Rear Finish Mower Woods RM660

[farmtrac360]

My neighbor keeps breaking the belt tension spring on his Woods RM660. I've replaced 3 already and am getting ready to replace number 4 as soon as I get a chance to stop and get one for him. I cannot figure out why these things keep breaking, he's experienced (very) and says that the RPM's are where they should be. The yard is not overly rough and there aren't any objects that would catch the blades. It's happening while moving, not when the PTO is engaged. What am I missing and what could possibly cause this. Oh, and it's well lubed with a new belt. BTW, those darn Woods belts aren't cheap are they.......