Dunc,



I had this very problem. I agree with Ian and Jon.



The force on the lower shaft might well be distorting the fan frame a little, and it only takes a bit of movement in the frame to give an amplified movement at the pulley - the 3x effect.



Have a look at the pictures on my website:



http://www.rhevans.co.uk/january2004.htm



to see the extra diagonals I added on to brace it properly.









Regards,



Richard E.