A grinding machine in which balls rotate under pressure to crush materials, such as coal, to a fine consistency. The material is usually fed through a chute to the inside of a ring of closely spaced balls. In most designs the upper spring-loaded race applies pressure to the balls, and the lower race rotates and grinds the coarse material between it and the balls (see illus.). The finely ground material discharges along the outer periphery of the ball races. For the pulverization of coal, hot air, introduced between the lower race and the pulverizer housing, lifts or carries the fines to a cyclone classifier at the center of the pulverizer. There the finer particles discharge from the pulverizer while the larger particles return to the grinding zone for further reduction in size. Two or more rings of balls can be cascaded in one machine to obtain greater capacity or output. Counterrotating top and bottom rings also are used to increase pulverizer capacity. Such pulverizers are compact and the power required per ton of material ground is relatively low.