design of sand grav 00 kum b

7/27/2019 Design of Sand Grav 00 Kum b

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Digitized by the Internet Archivein 2009 with funding from

CARL!: Consortium of Academic and Research Libraries in Illinois


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PREMCE.

!Clie object of the writers, in this design,has "been to make as theoretical, yet pract-ical, a design of a Sand and Gravel ?/ashingPlant as possihle. There have been no workspublished, to the best Imowledge of the writ-ers, on this subo'ect and the writers havehad much difficulty in obtaining what littletheory has been used in this design.

The writers wish to convey thanks to all whoassisted in their undertaking. To Prof. E. I.Stevens, of the Armour Institute of Technology,the writers are indebted for the assistancerendered in the design of the bins. A. W.Burns, of the Smith Engineering Works, supp-lied information regarding the selection ofthe proper machinery. B. W. Huntington, ofthe link Belt Company, gave us access to thetheory underlying the analysis of screens andbelt conveyors.


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MDEZ.

Introduction iScope .... 4Product .... 5Description and operation of plant 8Design of Plant location and Eailroad connections ... 13Bins .. 15Conveyors . 22Design of Belt Conveyors >. 23Distribution and installation ofPower * .... 36Comparative Analysis of Gravel andSand Screens 45The Cylinder Screen 48The Overhung Conical Screen 52Dimensions of Screens 59Brief of the entire Thesis 60Drawings of the Plant, layout, etc.,. 64.


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DESIGU OF A SAIID AUD GRAVEL 7/ASHIITG PIAHT.

Present day concrete construction re-quires the 'use of materials which will give thehighest attainable safe working strength. To ob-tain this point, it is necessary to properly propor-tion the ingredients which enter into the concretemixture and remove any materials that may tend toweaken the concrete after its final setting^ Theonly practical way yet found to meet the aboveconditions has been to wash the materials. Thewashing of sand and gravel being a comparativelynew field, there is very little to be found onthe theoritical design and economy of washingplants.

In our design, the object is to washand grade the materials in such a manner thatthe materials can be proportioned to conform withthe theoritical analysis of the materials enter-ing into concretes giving the highest strengths.

Fullers Curve, which is the most pract-ical, as well as theoritical, analysis of sands


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we have to work with. The curve will give the read-er a fair idea of an ideal composition of a sand.The curve is reproduced on another sheet for fur-ther reference. Curves for other sands are alsoreproduced on the same sheet.

It can "be seen, from the curves, thatordinarily sands vary greatly from fullers curve.It may also be reasoned that no two sands arealike and therefore each particular sand shouldperhaps "be screened into different grades. Butthis would not "be practical "because sands varyin composition with their depth under the sur-face of the earth.

]?uller*s curve, which is a comhinat-ion of ellipse and straight line, is plotted withpoints obtained from an equation for which WilliamB. Fuller is authority (See Taylor & Thompson's ""Concrete, Plain & Reinforced", page E06). Thiscurve is taken as the ideal which it is desiredto approach hy a comhination of materials inthis particular case, the "B" sand and the stonescreenings.


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ordinate. It is evident, also, that on sucti an or-dinate, the percentages desirable to pass any givensieve can be determined from its intersection v/ithPuller's curve. Subtracting actual and ideal per-centages then gives the variation of actual mater-ials, from the ideal; and knowing this difference,the proportions of one needed to supply the lack ofthe other may be determined. With the results ofthis analysis it is possible to design screeningmachinery which would yield a washed sand of anideal composition. The design of the screens willbe considered, in great detail, later.


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It is the object of the writers to designa plant to satisfy one certain set of conditionsv/hich will he enumerated later in this treatise.It is plainly seen that it would be impossibleto design a plant which would satisfy all condit-ions.

In as much as there has never beenany work published covering the design of thisparticular subject, the writers hope to incor-porate such methods in the design of this plant,as have been justified by good engineering pract-ice in other fields.

The Y^rriters do not undertake to designeach and every separate machine used in the design.The machinery used in this design is well standard-ized in every detail and is found on the marketof several different makes.

The capacity of the plant will be onethousand cubic yards per day. The writers feelthat in choosing a capacity of one thousand cubic

yards per day, the capacity and dimensions will


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be that of the average modern plant. Plants ofsmaller capacities and all plants of larger cap-acities can he designed hy us^ing the same meth-ods used in this design.

SIZES TO BE MAEZETSH.

Gravel and sands prepared for market-ing vary greatly in their sizes depending uponthe purpose they are to he used for. The great-er part of the materials are used in concreteconstruction. A large amount of sand is usedin the mixing of various kinds of plasters anda faiily large amount of pea size stone is usedin the finish of hard surfaced roads-

Banlr run gravel varies greatly in itssand content. Prom investigations conducted hythe writers, some gravel pits have been foundto have as much as sixty percent sand whileothers have been found to contain as little astwenty percent. Beside the varying sand contentsthat gravel banks may contain, layers of clay


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and moulding sand are often uncovered. Theclay layers are usually of small thicknesses,seldom exceeding four or five feet. While thereis a marlret for clays of certain structures, theclays are seldom marketed on account of longhauls to points v/here they may he utilized*Moulding sands are more easily marketed, hutthey require great care in preparation. Althoughthis plant will not he designed to market mould-ing sands and various clays, the two materialsare mentioned merely to enumerate the compos-itions of various gravel hanks.

Generally, gravel sands have the com-position as shown in J'uller's curve. In thisplant, all stones that will not pass througha screen with two and one quarter (E-l/4) inchperforations will be sent to a crusher andreduced to such size that will pass throughthe above mentioned screen. An^ material pass-ing through a two and one quarter inch screenhut retained hy an one and one half (I-1/2)inch screen will he the largest size marketed.


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Ihe next size marketed, will te that size whichpasses through a one and one half inch screenhut is retained by a threequarter (s/4) inchscreen. The three sizes screened out thus farare marketed as coarse aggregate used in con-crete construction, although sometimes used in"building hard surfaced roads* The next sizescreened out is that retained by a one quarter{1/4) inch screen. It is known commercially aspea size sand.

The remainder of the material is div-erted to settling tanks in which the finer andlighter particles are held in suspension "bywater currents while the heavier and coarserparticles settle to the bottom of the tank. Thetank is constructed with a weir on one sidefrom which the water containing the suspendedmaterial is drained off. The tank also has anopening at the bottom which dispenses with theheavier and coarser particles as they settle tothe bottom of the tanks.


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DESCHIPTIOIT OP OPERATION OP PIAUT.

An outline drawing of the plant issubmitted on page 1 of the appendix.

The gravel is "brought to the planthy means of a train operating between the plantand the gravel pit. Ihe gravel is usually loadedon gravel cars "by means of a steam shovel inlarge plants; in smaller plants, the use of agravel train is dispensed with, and the gravelis brought to the plant directly from the pitby means of drag lines or grab buckets operat-ing from a cable way. Some small plants haveused drag hoes successfully*

TThen a gravel train reaches the plant,

the gravel is uhloaded into a hopper. Graveloars are always provided with mechanical dump-ing devices. This hopper allows a constant streamof gravel to flow onto a conveyor belt whichcarries the gravel to a cylindrical screenabove the crusher. This screen removes all theoversize; the oversize being lead to the crush-


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material passing through the screen is diverted,through another chute, to the main conveyor whichcarries the material to the plant proper. Thecrushed material is conveyed from the tot tornof the crusher to the oversize screen and allthe material over two inches in diameter removed.Should any oversize pass through crusher, it wouldbe retained hy the oversize screen before reach-ing the conveyor running to the plant.

The main conveyor carries the materialto the top of the plant. Here the material isdivided into tv/o equal streams. One stream pass-ing through each of the two sets of conicalscreens. The material after being split into twosections, is conveyed through chutes to the screens.Dry material will generally not flow through chutesunless the inclination is great. A very large in-clination of the chutes leading from screen toscreen would mean a great waste of space and woulddiminish the capacity of the bins. The inclinationof chutes is usually about twenty degrees; neverany greater. This inclination is smaller than theangle of repose of the material and, the critically.


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the material will not flow at this angle*At this pdint, a three inch water pipe

is introduced and water is added to the material.In a thousand yard, duplex plant, about one hun-dred gallons of water per yard of material isadded at this point. The material is carriedthrough the chute, by the high velocity of the waterinto the one and one half inch conical screen*Some plants are equipped with scrubbers at thispoint. A scrubber is a cylinder equipped withbaffle irons on the inside and has no perforat-ions. The duty of the baffle irons is to toss thematerial around sulficientifcy so that the entiresurface of the material will come in contact- withwater* Upon entering the screen, the water willcarry all the material smaller in size than oneand one half inches directly through the perfor-ations in the screen. The larger sizes being re-tained by the screen. A one and one half inchwater pipe enters the screen and water, at ahigh spouting velocity, is forced against thematerial retained by the screen. The nozzle on


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ed over the material. This action of the watertends to oompletely wash the material retainedhy the screen. The material retained is thenlead through chutes directly to receiving "binsbelow. The material passing this process isseparated, in a similar manner, in the three*quarter inch screen. The material passing thr-ough the one quarter inch screen is divertedto the settling tanks. These tanks are providedv;ith a weir. The material enters at one end ofthe tanks and the heavier particles settle tothe hottom of the tanks. The material whichremains in suspension passes off through theweir with the discharge water. The materialwhich settles to the "bottom of the tank is all-owed to pass through an opening in the "bottomof the tank into the bin; the settling tankbeing of hopper like construction and station-ed directly above the Sand bin.

The discharge water is lead throughtroughs to a storage reservoir near the plant.This reservoir is an escavated pit of such


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Its walls as fast as the water flows into it.The "bins are hox-like in shape with

chutes in their side v;alls. The chutes are pl-aced high enough ahove the level of the rail-road track so as to allow the material to flowfreely from the "bin into railroad cars; a load-ing track runs parallel and directly beside the"bins. The material in the hins is kept suff-iciently wet so that it flows freely throughthe chutes into the cars.

Uo switch engines are used on theloading track. The track is so graded thatthe cars can be coasted from their positionon one side of the plant to the place where the loaded cars are stored*


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DESIGl OP PIAM).

LOCATIOIT Aim RAILROAD COUIIECTIOITS:The location of the plant depends on

mainly on two things, namely, railroad connect-ions and location of gravel bank. In choosinglocations, the market must "be one of the mainconsiderations. A plant huilt some where outin the wilderness a long distance from the mar-ket would not be very economical, in that thecost of hauling would be too great. Also, theplant should be built on or near the main lineof some railroad. The construction of a privaterailway to a plant will cost more than the costof the entire plant if the railway was of verygreat length. The connections of the existingrailTTay, if there is one, should be investigat-ed as to the possible marketing area it wouldcover.

The location of the loading tracksdepends mainly on the position of the plant.The only requirement being that it be built


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parallel to the plant; so as to facilitate load-ing from the different hinSf The part of the sidetrack passing the plant should he inclined sothat in the process of loading, the empty carscould he started downward,on a small grade, hyhand hy means of a car jack and controlled hymeans of the friction brakes on the cars. Thiseffect will dispense with a switching engine.A thousand yard plant, would he capable of load-ing about thirty carloads a day. The empty carswould be stationed on the upper end of the incl-ine and would be coasted to the plant, singular-ly or in groups, for loading. After being load-ed, the cars would be coasted to a place fartheron the side track to be stored until removedby the railroad. A seven tenths percent gradehas been employed successfully in practice forthe coasting of railroad cars. The distance thisincline should extend on each side of the plantwould necessarily be equal to the length ofthirty ears. This distance would be twelve hund-red feet. The length of the side track would of


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empty oars greater than the numher of carsused in any one day. In this particular design,the writers have decided to have a side track

*capable of accommodating at least sixty emptycars and as many full cars* This would callfor a side track extending twenty four hund-red feet on each side o;? the plant, which wewill adopt.

Bins:

The capacity of the hins will hemaintained small because of the fact that thematerial is loaded into cars almost as quick-ly as it is washed. The cost of bins for storagepurposes is so great that is found more econon-ical to store the material in piles near theplant and load from these piles by means of adrag line. From this stand point, the writersfind the most economical size of bin to be onewith a capacity of from to to three carloads.A sixteen foot square bin with a height oftwenty five feet (the height being measured


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ge ther of wood with no super structure. TOiileit may be argued that "by "building a super-structure, a great deal of lumber may be saved,the writers find the most economical kind ofstructure to be one built as a bin from thesurface of the ground up, with no floor dir-ectly beneath the mouth of the loading spouts.Some may argue that the bin should be const-ructed as a hopper in view of the fact thatthe loading is performed by means of spoutsplaced into the sides of the bin^, but the writ-ers find that the material will act as a hopperby itself; the material acting along planeseffected by its angle of repose, and the anglesof repose may be controlled by the degree ofwettness of the material.

The bins are built as water tight asis found practical in the process of construct-ion. It is desired that they be water tight inthat the angle of repose, and the flow, of thematerial both depend upon the degree of wetnessof the material. The water system is construct-


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ed in suoh a manner that any desired quantityof water may be diverted into the "bins when itis found the material is not wet enough tocause sufficient flow from the bins*

In the design of the bins structure,the pressures at various depths in the bin, dueto the material in the bins, have been takeninto account as shown in the data on the foll-owing pages. The structure was designed accord-ing to the same methods used in the design ofretaining walls. The writers have taken economyinto consideration throughout the entire design.

The foundation has been designed towithstand a maximum loading, and also for de-pendable service. Prom investigations of plantsnow operating, made by the writers, it wasfound the lower parts of the bins are usuallythe places where the first failures of the plantstructures occur. The lower parts of the bins,and also the foundations, were usually const-ructed too weak. The cause of the failures inthe foundation was found to be lack of reinfor-ceing steel the


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some cases, the two sides of the foundationswere not tied together, Uithout tie walls,"bulging out of the foundation is certain toresult* The writers have designed this found-ation with an ohject of perventing any suchfailures* The foundations being of reinforcedconcrete v/ith tie walls every sixteen feet;the walls of the bins being Sixteen feet apart.

The screening machinery and motorsrest directly upon the bin structure and thebins are designed with that end in view. Thefollowing page will give a demonstration ofthe method used in the design of the bins.


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DESIGII OF Bins FOR GRA7E1

The following is a ty^pical design ofmem'bers used in the construction of the "bins.

Assnmptions:Angle of repose of material: 30.i;7eight of material: 120# per cti ft*u-j_ = .40 for sand or gravel.u - .57774. Reference: Ketchum.tanx = ufyu-=^^-^-r Txt uj_

/.5774(ry-.35S)tanx = .5774/-/---/ 57774^-. 40

tanx = .57774 y-y = 1.4647..97774T/h^ tanx - up r XEtanx 1 - uuq^-^ (u -/-uiltanx

P 5 Total pressure on v?all at depthdesired.h = Depth*W ^ Weight of material.


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P taken at a depth of 40' in thisillustration.P = 120s.40Sxl60o/2.9294 =26,400#p = pressiure at any particular point,p at 40* equals P at 40.5' less theP at 39.5'.

DESiaiT OF SQHJDIIIS.

Studding placed 28-0^^ c-cDesign of studs from 40* to 37.'M = (VJi-f VI2)11^/20 for end.

iM = {W^'r-W2)LV24 for intermediate.W-,^- 1250 -t- Wg = 1,325.

M = 26,900"#Assimie 2"s8" studs.

M = Sl/C = 213S33 S.S = 1,200# which is within the allowed

limits.


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DESIGU OP BBACHTG.

Horizontal bracing 57' from top.Wi ; 75 Wg = 1E50.

Rl = 1800# 37 - 34 feet.Rl ; I7-lL/3 * Wgl/SRl = 1,913# for 40 - 37 feet.

Total R is 3713# per linear foot.Reaction for two foot interval is7, 426#

Et = 84,700"#.

M = Sl/CS = 995# whicli is within the allowedlimit of stress.

i'^?se 1 TIE rods spaced 4'-0" c-c.


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OOUVEYORS

Where it is necessary to elevate thematerial handled, two methods may be employed:1 st, where space is limited, place a fiseddtunp at the end of the belt and install a buck-et elevator: 2 nd, run the belt on an inclinedplane. The second method is approved where spacepermits. The angle of elevation is, however,limited under the vary best conditions itmust not exceed twenty five degrees and ord-inarily from eighteen to twenty degrees is thelimit. large particles tend to roll back, and,if the feed is irregular, the tendency is forthe whole load to land-slide. Therefore, asteady, well mixed load is advantageous.

Special consideration must be made whewhere the belt runs along horizontally for aspace and then starts on an incline* Here thebelt tends to pull away from the idlerw if theangle is too great. The allowable angle dependsupon whether the belt is loaded or not, thenature of the load, the weight and tension of


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DESIGII OP BELT COMEYORS.

Patting a "belt over two pulleys, pull-ing and tightening until it is perfectly flat,and installing guide pulleys to "steer'^ it, doesnot by any means constitute the successful inst-allation of a "belt conveyor^ On the contrary,

,there are a number of points, each small in it-self, but which in the aggregate make or uiimakea belt conveyor. Some of these points I willtry to explain.

TROUGHIHG IDISRS:The troughing idlers are often cons-

tructed more for the iinmediate convenience oftheir individual manipulation than with thou-ghtful consideration for the wear and tear ofthe belt. As this is first, last and always anitem of the greatest importance, it ought to beconsidered first. Pulleys in line may have atendency to act lilie a pair of shears and thisis continuous and intensified when the loads


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telt. As a rule. In a "belt conveyor of anyappreciable length, the telt represents atleast two thirds of the initial cost of inst-allation, and it would hardly seem the part ofwisdom to endanger the two thirds which issuhject to constant wear to effect a saving in tthe one third which, owing to the nature of thematerial and the character of the work that itperforms when under proper care and manage-ment should last almost indefinitely.

When troughing idlers were first"built they were designed for an angle of trou-ghing of 45, 30, 25, and 20 degrees, and finallythe five pulley idler in contradistinction tothe three pulley idler, was made with the diff-erentiation in the pulleys of 15 degrees. Thereason for this is self evident. The lattertype more nearly conforms to the circular, elim-inating angular "bending and "being a compromise,although a bad one "because not necessary.

ABRASIOF OP BEIT:


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to take them up individually.First: A conveyor belt should never

be called upon to do any other v;ork than tranS'-fer the material from point to point. But oftenit is called upon to stop the velocity of the ma-terial, producing abrasion that is illegitimateand ought to be taken care of in the design oftransfer or feeding chutes, but vyhich unfortunate-lyis not usually the case. There are two methodsof making transfers: One useing gravity, whichis always preferable when conditions make itpossible, and the other, in cases of absolutenecessity^ utilizing velocity.

Second: Much depends upon the correctdesign of the transfer chutes. Too often we seeskirt boards used, running parallel with thetravel of the belt, to control the spill of thematerial due primarily to the inability of thebelt to conform to the outline of the troughingidler, which in many cases is running almost flat,and in these cases the material catching underthe edge of the skirt boards has to drag and


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these retaining devices. This is wrong and shouldhe avoided. If the transfer is properly designed,with the center sufficiently contracted and thenoses or leads given the necessary freeing angle,skirt hoards can he entirely eliminated, and thehelt relieved of this wear and tear.

Third: Installing elevating he Its attoo steep an angle or running them too fast pro-duces slip and scour. There is a relation hetweenthe speed and angle of elevating which must nothe overlooked.

TEE DRIVER or HEAD PUIIEY:In order to conform to false economic

ideas and to reduce first cost, the hear or draw-ing pulley is often made too small, increasingthe necessary tension inorder to ohtain traction,whereas the true common-sense of. conveyor heltdriving is to have the traction without tension.In dynamo driving-, as well as in conveyor driv-ing, tension must he eliminated if you wish toget the hest results in longevity of your helt-ing. This is sometimes overcome, or attempted


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tension. In no part of the design of belt convey-ors is more damage done than in this one part-icular feature, for it is true that the usualtjrpe of conveyor "belt requires absolutely thisexcessive tension in order to make it tract orapproximately run true.

CEBAETHG A C03WEY0R ESIT:The method that is commonly used is that

df Employing a brush to clean the conveyor beltand often using it on a tripper between the twopulleys to prevent the material on the workingside being imbedded in the cover of the belt bythe pressure of the snub pulley. This is a verypoor method. Both pulleys on a tripper are crownpulleys, v;hich prevent the brush from having alevel or flat surface to operate against. If thematerial is wet, as is usually the case, it clingsand cannot be removed except v/ith the use of somuch pressure by the brush on the belt as to doserious damage to the cover, as well as rapidlywearing out the brushes, necessitating an almost


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ing a "belt is to have a driven shaft with looseatDciliary shafts pivoted together and thirownout "by centrifugal force, which strike the beltwith a continuous and incessant series of "blows.Jarring the material loose and necessitating noadjustment. This method is equally efficientand divested of all the disabilities inherent inthe use of the complicated mechanism required tooperate the brushes*

EIPROIES HAJTOLIUG:In operating belt conveyors in series,

especially if any of the sections are elevating,as is the case in this design, there should be agradual stepping up of speed from the initialreceiving conveyor to the ultimate distributingconveyor, if it is desired to prevent breakage*If the item of breakage does not enter into theproblem, the speeds can be uniform. To producebreakage, choke a thin rapid stream with a thickslow one by incorporating a large slow-movingconveyor into a series of conveyors with faster


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Wlien starting, the conveyor whould "be at fullspeed "before any material is fed onto the belt;and 7/hen stopping, the conveyor should be run atfull speed at least five or ten minutes after thefeed has been stopped, in order to have deliveredthe laggards or those pieces which may have a ten-dency, of all other material, to roll back.

SmJB FUIIiEYS:It is preferable, where it is possible to

eliminate snub pulleys to do so, rather increas-ing the diameter of the hear or tail pulley andmaking a long lead to the first return idler;as the tendency of all snub pulleys is to bring theworking surface of the belt with the material cling-ing to it against this snub pulley embedding thematerial, which cuts its way into the body of thebelt and thereby does serious damage.

EEEDS:Correct feed is often the most essential

adjunct of a belt conveyor, for its successful


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conveyor system, and the one too frequently over-looked, is the necessity for protecting the lowerreturning belt from any material dropping on it;and this is practically true when the feed is veryclose to the end pulley, as is usually the case.IPhis is very close to the end pulley and the mat-erial goes between the inner and unprotected sur-face of the belt and the pulley, and is embedded,destroying the thin film of protective cover, bar-ing the fabric and allowing absorption of moistureand grit, producing rapid destruction and disint-egration of the belt*

SPACmG OF IDLERS:Yi/hen it is considered that the belt us-

ually represents two thirds of the cost of inst-allation, the machinery one thid, and the trough-ing idlers often the least proportion of the onethird, it would seem the part of wisdom not tospace them too far apart, producing an unnecessarysag in the belt and causing a flexing that doesno good and often much harm. A little closer spae-


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is required, as more power is saved by the elim-ination of the sag than is required to operatethe additional pulleys. A belt conveyor shouldembody maximum capacity, minimum expense, deeptroughing and a clean carry, minimum tension anda perfect alignment, and absence of edge wearand corner system to secure the desirable featuresmentioned and eliminate the objectionable ones.

BELTIUG:The ultimate strength of the average

rubber belt is about 360 pounds per inch width ofply; the safe working tension (using a factorof safety of 12) is about 30 pounds per inchwidth of each ply. The pull required to move abelt over its carriers upon the level, is approx-imately 20^ of the weight of the belt plus 10^of the weight of the load upon the belt. Theproper flexibility for troughing idler belts is oneply for each four or five inches of belt width,with 12 inch 3 ply as a minimum, and 48 inches8 ply as a maximum*


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POXIEYS:It has Taeen found to "be good practice

to njake the diameter of all drive pulleys five timesthe number of plies of the helt, and all otherpulleys four times the number of plies. Bend orSnub pulleys of a three or four ply belt shouldbe ordinarily about eight inches in diameter;;for five ply belts, twelve inches and for sisply or over, sixteen inches in diameter.

Hubber lagged pulleys increase thetractive effort of the plain driving pulley ofa belt from ten to twenty percent, where contactbetween the pulley and the belt is clean or wherethe dust from the materials is damp. However indry and very dusty conditions of clays and sim-ilar materials which are smooth, the tractiveeffort may be decreased by useing rubber laggedpulleys.

Snub pulleys are recommended at each endpulley in ordinary practice, to relieve the str-ain on the end return idlers where the belt bendsto encircle the pulleys.


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DRIVES:Tl/hen greater tractive force than usiial

is required a greater length of telt is broughtin contact with the driving pulley hy means ofa snub pulley. The arc of contact may be increas-ed fro& 180 degrees to 240 degrees, which increasesthe horsepower pull 20fo and makes it possible touse a belt with one fifth as many plies. Thisestra tractive pull may also be used to extendthe masimum length of the loaded belt 20fo if theshafting and gears are increased in proportion.The average diameter of a plain snub shaft isabout one and fifteen sixteenths inches*

A yet greater tractive pull may be secur-ed with 240 degrees contact if the driving and firstsnub pulley are of the same diameter and are conn-ected by gears as shown in the drawing of thisplant,, thus making the cont^-ct on the gearedsnub available for doing a part of the driving .This is known as a multiple drive. With 240degrees around the geared snub and 180 degreesaround the driving pulley, the tractive effort


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decrease In telt plies within proper trotighingand service limits oer permitting an extensionof the conveyor with heavier drive.

The disadvantage of the snub in any formis that the reverse action of the "belt over thesntth causes some internal wear between the pliesof the belt, while any reduction of plies belowthe number required for proper flexibility tendsto crease the belt longitudinally and make thebelt of not sufficient durability for handlingabrasive materials*


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DISTHIBUTIOU AITO lUSTAILATIOl OP POWER.

The first consideration, is necessarily,the horsepower required. In this design, threepower units will be employed.

The first unit will supply the powernecessary to operate the screens and the beltconveyor. From experience and experimentaldata, obtained by the writers upon the invest-igation of numerous plants near Chicago, it hasbeen found that the power necessary for the op~eration of screens of the dimensions and capac-ity used in our design was approximately fivehorsepower per screen. The power necessary tooperate a carrier belt of the capacity and dim-ensions used in our design was found to be approx-imately five horsepower per twenty foot lengthof belt, or thirty horsepower for the entire belt;the belt carrying maximum load. The conveyor beltand two sets of "three screens on a singleshaftare the only machines driven by this power unit.The over all horsepower necessary will besixty


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The second imit supplys tlie necessarypower to operate the necessary water supplysystem. The horsepower needed in this tmit willdepend upon the amount of water used and theheight it is to "be lifted. IProm the calculationsmade for the water supply, and the efficiencyof the pump as given elsewhere in this thesis,a 16 horsepower motor will he required to operatethe water system.

The third unit will operate the cinish-ing plant and the preliminary screens. The motormust he of sufficient size to operate the crush-er at maximum capacity. The power required tooperate the crusher in this design will hefifteen horsepower. Beside the crusher, thismotor operates a twenty four foot "belt conveyorconsuming eight horsepov/er when carrying a max-imum load. In as much as motors have even rate-ing only, a twenty five horsepower motor will herequired.

In choosing the kind of motive power toused, the factor of economy must be considered.


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Many plants have steam power plants and all themachinery is driven "by "belt from the centralunit. This power system is not only inefficient"but has a very high rate of maintainance. Itis not as dependable as electrical power. It isa well known fact that small steam plants can-not compare in efficiency with electrical power.

In this work, we will use electricalpower only. There is uaually a transmission linein, or near, the vicinity of the plant. Althoughthe common voltage used in transmission lines isfar too great for use in driving motors, theamount of power used hy this plant is sufficientto warrant the installation of a transformer sta-tion. The line leading to the plant, and the tans-former station are always installed by the powercompany; so there is no need of our dwelling onthat subject*

The transformer station being at theplant, it is not necessary to operate the motorsat high voltages. In this plant two hundred andtwenty volt (220) motors will be used. The plant


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vrill also "be wired for lighting on the two twentyvolt circttit. Two twenty volt circuits are justas dependable as one ten volt circuits when usedfor lighting. The only disadvantage being thatof the cost of appliances for two twenty voltcircuits; the appliances cost a little more.

Considering the two twenty volt circuitfrom the motor stand point, the two twenty cir-cuit has an advantage in that a small sized wirecan he used; the current consumption of two twentyvolt motors being less than that of the one tenvolt motors.

Five hundred and fifty volt motors mayalso be used. This voltage, although it mightincrease the power efficiency of the entire sys-tem (transformers included) has its disadvantage inthat it would have to be stepped down for thelight circuit. The dangerof common labor aroundfive fifty volts is also to be considered. Al-though an ordinary mechanic may be able to makerepairs on a tv/o twenty or one ten volt circuit,it would be a dangerous undertaking for him to


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attempt working with five fifty volts aromida plant which is wet all over, if he was notan electrician. The insullation for a fivefifty volt circuit would also have to be greatand the cost of the entire circuit would bemuch greater.

In choosing the motors, it is necess-ary to know the voltage and the frequency of themain line. The common frequencies in this count-ry are twenty five and sixty cycles, but thevoltage vairys over a large range. Standard motorsare usually built for 110, 220 and 550 volts.

So far we have anly considered alter-nating current as our electrical power. Directcurrent may also be used, but direct current isseldom available. The efficiency of transmissionlines depends upon the copper losses in the lines,which in turn depends upon the voltage of trans-mission. It is possible to generate direct curr-ent at high voltages, but the disadvantage isthat the direct current voltages cannot be trans-formed to a lov;er value in the manner alternating


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which are a"ble to operate on the high voltagesused in the transmission of power, and as a re-sult, the direct current is seldom used in powertransmission. Electric railways are the greatestusers of direct current. They operate the carmotors on fi*e fifty and seyen fifty volts* Aboutthe only time direct current coult "be consideredin sand and gravel washing plants is when anelectrical generating plant is to be installed atthe washing plant.

Direct current motors have an advantageover the alternating current motors in thatthey have a very wide speed range. That advanta-ge is offset by their cost. The cost of a directcurrent motor, per horsepower, is usually somuch greater that that of alternating currentmotors that some manufacturers have found it econ-omy to install converters and convert direct curr-ent to alternating current bo as to be able to usealternating current motors. Alternating currentmotors are much simpler in construction. Alter-nating current Squirrel Cage Induction Ho tors are


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ranges with that of the "best motors. The speedof Induction Ilotors depends on the nmnher ofpoles of the motor and the frequency of thealternating current. "The range of speed forthe motors ds very small. They have a constantspeed. V/here peak loads are to be encountered,it is desirable to have a fly wheel when use-ing Induction Motors. An Induction Motor, whenloaded, has a certain percent of slip, that is,with a certain frequency, the speed of the motorshould be a certain number of revolutions perminute, but this is the case. The motor doesnot attain the theoritical speed. The speed isusually ten or twenty revolutions less. Thisamount is called the slip. It tends to lessenthe efficiency of the motor. This factor shouldbe taken into consideration in selecting a motor.It is desirable to has as low a slip as possible.

The one disadvantage an Induction" Motorhas, compared to other motors, is that the Ind-uction Motor has no accelerating torque. The mot-or requires the use of some kind of a starting


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taken into consideration in the selection of amotor. Man^' kinds of devices are "being liuilt "bythe various motor manufacturers* They are allgood devices as long as the can serve the purposeintended for. The same style of devices may beused for several different conditions, but itmust be built to take care of each separate setof eanditions* In selecting a motor, the condit-ions to be meib with must be specified for themaker. Makers guarantee their motors to carrya certain load for a certain length of time \7hitha certain rise in temperature during the time ofthe run. Motors are built to carry heavy overloadsfo'rshCKT'l; intervals of time. A ten horsepowermotor wotild be capable of carrying ^ fifteen horse-power load for a short length of time. Therefore,in selecting a motor, the motor need ohly be largeenough to carry the average load.


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COMPiiEATIYE AITAIYSIS OF GRAYEI SCi?EEirS.

IHE mCLIHED OR GRAVITY SCRSEH,A screen in its simples form is the in-

clined plane or gravity screen. The main diff-iculty with this type of screen is that the mat-erial after traveling a few feet accelerates tosuch a velocity that the holes in the screen arepassed over by particles which should pass through.The length of the effective travel is very shortand the results obtained are very unsatisfactory.

Engineers might say, why not make theshape of the screen or curvature of the path suchthat the velocity is constant or uniform and with-out acceleration? Fig. 1, on page 46 is referredto as a possible curve for such a screen. let A-Brepresent the curve and our problem is to find theshape of the curve or its equation.

The acceleration of a particle passingdownward on an inclined plane without friction isgsine where 9 is the angle of the inclination withthe horizontal. The retardation due to friction or


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Fig. 1. Fig. 2. Fig. 3. Fig. 4.

Fig. S. Fig. 6. Fig. 7.

Fig. 8.


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ieient of friction. Therefore, the accelerationof the particle passliig dovm the plane at anyinstant is:

g{sin9 fcose)ISow since we desire maintaining a uni-

form velocity T/ithout acceleration:gf sinG fcos9) =

sinG = fcosQ

'The result shows that the curve resolvesitself into a straight line the tangent of v/hichis equal to the coefficient of friction. In otherwords, if the screen is set at the angle of frict-ion the particle will slide dov/n the screen atthe same speed at v/hich it started. Unfortunate-ly, we have quite a variation in the coefficient offriction, for some particles will slide downwhile others will roll down. Therefore, hy sett-ing the screen for the former will result in giv-ing the later considerahle acceleration. Another


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tlock the screen unless removed ty some means.

TER CYLIUDER SCESEII.

The cylinder screen is the oldest tjrpeof revolving screen in use. They are set at slight-ly downward slope to cause the material to travelthrough the screen. The path of the particles onthe screen surface is a heliz. In order to deter-mine the pitch of the helical path of the particlein a cylinder screen we will assume a certain angle"a" as the inclination of the screen v/ith the hor-izontal. Every element of the screen surface parall-el to the axis will have this angle with the horiz-ontal, assuming that these angles are taken in ver-tical planes.

Pirst assume a horizontal plane with theline A-0 in that plane. (See Pig. 2, page 46).Assume that the vertical plane through A-0 is rot-ated ahout A-0 through an angle of 90 or into thehorizontal palne. ITow draw C-A-0 equal to angle "a";the C-A line represents an element of the cylind-er. Next draw C-B-A equal to*.


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We know that the particle will travel insome direction in the normal plane which has aninstantaneous angle of 9 with the horizontal plane.Rotating BCA back again into the vertical planeahout BOA as an axis, we will determine the inter-section of the normal plane with the horizontalplane and rotate the normal plane anout the line ofintersection into a horizontal, and determine thetrue angle which the path makes with the cylinderelement. Rotating OB ahout vertical axis, making tthe arc BD, and draw AD tangent to the arc. Ad isthe line of intersection of the normal plane andthe horizontal plane.

In order to get the true angle with thecylinder element it is necessary to lay out the ang-le 9 as ODE and rotate the normal plane into thehorizontal plane about AD as axis, which gives usAPD of i as the true angle which the path of the part-icle makes with the cylinder element.

To reduce this analytically fn terms of thegiven angle, we have:

GA = AP = 5?sin a


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CB = DP = ^^sin aCO

AI? "* _0_ sin4>sin a

// - /^ -1 sin aCos -^sin 9

In order to compute the length of the helixwe will develop the siirfaee of the screen and indi-cate the path of the particle. (See Pig. 3, page 46)The pitch P of each turn of the spiral is:

tan i " sin^

The length of each turn of spiral is:^Dcos^

T - P ilB?ri - tPcos^ cos? sin?

The nrunher of turns is:

P " " sin?" ~ T^DcosS


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The total length of spiral is equal to the lengthof a ttirn multiplied by the numlDer of turns:

Total Length I TtV^-^-'^S- = -^^fsa"sin0 DcosP GOS$But Cos j^ = -i--sinTotal length of spiral = i-Si^J-^-sin aAssume Z 45^ and a = 7

And we have 707L/.1219 = 5.8L for length ofspiral*

It therefore follows that the length oftravel in a cylinder screen is independent of thescreen diameter, but s imply depends on the lengthof the screen and the inclination of its asis. Thisfact no doubt will be a great surpirse to many, in-cluding screen manufacturers, for screens of largediameter are usually given considerably higherrateing than they should have. The length of travel

of a particle in a screen


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36 inches in diameter and 12 feet long is ident-ical to the path of a particle in a screen 72inches in diameter and 12 feet long, if "bothscreens are set at the same inclination. This pathis ahoTit 70 feet long. The advantage of the largescreen, however, is that the distance between thecoils of the helix is greater, v.hich allows the mat-erial to spread out more and make a wider helis.(See Fig. 4 page 46).

THE OVERHUffl COITICAI SCBEEIT.

We will next take up a type of screenused a few years ago quite extensively in gra-vel" washing plants. It is known as the overhungconical type. Figure 5 illustrates the screen andthe manner in which they were used.

To determine the angle of advance of thespiral, we will assume line AB, Fig. 6, as the in-tersection of the normal plane drawn tangentto the screen and forming angle of friction T^ith a horizontal plane. Draw AC equal to MO of Fig. 5,


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of advanoe. Draw DE parallel to AB, making length AEequal to ME in fig. 5, Then EAB, Pig. 6, 4s equalto angle of advance of the helix. This is evident"hj assuming that the normal plane is rotated intothe horizontal plane. The element AE and the axisare then in a vertical plane through AE and theperpendiculars to Ab and AE give the angle of ad-vance and hy geometry would be equal to angle EAB.

To trace the path of travel it is nec-essary to develop the surface of the screen cone.The path then takes the curve of the logarith-mic spiral as shown "by Pig. 7, and when drawn inon the cone takes the form as shovm hy Fig. 5,

The Length of the spiral is:

where R is the length of the cone element AS fromthe apex to the large diameter of the cone and rthe cone element from apex to small diameter of coneand (^ the angle of advance of the spiral justfound.


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Assuming the same slope for the screenas taken for the cylinder screen, we find that theangle of advance is approximately 10*^ and R = 18.18ft., r = 12.1 ft and cos 80 = .1756, and the len-gth of spiral is 347 ft for a cone screen 72 inchlong and I-I/2 inches per foot slope. This ispractically the same length of travel for a conescreen as cylinder screen 72 inches long. Thiswould naturally "be expected since we found thatthe diameter of the cylinder screen did not affectthe length of the path of the travel of the part-icle.

The form of the logarithmic spiral foriyi-ula is r = ae^ and "by use of hyperholic logarith-ms the curves of Pig. 7 are plotted.

rUCLIBED COITICAL SCSEEIT.

The screen which we will next discussis the well known inclined conical screen, whichis mounted on the same shaft as shown in S'ig. 8.The material in this screen travels ahead while


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the screen shown by 5'ig, 5, has a iDaclcward flow.These screens have an entirely different taper thanthe other style and are designed to give the "bestpitch of the spiral and minimum pitch for the flume,from screen to screen. These two angles, togetherwith the given diameter of the large end of thescreen, determine the shape of the cone.

A different diagram is necessary to det-ermine the angle of advance of the helix as well asthe path of the particle.

Assume line OA as the intersection ofa vertical plane through the screen or cone axisand a horizontal plane Jjhrough the apex 0. DrawOB, making angle of friction with line OD, whichis at right angles to OA. We next locate E, know-ing that it is equal to UP, Fig. 8, from Bo anda distance IE from DO. ITow drop a perpendicularEH to BO and rotate HO into OD* We know have thescreen element in the hori^^ontal plane antl canlay off element 1I, as DJ or as HK tefore rot-ation ahout OA. The axis of the screen is inthe vertical plane through 01. Rotate the vert-


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diagram and we have 01 as the axis and KO as theproj'eeted co-ordinate of the element. Assumea point as M and draw MU perpendicular to OLor cut the element with a perpendicular plane,which gives us the true angle of advance of thehelix or ^.

The helix can then he drawn as shov/n byFig. 10, in same manner as before except in thereverse order. T/e find that the length of thishelix is the same, "but usually the pitch is a littleless and the path a little longer.

^e now come to the most interesting anal-ysis of these paths.

The cylinder screen has a spiral whichhas uniform distances between the coils, When aquantity of material is introduced into the screenit can spread, depending on the distance between thespirals. The greater the diameter the greater thespread. ITow as the material travels through thescreen and the amount gets less and less sincethe greater portion goes through, v/e must over-crowd the screen at the start or we are not using


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the full width "between the coils at the lowerend.

The screens illustrated in Fig. 5 havethe helical oaths close together at small end andthe discharge between the corresponding points onthe helis:, or pitch of spiral increases toward thelarge end of the screen. The material is deliveredto the small end of the screen where the materialcan spread the least amount, yet this is the pointof maximiim quantity of material in the screen.

It is, therefore, necessary to crov/d thesmall ends of these screens to make use of thevoider path at the large end of the screen. In otherwords, the increased pitch of the helix is in thewrong direction. The piling up of the material atthe small end interferes with the pehbles passingthrough the openings and a large percent of thescreen area is lost.

The screens shown in Fig. 8, have the widehelical path at the large end where the materialis received. The paths getnearer together as thematerial travels along, The quantity also "becomesless and less. In this screen the spirals are in


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the right direct ion and the efficiency is muchhigher

!Ehe cost of screens is proportional tothe area of screening surface. Another consider-ation in comparing screens is the correct speed forscreens. It is Imown, from results found "by exp-eriments, that a surface speed of from 175 to 00feet per minute is the most efficient; if slov/er thanthis the capacity is cut down and if faster the screen-ing efficiency is lowered because the pehbles donot have sufficient time to go through. V/ith thecone screens the speed should be computed for thelarge end, for if speeded according to small endthe speed of large end would be excessive.

This results in a further cut of effic-iency in screen , Fig 8, because the bulk of thematerial is at the small end where the screenspeed is greatly reduced, while with the screen ofPig. 8, the correct speed is at the large end wherethe greatest quantity of material is received whilethe decreased speed of the small diameter does notcut down capacity much because the quantity of


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material is then so small.!Dh screens used in this design conform

with the theory set forth in the preceeding pages*!nhey were "built "by the Smith Engineering I7orlcs, ofMilwaukee, Wisconsin and are illustrated on page#0, Catalog 254, published ty the Smith EngineeringWorks.

The descriptipn of the screen is asfollows:

Diameter of screens:small end 36 incheslarge end 54 incheslength 84 inchesAverage peripheral speed 140 feet/MinCapacity of screens 15 cu. yds/fir.

IThe screening is performed hy three screens;each screen having different size perforations* In thisdesign, the three screens have perforations of I-1/2",3/4" and 1/4" in diameter.


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BRIEP.

Poimdation:Reinforced concrete.Depth below surface of ground:- 2'-0"length 66 '-6"Width 18 '-6"Thiclmess . 2'-0"Three Tie Walls ) o^ ^4Two End Walls ) ^^^ dimensions.

Bins:IJiamher of "bins 4

' Length (Overall) 64 '-0" c-clength (Bins) 16 '-0" c-cStructure *.. Yellow Pine.Stress (Working) 1S00#Studdings 2"x8" 2'-0" c-cSheeting 2"3:8"Corner Posts

6"2:8" and 8"-8" 16 '-0" c-cWhalers 6"x8"Tie rods .... 18 '-6" long 1" diameter.See drawings for Detail.


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Super- structureMaterial - yellow pinePosts 6 "26" and 6"s:8"Bracing 2"x8".

ConveyorBelt - Duck with ruTDber coaering.length - 120 '-0"24"-looton Conveyor Complete as manufact-ured "by the Smith Engineering V/orks.ofMilwaukee Wisconsin ; their tjrpe H"o. 3-A,Pive pulley troughing idlers spaced 4'-0".Maximum size, of lump material: 8 inches.

One conveyor, same as above, 30 feet long.

Motolrs:Manufactured hy the General Electric Com-pany of Schenectady, H.Y.One 25 Horsepower 220- Volt Squirrel CageInduction Motor; lilakers type 16-4.One 15 Horsepower 220 Volt Squirrel CageInduction Motor type 16-6.One 60 Horsepower 220 Volt Squirrel Cage


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Drive Palleys and Gears:Two - IS" - 60 Horsepo7/er Palleys,Two - 36" - 60 " "Two - 8** - 20 * "Three 18" - 30 " "Pour 8" - 30 B 45 degreeteveled gears*

ScreensSix Telsmith Conical GillDert screensas manufactured by the Smith Engineer-ing Works.Diameter - Small end - 36"

" Large end - 54"length 84"Total capacity: 50 cutiic yards per hour*

Screen Troughing:Eight 1/4" sheet iron, 3 '-10" semi- circul-ar troughs as detailed on drawing.

Settling Tanks:Two 60" Conical sand separators as man-ufactured by the Smith Engineering Works.


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Water Sttpply:Main llnei 4" piperBranches: 1 - 1/2" (One and one half) pipe^

Pomp:Capacity: ^0 gallons per minTite.lift - 60' alDove surface of grormd.Manufactured by Allis Chalmers Comp-any of Milwaukee,7/isconsin.Makers llo. Typer ll-Ea.

Car loading Gates and Chutes:Four 12'^xl4"x6'-0" cast iron car load-ing gates and chutes as manufactured"by Smith Engineering I7orks.

Receiving Hopper for Main Conveyor:Top dimensions 8'-0"zl4'-0".Depth: 6'-0".One side vertical.Structured supported on 8"x8" cornerposts fabricated with 2"x8" yellowpine with sheet iron lining.


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BIBIIOGMPHZ.

Concrete Cement Age 2:129-31 March '134:77


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il b "i)>^ !

design of sand grav 00 kum b

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