A B737-300 WAS DISPATCHED AFTER A #2 ENG FAN LUBRICATION WITH 1 DAMPER MISSING FROM #20 BLADE CAUSING ENG VIBRATION.

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DURING A ROUTINE FAN BLADE LUBE; DURING AN OVERNIGHT CHK; 1 DAMPER WAS LEFT OUT UNDER #20 BLADE. THE ACFT LEFT THE STATION (ZZZ) ON A REVENUE FLT THE NEXT MORNING; AND UPON REACHING XZX; THE PLT WROTE UP A VIBRATION ON #2 ENG AT 83% N1 AND 75% N1. A MECH IN XZX PERFORMED ANOTHER FAN BLADE LUBE (SUSPECTING THE DAMPER MISSING FROM THE PREVIOUS NIGHT'S FAN LUBE). FOUND THE MISSING DAMPER; INSTALLED A NEW ONE AND CORRECTED THE PROB. 2 OF US WORKED TOGETHER THAT NIGHT ON THE FAN LUBE. MY PARTNER CLEANED AND RESPRAYED THE DAMPERS. UNKNOWN TO HIM; 1 DAMPER FELL OFF THE SHELF WHERE HE WAS WORKING AND LANDED ON A LOWER PORTION OF THE WORK TABLE; OUT OF SIGHT. UPON REASSEMBLY; I SAT IN THE INLET INSTALLING BLADES AND DAMPERS. APPARENTLY I MISSED INSTALLING A DAMPER UNDER #20 FAN BLADE. THIS OCCURS OCCASIONALLY; AND YOU'LL HAVE AN EXTRA DAMPER LEFT OVER; AFTER ALL THE FAN BLADES ARE INSTALLED. THEN YOU SPIN THE FAN SLOWLY AND FIND WHERE THE MISSING DAMPER IS; AND INSTALL IT. BUT IN THIS CASE; WE HAD NO 'EXTRA' DAMPER BECAUSE IT HAD FALLEN TO A LOWER SHELF ON THE WORK TABLE UNKNOWN TO US. INSPECTION LOOKED OVER THE BLADE INSTALLATION (THEY DON'T LOOK AT DAMPERS) AND GAVE US AN OK TO INSTALL FRONT SPINNERS. I THINK THE MAJOR FACTOR IN THIS INSTANCE WAS ALERTNESS. THE HUMAN BODY IS NOT DESIGNED TO WORK FROM XA00 TO XI00. I CANNOT FUNCTION AT MY BEST DURING THE NIGHT. I ROUTINELY GET 4 HRS OF RESTLESS SLEEP A DAY; AND I'M CONSTANTLY TIRED AND IRRITABLE. THE SECOND FACTOR IS COMPLACENCY. PERFORMING THE SAME TASKS NIGHT AFTER NIGHT IN A THIRD SHIFT ENVIRONMENT LEADS TO COMPLACENT PERFORMANCE. IN THE FUTURE; HOWEVER; I WILL ENDEAVOR TO PERFORM MY TASKS IN A MORE DILIGENT AND ALERT MANNER. THIS EVENT DID NOT CAUSE ANY ACFT DAMAGE; PERSONAL INJURY OR FLT DIVERSION. JUST REWORK AT THE NEXT STATION.