Gyratory crusher is principally used in surface-crushing plants, although a few currently operate underground. The gyratory crusher 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. At any cross-section there are in effect two sets of jaws opening and shutting like jaw crushers. In fact, the gyratory crusher can be regarded as an infinitely large number of jaw crushers each of infinitely small width. Since the gyratory, unlike the jaw crusher, crushes on full cycle, it has a much higher capacity than a jaw crusher of the same gape, and is usually favored in plants handling very large throughputs. In mines with crushing rates above 900t h -1, gyratory crushers are always selected.
Some gyratory crushers have a hydraulic mounting and, when overloading occurs, a valve is tripped which releases the fluid, thus dropping the spindle and allowing the "tramp" material to pass out between the head and the bowl. This mounting is also used to adjust the set of the crusher at regular intervals so as to compensate for wear on the concaves and mantle. Many crushers use simple mechanical means to control the set, the most common method being by the use of a ring nut on the main shaft suspension.