Shaft alignment tolerances

Historically, shaft alignment tolerances have been governed by the coupling manufacturers’ design specifications. The original function of a flexible coupling was to accommodate the small amounts of shaft misalignment remaining after the completion of a shaft alignment using a straight edge or feeler gauges. Some coupling manufacturers have designed their couplings to withstand the forces resulting from as much as 3 degrees of angular misalignment and 0.075 in. (75 mils) of offset misalignment, depending on the manufacturer and style of the coupling.

Another common tolerance from coupling manufacturers is the gap tolerance. Typically this value is given as an absolute value of coupling face TIR (as an example, a specification migh read “face TIR not to exceed 0.005 in.”). This number can be deceiving depending on the swing diameter of the face dial indicator or the diameter of the coupling being measured. In fairness, it should be noted that the tolerances offered by coupling manufacturers are to ensure the life of the coupling with the expectation that the flexible element will fail rather than a critical machine component.

If this angular tolerance was applied to a 5 in. diam coupling, the angular alignment result would be 1 mil/1 in. of coupling diameter or 1 mil of rise per 1 in. of distance axially along the shaft centerline. If the coupling was 10 in. in diameter, the result of the alignment would be twice as precise (0.5 mil/1 in.). This would lead one to conclude that an angular alignment tolerance based on mils/1 in. would be something that could be applied to all shafts regardless of the coupling diameter.