Hammer mills consist of a series of hammers (usually four or more) hinged on a central shaft and enclosed within a rigid metal case. It produces size reduction by impact. The materials to be milled are struck by these rectangular pieces of hardened steel (ganged hammer) which rotates at high speed inside the chamber.
These radically swinging hammers (from the rotating central shaft) moves at a high angular velocity causing brittle fracture of the feed material. Hammer mills crush materials into two (2) stages.
- Size reduction which occurs by dynamic impact.
- Sizing which occurs by attrition and shear in the second zone were small clearance exists between the hammer and the screen bar producing a particle range of 15-50 micrometer.
The material is crushed or shattered by the repeated hammer impacts, collisions with the walls of the grinding chamber as well as particle-on-particles impacts. Screens for optimum capacity and excellent grind quality are fitted at the bottom of the mill, which retains coarse materials while allowing the properly sized materials pass as finished product.
- Produces specified top size without the need for a closed-circuit crushing system.
- Produces relatively numerous size distributions with a minimum of fines due to self-classification.
- Has high reduction ratio and high capacity whether used for primary, secondary or tertiary grinding.
- Reasonable energy requirements.
- Brittle materials are best fractured by impact from blunt hammers.
- Capable of grinding many different types of materials.
- The machine is easy to install and operate and its operation is continuous.