Why shaft alignment matters
Aligned shafts load the bearings and seals evenly. Misaligned ones push the coupling force straight into them, and the damage shows up months later as a failed bearing, a leaking seal or a coupling insert that keeps wearing out.
What misalignment actually is
Two machines joined by a coupling have two shaft centrelines. Alignment means making those centrelines one straight line when the machine is running. Any error is a mix of two simple things, each in the vertical and the horizontal plane:
| Type | What it is | What it does |
|---|---|---|
| Offset (parallel) | The centrelines are parallel but one is shifted sideways or up | Pushes the coupling and both shafts sideways, loads bearings radially |
| Angular | The centrelines cross at the coupling but are not parallel | Adds a bending and axial push on bearings and seals |
Shaft tolerance is not coupling tolerance
This is the most common misunderstanding on site. The coupling maker publishes a maximum misalignment, usually in a table next to the speed limit. That is the most the coupling element will survive. It says nothing about what the bearings, seals and gears can take.
What bad alignment costs
Bearings and seals
- Extra radial and axial load shortens the L10 life, which rises with the cube of the load for ball bearings.
- Mechanical seals have very close tolerances. Vibration from misalignment is a leading cause of seal failure.
Couplings and shafts
- Inserts, grids and disc packs wear out early and are replaced again and again.
- Fatigue cracks can start at the shoulder or keyway of a shaft that bends every turn.
Energy and heat
- Studies report anywhere from about 1% to 10% less power after precision alignment. Treat the saving as real but modest and hard to measure.
- Heat at the coupling and bearing housings is a quick sign of a machine fighting itself.
Typical limits by speed
Many sites use a table like this one. It is a common starting point for a short flexible coupling, expressed at the coupling centre. The faster the shaft, the tighter the limit.
| 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) |
Values shown are typical industry tolerance tables, converted to metric. Your machine maker or site standard overrides them.
Do not assume a machine is aligned
- A new machine on a skid is not aligned just because it left the factory aligned. Transport, lifting and piping all move it.
- Checking alignment as soon as the machine is set down lets you fix bolt-bound and base-bound problems before pipework and conduit lock the machine in place.
- Check again after the pipework is connected.
The next chapters take the job in the order you do it on site: remove the error sources first (soft foot, looseness, hardware), then measure and correct.
Questions
How accurate does shaft alignment need to be?
It depends on speed and the machine. A common table asks for about 0.05 mm offset and 0.05 mm per 100 mm angular error at 3,600 rpm, easing to about 0.10 mm and 0.07 mm per 100 mm at 1,800 rpm. Use your machine maker figures where they exist.
Is a flexible coupling not meant to cope with misalignment?
It tolerates misalignment without breaking, but it does not remove the load. The coupling pushes that load into the bearings and seals, so the shafts still need to be aligned.
What is the difference between offset and angular misalignment?
Offset is a sideways shift between parallel shaft centrelines. Angular misalignment is when the centrelines meet but point in different directions. Real machines have both, in two planes.
Does alignment really save energy?
Published studies disagree, from roughly 1% to 10%. The saving is hard to isolate, but a machine that is not fighting itself runs cooler and needs less repair, which is usually the bigger gain.
General good practice for horizontal machines driven through a coupling. The machine maker's manual, the coupling data and your site standard come first. Lock out and isolate the machine before any alignment work.