Gear Couplings is a device used to connect two shafts at their ends together for the purpose of transmitting power. The main and primary purpose of couplings is to join two rotating pieces while allowing some degree of misalignment or movement at one or both ends.
A full gear coupling has a toothed hub which is fixed in both driver and driven shaft, and sleeves which has internal teeth meshing with the hub. In both the figures, O-ring is for holding the lubricating grease inside the coupling and preventing the ingression of outside air and moisture.
Gear couplings are commonly employed on large, industrial machines such as pumps, blowers, compressors, mixers, etc. They can be used in smaller motion control systems, mostly with a polymer sleeve as a means of reducing backlash but other zero-backlash coupling designs are usually better suited to these applications.