Zero Turn Mowers

I bought a RZ4219 3 weeks ago. After the 3rd one hour mowing the back side of deck belt wore out. I suspect the tensioner pully pivot is haging-up. So it did not tension the belt properly causing the belt to wear out on backside due to rubbing on the keepers. I have bought a new belt. What do I need to do to loosen the pivot on tensioner pully so it won't hang-up? Do I put some oil on the pivot or is the nut to tight on the pivot? I need an answer before I put on the new belt and ruin the belt. The belts are so expensive.