Lesson 2/Learning Event 2
Learning Event 2
CALIBRATING A ROCK CRUSHING PLANT
In planning for crushing and screening plant operations, there are two basic objectives. The first is the provision
of the aggregate in the required size. The second is the provision of the aggregate in the required quantities. The
first objective can be easily met through selection of screens which will separate the material into the sizes
required. Achievement of the second objective involves not only the selection of the proper equipment but proper
adjustment of the equipment as well. This Learning Event will outline the steps for selecting screens for the
crushing plant and adjusting the crushing equipment for maximum efficiency.
SCREEN SELECTION
Before any adjustment can be made to a crushing and screening plant, it is necessary that the desired gradation of
material to be processed be obtained from appropriate specification. Once the gradation has been determined,
screens must be selected that will segregate the materials into appropriate sizes. The number of screens to be
selected is dependent upon the number of size ranges into which the material must be segregated and the type of
equipment available for screening. Table 2 gives the screen sizes of vibrating screens which will produce specific
sizes of aggregate.
EXAMPLES OF SCREEN SELECTIONS
The selection of screen sizes is dependent on the size product desired. The maximum and minimum sizes of the
product are expressed in the specifications. For example, consider the following specifications:
Size
Percent Passing
1-inch
100 percent
3/4-inch
90-100 percent
1/2-inch
20-50 percent
3/8-inch
0-15 percent
#4
0-5 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 #4 sieve, it is generally understood that the
#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 inch. The
reasoning behind calibrating the plant to produce a 3/4-inch to 3/8-inch aggregate rather than a 1-inch to #4 size
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