I am having trouble with some extraction fans on a ATI Heat treament Furnace when they are measured cold they run fine but when the area is up to temperature vibration analysis isn't needed because they vibrate that much that if i unbolted them they would vibrate to Dublin and back. I have replaced motors had them balanced made new frames and am currently waiting on new fans as i think the heat has warped them to mush ANY IDEAS WOULD BE GREAT.

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Robert,

You should post this question in vibration discussion board. I see these are welded impellers and possibly no stress relief was carried out after manufacturing. They should twist after temperature is applied. Do you know that vibration is only at 1xrpm? Possibly the impellers are getting loose on the shaft.

on one occasion i discovered the impellar had completely worked itself loose but do you think once at temperture its becoming loose then once cooled down ok again and if so what would u recommend

If the fan is coming loose, that's a serious issue. The question is...is it due to thermal growth of fan hub or is it movement from the fan wheel causing it to come loose. If you are over 250F the fan wheel should use a taper lock type connection to the shaft, along with a shaft key, or a bolt in the end of the shaft with lock washer.
Have a couple of fans that react the same way, balance cold and they run great, bring the fan up to temp (650)F and it is out of spec. We balance them hot, pain in the butt!
Have you performed any resonance checks, like bump test, phase, startup coastdown?

How do you balance them hot, and thanks for the input

Get close first while fan is cold. It takes about 15-20 min for our oven to heat up, once up to temp take your new reference run. Shutdown and let cool to a tolerable level, add trial weight, heat back up repeat normal balance procedure, just make sure oven is up to temp before storing data...

Thanks Dave i will try this