EE WAS CLEANING UPPER RETURN IDLER ROLLER WHILE STANDING ON RAISED BUCKET OF 950E PAYLOADER. UPPER LEFT ARM WAS CAUGHT BETWEEN IDLER ROLLER AND CONVEYOR BELT ON RETURN SIDE OF POWER SCREEN MARK II. MACHINE WAS TURNED OFF, 911 CALLED, ROLLER REMOVED. HE WAS TAKEN VIA AMBULANCE TO HOSPITAL.