Lesson 1/Learning Event 2
into bins, waiting trucks, or onto stockpiles. Rubber belt conveyors are of the fixed or portable type. Fixed
conveyors are attached to and are a component of the crushing unit. They are similar in design and work on the
same principle as do portable conveyors.
Three factors--speed, loading, and incline--affect the efficiency of conveyor
Speed. Most conveyors operate at a speed of approximately 300 feet per minute and have a capacity
of approximately 300 tons of material per hour. A reduction in speed will obviously reduce the conveyor capacity
while an increase in speed would theoretically increase the capacity. An increase in conveyor speed may also
increase wear on the conveyor belt due to increased slippage of the material at the loading point. An increase in
speed will also increase the "throw" of the material at the discharge end of the conveyor. In some cases it may be
necessary to fit the end of the conveyor with a box or "bang board" so that the material from the belt will fall
Loading. Proper loading of a belt conveyor is mandatory for efficient operation. This includes
placing the load so that it is centered on the conveyor belt. This is a most common problem. It is always good
practice to load a conveyor so that the material strikes the belt in the direction of travel. When material is to be
delivered from a spout or belt to another belt from one side, a transfer box or "bang board" should be provided to
facilitate proper delivery on the belt. Loading a conveyor belt on one side will cause it to lean to the opposite side
of the support rollers causing excessive belt wear.
Incline. Portable conveyors can be adjusted to operate at various inclines required by job conditions.
The maximum incline is determined by the material carried on the conveyor and varies from about 12 degrees for
washed gravel to 20 degrees for loose earth. When the maximum angle is exceeded slippage may occur.
Construction aggregates sometimes require washing to remove undesirable materials.
equipment includes revolving scrubbers, wet screens, log washers, and dehydrators.
Scrubbers are used to move clay, silt, and other undesirables from crushed stone. Wet screening is used to
initially wash easily removed material, such