The selection of screen sizes depends upon the size of the product desired. The maximum and minimum sizes of the product are expressed in the specifications. Consider  the  following  specifications: Size Percent  Passing 1 inch 100  percent 3/4 inch 90 to 100 percent 1/2 inch 20 to 50 percent 3/8 inch 0 to 15 percent No. 4 0 to 5 percent By this specification, the largest size aggregate that is acceptable is 1 inch. Since 100 percent must pass through the screen for 1-inch aggregate,  0 percent  can be larger than 1 inch. Although the specification says that the lot is acceptable with up to 5 percent of the total product passing through a No. 4 sieve, it is saying that the No. 4 aggregate is the smallest size desired. Based on these specifications, the plant should be calibrated to produce a product between 3/4 and 3/8 inches.  The  reasoning  behind  calibrating  the  plant  to produce a 3/4-inch to 3/8-inch aggregate rather than a 1-inch to No. 4-size aggregate is because somewhat oversize particles can pass a given screen opening size when  oriented  diagonally. By calibrating the plant to produce a top size of 3/4-inch aggregate, you can provide a safety margin of 10  percent  in  the  event  the  product  stockpile  may contain aggregate over 3/4 inch, but less than 1 inch. The same reasoning holds true on the minimum product size. Calibrate the plant to produce 3/8-inch aggregate as the minimum size, thereby creating a 15-percent leeway for particles  in  the  stockpile  being  less  than  3/8  inch. Therefore,  you  should  choose  the  next  smaller  size screen than the given maximum specification size for the upper size calibration. Smiliarly, you should choose the next larger size screen than the given minimum specification  size  for  the  lower  size  calibration. Remember: It is necessary to select screens with openings  slightly  larger  than  the  size  products  desired. This  is  due  to  screen  inefficiency  caused  by  the tingle  of  inclination,  throw,  and  angular  particle shapes. Table 6-1 is used for the selection of screens and the relationship between product size and screen size. For example: If your plant has a two-deck vibrating screen, you should select the following screens: Product Size Desired Screen Size Selected Top 3/4 inch 7/8 inch Bottom 3/8 inch 7/16  inch The relationship between aggregate size and screen size is important. The required screen sizes are slightly larger than the product sizes. If a 7/8-inch screen is not available, a 3/4-inch screen should be selected to be put in the unit. If a 7/16-inch screen is not available, a 3/8-inch screen should be used. Conveyers Conveyers (fig. 6-10) are used to move material behg processed from one component to another and [o convey  the  finished  product  into  bins,  trucks,  or stockpiles. Rubber belt conveyers are of the fixed or portable type. Fixed conveyers are attached to and area component of the crusher unit. They are similar in design and work on the same principle as do portable conveyers. Before operating conveyers, check to see that belts are aligned and do not rub on the frame or other parts of the plant, As each conveyer is operated, check to see if the belts are ruining true. If the conveyer belt is running to one side on the head or tail pulley, then tighten the end adjustments on that side until the belt is running in the center of the pulleys. On new assembled conveyers, this procedure must be performed quite often during the first few days until the belt is trained. Even after the head and tail pulleys are set and the belts are ruining centrally over them, a conveyer belt may ride to one side and off the end of the troughing rolls. Should this occur, loosen three or four troughing roll assemblies and slide the ends upward about one-fourth inch on the side to which the belt is climbing. his action forces the belt toward the center of the troughing rolls. The (roughing roll frames have slotted holes to allow for this adjustment. The return roll frames have slotted holes for adjustments and may be adjusted in the same manner as the (roughing rolls. Make sure all troughing and return rolls are rotating. Rolls can be worn through and belts damaged if the rolls become   jammed.   The   belts   should   be   checked frequently to ensure the fasteners are holding and the belts are not slashed or torn. Operating a conveyer with defective Fasteners or belts is dangerous to personnel operating the plant and could also cause damage to other equipment. All conveyers are provided with wipers located near the tail pulley on the return run of the belt. Keep the 6-10