Shaft alignment: the practical guide
Nine short chapters, from why alignment matters to the order of work on site. Written for maintenance and reliability teams, in plain language, with original diagrams.
Start here
The short version
- Align the shafts, not the coupling. A flexible element hides the error but passes the load on.
- Fix the machine before you measure: soft foot, loose or cupped hardware, shims, pipe strain, bearing play.
- If readings will not repeat, something is moving. Stop correcting and find it.
- Move vertically first, then horizontally, tighten in a fixed pattern, then check again.
- Machines run hot. Set a cold offset from maker data or a hot check.
Quick reference: typical tolerances
| Speed | Angular error at the coupling | Offset at the coupling |
|---|---|---|
| 3,600 rpm | 0.5 mil per inch (0.05 mm per 100 mm) | 2 mil (0.05 mm) |
| 1,800 rpm | 0.7 mil per inch (0.07 mm per 100 mm) | 4 mil (0.10 mm) |
| 1,200 rpm | 1.0 mil per inch (0.10 mm per 100 mm) | 6 mil (0.15 mm) |
Typical industry values for short flexible couplings; your maker data and site standard come first.
Related
Questions
What is shaft alignment?
Making the centrelines of two coupled shafts one straight line when the machine runs, so the coupling does not push load into bearings and seals.
What is soft foot?
A machine foot that does not sit flat on its base, so tightening the bolt distorts the casing and moves the shaft. It is the commonest reason alignments will not repeat.
How accurate must alignment be?
It depends on speed: a common table is about 0.05 mm offset at 3,600 rpm and 0.10 mm at 1,800 rpm for short flexible couplings. Use your maker data first.
Is a flexible coupling enough to cope with misalignment?
No. It survives misalignment but passes the load into the bearings and seals. Align the shafts, not the coupling.
General good practice for horizontal machines. Diagrams are original and illustrative, not to scale. The machine maker's manual comes first. Lock out and isolate before any work.