A hammermill is a common type of equipment currently used for size reduction of solid waste. A hammermill consists of a central rotor or shaft with radial arms (hammers) protruding from the rotor circumference. The rotor is enclosed in a heavy duty housing, which may be lined with abrasion resistant steel members. Some hammermilIs may also have stationary breaker plates or cutter bars mounted inside the housing. Input material is reduced in size by impact, attrition, and shearing forces induced by the hammers.
There are two basic types of hammermiIls: the horizontal shaft type and vertical shaft type. The horizontal shaft hammermill is the more common type. As the name implies, the rotor or shaft is horizontal and supported on each end. Input is usually at the top and material flows through the machine assisted by gravity. Force feed at the side of the hammermill has also been used for solid waste shredding. Most horizontal hammermiIls have a grate placed across the output opening. Input material cannot pass through this grate until it has been sufficiently reduced in size in at least two dimensions. Output particle size is controlled primarily by the size of openings in the grate. Some manufacturers also offer reversible rotor horizontal hammermiIls.
The vertical shaft hammermills have the rotor placed in a vertical position. The input material moves parallel to the shaft axis and flow is assisted by gravity. The lower shaft bearing must be.a thrust bearing; that is, a bearing capable of supportinq the weight of the rotor. The machines may have a decreasing clearance between the hammer tips and the stationary housing from top to bottom, thus effecting progressively finer size reduction as the material moves through the machine.
There are two basic variations of both the vertical or horizontal shaft hammermills--the swing hammer type and the rigid hammer type.
The swing hammer type is the most common in solid waste processing and has hammers mounted on pins and free to pivot. The swing hammer concept reduces damage to the machine when it encounters a very hard piece of input material. However, the hammers can create a severe imbalance problem if they become entangled with the input material and are not al lowed to swing freely. The rigid hammer type is more typical of smaller sized machines not specifically designed for sol id waste. For both the fixed- and rigid-hammer types hammer shapes vary from sharp choppers to blunt rectangular beaters. The latter are often used in machines for processing solid waste materials.