Hammermills have long been used for particle size reduction of materials used in the manufacture
of animal feeds. It could be said the hammermill has been the most studied and least understood piece of equipment in the feed manufacturing
plant. Much of this confusion has come about
over the years as a result of hit or miss problem
solving and by treating symptoms rather than
addressing the root causes when treating
A well-designed hammermill grinding system will
offer good long-term performance and require
a minimum amount of attention if a few basic
considerations are made at the time the equipment
is selected. The following discussion will explore
the basic theory of hammermill operation and
offer some principles on which systems may be
A hammermill consists of a rotor assembly, consisting
of two or more rotor plates fixed to a main shaft,
enclosed in some form of grinding chamber. The actual
working mechanisms are the hammers and the screen
or grinding plates that encircle the rotor. The rotor may
be supported from one end only (overhung) or
supported on both ends by the shaft and bearings.
For modern, high capacity machines in widths of 12"
up to 48" , the rotor is normally supported on both
ends. This provides a more stable running mill, and
reduces the tendency for a rotor shaft to "wind up"
or run out under load. The hammers are simply flat
metal bars with a hole at one or both ends and usually
have some for of hardface treatment on the working
end(s). The hammers may be fixed, fastened rigidly to the rotor assembly but much more common
is the swinging hammers where the hammers float on pins or rods. This swinging hammer design
greatly facilitates changing hammers when the working edges are worn.
Reduction in a hammermill is primarily a result of impact between the rapidly moving hammer and
the incoming material. There is some attrition (gradual reduction by particles rubbing) between the
particles, and between the hammers and the screen.
The efficiency of the grinding operation will depend on a number of variables including but not limited
to: screen area / horsepower ratio, screen (hole) size and open area, tip speed, hammer pattern (number
of hammers), hammer position (coarse or fine), uniform feed distribution, and air assist. In addition, the
nature and quality of the material(s) being processed will affect the performance of the hammermill.
Basic Machine Characteristics
Hammermills used in feed processing have some common characteristics but equipment manufacturers
differ significantly in how they achieve those same characteristics. For the purpose of the discussion,
here a number of basic design principles will be reviewed as they apply to maximizing the performance
and minimizing the cost of operating a hammermill system.
Full Width Top Feed - the modern hammermill design must include a full width top feed in order to
achieve maximum efficiency and minimize the cost of operation. A full width feed insures the entire
screen area can be utilized, and that the work being accomplished will be evenly distributed across
the full hammer pattern. The top feed permits the direction of rotation to be changed, allowing two
corners of the hammer to be utilized before a physical change of the hammer is required.
Grinding Chamber - a tear shaped grinding chamber is necessary to prevent material
from circulating within the grinding chamber. Most well designed modern hammermills have some
sort of flow director in the top of the hammermill to properly feed the hammermill and to stop any
materials that are circulating within the grinding chamber.
Split Screen / Regrind Chamber - the screen should be split in two pieces, with some device at the
bottom of the mill to disrupt the flow of materials within the grinding chamber. This device must be
large enough to take products out of rotation but should not be so large as to subtract from the
screen area available for grinding. The application of a split screen design will permit the user to adjust
the screen sizing on the down side and up side to maximize productivity and product quality.
Supported Rotor - As noted earlier, the rotor should be supported at each end, preferably
with standard bearings and bearing housings. This will provide a degree of rigidity not available with
an "overhung" rotor design and reduce any problems with rotor shaft "wind up" even if the mill
operates with an out of balance rotor. Adequate support for the rotor is particularly important with
todayâ€™s increased capacity demands requiring wider machines..
Rigid Rotor Support - in order to maintain the relative position of the rotor to the grinding chamber
the foundation of the mill must be extremely rigid. A rigid structure positively maintains the clearances
between the hammer tips and the screen through the full rotation for consistent, efficient processing.
This must be accomplished without sacrificing the accessibility to the grinding chamber, as routine
maintenance of the hammers and screens will be required.
Replaceable Wear Items - one final rule for a good hammer design is if it can wear, it should be
replaceable. Beyond the hammers, screens, and pins, every component within the hammermill will
be subject to wear. Accordingly, these components should be fabricated from wear resistant materials,
heavy enough to provide good service life, and ultimately should be reasonably simple to replace.