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Refining action is caused by passing blown digester material, the cooked wood chips, and black liquor between two large metal disks with ridged surfaces. Typically, one of the discs rotates and one is stationary causing a shearing action on the blown material as it passes through the gap between the disks. The amount of energy that goes into the pulp is controlled by the size of the gap the material passes through. Decreasing the gap increases the energy applied, and the amount of work done on the material. By applying different amounts of energy to the material, the effects on the pulp can be controlled. The gap is controlled by positioning devices that move the rotor towards or away from the stater or by a motorized screw drive.