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Although the term "crushing rolls" might, logically, include roll crushers of the sledging type, the name, by popular usage, is restricted to the double-roll machine (with either smooth or corrugated shells) which crushes entirely by pressure between the surfaces of the roll faces. The sledging type of either single- or double-roll arrangement is usually differentiated by such titles as "roll crusher" or "sledging rolls." The only point of similarity in the actions of the crushing rolls and the gyratory and jaw types is that both do their crushing by pressure. As contrasted to the action of gyratory and jaw types, rolls have a continuous "one-bite" action; once a particle of material is firmly gripped, it is drawn down between the converging shell faces in one quick, continuous "squeeze," until the discharge point is reached. Another point of difference is that the rolls do not rely upon gravity to work the material down through the crushing zone; the action is a forced, mechanical one. Still another difference between the types is that, in the crushing rolls, there is no "close-side" and "open-side" discharge setting; the distance between roll faces--on a line between the two shaft centers-establishes the discharge opening, which remains unchanged during normal operation. Mechanically, a pair of crushing rolls is a simple machine.
 Fig. 1 and 2 show all of the essential details of construction of a heavy-duty machine.
A heavy and rigid cast-iron frame supports the two-roll assemblies, each of which comprises a shaft, a roll center, and a shell of wear-resisting metal, such as high-carbon steel, or manganese steel. Each roll is independently driven by a fly-wheel type, flat-belt pu11ey, or V-belt sheave. One of the two pairs of bearings is arranged to slide horizonta11y on the side frames. These movable bearings are spring-Ioaded to provide a safety relief for excessive pressures, such as are caused by tramp iron, etc.
They are drawn up against locating shims which establish the spacing between roll faces (discharge setting), and are held in that position by the springs, which are pre-set to the working pressure for which the particular machine is designed.
This working pressure may vary from as low as 500 lb. per lin. in. of roll face, for light-duty rolls, to as high as 15 tons per in. of roll face for extra-heavy-duty rolls.
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