Hammer Mill Application Focus: Grinding Grains for Animal Feed

Producers of animal feed all know a simple truth: when processing cereal grains for animal feed, finished particle size matters.  A lot.

Blendability, and digestibility – even the economics of feed production – are all affected by the finished particle size of the grains.  But this is not a one-size-fits-all situation.¹  The ideal finished particle size varies by the grain being processed, and depending on the species, the life stage of the animal.²  With all of this in mind, selecting the proper method of size reduction is a critical decision.

Roller Mill or Hammer Mill

Unlike the angular shaped particles produced in a roller mill, the pulverizing action of the hammer mill results in particles that are more rounded. These smoother surfaces make it easier for the grains to be blended with other components of the feed.²

So Many Hammer Mills

A hammer mill is essentially a steel box surrounding a rotor. Swinging hammers attached to the rotor, flail out when the rotor spins. A screen covers the discharge opening to regulate the finished particle size.

As material enters the hammer mill, it is pulverized by a combination of hammer blows, particle on particle contact, and impact with the walls of the hammer mill. The material remains in the grinding chamber until it is able to pass through the screen covering the discharge area.

But those are just the basics. There are myriad styles of hammer mills: industrial mills, full circle screen hammer mills, horizontal grinders, tub grinders, and more.  As for size, hammer mills can range in size from 6” to more than 60” rotor widths.  Choosing the right hammer mill for your purposes depends greatly upon the material you are processing, and your production goals.

The Full Circle Solution

The design of the full circle screen hammer mill makes it the ideal method of size reduction when processing cereal grains for animal feed.  Let’s take a closer look:


With swine operations reporting that up to 75% of their budget is allocated for feed production,³  the need for energy efficiency cannot be overstated.

As its name suggests, the full circle screen hammer mill differs from the traditional industrial hammer mill because it has nearly double the screen coverage of the rotor.  The greater screen coverage of the full circle mill means that there is more area for the material to evacuate the grinding chamber. As a result, a higher throughput rate can be achieved at a lower horsepower in comparison to industrial hammer mill models.

Particle Size Flexibility

Precision and consistency of the finished particle size has a strong effect on the digestibility of the feed.  A finished particle size that is either too large or too small will have adverse effects on animal performance.²

The primary factor determining finished particle size is the measurement of the perforations in the screen covering the discharge area. The size of these perforations is equal to the maximum finished particle size of the material processed.

The ideal particle size range of 650 to 750 microns³ can easily and consistently be achieved through proper screen selection. In addition, screens can easily be changed to accommodate the processing goals of a variety of grains using the same hammer mill.

The secondary factor in determining finished particle size if the speed of the hammer mill. When the rotor spins, the hammers flail out and impact the material with great severity causing it to break down.  As a result, the higher the rotor speed, the greater the number of hammer mill blows, and thus a finer end product.  An optional component for the full circle screen hammer mill is a variable frequency drive electrical control. This control panel allows the operator to raise or lower the rotor speed as needed for different applications.


The pulverizing action of the hammer mill produces a finished particle that is easily blendable. The nearly 360 degree screen coverage of the full circle screen hammer mill allows for the greatest throughput to horsepower ratio of all hammer mill designs. A consistent and precise finished particle size can be achieved by screen selection. Finally, the ability to change screen size, and the option of a variable speed frequency drive control, enable processing a variety of grains using the same hammer mill.


¹ Steinhart, Terry L., Iowa State University (2012). Swine Feed Efficiency: Influence of Particle Size. Retrieved from  http://www.swinefeedefficiency.com
²Pork Production in Mississippi is Particle Size Important for Swine Diets?(2013). http://www.msucares.com/livestock/swine/particle.html
³Sayler, Julie. (2012). Small Particle Size Makes a Big Difference.http://swineperformance.vitaplus.com/2012/04/small-particle-size-makes-a-big-difference/

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