sears best / craftsman 32 cc blower. vac mulcher 358.798980 got a loose nut

I was out cleaning up leaves the other day when the blower decided to quit, after checking things out, I found the nut that tightens up the fan had come loose. I did the rope in the spark plug hole to tighten the nut ( three times to get the yard done). Why is it coming loose? and how can I keep it tight, its been good for 30 some years until now.