EE WAS USING A C-CLAMP TO PRESS A WEAR PLATE AGAINST THE BLADE OF A D9 DOZER. THE CLAMP HAD A LOT OF PRESSURE ON IT - WHEN HE REACHED UP TO RELEASE THE CLAMP IT POPPED OFF. THEN HE JERKED HIS HEAD OUT OF THE WAY, HIS HARD HAT CAME OFF & THECLAMP HIT HIM ON TOP OF THE HEAD.