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Impact Crushers - Hammermills



Hammermills

A hammermill crushes material that is friable, by impacting it against a rotating hammer (typically traveling between 750 RPM and 1800 RPM). Then the material is forced against a rugged solid plate called a "breaker plate" which further degrades the particle size. Finally, the material is forced over a discharge grate by the hammers, where crushed finer particles drop through the discharge grate and larger particles travel around for another crushing cycle, until they fall through the discharge grid. During the entire time the material is traveling around in the hammermill it is constantly being impacted by the hammers, and the side of the mill casing, causing breakage of the particles. This sequence repeats itself between 750 - 1800 times each minute, until the particle is ground fine enough to fall through the discharge grid.

One big drawback to hammermills is they wear, due to the abrasiveness of the material being crushed and the high speed of the mill. It should not be used to grind any material harder than a medium hard limestone. They are used most in crushing coal, but are also common in limestone operations. Large particles can not usually be crushed in hammermills, so they are almost never used for run of mine material or primary crushers, but secondary or tertiary crushers. Six to eight inches is a typical top size of feed for hammermills.

Another possible drawback for using hammermills, is that they tend to produce a lot of fine material (50 mesh to -100 mesh). Crushers like jaw crushers, cone crushers, and roll crushers, which operate a much lower speeds, tend to produce very few fines (generally less than 1% finer than 50 mesh). For some processes, excess fine material is a drawback, for others, it is not a problem.

On the positive side, hammermills are relatively inexpensive, as crushers go, can produce relatively fine material (1/4" to -10 mesh) from 6" or 8" feed in one step, and they take up relatively little floor space. The are easily repaired, and simple to operate. Production size hammermills can produce products ranging from 1" to as fine as 10 mesh, or finer, depending upon the material being crushed.  


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