GoldenGears GT2 Belt Tensioner

thingiverse

This is a fresh take on adjusting belt tension for delta printers, especially those with Kossel type configurations - it may also apply to other designs. As shown in the picture, it attaches at the regular bolt and attachment point where the pulley usually sits (often on top, but in my case, I'm using a modified design with the pulley positioned at the bottom). This setup essentially adjusts the pulley's location by taking advantage of the space between two M3 screws that secure the two components together. You can fine-tune the adjustment flexibility by shortening or lengthening the M3 screws - just keep in mind that going too long will compromise build height.