1994 C1500 Chevy PK 6cyl

My 1994 chevy c1500 pk 6cyl engine needs a part but I don't know what it's called, The wheel directly in front of the alternator in which the serpentine belt loops below in the grooves of this wheel, is there a bolt to secure this wheel? Because it came off as the belt and can't find the bolt and the wheel had unscrewed itself from in front of alternator, how to fix and what is this called? Do I just buy a bolt?