Since I don't feel too confident about running the belt in either oil or grease, and since I'm positively sure it was not over-tensioned, and since some others run those things for 15000+ miles without problems (hi groily
), I wanted to get at the bottom of this, so today got Conti's design tool and fed the geometry and power data into it to see whether the belt is basically up to the intended purpose.
So for those interested, that's the calcs and results:
belt type: Conti AT5 GenIII, 420mm length
pulley1: driving pulley (intermediate pinion at half engine speed), 46T
pulley2: driven pulley, 17T
The calculations are based on a power of 100Watt (0.1kW) on the driven pinion (assuming 60Watt electrical power output and 40% power loss in the dynamo, which I think is quite reasonable for a DC brush-operated design) at roughly 1900 generator rpm (this being the design rpm stated in the Lucas service sheet), thus resulting in the highest driving torque over the operating rpm range.
The results show a maximum transferable power of 0.52kW on the driven pulley, so we are facing a safety factor of 5.24 here. The belts are designed to work in temperatures up to 100°C, reduction of maximum power above 50°C is not taken into account in these calculations, but with a safety factor of more than 5 there is plenty of room there.
So the good news is that these belts should be more than up to the task and failure actually is very unlikely.
What is interesting though is the required pre-tension force in the belt, which is 18.5N (about 4.1pounds) in each span of the belt. This is quite some tension, imagine an 8.2 pound weight hanging on the belt, and the "easy 90° twisting-thing" from the mounting instructions misses the point here imo.
So I'm led to believe that my belt failed because it wasn't tensioned sufficiently, I shall give it another try with a new one and this time rather trust my own judgement than the instructions (let's face it, probably t'was my fault this time)
Cheers, Markus
edit: Lacking equipment to properly measure belt deflection or frequency of oscillation, this could be a neat idea to get the pre-tension about right: Hang an 8.4pound weight on the belt using two discs similar to the pulleys in diameter and twist the belt by 90° to get a feeling for the necessary force, and then apply same to the mounted belt.