These herringbone spur gears fit the National Geographic Professional 2 Pound Rock Tumbler Kit, which is known for being much too fast compared to other popular tumblers on the market. This gear reduction slows the speed from about 80-85 rpm to about 30-35 rpm on the lowest setting.

Remove original pulleys by simply pulling them off. Small motor pulley may need to be pried with help of a screwdriver, as it is press fit onto the shaft. New gears can be put on the same way, press fitting the bottom small one while aligning the D-shaft on the top large one. Both gears must be installed at the same time due to the herringbone patterns, which helps keep them aligned. I have not tested it in PLA or other materials.

NOTE on NOISE

This mod will slow your tumbler down to a better speed, but will also make it louder due to the contacting nature of the gears compared to the pulleys. I recommend using a small soundproof box for your tumbler if using these gears.