Lesson 1/Learning Event 2
Function and Operation. The jaw crusher receives raw material from the pit or quarry and reduces it to a smaller
particle size. Material to be crushed is introduced into the jaw cavity and is gradually reduced by a series of
elliptical-downward crushing strokes and is discharged out the bottom of the jaw assembly. The product size is
determined by the product setting adjustment made at the discharge end of the jaw plates.
Production Considerations. The hourly rate of production to be obtained from a given size jaw crusher is
dependent upon a number of variable factors:
The toughness of the raw material.
The product setting.
The reduction ratio. (The size of the material fed to the crusher compared to the product size.)
The gradation of raw material.
The extent of wear to the corrugated surfaces of the jaw plates.
The rate of feeding.
Due to the characteristic irregular shapes and unwieldy nature of quarry run rock, several feeding problems may
develop which will drastically reduce production. Some common operational problems are blocking, bridging,
choking, and packing.
Blocking occurs when an oversize rock particle settles over the jaw cavity opening and stops the flow of
incoming material. The jaws continue to operate but no crushing takes place. To prevent blocking, the maximum
size of material should be 2 inches less than the jaw crusher size. Blocking can best be controlled or eliminated
by using a scalping grizzly to prescreen material. For optimum production, the ideal feed size is 75 percent of the
jaw size.
Bridging occurs when two rock particle, within the maximum size limitations, arrive at the same time. The two
rocks interlock and bridge the jaw cavity opening. When this occurs, all production stops. When large pieces
approach the opening, they should be fed individually to prevent bridging.
Choking occurs when the jaw chamber is continually overfilled. This creates an overload condition and causes
the engine to lug down and may damage equipment. For optimum production, the operator should try to keep the
jaw chamber 75 percent full.
Packing occurs when feed material cakes and packs in the crushing chamber. Plastic material, such as clay, may
become sticky and cause this problem. Packing can become so severe as to completely stop production. This will
severely damage the jaw crusher assembly. Packing can be reduced or eliminated by prescreening or prewashing
the material. The most practical solution in most cases is to thoroughly wet down the material. Then process the
material, though it's almost in the form of a slurry.
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