Ok this is a little more clear but if I may follow up with this. I went to this site mentioned above:
http://www.translatorscafe.com/cafe/EN/units-converter/moment-of-inertia/12-1/
and 1 pound-force inch second² = 386.0885865302 pound inch². So if I have a control system with several components...
Why are some inertia units lb-in-s^2 and others lb-in^2. what the difference? The first one is from a gear box spec and the second is from a motor rotor.