Many people believe that all they need is a good flexible coupling. They believe that flexible couplings will eliminate the need for performing proper machinery alignment. Nothing could be further from the truth!
Coupling manufacturers provide the allowable misalignment values of the couplings they provide. These tolerances only tell you what the couplings can withstand but do not take into consideration what the coupled machines can withstand. The coupling may be able to tolerate a lot of misalignment, however, components in the machine-like bearings and seals may not be able to tolerate the same amounts of misalignment. Even though the coupling is flexible and may be able to withstand a lot of misalignment, the stresses of misalignment are still transmitted to the components in the machine causing premature equipment failure. Good equipment health means completing best practice machinery alignment. The reason why some flexible couplings are built to withstand so much more misalignment than what the machines can withstand is so that machines that experience substantial positional changes from operational load stresses or undergo a lot of thermal growth can be misaligned in the cold and stopped condition to compensate for these anticipated changes.
Alignment by Trent Phillips