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A hammer mill is a machine used to shred or crush materials into smaller pieces. The basic design of machines of this type involve a rotating shaft to which free-swinging hammers are attached. This assembly is enclosed by a drum which holds the material to be pulverized. A hopper feeds the source material into the drum. The hammers gradually break up the material until it is small enough to pass through a screen with openings corresponding to the desired particle size.
Many types of hammer mill are used for many industrial, research, agricultural and residential applications. A hammer mill can be small enough to sit on a counter top and run on household current, or large enough to shred entire cars. Some of the largest hammer mills may use a large diesel engine of 2,000 horsepower or more. The axis of the rotating shaft of a hammer mill may be constructed in either a horizontal or vertical position, although a horizontal configuration is more common.
Farmers sometimes employ a hammer mill to pulverize grain for livestock feed. These machines have the advantage of being able to process many types of materials as well as producing particles of various sizes by simply changing the output screen. A hammer mill of this type may operate at around 1,800 rpm with the tips of the hammers moving 60 mph (90 kph)or more. Particles produced this way will be very uniform and tend to have a spherical shape.
Industrial uses for hammer mills include many types of recycling operations, including producing pulp for paper production, bio-fuel production, and stone crushing. Grinding grain for ethanol production or pulverizing fruit in juice production are other possible uses for a hammer mill. A sawmill may employ a hammer mill for the purpose of grinding scrap into mulch or material for manufacturing wood pellet fuel.
Metal recycling operations like scrapyards often employ large hammer mills for grinding scrap metal into small pieces for separation and processing. Large hammer mills are able to grind up entire cars at one time, pulverizing them into fist sized pieces, and can process many tons of scrap metal each day. Other types of hammer mills are used to reclaim recycled asphalt, electronic components or old tires.
Small hammer mills are sometimes used in laboratories for grinding the materials for growing media or experiments. Other small hammer mills can be employed in grinding foods or spices, and food waste is often processed by these machines. The common garbage disposal is a variation of a hammer mill with a disk with fixed teeth and grinding surfaces instead of free-swinging hammers.