Case story: Stone recognition

A seam of brown coal can be as deep as 100 m below the surface of the earth. In the majority of cases bucket wheel excavators are used to recover the coal. The so-called stone excavation involves a certain risk.  The bucket wheel can become blocked if a tooth of a bucket comes up against a pocket of firmly entrenched stone. The increased load moment can result in the speed of the bucket wheel dropping within a very short period of time. At the same time the blockage causes the bagger wheel boom to lower. The entire construction of the bucket wheel excavator performs a dipping motion in the opposite direction to the rotary movement of the bucket wheel. This results in a further drop in the rotary speed of the bucket wheel. A risk is posed to the safety of personnel if the tooth of the bucket wheel slips from the stone and causes the bucket wheel boom to swing upwards.