What is this LEGO part (kind of wheel or gear) number?
It is a bit tricky to find this piece on Bricklink. Even though I knew the part it took me a little to find it.

The part you are interested in is Hero Factory Weapon Barrel with 2 Pin Holes and 3 Axle Holes. Default item part number is 98585, but 31511, 31520 could also be used for other colors by LEGO.