When installing a new belt/chain; adjust the belt/chain tension then rotate the assembly a bit, and check tension in several spots. If you throw the belt on and set tension in one spot it may be good, but real tight or loose in another. If there's a tight spot somewhere it will wear, and just keep wearing rapidly in that spot. Replace belt and check tension in multiple locations. Also check for an out of round pully (generator, water pump, harmonic balancer).
