We will continue to add to this blog post, but here's what we've tried for repairing the tumblers we have.
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National Geographic professional tumbler
National Geographic belts are easy to replace.
A good source for belts is Amazon. Specifically, these:
National Geographic tumbler motors are easy to replace.
Just flip the tumbler over and peel off the little round sticky screw covers. Keep a rag handy to wipe away gray dust. Use a phillips head screwdriver to remove the screws. Pull off the bottom to peek inside.
How to replace Nat Geo: https://youtu.be/QIrNpuBIqUQ?si=b1BT94Dg2gLmNKzw
Lortone belts are easy to replace:
The tumbler stopped turning with both barrels, then stopped with one barrel. When I had no barrels on it, the rollers would spin, so the motor was working, but when I put a finger on the roller, it would stop moving while the engine purred on. So I removed the cover with a 9 mm ratchet wrench and felt the belt. It was loose.
Then I used an 8 mm ratchet wrench to adjust the two bolts to the right to tighten the belt. By shifting them right and tightening, the belt had enough tension to continue operating.
With the new belt in hand, I loosened the screws again and brought the motor to the left to create slack to remove the old belt and put the new one on. Then it's just a matter of moving the motor right again and tightening the screws to make sure there is enough tension on the belt. Both barrels are rolling happily now.