screen out fines. This enables you to direct certainselected material to receive special or additionalprocessing. Certain material may also be directed tobypass processing that is not required.Screens consist of two, three, or four layers or decksof open-mesh screen wire cloth, mounted one above theother in a rectangular metal box. The screen surfaces arevibrated to aid sorting. Material is fed at one end and isseparated into size ranges as it passes over the screeningsurface. The screening process is based upon the factthat particle sizes smaller than the screen cloth openingsize passes through the screen and oversized particlesare retained.Stratification of the feed material must occurrapidly as the material is passed over the screen surfaceto obtain good efficiency and high capacity. This ensuresthe smaller particles can move quickly to the bottom andfall through the screen openings, and the largeroversized particles are carried to the top of the feedstream where they are retained and directed off the endof the screen. The desired performance of specificscreens is obtained by varying the degree of inclination,frequency and amplitude of stroke, and the direction ofthrow. Screens may be horizontal or inclined up to about20 degrees and vibrate at 850 to 1,250 strokes perminute, depending upon the particular application.CAPACITY.– Capacity is the rate in tons per hourat which a screen produces (passes) the material desired.The capacity of a screen is the rate at which it separatesdesired material from the feed.FEEDING MATERIALS TO SCREEN.– Caremust be taken to spread the flow of material evenlyacross the full width of the screen. The thickness of thebed of feed material should be approximately four timesthe screen opening size. If this is exceeded, the screenbecomes overloaded and the vibrations are dampened.This results in the finer particles being unable to findtheir way to the screen wire opening. When aninsufficient amount of material is fed, the total capacityof the screen is not used.VARIABLE FACTORS.– Numerous factors canaffect the performance of screens. Some examples ofthese are as follows: shapes, weight, and gradation ofparticles; degree of inclination and vibration; type ofwire screen cloth; position of the screen within the deck;and wet screening (washing).Screen SelectionOnce the gradation of material has been determined,screens must be selected that can segregate the materialsinto appropriate sizes. The number of screens to beselected is dependent upon the number of size rangesinto which the material must be segregated and the typeof equipment available for screening. Table 6-1 givesthe screen sizes of vibrating screens used to producespecific sizes of aggregate.Table 61.-Vibrating Screen Selection Chart6-9
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