NH hayliner 420 small square wire tie

I have a NH hayliner 420 small square baler.  Recently, the idler pulley for the needle and wire twist timing keeps falling which causes the chain to come off and throw the needles/twist to jump time.  I have to reset and time it every time costing valuable production time.  The bolt/nut is not wobbling lose; it has to be the metal is getting worn down.  I have also had trouble finding a mechanic that knows how to work on this old machine.  I purchased the baler ten years ago and this is the first trouble it has given me.  I need to figure out what is causing enough tension to to gradually pull the idler pulley down.  I have had great success with this machine and want to keep it operating.  Anyone have any experience with this machine?

NH hayliner 420 small square wire tie