From your pics I would suggest that your belt issues are due to your paddles that the chains fix to. There is minimal clearance between it and the ends of your drum. this gives a place for the stone to catch and it will cause much havoc and wear. As well. Have you tried to space your chains. If the rock cant veer it will only tumble. so in theory, you are wash machining the rock. I have found that in a cylinder style drum that 90 degree variations instead of 180 degree allows for the rock to tumble and bounce between chains. You are 2 sets of chains. Go to four. an X , You will still use the same amount of chain because they will be offset. This will keep the shaft much tighter aligned as well as give the stone separation from one chain to the next. it also keeps momentum up, drum wear down, with other big benefits.
As well, You must allow a small amount of slip. If your tension is too tight on the pulley then the jars and impacts are destructive to your equipment, from chains, bearings, shafts and motor.
Oh, and I will double or even triple classify depending on the size of gold in the host rock. In the end, I pulverize everything. but when the vein has nice flakes its easier to catch than micron gold. and seems to add up faster. Although, remember, if there is big gold, then there is small gold. And every flake and speck adds up. This is why I went water table instead of cube. I always get an additional 30 percent plus of micron as I do flake or larger. And If we are already crushing, well, it only makes sense to get all of it!
