Shaft Alignment Offset and Angularity (Rim-and-Face)

Shaft misalignment is the largest single cause of premature coupling, seal, and bearing failure on rotating equipment, and rim-and-face is how most of it still gets corrected in the field. The rim reading sees pure offset and reads TWICE it, because the indicator crosses the misalignment on both sides of the sweep -- forgetting the divide-by-two is the classic doubling error. The face reading sees pure angularity, which becomes a slope once divided by the diameter the indicator actually swept. Both then project out to the feet, and that projection is why a small angle is worse than a large offset: angularity multiplies by the distance, so a 0.001 in/in slope becomes 0.020 in of shim at a foot 20 in away. EVERY MOVE HERE CARRIES AN EXPLICIT CONVENTION, stated in words rather than left to a sign: positive raises the foot and adds shim, negative lowers it and removes shim. A rim TIR of -0.020 in with a face TIR of +0.006 in on a 6 in sweep gives a -0.0100 in offset and a 1.00 mils/in slope, which projects to LOWERING the front foot 2.0 mils and RAISING the rear foot 10.0 mils -- feet moving in opposite directions, 12.0 mils apart, and that difference IS the angle. A crew that shimmed both feet equally to the offset would leave the entire angular error in place, which is the error this arithmetic exists to prevent. Correct the angle first and the offset second, because moving to fix an angle changes the offset and not the reverse. This is the vertical solve on readings the user takes. It assumes the readings were taken with the shafts rotated together at the same angular positions, without which the numbers describe a bent shaft or a runout rather than a misalignment; it does not correct for indicator bracket sag, which on a long bracket is easily larger than the misalignment being measured and must be measured and subtracted first; and it does not do the horizontal solve, which is the same arithmetic on the side readings and is corrected by jacking rather than shimming. It does not check the result against a tolerance, apply a thermal growth target, or verify soft foot -- and every alignment number is meaningless until soft foot is zero, because a machine that rocks changes shape when the bolts come down. The machine manufacturer's alignment specification, the coupling manufacturer's data, and the plant's own precision maintenance procedure govern.

offset at the coupling = rim TIR / 2; angularity = face TIR / the diameter the face indicator swept; move at a foot = offset + angularity x the distance from the coupling face to that foot. Positive raises the foot and adds shim.

The rim-and-face alignment relations as standard millwright practice, by name. Bracket sag must be measured and subtracted before the readings are used. The machine manufacturer's alignment specification, the coupling manufacturer's data, and the plant's precision maintenance procedure govern.

Arithmetic on dial indicator readings the millwright takes; no manufacturer table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: rim_tir_in, face_tir_in, face_diameter_in, front_foot_distance_in, rear_foot_distance_in, offset_in, angularity_in_per_in, front_move_in, rear_move_in, foot_move_difference_in

• Readings must be taken with the shafts rotated together otherwise the numbers describe runout or a bent shaftmillwright practice
• Bracket sag is not corrected here on a long bracket it easily exceeds the misalignmentmillwright practice
• Soft foot comes first an alignment number is meaningless while the machine rocksprecision maintenance practice