Scag Mower Drive Belt Coming Off

I have an older Scag mower (SSZ 22CV 61") that came with our new house in 2016. Today while mowing it threw the main drive belt from the engine to the mower deck. I re-installed the belt and went back to mowing and it threw it again. Re-installed a 3rd time to try to get the lawn finished and it threw it almost immediately.

The belts looked rather old and worn, so I've got new ones on order. No debris in the belts anywhere that I've seen and all 3 spindles are pretty tight (one has a barely perceptible about of play in it).

But, both idler pulleys have pretty significant (about 1/8") vertical play at the pulley. The play is in the pivot at the end of the arm, not in the pulley bearing. Is this normal or should I replace the bushings in the arms too?

I'm just not sure if it's the belt or something else making it jump.

 

On my older mower, each pully has a guard that tightens to the pulley when you engage the belt to keep it from throwing. The main pulley off the engine has a stationary guard. When the tension is released from the deck to disengage the belt, the guards loosen. Have someone cycle the engagement arm while you observe what goes on underneath.

I just went through an issue where the deck blades would engage but would not disengage. Turns out from years of service, one of the locking pins that holds the linkage engaged had rubbed almost half of the metal away such that everything got hung up. Flip flopped he primary with the secondary and orientated the worn part so it was not facing any parts and got the mower back up and running. If yours is older, may have had a similar failure.

 

No guards like this on my Scag. Also, belt is under tension constantly mower engagement is done electrically via a toggle switch.

I took apart one of the idler arms and found no evidence of wear. I think the play I'm seeing is inherent in the design of the arm. Because they are retained with an e-clip, there has to be a little vertical slop in order to get it assembled. A little play at the pivot produces 1/8" at pulley.

Hopefully the new belts do the trick. I'll know later this week.

 

The new belts seem to have fixed it. Once I pulled the old belts out and got a good look at them, they were in even worse shape than I thought. Several cuts and splits in the belt that was coming off. Was able to mow all 1.4 acres today without an issue.