by George Obeng-Akrofi, N.A.M. Kusi Fordjour, and Dirk Maier 

AMES, IOWA, US — In feed manufacturing, hammer milling commonly is described as a particle-size reduction operation, one of the most closely monitored outputs of the grinding process. However, the objective of grinding is not simply to reduce grain to its ground form; it must be reduced to the desired particle size while maintaining acceptable throughput and minimizing energy consumption. 

When adjustments are needed, the typical response is to change only the screen size or the feed rate. However, another variable combination, such as hammer tip speed as a function of motor speed and the hammer mill diameter and screen size, plays a critical and often underutilized role in determining grinding performance and achieving the desired particle size.

As discussed in part one of this series (in the May 2026 issue of World Grain), proper evaluation of hammer mill performance requires steady-state operation and consistent measurement practices. Building on that foundation, this article examines how motor rotational speed (i.e., hammer tip speed) and screen size (i.e., screen hole diameter) affect particle size, throughput and energy efficiency under steady-state conditions.

Motor speed often overlooked

In most feed mills, grinding adjustments are made by changing screens to achieve the desired particle size, while the motor rotational speed remains fixed. This results in production downtime whenever a screen change is required. Many hammer mills ramp up to 100% operational speed, resulting in the highest hammer tip speed during startup and operate at that level throughout production runs.

As a result, motor speed often is treated as a design parameter rather than a controllable variable. Even in systems equipped with variable frequency drives (VFDs), speed adjustments are not always used strategically. This limits the ability of operators to fully understand how hammer tip speed influences grinding performance.

This gap highlights the need to evaluate hammer tip speed more systematically, especially under steady-state conditions, and to use hammer tip speed in combination with different screen sizes to achieve a desired particle size.

Motor speed directly determines hammer tip speed, which governs the intensity and frequency of particle impacts within the grinding chamber. The hammer tip speed is a function of the hammer mill diameter and the rotational speed of the motor. Hammer tip speed for any hammer mill is calculated as a function of the following equation:

Tip Speed (ft/min)=(π×hammer mill rotor diameter (inches)×motor speed (rpm))/12

The main idea is that increasing motor speed increases hammer tip speed, resulting in more frequent and more energetic impacts between the hammers and grain particles. These repeated impacts accelerate particle breakage, producing finer particles. Conversely, reducing motor speed decreases hammer tip speed, and thus impacts energy and reduces the frequency of particle collisions. Material experiences fewer breakage events before exiting the grinding chamber, producing coarser particles.

While this relationship is well understood in theory, its practical implications extend beyond particle size alone. Changes in hammer tip speed also affect residence time in the grinding chamber, machine throughput, and overall system efficiency. Understanding these interactions is essential if motor speed (and hammer tip speed) is to be used as a practical optimization tool rather than simply another machine setting.

Credit: ©IOWA STATE UNIVERSITY

ISU grinding trials

Using the performance evaluation methodology presented in the previous article, controlled grinding trials were conducted at the Iowa State University (ISU) Kent Feed Mill and Grain Science Complex (FMGSC) using a 100 hp CPM 44912 Champion hammer mill.

Motor speed was evaluated at five operating levels: 100%, 80%, 70%, 60% and 50% of the rated motor speed of 1,775 rpm. These speed settings corresponded to hammer tip speeds of 17,658, 14,127, 12,361, 10,595 and 8,829 feet per minute, respectively. Two screen sizes (10/64 and 12/64 inches) were evaluated, with two replicates for each motor speed and screen size combination. Key performance indicators, including hammer tip speed, throughput, average particle size and specific energy consumption (SEC), were measured to assess the effects of hammer tip speed and screen size on hammer mill grinding performance. Table 1 summarizes the results from this trial.

One obvious observation from the study is that hammer tip speed consistently influences particle size, regardless of screen size (Figure 1).

As motor speed decreased from 100% to 50% (i.e., 17,658 to 8,829 ft/min), average particle size increased for both screen sizes. Using the smaller 10/64-inch screen, particle size increased from 259 μm to 532 μm compared to 382 μm to 764 μm with the larger 12/64-inch screen.

While the influence of screen size on particle size is well established, these results demonstrate that motor speed provides an additional degree of control within each screen size. Rather than relying solely on changing screens, operators equipped with VFD-controlled hammer mills may be able to fine-tune particle size by adjusting motor speed.

Overall, reducing motor speed from 100% to 50% reduced the hammer tip speed from 17,658 ft/min to 8,829 ft/min and produced a coarser grind across the different screen sizes, as expected due to reduced hammer impact energy, hammer tip speed, and impact frequency during rotation. This approach has practical value because changing motor speed is significantly faster and less disruptive to the production schedule than physically replacing screens.

Particle size alone does not determine grinding performance. Throughput and energy consumption also must be considered.

As shown in Table 1 (above) and Figure 2), reducing motor speed affected the two screen sizes differently.

For the 10/64-inch screen, throughput remained relatively constant between 100% and 70% motor speed, then declined at lower speeds. Specific energy consumption decreased from 4.29 to 3.37 kWh/ton as motor speed was reduced to 60% and then increased to 4.10 kWh/ton at 50% because throughput decreased substantially. The 12/64-inch screen behaved differently. Throughput remained nearly constant across all motor speed treatments, while specific energy consumption steadily decreased from 3.76 to 2.62 kWh/ton as motor speed decreased. These results illustrate that the effect of hammer tip speed on energy efficiency depends not only on motor speed but also on its interaction with screen size and production rate.

Credit: ©IOWA STATE UNIVERSITY

No universally optimal motor speed

One of the most important findings from the study is that there is no universally optimal motor speed. Instead, an optimal operating window exists when motor speed (and hammer tip speed) may be combined with the right screen size to achieve a range of particle sizes for feed rations without sacrificing production capacity (including downtime to change screen sizes).

For the smaller 10/64-inch screen, reducing motor speed below approximately 60% resulted in a substantial loss in throughput, causing specific energy consumption to increase despite lower motor power demand. For the larger 12/64-inch screen, reducing motor speed had little effect on throughput, allowing energy savings to be realized across nearly the entire operating range.

In the case of the hammer mill installed at the ISU Kent Feed Mill and Grain Science Complex, the optimal screen size is 12/64 inches for the range of feed rations requiring average particle sizes of 400 to 600 µm because by adjusting motor speed (and thus hammer tip speed), an average particle size range of 382 to 685 µm may be achieved with specific energy consumption ranging between 3.76 to 2.61 kWh/ton while maintaining 10 TPH throughput at an 88% feeding rate of 14% to 16% moisture content corn. 

These findings suggest that motor speed should not be adjusted simply to reduce electrical demand. Instead, operators should identify the operating window that balances particle size, throughput and energy efficiency for the specific screen size and production objective.

Credit: ©IOWA STATE UNIVERSITY

Practical implications for feed mills

For mills operating conventional fixed-speed hammer mills, screen selection, feed rate and airflow will remain the primary parameters for controlling grinding performance.

However, mill operators with VFD-controlled hammer mills may use this practical guidance to adjust hammer tip speed and improve grinding performance. Rather than operating the mill at full motor speed continuously, reducing motor speed may increase average particle size toward a desired particle size target while utilizing the same screen size, and within an effective operating range, reduce the energy required to achieve an acceptable throughput. 

Any speed adjustment should be evaluated using the systematic methodology presented in part one of this article series. Reliable optimization requires reliable performance data.

Conclusion

The first article in this series demonstrated that hammer mill performance cannot be properly evaluated without first establishing steady-state operating conditions and standardized measurement procedures. This second article shows how that evaluation framework may be used to investigate one of the most overlooked operating variables in hammer milling, i.e., motor speed and hammer tip speed.

The results demonstrate that motor speed influences particle size reduction more than changes in screen size alone. It affects throughput, specific energy consumption, and ultimately the overall efficiency of the grinding process. More importantly, they show that no single motor speed is universally optimal. The best operating condition depends on the interaction between motor speed, screen size and production rate.

As feed mills continue to adopt VFDs and more advanced process control systems for their hammer mills, motor speed has the potential to become a practical optimization tool rather than simply a fixed machine setting. The objective is not to maximize or minimize motor speed, but to identify the operating conditions that provide the best overall balance between product quality, production capacity and energy efficiency.

A 10-tph CPM hammermill is used in the ISU feed mill.

Dirk Maier has written Grain Operations articles for World Grain since 2016. He is a post-harvest engineer with the Iowa Grain Quality Initiative at Iowa State University. Prior to that, he was a professor in the Department of Grain Science and Industry at Kansas State University and in the Department of Agricultural and Biological Engineering at Purdue University.