|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