In these machines, the crushing takes place due to the impact of a single element that is violently thrown, by a rotor with a high speed of rotation (400 -1500 rev/1”), against the internal walls.
Much of the disintegration also occurs as a result of clashes between aggregates.
The adjustment is carried out by measuring the rotational speed of the rotor and the distance between the internal walls. Impact mills produce polyhedral materials suitable for the production of concrete or bituminous conglomerate as well as for the formation of substrates and road surfaces, fillings, drainage layers.
They are divided into:
· Stationary hammer mills: The feed port is protected by a chain grid that allows only inbound passage. The material, proceeding along the slide, reaches the crushing chamber, where it meets the impact bars of the rotor (the hammers); the material is therefore projected outwards and collides with the impact walls covered with interchangeable armor or with other incoming material. They are mills generally intended for primary or secondary crushing.
· Free hammermills: they are similar to fixed hammer mills, that is, formed by a rotor with the function of projecting the material fed against the walls, causing it to disintegrate. They consist of a steel housing that encloses the rotor on which oscillating (free) hammers are fixed, a housing against which the material hit by the hammers hits, a grid on which the material is further crushed until it passes through the relevant holes. Anything that has not undergone such a reduction as to pass the lower grid is fished out by the hammers and put back in motion. They are mills generally intended for primary or secondary crushing.
Since only the maximum output size is governed when crushed by a crusher, the result that is obtained, as produced by the individual crusher, is a material that is not selected, that is, separated into very specific granulometric classes. To obtain a material divided by size, it is necessary to resort to screening.
