Lesson 2/Learning Event 3
the plant with the tires clear of the ground. Twelve by twelve-inch timber cribbing or other suitable
material may be used for this purpose. Before crushing operations begin, the plant should be leveled
from end-to-end and side-to-side. Otherwise, material will be channeled along the low side or end of the
plant and increase wear and tear on the equipment.
If the material is to be dumped directly into the apron feeder of the primary crusher unit, extensions
must be welded onto the hopper. These extensions should increase its width to approximately 1.25
times that of the truck beds.
A loading chute is used when a suitable bench greater than 18 feet (5.5 meters) in height is used. It is
also used when you have a grizzly at the crusher site and a suitable size bench or loading ramp is to be
constructed. For very high headwalls, the use of an inclined chute will allow equipment to operate at a
greater distance from the edge of the wall. This reduces the earth pressures against the wall. Whenever
chutes, bins, or hoppers are used to handle large rocks they should be kept one-third full so the impact of
falling rocks is absorbed without damaging the structure or equipment.
Grizzlies and Scalping Screens
Grizzlies are essentially very coarse screens for sizing a quarry run rock before it reaches the crushing
plant. They consist of a durable, rigid, rectangular frame with steel rails spaced at intervals determined
by the crusher opening. As a guide, the optimum size material fed into a jaw crusher is 75 percent of the
jaw dimensions. For a 20 x 36-inch jaw crusher, this would be 15 x 27 inches. Grizzlies may be
constructed for use at the crusher site. They may also be used in connection with loading ramps or
constructed on skids for use in the quarry. When the quarry run rock contains excessive fine-sized
material, scalping screens may be needed. These are constructed like grizzlies, but use fine screens. In
this case, the screen should be sloped in the direction toward which the select material is desired to be