A vibratory screening apparatus has two vibrator motors (19, 20, 19', 20') having respective out-of-balance weights arranged to produce substantially linear vibratory movement when, in use, these motors (19, 20, 19', 20') are running in mutually opposite directions, an electrical control means (37) connected...http://www.google.es/patents/US5896998?utm_source=gb-gplus-sharePatente US5896998 - Vibratory screening apparatus
