Jaw Crushers are designed to impact on a rock particle placed between a fixed and a moving plate (jaw). The faces of the plates are made of hardened steel. Both plates could be flat or the fixed plate flat and the moving plate convex. The surfaces of both plates could be plain or corrugated. The moving plate applies the force of impact on the particles held against the stationary plate. Both plates are bolted on to a heavy block. The moving plate is pivoted at the top end (Blate crusher) or at the bottom end (Dodge-type crusher) and connected to an eccentric shaft. In universal crushers the plates are pivoted in the middle so that both the top and the bottom ends can move. The Blake crushers are single or double toggle drives. The function of the toggle(s) is to move the pivoted jaw. The retrieving action of the jaw from its furthest end of travel is by springs for small crushers or by a pitman for larger crushers. As the reciprocating action removes the moving jaw away from the fixed jaw the broken rock particles slip down, but are again caught at the next movement of the swinging jaw and crushed. This process is repeated until the particles sizes are smaller than the smallest opening between the crusher plates at the bottom of the crusher(the closed set). For a smooth reciprocating action of the moving jaws, heavy flywheels are used in both types of crushers. Fig. 4.1 shows a sketch of a Blake Crusher operated by double toggles and controlled by a pitman. These are commonly used as primary crushers in the mineral industry. The size of the feed opening is referred to as the gape. The opening at the discharge end of the jaws is referred to as the set. Fig. 4.2 is a sketch of a Dodge type of crusher. They are comparatively lower in capacity than the Blake crushers and are more commonly used in laboratories.
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