SETTING BY CENTER SECTION OF RC21 TIGHTENING TRUNNION NUT WITH BOLTS AND 5 TON CRANE, WHEN TRUNNION NUT TIGHTENED COMPLETELY WITH FORCE BY CRANE BOLT. SNAPPED OFF BECOMING AIRBORNE WITH FORCE AND STRIKING EE ON SAFETY GLASSES AT LEFT EYE CAUSING CUT FROM NOSE PIECE AND LOWER EDGE OF SAFETY GLASSES. EE LEANED OVER CENTER SECTION WITH TENSION STILL ON STRAP.