Lesson 2/Learning Event 3
TABLE 12. SCREEN CORRECTION FACTORS
gregate. As the percentage of 1/2 size aggregate in the feed is increased, the capacity of the screen will
increase. This is because 1/2 size aggregate will pass through the screen openings much more readily than
aggregate greater than 1/2 size aggregate.
O = Oversize material. This is the aggregate (Table 12) that is too large to pass through a screen. As the
percentage of oversize aggregate in the feed is increased, the capacity of the screen will decrease. This is due
to the fact that the oversize particles, being heavier than the smaller particles, will fall to the bottom of the bed
of rock going across the screen. Since they are too large to pass through the screen, they tend to prevent the
smaller parties from passing through.
W = Washing factor (Table 12). When aggregate is sprayed with water as it passes across a screen, the
capacity of the screen will be increased. This is because the water will lubricate the outer surfaces of the
aggregate, thereby causing the aggregate to pass through a screen more readily than if it were dry. When
washing is not employed, a factor of 1 is used for the washing factor. Washing is most effective in the
smaller size ranges. As the aggregate size is increased, the advantage of washing is decreased. Care should
be taken in washing operations in that too little water will not flush out the dirt and mud washed off the rock.
If too little water is used this dirt and mud may clog up the screens thereby decreasing the screen capacity.
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