On my JK, one of the idler pulleys literally split in half, causing the pulley to have a narrow, sharp edge that caused the belt to eventually split in two and subsequently disintegrate into what seemed like fine pieces of black string.
Upon inspection, you at first don't easily pick this up, as all seems fine. In fact, after the belt first disintegrated, I replaced it with a second one without noticing that one of the pulleys I routed the belt through, was in fact much narrower than it supposed to be. I only started noticing it when a ridge became visible on the belt itself as the sharp edge of the narrower pulley started eating into the belt.
These pulleys are made of some plastic compound, hence its weakness in this regard.
Bookmarks