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first to supply the needed processed material for its special use and the second to purchase its products. Relative Importance of Various Factors
I n an analysis of industry we find that the order of importance of these twelve factors is often changed, so that in one case one factor may predominate and in the other another. It is to be noted, however, that distribution, favorable freight rates, markets, and labor conditions become paramount, in basic industries, after it has once been established that favorable fuel or power costs and, a t least, competitive raw
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material costs are obtainable for that industry. That is, given favorable fuel and raw materials, then distribution, favorable freight rates, markets, and labor conditions are the next governing factors, as a rule, in the establishment of a basic industry. In the establishment of intermediate industries these same factors become paramount when it has been determined that these industries may obtain, under favorable conditions, those finished products which are their chief raw materials. For tributary industries these same four factors are the chief governing factors without any modifying conditions.
Industrial Measurements, I-Weighing1 Everett P. Partridge 1440 EASTPA= PLACE, ANN ARBOR,MICH.
IFTED foremen used to be able to squint one eye and estimate the amounts of materials going into a batch. Either the trait is dying out and foremen aren’t what they used to be, or else the old-time experts were really somewhat less accurate than the legends assert. At any rate, modern technology is no longer content with approximations in measurement. This article describes the developments which have been taking place in the weighing of solid materials in industry, both by manual processes and automatically. Future articles will deal with other types of industrial measurements, such as determination of gas and liquid flow, gas composition, temperature, and hydrogen-ion concentration. I n treating the subject of industrial weighing it will be convenient to take the viewpoint of the application of the equipment, rather than to discuss specifically the mechanical features of all the types of scales available. Some mention will be made of these mechanical details, but no attempt will be made a t a complete description of individual scales or a t a complete comparison of different types. A simple division may be made, for the sake of clearness, into the weigh-beam and pendulum types of weighing devices. I n the first type the weight of an object is transmitted by levers to a weighbeam, which is balanced by varying either the position or the amount, or both position and amount of weights upon it, while in the second type the weight of the object is transmitted to a lever, which lifts a constant weight in a vertical arc against the force of gravity. The first type of weighing mechanism may be operated manually, semi-automatically, or automatically. I n manual operation the load is balanced by hand, and the weight is read and recorded by the operator. I n semi-automatic operation the load is balanced mechanically, and the weight is indicated by the machine, but is read and recorded by the operator. I n complete automatic operation the machine not only balances the load, but records the amount of the weight by printing on a slip of paper or by actuating the mechanism of a mechanical counter or a continuous recorder of the familiar circular type. The second type of weighing mechanism is almost universally used as a semi-automatic installation, the weight being indicated on a dial and read and recorded by the operator. With this brief classification of the types of industrial scales for an introduction, attention will now be directed to the weighing problems occurring in manufacturing the
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Received March 15. 1929.
products of the process industries. These problems, when reduced to their simplest terms, may be grouped under three heads: (1) weighing of raw materials; (2) compounding and check-weighing; (3) packaging and product measuring. Some typical applications of scales in each of these three fields will be discussed in the following sections. Weighing of Raw Materials
No plant can afford to operate without an accurate check on the amounts of its raw materials, whether they are supplied in cargo, carload, or truck lots. Certain mechanical weighing devices are particularly well adapted to this type of service. Hopper scales and conveyor scales will handle a continuous flow of material from transportation to storage, or from storage to process, while scales of the platform type are built in models to weigh batch lots ranging from a loaded freight car down to a wheelbarrowful. The hopper scales adapted for weighing-in raw materials reverse the ordinary process of weighing, since they measure and pass loads of material against a predetermined standard. The weighing hopper is mounted on one end of a lever, which carried a t the other end a variable counterbalance which can be adjusted and locked a t the point necessary to give a certain weight discharge from the hopper. As the hopper fills with material to the desired weight, it starts to drop. The movement of the lever arms actuates the feed shutoff, while further travel opens the hopper discharge. Relieved of its load, the hopper moves upward again, closing its own discharge gate and opening the feed-gate again. An automatic counter records the number of cycles, and this value multiplied by the standardized weight of a single discharge gives the total weight of material passed through the hopper. Hopper scales are much used on dry materials which will not ball up and cling t o the hopper. Some of their applications are the handling of coal, of grain to elevator storage, and of such raw materials as crushed limestone or sugar beets from storage to process. Conveyor scales differ fundamentally from scales weighing a dead load in that they must integrate the instantaneous weight of the material on a section of conveyor belt against the rate of belt travel. This problem of mechanical integration has produced several ingenious devices. The most recent of these is the electrically operated system of the Telepoise. A short section of the conveyor is suspended from weighing levers working against precision springs. The levers and springs control the position of a contactor,
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INDUSTRIAL AND ENGINEERING CHEMISTRY
described below, which is connected to a registering device placed at any distance, and to a graphic recorder which shows the flow of material on the conveyor, including all variations and interruptions. The contactor mechanism of the Telepoise is shown in Figure 1. The small vertical drums at the lower right revolve toward each other, and are driven directly by chains and gearing from a traction roller riding on the inside of the return span of the conveyor belt. The left-hand drum has a
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load. This ground glass is set so that i t may be observed by the operator controlling the feed of material to the belt. Another clever integrating device for belt conveyor weighing is included In the Conveyor Weightometer, the essentials of which are shonrn in Figure 2. An aluminum disk with small bakelite rollers mounted around its circumference is suspended on a frame so that it can rotate freely and at the same time sming freely on ball bearings between a horizontal and a vertical position. A continuous small belt driven from a traction roller on the belt conveyor is pressed against opposite sides of the aluminum disk by small pulleys. When the disk is vertical, as it is when the conveyor is running empty, tlie component of the motion of the small continuous belt tending to rotate the disk is zero. When the conveyor is loaded, however, the weight transmitted through the lever systems from the weighing rollers under a section of the conveyor swings the disk from the vertical toward the horizontal position to a degree proportional to the load on the conveyor. I n such a position the movement of the small continuous belt exerts a rotating setion on the disk, and the rotation of the disk is recorded on a continuous counter. The more nearly the disk approaches tlie horizontal the greater becomes the component of the motion of the small continuous belt in the direction of rotation of the disk. Since the rate of travel of the small continuous belt is directly proportional to the rate of travel of the conveyor belt, and since the angle of the rotating disk is proportional to the instantaneous weight on the weighing section cf tlie belt, the resultant rate of rotation of the disk is a measure of the rate at which material is being conveyed.
Courleiy of John Choldon 6. Sons Figure 1-lnteeratine. Mechanism of Telepoise Convoyor Scale
ridged surface, the separate ridges acting as cams to push a contactor suspended from the weigh-beam against the righthand drum. The latter has a surface hall of which is electrically conducting and half non-conducting, the line between the two portions following a true helix. With the belt running, the contactor is pushed periodically several times a second against the right-hand drum by tlie revolution of the ridged left-hand drum. Wheri there is no load on the belt, the contactor hangs at its lowest point. where i t does not touch any of the electrically conductive surface of the riglit-hand drum. With a load on the belt the contactor is pulled up betwcen the drums, its position being dotermined bv the weieht on tho belt. The contacts with the conduetine pbrtion oflthe right-lmnd drum allow the pa.ssage of electric current impulses to the register and recorder. Increased weight on tile belt raises the contactor, resulting in a,n increased number of tra.nsmittcd impulses per unit of time, while decreased weight on the belt lowers the contactor, cutting down the number of transmitted impulses. Similarly, increased belt speed increases the numhcr of contacts per unit of time, because the rate of rotstion of the drums is proportional to the rate of belt travel. The number of electrical impulses to the register is therefore a combined measure of tlie rate of belt travel and of the weight of material on the belt. The Telepoise can be balanced directly by means of a test weight by merely shifting a lever, which automatically cuts out the recording mechanism and shows by an ingenious arrangement of moving pointer and dial whether the scale is in accurate balance. Another feature is a percentage load indicator consisting of a mirror system so arranged that the movement of the weieh-beam reflects a black line on a large illuminated ground &ss calibrated in per cent of full
Figure 2-Integrating
Courlrry of Herrick Scale M f t . Co. Mechanism of Conveyor Weiehfometer
Cut-away view shows mechaiical inlegratin8 device described io text.
The Conveyor Weightometer weigh beam is counterbalanced by a cylindrical steel float pmtially immersed in a bath of mercury. Calibration is accomplished by suspending a test section of roller chain of known weight on the weighing section of the conveyor while the conv&or is running a i normal speed. This simulates a constant known load. Ad-
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justment for the iveight of the empty belt is made with the conveyor running unloaded. Platform scales of the automatic type are applicable to the handling of a continuous flow of material contained in small plant trucks, narrow-gage rail buggies, or carriers on overhead monorail lines, the platform in the last case being replaced by a weighing section of the rail mounted on levers connected to tho scale mechanism. The automatic weighing
Covrlrsy
Figure 8-Hupper
01 Tdoicdo Scdc Cornpony
Scales Meaaurink Feed of Batch to Glass
and recording is carried out by mechanisms such as the Mechanical Weighman, which utilizes the disk-and-belt principle of the Conveyor Weightometer to operate the balancing weight on a weigh-beam, and automatically records the value of each load on a totsliaing counter. The same type of automatic weighing and recording mechanism may also he applied to hopper scales. Semi-automatic operation of platform scales of the weighbeam type may be obtained by the use of the Weightograph, a weight-indicating device of the pendulum type, which may be attached to the end of the weigh-beam to face in any convenient direction. The pendulum carries a weight scale, the numbers on which are transmitted to an illuminated ground glass easily read by the operator at a considerable distance. Manidly operated platform scales are useful for handling infrequent loads of material, but are not good practice for large plants operating on a continuous schedule of raw material input.
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the dial corresponding to the desired weights of the various components of the batch. Hopper scales are also used for measuring hatches of material to processes. Figure 3 shows in the foreground one of a battery of hopper scales feeding to glass furnaces. T h e operation of this type of hopper scale differs from that previously described for the weighing-in of raw materials, in that the amount of material taken into the hopper from the storage bin is indicated directly on the large dial on t h e operating floor, and this amount is directly controlled by the operator instead of being a constant value. Hopper scales of the type illustrated in Figure 3 are excellently adapted to any process which requires the feeding of materials in rather large quantities, but a t intervals rather than continuously. Convcyor scales on parallel conveyors, each of which carries one of the components of a batch, offer a check on the input of materials to a continuous process feed, as in the manufacture of cement. Simple conveyor scales are not adapted to controlling the rate of feed of the various compunents, however. The Poidometer goes a step beyond the conveyor scale and supplies this control. This equipment uses the movement of a series of lever arms, actuated by a section of the conveyor belt, to control the hopper gate feeding to t.he conveyor. Figure 4 shows poidometers arranged in batteries of two each for the purpose of proportioning raw materials in cement manufacture. For operations requiring the addition of definite proportions of water, such as the slaking of lime, a liquid-measuring device may be attached to the Poidometer, which feeds water to the hydrator a t a rate that is predetermined and is controlled by the same mechsnisni which feeds the lime t o the conveyor. Poidometers are particiilarly applicable bo the control of compounding in
Compounding and Batch Check-Weighing
For making up small batchos of material, as in the compounding of rubber accelerators or of dough in bakeries, small platform scales are used with ungraduated dials showing a center indication and a warning range above and below the correct balance. For larger industrial operations platform and rail s c a h find varied uses. In several large glass plants the components of the batch are weighed into a special truck running on a track below a line of storage bins, and thc complete hatch is then check-weighed on rail scales at the end of the run. These special batch trucks are merely containers mounted on platform scales, the whole arrangement being carried in turn on the truck base. Scales of the pendulum type with direct indiration of weights on a large dial are used chiefly for this work. If it is desired to keep the composition of a batch secret from the truck operator, the scale dial may be left blank, and adjustable indicators supplied instead, these being fixed at the points on
Fipure I-Poidomefer9 ProporfloninC Materials in Phoenix Portland Cement Company Plant
any process where operation is continuous and dry granular materials are to he handled in large quantities. They are generally equipped with automatic controls which will stop all of the conveyor scales in a battery if any one fails to receive its proper amount of material. Another attachment is provided for batch work which will stop the conveyor scale after a predetermined amount of material has been delivered b y it. The amount of material carried is calculated from the setting of the scale weights on the weigh-beam and the distance traveled by the belt, as recorded on a cnntinuous counter. Installations are in operation with as many as sixteen of these feeder conveyors operating in parallel for the cnmpounding of a mixture. They are in use for the extensive handling of material in the clay products, gypsum, lime,
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
fertilizer, cement, and similar industries, and for handling grain, as well as coal and ashes in power plants. The guaranteed accuracy for normal operating conditions is 99 per cent.
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package weight on equal-arm balanccs with scales graduated to show a certain small range of underweight and overweight. Here the amount of deviation from the desired constant value is the important thing rather than the actual total weight, so that scales of this type are frequently called difference weighers. A typical application outside of the field of packaging is the checking of rubber heels before vulcanizing. Overweight is trimmed off,the rubber scrap being returned together with underweight heels t o be re-used. Counting scales have a n important place in industries manufacturing a small uniform product, such as washers, fullcr balls, special bolts and nuts, bushings, small molded articles, and similar small specialties which are sold by quant.ity. These counting scales are so constructed that one object in one pan will balance a certain number, as fifty or one hundred, in the other pan. They are frequently referred to as ratio scales for this reason. Overhead monorail systems are used extensively in packing houses, t.extile mills, paper mills, and foundries for transferring products. Weighing sections in these monorail tracks connected to dial indicators allow the rapid determination of the weights of heavy articles in transit. An interesting development from the ordinary conveyor scale is a machine developed for weighing continuously a sheet product such as auto-top material, paper, or rubber tire tread. The strip of material passes over a weighing
CouiL~ryof The Frorl Wciglil S c d c Co Figture §-Fertilizer Sackin# Scale
Packaging and Product Measuring
The estimation of the amount of a product from the weighing of the raw materials entering into its manufacture allows too many loopholes for loss in processing to be tolerated by careful engineering executives. The product, not only of the final process of a plant, but also of the different stages in processing, should be checked to make possible the reduction of manufacturing losses to a minimum. In all of this weighing of intermediate and final products the conveyor scale is important for the plant in which production is arranged on a continuous interlocking basis, and the materials are such as can be handled well in conveyors of the belt, bucket, or pan type. Where plants are built for batch operation, the platform scale for the weighing of hand trucks or buggies, or for adaptation to a simple roller conveyor, is likely to be favored. The hopper scale may also be used at the end of a cont.inuous conveyor as a measuring unit discharging to storage or loading material for trausportation. For products distributed in packages, rather than in carload or cargo lots, there are numerous typcs of scales adapted for special conditions, such as sacking scales, and package obeckweighing scales or difference weighers. Sacking scales are of two types. The first is an ordinary platform scale, the feed hopper being arranged over the platform so that the sack when attached to i t rests upon the platform. The other type of sacking scale is illustrated in Figure 5, the bag being suspended from a feed hopper which is supported by the lever mechanism of the scale. Such scales are used largely for sacking fertilizer, grain, flour, cement, plaster, salt, and similar materials. Where products are put up in small cartons by automatic filling machinery, a checking scale is necessary to insure against a continued run of underweight or overweight packages. Check-weighing is usually done by hand, sample packages being taken at intervals from the line leaving the filling machine and rapidly compared with the standard
. . Figure 6-Contlnuous Sfale Recordine Weight and Checking Uniformityof Sheet Product sf End of M a n u facfurinll - Process Weighing roller at center of spa? between upper I?me rollers is suspended at one end on bearing suppoifed by weizhbeam of scnie.
roller connected to a lever system as in tlie conveyor scale, and records on a dial the weight of the moving material. Such scales give a constant check on the uniformity of the product. Figure 6 shom a typical installation and emphasizes the small amount of space required by the actual weighing mechanism. The urcighing roller is the small-diameter roller in the center of the illustration, one end of which is supported in a bearing mi the weigh-beam. Important Factors in Industrial Weighing
The chief requisites of an industrial weighing mechanism are accuracy, ruggedness, speed of weighing, and eliminat.ion
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of errors in reading. A very good degree of accuracy may be obtained with any of the types of scales mentioned; conversely, any of these types may become very inaccurate if not consistently checked. I n general, the accuracy of most industrial installations should be not lower than 99 per cent. It is stated previously in this article that one type of conveyor scale is guaranteed t o meet this standard, while a recent English article (1) claims that a good hopper type scale should be accurate to within 0.2 to 0.5 per cent. Most industrial scales are constructed to stand punishment, but the engineering excellence is likely to vary directly with the price. Particular attention should be paid to the manner of suspending knife edges to eliminate scraping over the surfaces upon which they bear, and to the maintenance of constant belt tension in conveyor scale installations. The factors of speed of weighing and elimination of errors in reading are closely related, and are not, as in the classic dilemma of efficiency and economy, mutually incompatible. Weighing by completely manual operation is the slowest method and the one which introduces the greatest number of opportunities for error. Semi-automatic weighing, in which the balancing is done automatically and the weight is merely read off by the operator, is a great improvement both in speed and in elimination of errors, but complete automatic weighing, in which the final step of recording the weight is also carried out by the machine, represents the most rapid and foolproof system of all. It is absolutely necessary, of course, that automatic weighing devices be checked frequently to detect possible errors of adjustment, but if this is done they provide an invaluable measuring instrument for the large-scale operation of industrial processes. The trend in the United States a t the present time is predominantly in favor of the automatic handling of processes, because it has been very definitely proved that a machine can always surpass a man a t doing anything that
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has to be done repeatedly. The trend toward automatic equipment is reflected in the replacement of the hand truck by the continuous conveyor, a change which carries with it the replacement of the manually operated scale by the automatic conveyor scale or the hopper type of weighing equipment. Even in plants where transfer of material is still, for some good or bad reason, carried out largely by hand, the semi-automatic scale is replacing the old-fashioned platform scale where the weighmaster juggled weights all day. Complete automatic weighing equipment is, of course, not particularly applicable to the small-scale or discontinuous operation. The process in question must involve sufficient amounts of material and labor to overbalance the cost of automatic equipment by the savings in time, labor costs, and accuracy of measurement made by the equipment. Nothing in this article should be taken t o indicate that automatic weighing is the type indicated for every industrial problem of material measurement. This article has attempted merely t o indicate that the manually operated scale is not particularly applicable to modern large-scale production, and that either semi-automatic or automatic equipment is the better engineering choice. Acknowledgment
Information supplied by the following firms has been used in the preparation of this article: Exact Weight Scale Company, Columbus, Ohio; Howe Scale Company, New York, N. Y.; hlerrick Scale Manufacturing Co., Passaic, N. J.; Ralston Scales Corporation, Columbus, Ohio; Schaffer Poidometer Company, Pittsburgh, Pa. ; Toledo Scale Company, Toledo, Ohio; E. J. White, Sales Agent, Telepoise Conveyor Scale, New York, N. Y. Literature Cited (1) Benton, Chemistry Industry, 46, 741, 764 (1927).
Making the Glass Disk for a 70-Inch Telescope Ref lector',' A. N. Finn BUREAUO F STANDARDS, WASHINGTON, D . C.
Details are given of the procedure followed in making is increased. The most seriHE m a n u f a c t u r e of a glass disk 70 inches in diameter and 11 inches thick ous difficulties met in proglass for o r d i n a r y for use as a reflector in an astronomical telescope. ducing large pieces of glass optical instruments is These include the making of the pots, the modificainclude properly melting the a well-established but limited tions in the construction of the melting furnace, the amount of glass required, industry, there being probconstruction of the mold and annealing furnace, the transferring it to a mold t o ably not more than ten optimethods of measuring and controlling temperatures, produce the desired shape cal glass plants in the world. the melting and casting of the glass, essential data on while maintaining the necesThis industry is, however, annealing the glass, the determination of the quality sary quality, and finally coolvery necessary for the mainof annealing (the amount and distribution of the ing it a t rates such that it tenance and development of residual strain), and the drilling of the hole to accomdoes not crack and that it will our commercial and scientific modate a Cassegrainian mounting. be well annealed (free from enterprises, since a lack of disturbing internal stresses). suitable glass for making The proper cooling (annealing) of large pieces of glass is microscopes, cameras, surveying instruments, range finders, etc., would greatly hamper all work in which such instru- probably the most exacting and tedious phase of the work, and this cannot be successfully accomplished without knowments are used. Difficulties which are of minor importance in making glass ing the thermal properties of the glass and adequately confor smaller optical instruments become more serious as the trolling the temperatures of the annealing furnace. size of the instrument, and consequently the size of the glass, Plan of the Work
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Received May 21, 1929. Publication approved by the director of the Bureau of Standards of the U. S. Department of Commerce. 1
On methods Because Of the lack Of definite of making satisfactory pieces of glass which could be used
