Coupling and Shaft Misalignment Tolerances – What is the Difference?

It is important for the person maintaining rotating equipment with flexible coupling not to be confused by the term ‘‘allowable misalignment’’ in a coupling. The coupling manufacturers will often quote information on allowable misalignment for the coupling and not necessarily the equipment it is coupled to.

These tolerances seem to lull the user into a sense of complacency, leading one to believe that accurate shaft alignment is not necessary as ‘‘ . . . the coupling can take care of any misalignment’’ (famous last words).

It is imperative that the user can differentiate between coupling tolerances and alignment tolerances. Coupling misalignment tolerances quoted by flexible coupling manufacturers typically specify the mechanical or fatigue limits of the coupling or components of the coupling. These misalignment tolerances are frequently excessive compared to the misalignment tolerances, which deal with the rotating drive system as a whole. The misalignment tolerance guide is concerned with the survivability of not only the coupling, but also the shafts, seals, and bearings of the machinery over long periods of time.

(Shaft Alignment Handbook, Third Edition – John Piotrowski, 2006)