be used at the knife edge.
A movable weight F is provided at the left side of the fulcrum to counterbalance the weight of the long arm of the beam and the load pan G at its end. It consists of two machine-steel blocks held in place by screws; which may be moved horizontally and vertically; its construction is shown by the top view and the section at X-X. As this weight is movable vertically, it is possible to increase the sensitiveness of the instrument by bringing the centre of gravity of the loaded beam close to the knife-edge. This weight should be a neat fit on the arm on which it slides. After adjustment it should be tightly locked in place by the clamping screws shown. A finer adjustment may be obtained by means of the knurled nut H travelling on a fine threaded screw I immediately above the knife-edge B. The movement of this nut raises or lowers the centre of gravity of the loaded beam in its relation to the knife-edge. The movement of the brass nut K that travels on the screw L inserted in the end-of the beam is usually the only adjustment necessary when the balance is in use.
The pillar M that carries the hardened fulcrum bearing D should be firmly attached to the base of the balance. Near the bottom is placed a scale N; in the case shown, this has ten graduations, spaced 1/16 inch apart, on each side of the zero mark. The pointer P that travels over this scale is a slender rod that is attached directly above the knife-edge. The base O may be made either of well seasoned, varnished wood or of a light hollow casting. The one requirement is that it shall be rigid and firm.
The load pan G, an end view of which is shown, carries both the load to be weighed and the set of weights. Its construction should be as light as practicable in order that the load on the cone pivot C may not be unnecessarily great. In the present case it is made of brass wire 3/32 inch diameter passing through a piece of hardened tool steel that acts as a bearing for the cone pivot C. The load to be weighed is placed on the upper tray and the weights on the lower. The upper tray may be flat or slightly concave; a large watch glass, about 2 3/4 inches in diameter, for instance, serves the purpose very well. At a convenient distance below is a brass or aluminium tray or rack, which carries the set of weights. This should provide for the arrangement of the weights in the order of their value.
In order to protect the bearings, knife-edge, and cone from wear when the balance is not in use, an arrestment rod R is provided directly under the counterbalance weight F. This is a straight rod, with a flat cap at its upper end that is screwed into the base of the balance with a coarse-pitch thread. Turning the rod, by the knurled part, raises it into contact with the bottom of the counterbalance weight and lifts the latter so that the pan is brought down upon the base and the cone is relieved from contact with its bearing. When it is desired to use the balance, the rod is simply screwed down again so that it is no longer in contact with the beam.
The weight denominations should be so chosen. that any weight value up to the weighing capacity of the balance can be determined. For example, if the weighing capacity of the instrument is to be about 0.5 lb. and a balance of this type will serve very well for that load the set of weights may be the following decimal fractions of a lb.: 0.2, 0.1, 0.1, 0.05, 0.02, 0.02, 0.01, 0.005, 0.002, 0.002, 0.001, 0.0005, 0.0002, 0.0002, 0.0001, 0.00005, 0.00002, 0.00002, 0.00001. For much of the work for which a balance of this sort will be used, however, it will probably be advantageous to have weights in the metric system; in this case the weight set should include 100 grams to 10 milligrams, in the same 5-2-2-1 series. The weights should be handled with wooden or horn-tipped tweezers or pincettes. The Bureau of Standards will supply suitable specifications for weights of this sort if the inquirer will state the nature of the work for which they are to be used.
Adjusting and Using Balance
The adjustment of this balance, unlike that of the usual type of precision balance, is very simple, and consists of adjusting the position of the centre of gravity of the loaded beam until it lies vertically below and very near the knife-edge line. This operation is performed in the following manner: After the parts are assembled, the weights are placed on the pan, and the whole brought into approximate equilibrium by moving the balancing weight F to the left or right as required. If the equilibrium is unstable, the centre of gravity is too high, and may be lowered by lowering the balancing weight without changing its position in a horizontal direction. If, on the other hand, the balance oscillates quite rapidly, the centre of gravity is too low, and may be changed by raising the balancing weight F without moving it horizontally. When the balance oscillates with a period of five seconds, or more, after the large balancing weight is securely clamped upon its stem, the final adjustment can be made with the adjusting nut H above the fulcrum. The occasional adjustment of the balance which may be required, in service, to cause the pointer P to oscillate about and come to rest at the zero of the graduated scale will be made by changing the position of the nut K at the right end of the beam.
To use the balance, bring it into equilibrium by turning the adjusting nut K at the right end of the beam until the pointer vibrates through equal distances on each side of the zero graduation. Then carefully place the object to be weighed in the upper tray and remove weights from the lower tray until the pointer again swings about the zero graduation. The sum of the values of the weights removed from the pan to produce equilibrium will be the weight of the object sought. (This method of weighing is called the method of substitution.). The value of the weight of the object should be recorded, the weights again placed in the lower tray, and the object weighed, removed; the pointer should again swing about the zero graduation, affording a check on the determination.
Care must be taken in the use of the balance to avoid all unnecessary shock and jar, thereby reducing the wear and dulling of the pivots. In addition, reasonable care should be taken to protect the balance from sudden and irregular changes of temperature, especially just before or during the time it is in use. The balance must also be carefully protected from dust and moisture, as these will cause rapid deterioration of any knife-edge or pivot.
If much careful work is to be done on the balance, it should be protected from dust and air currents, and to some extent from temperature disturbances, by a glass case. It will be impossible to weigh more closely than to about 1/5 grain (or 15 milligrams) on any balance in which the moving parts are not protected from air disturbances and radiation. A rider may be added to the beam to provide a more convenient method of determining the last figure or the last two figures of the weight. This is a modification that can be easily worked out to suit any particular case, as the rider scale is simply a uniformly divided linear scale, securely mounted on the beam, that carries a sliding weight the value of which will be adjusted by trial against a weight in the pan. The value of such weight is equal to the total interval of the rider scale.
Advantages of Balance
This type of balance has the advantage of case of construction and adjustment. The common or so-called "equal arm" balance has three knife-edges in the beam, and the accuracy of weighing depends on these knife-edges being constructed and maintained straight and parallel; also, if' the balance is to be used in the usual manner, they must be spaced exactly equal distances apart. The satisfactory attainment of these conditions is a matter of great manufacturing difficulty and requires considerable skill and care. In balances for careful work, the spacing of these knife-edges will be adjusted to within one or two parts in one hundred thousand. In the highest grades of precision balances, such as are used for standardization work, equal spacing is sometimes attained to a precision of one part in a million, or less. This adjustment is obtained by almost infinitesimal shifts of the several knife-edges through a long and tedious process of trial and adjustment. The difficulty and expense of these adjustments will be readily appreciated.
In the balance here described, the knife edge need only be straight and the cone point sharp and reasonably free from deformation under the load it carries. The effective length of the balance arm is always the perpendicular distance between the apex of the cone and the contact line of the knife-edge. This distance cannot change, as when three knife-edges are used, by the shifting of the point of resultant pressure along a knife-edge which is a trifle warped or out of parallel. (It should, however, be noted that for much work the cone pivot is not at all satisfactory; if the service is frequent and severe and the loads applied-quite large, the high local stresses developed will cause rapid deterioration.) Another and very important advantage is that the sensitiveness of the balance is constant for all loads within its capacity, since the gross load carried by the beam is. at all times constant during weighing.
With reasonable care, the balance should last a long time; but if after a period of use the cone pivot becomes dull, it is easily taken out and resharpened; at the same time the cup bearing should be repolished. When the pivot is replaced the balance is ready for use as soon as it is rebalanced to compensate for the changes which have been made in the weight of the parts by the repair. This easy repair, without the necessity of a complete realignment of parts, is a great advantage. The cone point when replaced should project the same distance from the beam as before, so that the balancing plane and sensitiveness will not be altered too muck Results obtained with Balance
An experimental balance of this type, made in accordance with the foregoing description in the instrument shop of the' Bureau of Standards, showed a surprising precision and convenience. Its sensitiveness, as it was first adjusted, was so high that its readings were seriously affected by the slight air currents which prevailed in the room, although kept at a very nearly constant temperature and protected from draughts of every sort. It was necessary to remove the disturbing influences occasioned by the heat convection and radiation from the observer's body by reading it through a telescope mounted on a stand about five feet away. The readings then checked very well and weighing could be made and repeated within 0.006 grain (0.4 milligram, approximately). This difficulty due to air currents would have been practically eliminated, of course, had the balance been provided with the enclosing case usually supplied with balances used for accurate work.
Of course, for much work such precison is not required, and the balance can be adjusted to a lower sensitiveness and used in the open air, if reasonably well protected from draughts, serving even under these conditions a very useful purpose. However, for those who may wish to obtain the highest possible accuracy, the writer recommends the construction of a case of wood and glass, having a single sliding glass door in the front which can be opened for the removal and addition of weights. There is no reason, if this is done, why this balance will not perform as well, though perhaps lacking somewhat in convenience, as any analytical balances save those of very high grade and cost.
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