Hammermills can be divided into two generic types on the basis of orientation of the rotor -- namely, horizontal and vertical. Both types have hammers that rotate within the shredder and cause particle size reduction through collision with the infeed material. The hammers may be mounted to the shredder rotor in a fixed or freely swinging manner. The horizontal swing hammermill is commonly used in mixed waste processing. Its principal parts are the rotor, hammers, grates, frame, and flywheel. Its rotor and flywheel are mounted through bearings to the frame. A set of grate bars or cages through which size-reduced materials exit the machine is held in the bottom of the frame. A diagrammatic sketch of a horizontal hammermill is shown in Figure VI-1.