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A crusher is a machine designed to reduce large solid chunks of raw material into smaller chunks. Alternately, the term may be used as a synonym for compactor[?].

Crushers are commonly classified by the degree to which they fragment the starting material, with coarse crushers not reducing it by much, intermediate crushers fragmenting it much more significantly, and grinders[?] reducing it to a fine powder[?].

The two best known types of coarse crusher are the jaw crusher and gyratory crusher. A jaw crusher consists of a set of vertical jaws, one jaw being fixed and the other being moved back and forth relative to it by a cam and pitman[?] mechanism. The jaws are farther apart at the top than at the bottom, forming a tapered chute so that the material is crushed progressively smaller and smaller as it travels downward until it is small enough to escape from the bottom opening. The movement of the jaw can be quite small, since complete crushing is not performed in one stroke. A gyratory crusher is similar in basic concept to a jaw crusher, consisting of inner and outer vertical crushing cones; the outer cone is oriented with its wide end upward, and the inner cone is inverted relative to the outer with its apex upward. The inner cone has a slight circular movement, but does not rotate; the movement is generated by a cam arrangement. As with the jaw crusher, material travels downard between the two cones being progressively crushed until it is small enough to fall out through the gap between the two cones at the bottom.

One type of intermediate crusher consists of a pair of horizontal cylindrical rollers through which material is passed. The two rollers rotate in opposite directions, "nipping" and crushing material between them. A similar type of intermediate crusher is the edge runner, which consists of a circular pan with two or more heavy wheels known as mullers rotating within it; material to be crushed is shoved underneath the wheels using attached plow blades.

Hammer mills involve the use of impact[?] rather than pressure to crush material. They utilize heavy metal bars attached to the edges of horizontal rotating disks by hinges, which repeatedly strike the material to be crushed. The material is contained within a cage, with openings on the bottom of the desired size to allow pulverized material to escape. This type of crusher is usually used with soft material such as coal.

A typical type of fine grinder is the ball mill[?]. A slightly inclined or horizontal rotating cylinder is partially filled with balls, usually stone or steel, which grinds material to the necessary fineness by rubbing and impact with the tumbling balls. The feed is at one end of the cylinder and the discharge is at the other. Ball mills are commonly used in the manufacture of portland cement. Another type of fine grinder commonly used is the Buhrstone mill[?], which is similar to old-fashioned flour mills[?].

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