Hammer Tip: Go High Speed to Control Particle Size

In most cases, feed manufacturers primarily rate a hammermill's performance on its capacity to grind a certain amount of material in a given time. However, this practice ignores particle size, an important consideration in handling characteristics, pellet durability and animal performance. Traditionally, feed manufacturers who use hammermills to grind grains and other materials control particle size by changing the size of the hammermill screen holes. While screen size has a major influence on particle size, hammer tip speed is also an important factor. Generally, if all other factors are the same, hammermills with higher hammer tip speeds produce a finer grind than those with lower tip speeds.

To understand how tip speed affects particle size, it is important to know how a feed machine operates.

A hammermill grinds by two distinct forces. The first force is impact. As the grain drops into the grinding chamber, it is struck by fast moving hammers and accelerated toward the screen where a second impact occurs.

The particles that don't discharge through the screen immediately form a fluidized bed against the inside of the screen where they are ground by attrition, the second grinding force. With attrition, particles are ground as they are forced against the screen and against each other. Of the two grinding forces, impact is more efficient. Feed machinery manufacturers, therefore, have gone to great lengths to increase the amount of impact that occurs in the hammermill, resulting in steady improvements in grinding efficiency.

One way to increase the amount of impact between hammer and particle is to increase the tip speed of the hammers.

According to basic principles of physics, if motor speed is fixed, increasing the diameter of the rotor will increase the speed of the hammer tip.

Special design and material modifications are required to produce a machine that operates safely at rotation speeds of more than 25,000 feet per minute. The rotor shaft and plates are manufactured from materials that withstand tremendous forces. The assembly is balanced to prevent excessive vibration. Engineers and designers must also give consideration to the bearing mounts, hammermill base and frame to maintain proper alignment and support for the heavy rotor assembly. A hammermill with a higher tip speed will produce a finer grind than one with a slower tip speed, provided the two grinders have the same size screen. If a high-speed hammermill is equipped with a larger hole screen, ground material particle size will be the same for both machines.