The three abrasive rollers are distributed at the roller path of the millstone uniformly, and are fixedly to the loading rack.

Further, each of the side members and end cross members preferably has a section of a roller path and section of a ring gear attached to them while being transported and assembled.

By the way of improving roller dimensions, roller number, outer roller path diameter, stop groove diameter and the effective thickness, the using reliability of the bearing is advanced.