Looks like I found the problem. Looks very likely the lift belt was too slack. I used the club belt calculator and it said the belt should have a 7.8mm deflection with a force of 2.6kg. I checked the deflection at 2.6 kg and it was 15mm.
While I was at it I fitted a spare lift belt which I cable tied to the frame. That was quite a lot of work, so I’m pleased I done it at home rather than stuck on a beach somewhere


I also swapped one of the idler pulleys, as it had some play in the bearing and was noisy

I looked at that John but there is so little space on my thrust frame that the rollers would need to be tiny and spin at a zillion rpm - hence a major failure point.  Also concerned about having bearings right in the slip stream - I can only see touch pads as a possible solution but am concerned about wear on the belt ....  I would be interested in your design concept as/when it is finished !  Tx.  :-)

I've just put the numbers into the belt calculator.


I went with what was on the plans, which was 4.5lbs & 1/2" deflection.  Looks like I may of had the belt too loose.  Will check what my current tension is before removing the belt to rule out any other factors, but I suspect belt incorrect tension.



TensionNumber of belts required (calculated 0.65)[/size]1Belt tension deflection distance (mid-span)[/size]7.8 mmMinimum deflection force @ mid-span (used belt)[/size]2 KgMaximum deflection force @ mid-span (new belt)[/size]2.6 Kg[/size][/font][/size][/font]

As said, its' also possible (although very unlikely!) that the drive/driven pulleys or fan hub are slipping on the shafts (I would have thought that would be very obvious due to a screeching noise and very hot pulleys!).

Assuming belt slip due to wrong tension or belt type then the belt should be discarded as its been run slipping for a considerable time (the heat generated damages the tension cords and friction surfaces to the point the belt needs continuous tensioning).  The belt should be a wedge type (SPA) which increases margin by around 30% over standard 'A' section (and fits the same pulleys, etc.).  I'd also recommend the higher end no-maintenance belts (Gates Quad Power, etc.) from a reputable manufacturer.

There is a proper belt tension calculator HERE that will give the correct tension for a new belt (over-tension by 15% normally).  Note that for a mule drive you should use the driven-to-idler pulley centre spacing, instead of the full length, to calculate the tension).  Use the full centre spacing to calculate the load rating is OK (it is!). All simple basic stuff.