Lesson 2/Learning Event 2
Assume that your plant has a two-deck, vibrating screen. In this case the following screens would be selected:
Product Size Desired
Screen Size Selected
Top 3/4-inch
7/8-inch
Bottom 3/8-inch
7/16-inch.
Note the relationship between aggregate size and screen size. 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.
ROLL CRUSHER SETTING
The top screen in a deck of screen has a two-fold purpose. First, it allows aggregate that is down to product size
to pass through and fall down to the next screen. Secondly, it prevents material that is not down to product size
from passing through the screen. Material that is not down to the product size passes off the end of the screen and
normally is routed through the roll crusher. The roll crusher then crushes the oversize material down to the
required product size.
Therefore, the distance between the roll shells, to include allowances for roll shell corrugations, must be set equal
to the top product size. This distance is called the product setting. For example, as in the above problem, if the
top product size is 3/4 inch, the product setting on the roll crusher (the distance between roll shells) should be 3/4
inch. Top product size is the key to calibration of the roll crusher setting.
The top product size and the top screen size dimensions should not be confused. If a 7/8-inch screen is selected to
pass a top product size of 3/4 inch, as in the foregoing example, the product setting on the roll crusher is 3/4 inch
and not 7/8 inch. The 7/8-inch screen was selected to compensate for screen inefficiency and not to produce a
larger product size.
JAW CRUSHER SETTING
The last step in calibrating a plant is to set the jaw crusher. The product from the jaw crusher is directed to the
screens. The screens reject all the jaw crusher product that is not down to the maximum product size desired.
The oversize is sent to the roll crusher where it is further reduced in size so that it will meet maximum top size
requirements. Therefore, the maximum size aggregate produced by the jaw crusher is dependent on the maximum
size aggregate that the roll crusher can accept and crush effectively. After the maximum feed size to the roll
crusher is determined, then the appropriate jaw crusher setting can be made.
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