The operation principle of a hammer mill is quite simple. The machine is essentially made of a steel drum with an internal rotating shaft. The shaft may be either vertical or horizontal in alignment. Hammers are attached to the shaft and allowed to swing once rotation begins. Large pieces of material are fed into the drum where they are impacted by the swinging hammers. Finer pieces are expelled from the drum through screens of various sizes. The grade of final output will depend on the type of screen used and the size of grains it allows to pass through.
Applications of the hammer mill can be classified as primary, secondary, and tertiary. In primary applications, the machine is used for household activities. In such a case, it uses little current supplied from a single face source. Secondary and tertiary mills use larger current for operation. In fact, most of them use three-face current source or sometimes are driven by diesel. Their horsepower ranges between 2000 – 5000 horsepower. This is equivalent to 1.5-3.7 megawatts of power. Due to this large power capacity, they can be used to crash very large pieces such as timber into fine sawdust.
Inventions in the milling industry have led to the production of screenless hammer mills. They basically operate without a screen. They have sophisticated airflow systems that are used for the separation of ground material. They are much more reliable and can be used to generate better results than the traditional types of mills. The manufacturers also claim that they are energy efficient as compared to the traditional types hence have a low operation cost. Each of the mentioned types has its own advantages and disadvantages. Some are meant for small household jobs while others are made for heavy industrial applications.