Abstract: A materials grinder includes a rotary grinding drum and a shear bar cooperating with the grinding drum to grind various types of materials. The shear bar is mounted on a pivotal structure adjacent the materials grinding drum. A displaceable biasing member biases the pivotal structure toward the grinding drum to maintain the shear bars in close cooperating relationship therewith. If ungrindable materials are encountered, the pivotal structure pivots away from the grinding drum against the bias force of the biasing member to permit the ungrindable materials to pass between the grinding drum and the shear bar. Once the ungrindable materials pass, the biasing member moves the pivotal structure back toward the grinding drum. Materials to be ground are fed to the grinding drum by means of an infeed conveyor and an overhead infeed roller.