These are used to reduce hard bodies to small fragments. They are composed (Fig. 41)of movable cheeks between which the material is broken and reduced to larger or smaller fragments according to the distance between the cheeks. These regular fragments are afterwards passed under the wheels of mills to reduce them to powder.

Fig. 38. Crushing Mill with Movable Pan (Whittaker).

Another machine used to crush these bodies direct is Carr's universal crusher. It is composed of a certain number of concentric barred cages fixed upon a rotating plate. Other cages, also concentric and of diameter intermediate to the preceding ones, are fixed to another plate which revolves in the opposite direction. This second plate and its shaft are mounted on a support which can slide longitudinally, so that the cages fit one within the other (Fig. 43).

Fig. 42. Carr Universal Crusher - the Cages separated (Jager).

When the machine is working, the cages being thus fitted together, two contrary movements are produced. The whole is contained in a sheet-iron cover, shown on the right of the sketch, and the substances to be crushed are thrown in by the hopper fixed to it. They receive a shock against the bars of the first cage, and after passing through those bars, meet those of the second cage, which are turning in the opposite direction, then those of the third, and finally those of the fourth, each being opposite in direction to the preceding. At last they fall outside in the form of a powder the fineness of which depends upon the number of turns of the cages. These latter,. rotating within the cover, act as ventilators, and prevent dust in the workshop.

Fig. 43. Carr Universal Crusher - the cages together (Groke).

The production of this machine is much greater than that of any other used for the same object, but it requires a considerable motive power.