There are an unlimited number of hammer styles
available from many suppliers around the world.
At the same time, there are distinctly different
types of hammers used in different regions of the
world for a variety of hammermill grinding tasks.
For longest hammer life and most efficient
operation, a hammer with a flared hardface end
(or ends) is preferred. Hammers may be one hole,
with one working end (two corners) or two holed,
with four corners available for grinding. One hole
hammers are generally preferred to maintain
balance of the rotor and minimize the potential
for catastrophic hammer failure (Figure 5).
Hammer patterns (the number and distribution of
the hammers on the rotor) and positions (setting
the hammer closer to or further from the screen)
have a profound effect on the performance of any
hammermill. Because different materials grind
differently, the ideal number of hammers and
clearance to the screen will need to be adjusted
according to each application.
It is important to make sure the hammer pattern
covers the working screen, without having
hammers trailing hammers in line. In most cases,
the hammer pattern should include double
hammers on the outside rolls of at least two
opposing pins. This is because the material in the
grinding chamber near the sides of the mill moves
more slowly than material in the middle of the mill
due to friction on the sides, consequently the
outside rows of hammers must do more work
and are subject to more wear (Figure 6).
Most general application hammermills today are
equipped with a rotor designed for a 4 pin hammer
pattern or a 6 pin hammer pattern. Since the
rotors are normally drilled for two hammer positions
(coarse and fine) the rotors are actually fit with 8
or 12 sets of hammer pins. For fine grinding
applications it is often necessary to use an extra
heavy hammer pattern to achieve the very fine
finished products desired: in many cases, the
rotor will be equipped with hammers on all 8 or
all 12 sets of hammer pins. This way the total
number of hammers is increased significantly,
without putting an excessive number of hammers
on any individual pin(s), which could lead to high
stress and the possible failure of the pins or rotor
plates (Figures 7 and 8).
To insure the motor can start a machine with a
high inertial load, an electronic soft start may be
In terms of hammermill capacity and efficiency,
the best screen for any job is the thinnest material
with the most open area. Naturally, some sacrifice
in efficiency must be made for the sake of
endurance, yet the general rule applies. On the
other hand, for many aquaculture applications
there is a benefit to using screens with limited
open area to promote a finer finished product.
Indeed, many hammermills grinding in aquaculture
applications are equipped with extended wear
liners at the inlet of the machine to increase the
impact and grinding that occurs when the product
is first struck by the hammers. Some users go
so far as to "blank off" a portion of the down side
screen by placing a solid sheet metal plate behind the screen to prevent material from passing through
the screen openings. This promotes finer finished products, although the production rate will be
negatively impacted (Figure 9).
Factors effecting open area include hole size, stagger, angle of stagger, and hole spacing. There are
a few specialty screens being used in aquaculture applications to improve the grinding performance.
One type of screen material is known as the "Conidur" with the holes punched is such a way that
the surface is upset, almost like small louvers or the surface of a cheese grater. This type of screen
can work effectively, but is very expensive to purchase and will reduce the machine capacity. One
reason the "Conidur" type screen produces a finer finished product is the fact the open area is very
low when compared to a conventional round hole screen with a similar opening diameter.
Sealing at the edges of the screen is particularly important when fine grinding for aquaculture feeds.
The fit of the screen carriage and the wear liner must be precise to prevent any oversize particles
from bypassing around the edges of the screen. At the same time, the screen must be adequately
supported, as fine screens do not have the mechanical strength of thicker, heavier screen material.
Regular inspections of the screens must be made to monitor the condition of the screens and to
catch any worn or failing screens before they fail completely.