The selection of screen sizes depends upon the sizeof the product desired. The maximum and minimumsizes of the product are expressed in the specifications.Consider the following specifications:SizePercentPassing1 inch100 percent3/4 inch90 to 100 percent1/2 inch20 to 50 percent3/8 inch0 to 15 percentNo. 40 to 5 percentBy this specification, the largest size aggregate thatis acceptable is 1 inch. Since 100 percent must passthrough the screen for 1-inch aggregate, 0 percent canbe larger than 1 inch. Although the specification saysthat the lot is acceptable with up to 5 percent of the totalproduct passing through a No. 4 sieve, it is saying thatthe No. 4 aggregate is the smallest size desired.Based on these specifications, the plant should becalibrated to produce a product between 3/4 and 3/8inches. The reasoning behind calibrating the plant toproduce a 3/4-inch to 3/8-inch aggregate rather than a1-inch to No. 4-size aggregate is because somewhatoversize particles can pass a given screen opening sizewhen oriented diagonally.By calibrating the plant to produce a top size of3/4-inch aggregate, you can provide a safety margin of10 percent in the event the product stockpile maycontain aggregate over 3/4 inch, but less than 1 inch. Thesame reasoning holds true on the minimum product size.Calibrate the plant to produce 3/8-inch aggregate as theminimum size, thereby creating a 15-percent leeway forparticles in the stockpile being less than 3/8 inch.Therefore, you should choose the next smaller sizescreen than the given maximum specification size forthe upper size calibration. Smiliarly, you should choosethe next larger size screen than the given minimumspecification size for the lower size calibration.Remember: It is necessary to select screens withopenings slightly larger than the size products desired.This is due to screen inefficiency caused by thetingle of inclination, throw, and angular particleshapes.Table 6-1 is used for the selection of screens and therelationship between product size and screen size. Forexample:If your plant has a two-deck vibrating screen, youshould select the following screens:ProductSizeDesiredScreenSizeSelectedTop 3/4 inch7/8 inchBottom 3/8 inch7/16 inchThe relationship between aggregate size and screensize is important. The required screen sizes are slightlylarger than the product sizes. If a 7/8-inch screen is notavailable, a 3/4-inch screen should be selected to be putin the unit. If a 7/16-inch screen is not available, a3/8-inch screen should be used.ConveyersConveyers (fig. 6-10) are used to move materialbehg processed from one component to another and [oconvey the finished product into bins, trucks, orstockpiles. Rubber belt conveyers are of the fixed orportable type. Fixed conveyers are attached to and areacomponent of the crusher unit. They are similar indesign and work on the same principle as do portableconveyers.Before operating conveyers, check to see that beltsare aligned and do not rub on the frame or other parts ofthe plant, As each conveyer is operated, check to see ifthe belts are ruining true. If the conveyer belt is runningto one side on the head or tail pulley, then tighten theend adjustments on that side until the belt is running inthe center of the pulleys. On new assembled conveyers,this procedure must be performed quite often during thefirst few days until the belt is trained. Even after the headand tail pulleys are set and the belts are ruining centrallyover them, a conveyer belt may ride to one side and offthe end of the troughing rolls. Should this occur, loosenthree or four troughing roll assemblies and slide the endsupward about one-fourth inch on the side to which thebelt is climbing. his action forces the belt toward thecenter of the troughing rolls. The (roughing roll frameshave slotted holes to allow for this adjustment. Thereturn roll frames have slotted holes for adjustments andmay be adjusted in the same manner as the (roughingrolls.Make sure all troughing and return rolls are rotating.Rolls can be worn through and belts damaged if the rollsbecome jammed. The belts should be checkedfrequently to ensure the fasteners are holding and thebelts are not slashed or torn. Operating a conveyer withdefective Fasteners or belts is dangerous to personneloperating the plant and could also cause damage to otherequipment.All conveyers are provided with wipers located nearthe tail pulley on the return run of the belt. Keep the6-10
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