http://www.onlycubcadets.net/forum/showthread.php?t=5
Go to the technical library forum, implements, and take a look at the belt post I linked above...also the manual for your deck.
If there is no mule tensioning spring, I don't understand how you are getting proper tension on the belt....I am assuming you have the mule drive with one spring tensioned pulley and one with the bolt type adjuster. The diagram above shows a double bolt style adjuster but the end result of adjusting should be the same..
Your deck belt tensioner should pivot freely... is the deck belt coming off as well or just the PTO belt?