Lesson 1/Learning Event 2
parison, dual roll crushers have a very limited stage of reduction capability. Stage of reduction is the difference in
maximum input and maximum output size of material due to a single crushing action. Stage of reduction can be
expressed in inches or centimeters. A stage of reduction of 3 inches (7.6 cm) would indicate a 3 inch (7.6 cm)
reduction in maximum particle size. This reduction capability is a function of the diameter of the rolls, and the
nature of the roll shell surface. Therefore, every different size and combination of roll shells has a somewhat
different stage of reduction capacity. For this reason the maximum size of material to be fed to a dual roll crusher
is critical.
The maximum allowable feed size is the sum of the stage of reduction capability and the product setting. If the
maximum feed size is exceeded, several unsatisfactory results are likely to occur:
Retarded production.
Excessive roll shell wear.
Excessive long and flat particles.
SCREENS
Function and Operation. In all but the most basic and fundamental crushing operations, the crushed rock
particles are separated into two or more particle size ranges. This is accomplished by the use of screens. Screens
are also used to scalp off oversize rock and to screen out fines. This enables you to direct certain selected material
to receive special or additional processing. Certain material may also be directed to bypass processing that is not
required.
Screens consist of two, three, or four layers or decks of open mesh screen wire cloth mounted one above the other
in a rectangular metal box (Figure 3). The screen surface is vibrated to aid sorting. Material is fed at one end and
is separated into size ranges as it is passed over the screening surface. The screening process is based upon the
simple premise that particle sizes smaller than the screen cloth opening size will pass through the screen and
oversized particles will be retained.
Production Considerations. In order to obtain good efficiency and high capacity, stratification of the feed
material must occur rapidly as the material is passed over the screen surface. This ensures that the smaller
particles move quickly to the bottom and find their way through the screen openings while the larger oversized
particles are carried to the top of the feed stream where they are retained and directed off the end of the screen.
The precise performance characteristics desired of specific screens are obtained by varying the degree of
inclination, frequency and amplitude of stroke, and the direction of throw. Screens may be horizontal or inclined
up to about 20 degrees and vibrate at 850 to 1,250 strokes per minute depending upon the particular application.
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