Example: Resolving product blockages and adhesions in silos. Electric external vibrators generate non-directional, circular vibrations. To drive chutes, screens, or vibration tables, external vibrators are synchronized in pairs and operated in opposite directions to achieve a directional vibration. With an optional frequency converter, the vibration frequency and force can be adjusted and optimized for the specific process. Structure and mode of operation: Electric external vibrators are motors with unbalanced weights mounted on both ends of their shafts. The unbalance discs, and thus the torque, centrifugal force, and vibration amplitude, are adjustable.