These are flexible shaft couplings - particularly useful for joining motors to Z-axis threaded rods or lead screws. These help reduce or eliminate the dreaded Z-Wobble that plagues many machines.
Each has four M3 grub screws - 2 for each shaft, for the best concentricity and grip.
These 5mm - 8mm couplers are great for joining motor shafts to M8 threaded rod, or to certain leadscrews that have a turned down 8mm portion on them.
For the dimensions of this product please refer to the technical drawing in the image gallery above.