Results and Discussion
Hammer Mills
Hammer Mills accomplish size reduction by impacting a slow moving target, such as corn, with a rapidly moving hammer. The hammer tip is traveling at a minimum of 16,000 feet per minute and perhaps in excess of 23,000 feet per minute. Size of particles is a function of hammer tip speed; hammer design, placement, and wear; screen design and hole size; and whether or not air assist is used.
Table 1 describes screen sizes that producers use in on-farm portable grinders, mix mills, and modern toll mills and the resulting micron sizes. Screen sizes ranged from 1/8 to 3/8 inch for on-farm hammer mills. Micron sizes are categorized as lowest average microns sampled, average microns for all samples, and highest micron average. Factors influencing these differences in microns include flow rate of corn to the grinder, condition of the screen, number and condition of hammers, tip speed of hammers, moisture content of corn and in some cases, variety of corn. Figure 5 represents the values from Table 1 in graphic form.
When evaluating the data in Figure 5, it appears that screen size has a significant impact on final micron size achieved. If you follow the average particle size for a 1/8th inch screen up to the 3/8th inch screen, there is a nearly linear increase in average particle size, with the average ¼ inch screen not meeting the target of 700 to 800 microns. These data would indicate that if you want to consistently grind corn in a hammer mill to 700-800 microns, a 3/16th inch screen or smaller should be used. In addition, a large amount of variation is observed within a particular screen size. Even on the smallest screen size, particle size achieved under operational condition did not meet the target micron size. This would indicate that other factors are influencing the ability of the hammer mills to adequately grind corn including: hammer size and number, operation speed of the hammer, as well as screen and hammer condition. Another important note for producers is that, while using a 3/16th inch screen or smaller will increase grinding time and the energy requirements to grind corn, research at Kansas State has generally shown that increases in energy costs and labor can be more than offset by improved efficiency of gain. Producers should evaluate their cost of milling and the anticipated improvements in animal performance when deciding to utilize very small sized screens.