The materials to be crushed fall from the feeding mouth to the space between the two rolls. After being squeezed and crushed, the final products will naturally fall. When there are too hard materials or materials that cannot be crushed, the rolls of the roller crusher will automatically withdraw under the effect of a hydraulic cylinder or spring to increase the gap between the rolls, and the too hard materials or the materials that cannot be crushed will fall. Thus protecting the machine from being damaged. There is a certain gap between the two rolls that rotate in the opposite direction, and the maximum discharging granularity of the final products can be controlled by changing this gap. A double roller crusher's the materials by relying on the two round rollers rotating in the opposite direction, and a four-roll crusher crushes the materials by relying on the two pairs of round rollers rotating in the opposite direction.