Shaft coupling alignment is the practice of positioning a driver and a driven machine so their shaft centerlines coincide within tolerance at operating temperature. It is a high-leverage task in rotating equipment maintenance: misalignment is a leading cause of premature bearing failure, seal leakage, coupling wear, and elevated running-speed vibration. Unlike balancing, which corrects mass distribution on a single rotor, alignment corrects the geometry between two shafts joined by a coupling, a different problem with different fixes.
A coupling joining two misaligned shafts is forced to flex or slide on every revolution, transmitting reaction forces into both machines' bearings as radial and axial loads they were not designed to carry continuously. Over time this shows up as elevated bearing temperatures, seal leakage, coupling wear or cracking, and loosened foundation bolts. On a vibration spectrum, misalignment produces strong 1x and 2x running-speed components with elevated axial vibration, unlike unbalance, which is dominated by radial 1x with low axial content. See our guide to ISO 10816-3 vibration severity zones for related acceptance criteria.
Real-world misalignment is almost always a mix of two conditions:
Both must be corrected in the vertical and horizontal planes; fixing only one component or plane leaves residual misalignment that still generates vibration and bearing load.
Three method families are used in the field, in order of increasing accuracy and decreasing labor:
No alignment method gives a valid result if the machine has soft foot: one or more feet not sitting flat on the baseplate. Soft foot distorts the machine frame every time a hold-down bolt is torqued, shifting the shaft centerline and invalidating whatever readings were just taken. It must be checked and corrected, foot by foot, before any shimming begins. See our companion article on soft foot diagnosis and correction for the checking sequence.
Most rotating machines change shaft centerline height, and sometimes lateral position, as they heat up. A near-ambient pump may show negligible growth, while a hot pump, steam turbine, or hot-gas compressor can grow several tenths of a millimeter. Cold alignment must therefore target a deliberately offset "cold" value, not zero, so the machine arrives at true alignment once hot. Cold targets come from measured thermal growth data, OEM charts, or historical hot-check data. Aligning to zero cold is a common cause of alignment that fails once the machine is running.
Acceptable residual misalignment tightens as speed increases, since the same offset produces proportionally higher cyclic loading at higher rpm. Speed-graduated tolerance charts published by coupling and alignment-equipment makers express the offset limit per 100 mm of coupling span and the angularity limit as a slope, shown below. Treat these as typical field guidance, not a universal standard.
| Running speed (rpm) | Offset tolerance (excellent), per 100 mm span | Angularity tolerance (excellent), mm/100 mm |
|---|---|---|
| Up to 1000 | 0.09 mm | 0.09 |
| 1000 to 2000 | 0.07 mm | 0.07 |
| 2000 to 3000 | 0.05 mm | 0.05 |
| 3000 to 4000 | 0.03 mm | 0.03 |
| Above 4000 | 0.02 mm or per OEM | 0.02 or per OEM |
These bands illustrate "excellent" alignment as commonly used in laser alignment software; "acceptable" bands run two to three times looser. API 686 instead sets one fixed tolerance regardless of speed: 0.02 mm offset at the coupling center and 0.03 degrees angularity at each hub. Critical and high-speed machines should follow the OEM's tolerance, not a generic table.
Alignment checks work best as a scheduled, documented step, not a reaction to a vibration alarm: after any coupling disconnect, foot or baseplate work, bearing or seal replacement, and on a fixed interval for critical machines. Recording as-found and as-left readings against each asset lets the reliability team catch baseplate settlement before it causes a failure. Within a CMMS such as Fabrico, readings can be logged against the asset record alongside vibration history. Book a Fabrico demo to see how this fits a broader workflow.
Offset means the shaft centerlines are parallel but displaced from one another. Angular means the centerlines meet at an angle. Most machines have a mix of both, in both planes, and all components must be corrected.
Many machines, especially hot pumps, turbines, and compressors, grow at the shaft centerline as they heat up. Aligning to zero while cold leaves the machine misaligned once hot, so the cold target is deliberately offset to compensate.
Yes. Balancing and alignment fix different problems, mass distribution on a rotor versus centerline agreement between two shafts, so one can be correct while the other is not. The ratio of 1x to 2x amplitude and axial vibration level help distinguish which is present.
Soft foot distorts the machine casing every time the hold-down bolts are torqued, shifting the shaft centerline. Any alignment reading taken before it is fixed is invalidated once the feet are properly torqued, so soft foot correction always precedes alignment shimming.
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