Basic Operational Concepts
Particle size reduction in a hammermill occurs as a result
of the impact between a rapidly moving hammer and a
relatively slow moving particle. The particle breaks and
is accelerated towards the screen. Depending on the
particle size and angle of approach, it either passes
through the screen or rebounds from the screen into the
rapidly moving hammers again. As materials move through
the grinding chamber they tend to approach hammer tip
speed. Since reduction only occurs when a significant
energy is transferred from the hammer to the particle
(large difference in velocities), less grinding takes place
when the particles approach hammer tip speed. Many
manufacturers incorporate devices within their mills to
interrupt this product flow, allowing impact and reduction
to continue. Tear circle hammermills have a more positive,
natural redirection of product at the inlet than "full circle"
While the basic operational concepts are the same for all hammermills, the actual unit operating
conditions change rather dramatically depending on the materials being processed. Grains such as
corn, wheat, and sorghum and various soft stocks like soybean meal tend to be friable and easy to
grind. Fibrous, oily, or high moisture products like screenings, animal proteins, and grains like oats
and barley on the other hand, are very tough and require much more energy to reduce. Consequently,
the hammermill set up that works well for one will not necessarily work well for the other. The following
discussion covers such factors as tip speeds, hammer patterns and position, horsepower ratios (to
hammer and screen area), and air assist systems.
Tip speed, in addition to screen size has a
significant influence on finished particle sizing.
High tip speeds (>18,000 Ft/Min) will always grind
finer and produce more fines than lower tip
speeds. Low tip speeds (<13,000 Ft/Min), on the
other hand, produce a coarser granulation with
fewer fines. As a rule smaller screen hole sizes
should be used with higher tip speeds, and larger
screen hole sizes with lower tip speeds.
Tip speed is simply a factor of mill diameter and
motor RPM and is not easily changed on direct
coupled machines. There are a few V-belt drive
hammermills on the market today.
Tip Speed -Friable Products
For producing a uniform granulation with few fines
on friable products like corn, wheat, grain sorghum,
pelleted ingredients, and solvent extracted meals
an intermediate tip speed is normally desirable.
Hammermills with a tip speed of 13,000-18,000
Ft/Min will produce a high quality finished product
with excellent capacity and efficiency. 38" diameter
mills with 1800 RPM motors (17,800 Ft/Min) and
44" mills with 1200 or 1500 RPM motors (13,500
or 17,250 Ft/Min) are both used extensively in
the processing of all kinds of feed ingredients.
Tip Speed - Fine Grinding And Tough To Grind Materials
For fine grinding friable products and tough to grind materials like soybean hulls, mill feed, and mixtures
with animal protein products, a higher tip speed is indicated. Because more energy is required to
grind these kinds of materials a higher tip speed is beneficial. Normal tip speeds for fine grinding and
fibrous materials are obtained on 42" and 44" mills operating at 1800 RPM (19,500 and 20,000
Ft/Min), or 28" mills operating at 3000 RPM and 54" mills operating at 1500 RPM (21,000 Ft/Min).
Recent developments in hammermill grinding have included the use of 54" diameter mills operating
at 1800 RPM. This very high tip speed (>25,000 Ft/Min) is particularly well suited to fine grinding at
high capacities and high efficiency. Because a larger screen (hole) sizes can be used while maintaining
the fineness of the grind, operating costs are reduced as well.
There is 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. European feed
processors tend to favor a plain two-holed hammer, with no hardfacing
or edge treatment. North and South American feed millers tend to favor
a hammer with a flared hardface end (or ends). Each market tends to
find a hammer type that best suits their particular needs.
Hammer patterns and positions 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. At the same
time, it is important to make sure the hammer pattern covers the
working screen, without having hammers in line.
In most cases, the hammer pattern should include double
hammers on the outside rolls of at least two opposing pins.
Because the material in the grinding chamber near the sides
of the mill moves more slowly (dragging on the sides), the
outside rows of hammers must do more work and are subject
to more wear. Some manufacturers use thicker or longer
hammers on the outside rows.
There is also a relationship between the HP/hammer and the
wear on the hammer. Too much HP/hammer will tend to "rock"
the hammer each time the hammer swings through a bed of
material on the screen, leading to rapid wear of the hammer
hole and hammer mounting pin. In extreme cases, the bed may
the hammer wears above the hardfacing. If this
happens the correct solution is not to use a hammer
with more hardfacing extending up the hammer,
but to reduce the H.P., increase the number of
hammers, or reduce the feed rate to the mill. Too
little HP/hammer dramatically reduces hammermill
efficiency by consuming motor horsepower simply
to turn the rotor with its load of hammers. Too little
HP/hammer also tends to wear the hammers right
on the corner and does not effectively use all the
working surface of the hammer.