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 grub screws - 2 for each shaft, for the best concentricity and grip.
These 5mm - 8mm couplers are great for joining motor shafts to M5 threaded rod, or to certain leadscrews that have a turned down 5mm portion on them.
Inner diameter 1: 5mm
Inner diameter 2: 8mm
Outer diameter: 20mm