In brief
Alignment is corrected by moving the movable machine — usually the motor — until its shaft matches that of the fixed machine in both parallel offset and angle, and it is verified with dial indicators or laser equipment, never with a straightedge alone. Soft foot must be eliminated before aligning, because a foot that does not sit distorts the housing and falsifies every later reading. On equipment that runs hot, alignment is deliberately offset when cold so that thermal growth brings it to zero in operation. A flexible coupling tolerates misalignment but does not remove it: it converts it into load on the bearings and seals.
There is a scene that repeats in every plant: a motor bearing is replaced, the machine runs fine, and a few months later the same bearing fails again. The spare is not to blame, and neither is the mounting. What fails is what nobody measured: the alignment between the motor and the driven machine.
Misalignment does not break the coupling — that is what its flexibility is for — it transmits a cyclic load to the bearings on both sides. That load rotates within the bearing geometry and leaves a displaced load pattern, visible on the ring once dismounted. It also heats the coupling, punishes the seals and raises the motor's power draw.
The two kinds of deviation, and why they are corrected together
Parallel misalignment is an offset: the two shafts are parallel but not on the same line. Angular misalignment is a tilt: the shafts cross at an angle. In practice both almost always coexist, and in two planes at once — vertical and horizontal — so a real alignment corrects four values, not two.
That is why a straightedge laid across the coupling halves is not enough. It catches a gross offset, but it cannot see the angle, cannot separate vertical from horizontal, and lacks the resolution the tolerances of a machine running at 1,800 rpm demand.
Before measuring: soft foot
Soft foot is the condition where one of the machine's feet does not sit on its base, or sits on a surface that is not flat. Tightening the bolt distorts the housing. Aligning with soft foot means aligning a machine that changes shape every time a bolt is touched: the numbers never settle.
- 01
Check it
With all bolts tight, place an indicator on one foot and loosen only that bolt. The movement the indicator registers is that foot's soft foot. Repeat foot by foot.
- 02
Read it
Movement below 0.05 mm is usually considered acceptable in general industrial machinery. Above that, correct it before going any further.
- 03
Correct it
With shims of the measured thickness, in suitable material and size. Never with many thin shims stacked: a shim pack behaves like a spring and brings the problem back. Four at most, and flat.
How it is measured
| Method | What it solves | Where it fails |
|---|---|---|
| Straightedge and feelers | Catches gross deviation, useful for rough setting | No angle, no plane separation, insufficient resolution |
| Dial indicators | Accurate enough for most industrial machines | Needs careful setup and correction for bracket sag |
| Laser equipment | Fast, shows the correction at each foot and stores the record | Requires the operator to understand what the screen shows |
With dial indicators one detail ruins more jobs than any other: bracket sag. The arm holding the indicator bends under its own weight as the assembly is turned, and that bend adds to the reading as though it were real deviation. Measure it beforehand, with the assembly mounted on a rigid pipe, and subtract it.
Tolerances: they depend on speed
The faster it turns, the less deviation it tolerates. These are indicative values in widespread use in industrial maintenance for standard flexible couplings; the coupling and machine manufacturer's specification always prevails over any general table.
| Speed | Parallel offset | Angular deviation |
|---|---|---|
| Up to 1,000 rpm | 0.07 – 0.13 mm | 0.06 – 0.10 mm per 100 mm |
| 1,000 – 2,000 rpm | 0.05 – 0.09 mm | 0.05 – 0.08 mm per 100 mm |
| 2,000 – 3,000 rpm | 0.03 – 0.06 mm | 0.03 – 0.05 mm per 100 mm |
| Above 3,000 rpm | Per manufacturer | Per manufacturer |
Thermal growth: misaligning on purpose
A pump handling hot fluid, a compressor or a large motor do not operate at the temperature at which they are aligned. As they heat, housings grow and shaft centrelines shift vertically. Align the machine perfectly cold and it will run misaligned hot.
The correction is to leave a deliberate cold offset, equal in magnitude and opposite in direction to the expected growth. That figure comes from the manufacturer for their equipment, or is established by measuring cold and hot. It is why two technicians can disagree about whether a machine is well aligned: one is looking at the number, the other at the operating condition.
Closing the job
- Tighten anchor bolts to the specified torque and measure again: final tightening moves the numbers.
- Turn the assembly by hand through a full revolution before energising, checking nothing rubs.
- Take baseline temperature and vibration readings once the machine has stabilised. Without them the next reading has nothing to be compared against.
- Record in the work order the final alignment values, the shim thickness fitted and the soft foot corrected at each foot.
ISO 20816 is the usual reference for evaluating vibration in rotating machinery and setting alarm thresholds. Misalignment that survived the intervention typically shows in the vibration spectrum as a marked component at twice running speed, together with axial vibration: that is the signature to look for when the equipment gives trouble again.
Frequently asked questions
- Doesn't a flexible coupling compensate for misalignment?
- It compensates in the sense that it does not break, but the deviation does not disappear: it turns into cyclic load on the bearings and seals of both machines. A flexible coupling exists to absorb small residual deviations and thermal expansion, not to replace alignment.
- How often should alignment be checked?
- Always after dismounting, moving or repairing either machine, and whenever vibration analysis suggests it. On critical equipment it is worth checking again after the first week of operation, because the base and shims settle.
- Is dial indicator alignment good enough without laser equipment?
- Yes. Done properly, with bracket sag compensated and no soft foot, indicator alignment reaches the accuracy most industrial machines need. Laser adds speed, fewer calculation errors and a record that can be filed.
- Can I stack several thin shims to reach the thickness I need?
- It is the practice that causes the most trouble. A pack of thin shims compresses elastically and brings soft foot back. Do not stack more than four, use shims of the correct thickness, and check they are clean and free of burrs.
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About this contentThese guides collect industrial maintenance good practice and widely used standard references. They do not replace the technical documentation of the equipment or component manufacturer, which always prevails.