Shaft alignment: offset and angle, tolerances by speed, soft foot, laser and dial methods, and the correction at the feet
Shaft alignment for coupled rotating machines: offset and angular misalignment, a commonly published tolerance table by speed and the ANSI/ASA S2.75 approach, laser and dial indicator methods, the kinds of soft foot and why they come first, working out the shim change at each foot, thermal growth, and recording as-found values so repeat misalignment is caught. With a drawing of the correction at the feet.
Key takeaways
- Misalignment is offset and angle between two shaft centrelines, in the vertical and horizontal planes. It loads bearings, seals and couplings, and shows up as vibration, heat and early failure.
- Tolerances tighten with speed. A machine at 1,500 rpm is commonly held to about 0.05 mm offset and 0.05 mm per 100 mm angle, but the coupling and machine manufacturers' limits come first.
- Soft foot is checked and corrected before alignment. A machine that rocks or twists when its feet are tightened will not hold any alignment.
- The correction at each foot follows from simple geometry: the offset at the coupling plus the angle times the distance to the foot.
- Record the as-found and as-left values against the equipment. Repeat misalignment on the same machine is a base, pipe strain or thermal growth problem, not an alignment problem.
A pump on a process water duty goes through a mechanical seal every few months and a set of bearings twice a year. Each time, the fitters replace the parts and realign the motor. Each time, the readings are out by about the same amount in the same direction. Nobody records the as-found values, so nobody sees the pattern, which is the pipe pulling the pump out of line every time the system heats up.
This guide covers shaft alignment for coupled rotating machines: the kinds of misalignment, the tolerances commonly used, the measuring methods, soft foot, the correction at the feet, the steps of a good alignment and what to record. It sits beside the guides to bearing failure analysis and condition based maintenance. The tolerances shown are a commonly published rule of thumb, not a substitute for the manufacturers' limits.
Offset, angle and the correction at the feet
Two shafts can be out of line in two ways at once: offset, where the centrelines are parallel but displaced, and angle, where they cross. Both are measured at the coupling centre, in the vertical and horizontal planes. The stationary machine, usually the driven one, is left where it is, and the moveable machine, usually the motor, is brought into line with it.
The correction at each foot of the moveable machine follows from similar triangles. The required change is the offset at the coupling plus the angle times the distance from the coupling to the foot. In the illustration the motor sits 0.20 mm high at the coupling and slopes up by 0.10 mm per 100 mm, so the front feet, 250 mm back, come down 0.45 mm and the back feet, 650 mm back, come down 0.85 mm. A laser system does the same arithmetic.
Measure at the coupling, correct at the feet, and record what you found before you changed it.
The kinds of misalignment
| Kind | What it is | How it is corrected |
|---|---|---|
| Offset (parallel) | The shaft centrelines are parallel but displaced, measured at the coupling centre in mm | Vertical: shims under the feet. Horizontal: moving the machine sideways |
| Angular | The shaft centrelines meet at an angle, given as a gap difference in mm per 100 mm | Vertical: different shim changes at front and back feet. Horizontal: moving the front and back feet by different amounts |
| Combined | Both at once, in both planes, which is the usual case | Corrected vertical first, then horizontal |
Tolerances
There is no single tolerance for every machine. Faster machines need tighter alignment, because the coupling flexes more times a second and the forces it passes into the bearings rise. The table below is a rule of thumb widely published for short flexible couplings. ANSI/ASA S2.75 Part 1 takes a more rigorous approach, with alignment quality grades based on the angle at each flex plane of the coupling. Where the coupling or machine manufacturer gives tighter limits, theirs apply.
| Speed, rpm | Offset, excellent | Offset, acceptable | Angle per 100 mm, excellent | Angle per 100 mm, acceptable |
|---|---|---|---|---|
| Up to 1,000 | 0.07 | 0.13 | 0.06 | 0.10 |
| 1,000 to 2,000 | 0.05 | 0.10 | 0.05 | 0.08 |
| 2,000 to 3,000 | 0.03 | 0.07 | 0.04 | 0.07 |
| 3,000 to 4,000 | 0.02 | 0.05 | 0.03 | 0.06 |
Measuring methods
The method matters less than the preparation. A laser system will produce an accurate number on a machine with soft foot, and the alignment will still be lost when the bolts are tightened. Whatever the method, the readings are taken with both shafts rotated together, so runout on the coupling hubs does not appear as misalignment.
| Method | Suits | Strengths | Limits |
|---|---|---|---|
| Straight edge and feeler gauges | Rough alignment only | Quick, no special tools | Coarse, and reads the coupling hubs rather than the shafts |
| Rim and face dial indicators | Small machines, short couplings | Simple equipment, well understood | Bracket sag to correct, face readings upset by axial float |
| Reverse dial indicators | Most machine trains with room for brackets | Accurate, works with longer spacers | Bracket sag, more calculation, slower |
| Laser alignment | Most work today | Fast, calculates the foot corrections, records the result, live readings while moving | Equipment cost, care in direct sun and on very short spans |
Soft foot first
A machine with soft foot is twisted by its own hold down bolts. The frame distorts, the bearing bores go out of round, and the alignment changes depending on which bolt was tightened last. Soft foot is checked before alignment by loosening each foot in turn with an indicator or the laser reading, and any foot that moves more than the site limit is corrected. Induced soft foot from pipe strain is the one to watch: it means the pipework is holding the machine, and no amount of shimming fixes it.
| Kind | What it is | Correction |
|---|---|---|
| Parallel soft foot | One foot is short of the base, so it rocks | Add shims under that foot |
| Angled soft foot | The foot or base is not flat, so it touches on one edge | Machine the foot or base, or use step or tapered shims |
| Induced soft foot | Pipe strain, a bent foot or a twisted frame pulls the machine | Relieve the pipe strain or correct the frame before aligning |
| Squishy foot | Too many thin, dirty or damaged shims | Replace with a few thick, clean shims, commonly no more than four |
Doing it well
Alignment is usually done cold, but machines run hot. A pump handling hot fluid, a gearbox and its motor grow by different amounts as they warm up, so the cold alignment is set with deliberate offsets that bring the shafts into line at running temperature. The offsets come from the manufacturer or from measuring the machine hot. A work instruction for each critical machine train that holds its tolerances, thermal offsets and shim positions saves working them out again each time.
| Step | What it involves |
|---|---|
| Prepare | Isolate both machines, check the coupling, bearings and base, clean the feet and base, remove old damaged shims |
| Rough align | Bring the machines close by eye or straight edge so the measuring system can read |
| Check soft foot | Loosen each foot in turn with an indicator or the laser on it, and correct any foot that moves more than the site limit, commonly 0.05 mm |
| Measure | Take readings at several shaft positions, rotating both shafts together where possible |
| Correct vertical | Change shims at the front and back feet of the moveable machine by the calculated amounts |
| Correct horizontal | Move the front and back feet sideways with jacking bolts while watching live readings |
| Tighten and verify | Tighten the feet in sequence, measure again, and repeat until within tolerance |
| Record | As-found and as-left values, shims used and any thermal offsets, against the equipment in the CMMS |
Recording, and repeat misalignment
The as-found reading is the useful one. It shows how far the machine moved since it was last aligned, and a machine that keeps moving in the same direction has a cause: pipe strain, a cracked or settling base, grout failure, loose hold down bolts or thermal growth not allowed for. Recording as-found and as-left values against the equipment in the maintenance log is what makes the pattern visible.
Misalignment also shows in condition monitoring, typically as vibration at one and two times running speed with high axial readings. A condition based approach checks alignment when the vibration says so, and after any work that disturbed the machine, rather than on a fixed calendar.
Common mistakes
- Aligning without checking soft foot.
- Stacks of thin, rusty or damaged shims under a foot.
- Aligning to the coupling hubs instead of rotating the shafts.
- Correcting horizontal before vertical, so the shim change moves it again.
- Ignoring thermal growth on hot or large machines.
- Recording only the as-left values, so repeat misalignment is never seen.
- Realigning a machine that keeps moving instead of fixing the pipe strain or base.
Common questions
What is shaft alignment?
Shaft alignment is positioning two coupled machines, such as a motor and a pump, so that their shaft centrelines are collinear at running conditions. Misalignment is measured as offset, the distance between the centrelines at the coupling, and angularity, the angle between them, in both the vertical and horizontal planes. It is corrected by changing shims under the feet of the moveable machine and moving it sideways.
What are acceptable shaft alignment tolerances?
Tolerances depend on speed and the coupling. A commonly published rule of thumb for short flexible couplings allows about 0.10 mm offset and 0.08 mm per 100 mm angularity as acceptable between 1,000 and 2,000 rpm, and half to two thirds of that as excellent. ANSI/ASA S2.75 Part 1 sets alignment quality grades. The coupling and machine manufacturers' limits take precedence where they are tighter.
What is soft foot?
Soft foot is when one or more feet of a machine do not sit flat on the base, so tightening the hold down bolt distorts the frame. It can be a short foot, a foot or base that is not flat, or a distortion induced by pipe strain. It is checked by loosening each foot in turn with an indicator on it, commonly against a limit of about 0.05 mm, and corrected before the machine is aligned.
Is laser alignment better than dial indicators?
Laser alignment is faster, calculates the foot corrections, shows live values while the machine is moved, and records the result, which is why it has become the usual method. Reverse dial indicators are just as accurate when done well, but need bracket sag corrected and more calculation. The quality of the preparation, soft foot correction and recording matters more than the instrument.
What does misalignment cause?
Misalignment adds load to the bearings, seals and coupling of both machines. Typical results are vibration at one and two times running speed with high axial vibration, hot bearings and couplings, worn or failed coupling elements, seal leaks, cracked shafts and early bearing failure. Bearing failure analysis often finds misalignment as the cause of fatigue on one side of the raceway.
Key terms
Plain-language definitions from our glossary for the concepts this article leans on.
Standards and further reading
Related case studies and tools
- Reliability engineering and asset strategy program (case study)
Related reading
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