Type SG Rollers with molded rubber spirals function the same way as molded disc rolls rollers. They also keep the belt clean and centered. Two non-abrasive molded rubber couplings are fitted on the roller with a left-right molded spiral and blocked at the ends with stop rings. Belt cleaning is by an Archimedian screw effect which rotates the roller and expels material from the sides. The molded spiral also keeps the belt centered through its motion.