Gyratory crushers are principally used in surface- crushing plants, although a few currently operate underground. The gyratory crusher (Figure 6.7) consists essentially of a long spindle, carrying a hard steel conical grinding element, the head, seated in an eccentric sleeve. The spindle is suspended from a "spider" and, as it rotates, normally between 85 and 150rev min -~, it sweeps out a conical path within the fixed crushing chamber, or shell, due to the gyratory action of the eccentric. As in the jaw crusher, maximum movement of the head occurs near the discharge. This tends to relieve the choking due to swelling, the machine thus being a good arrested crusher. The spindle is free to turn on its axis in the eccentric sleeve, so that during crushing the lumps are compressed between the rotating head and the top shell segments, and abrasive action in a horizontal direction is negligible.