Shaft Alignment and Couplings
Methods
Rim-and-face uses one indicator on the coupling outside diameter and one on the face. Reverse-indicator mounts dials on both hubs and reads both, which cancels much bracket sag and is preferred on close-coupled machines. Laser systems compute the same geometry with fewer reading errors.
Procedure
- Correct soft foot. 2) Remove pipe strain. 3) Check shaft and coupling runout - a bent key or eccentric coupling gives false readings. 4) Rough align within indicator travel. 5) Rotate both shafts together through 0°, 90°, 180°, 270° and record. 6) Correct vertical with shims and horizontal with jacking bolts, angular first then offset. 7) Re-check after bolting down and torque in stages.
Reading the numbers
Bracket or bar sag is measured by mounting the indicator rigidly and rotating; record and subtract it. Rim readings give offset: total indicator reading (TIR) is twice the offset, so offset = TIR ÷ 2. Face readings give angularity - convert to a slope in in/in and project it to the feet.
Thermal growth
Hot-running equipment is aligned to a cold offset specified by the OEM, usually a vertical offset of a few thousandths plus a slope.
Traps
Not rotating both shafts, ignoring sag, aligning to a coupling with runout, treating the coupling's misalignment tolerance as a target rather than a limit, and skipping the post-piping re-check.
Rule of thumb
Many OEMs accept 0.002 in offset and 0.0005 in/in angular at 1800 rpm and roughly half that at 3600 rpm. The flexible coupling tolerates misalignment - the bearings and seals pay for it.