Re: 2015 Craftsman GT5500 Review
There's only a few reasons why a deck belt will keep coming off.
1: belt old, heat cracked or stretched too much that it slings off when engaged or after a few moments of usage. I like Kevlar belts for all the belts on my mowers as Kevlar is a product that holds up to heat better than regular rubberized cloth bound belts.
2: Deck idler pulley(s) spring or springs have gotten weak or broke and don't hold the belt in tension anymore. Most of the springs have got different tension slots that one can use to help with spring belt tension. Another backyard trick if springs can't be tensioned tighter is to get some strong wire and wrap a couple spring coils together to not separate, which will make the other coils tighter.
Something I've done with good results is get another spring of smaller size, but with good strength and just a tad shorter and put this smaller spring into the center portion of larger spring to make it a double spring to make a weak and no-longer-available (NLA) spring usable again. Most home/tractor supply stores will have a decent supply of utility springs to see if they'll work. If not one store, go to another.
3: Belt v-pulleys, either one or all are worn and need replaced. Or, by spinning each v-pulley by hand one can see if that pulley is bent or loose on spindle shaft the pulley attaches to.
4: Spindle shaft(s) of one or more need rebuilt with new bearings, or whole mandrel assembly needs replaced as something has egged-out the assembly and it can't be rebuilt.
5: Deck bent from hitting something and thrown a pulley off kilter with alignment to other pulleys.
6: If something was changed on that deck, either a new mandrel assembly or a new pulley of incorrect size/shape, its possible that whatever it is, it's maybe out of alignment with the other pulley's. Even a bolt washer in wrong spot could tilt a pulley off and throw the belt. Everything needs a close inspection to find the cause.
These are some of the things I can think of and I'm sure there's more, but without seeing where the problem lies, one needs to go through the process of elimination and it takes time.
7: Don't let that Sear's tech leave after saying they've fixed the problem, without you using the mower for at least 5-10 minutes if belt comes off like you say.
1967 Cub Cadet 102-Kohler K241(10hp) Kept as original as possible.
1990 Kut Kwick CL20-60-Onan P220(20hp) Front deck/rear steer industrial/commercial mower
2003 Craftsman GT5000-B&S engine(24hp) Manual transaxle