A torque bearer (23), acting as an anti twist device, is installed on the gear motor (22) radially to the drive shaft (6), and is connected to a base support (G4) by a length adjustable component (24). The latter component is located preferably at right angles to the torque bearer and has a pneumatic cylinder (25) The cylinder's force is adjustable by means of air pressure. If obstacles induce external forces exceeding the set force of the pneumatic cylinder the piston rod and cylinder move relative to each other to generate a signal which disengages the drive shaft and main drive for the conveyor chain.
