Shaft Couplings serve as components to time, join, or align shafts at lower speeds and torque, especially when zero backlash is desired. They are not intended for use as a critical part of a drive line or as a substitute for flexible or universal joints or other power transmission devices. Rigid couplings are designed for aligned shafts only, and are the most basic but often the most cost-effective coupling.
For more information check out the following: Rigid Couplings Infographic: