By Yossi Tuchner, vibration specialist certified to
ISO 18436 CAT III
Fundamentals of Shaft Alignment
Shaft alignment is the process of bringing the rotational axes of two machines into a single, colinear axis
in all planes. Proper alignment is a basic requirement for reliable operation of rotating equipment.
Misalignment introduces radial and axial forces that increase friction and load on bearings and mechanical seals.
The result is accelerated wear, higher vibration, unplanned downtime and higher maintenance cost.
Soft Foot and Base Conditions
Misalignment can stem not only from shaft-to-shaft offset or angle. A warped or loose baseplate, uneven foot heights,
or unequal bolt tightness can create soft foot. Soft foot must be corrected before alignment,
otherwise the machine will not hold alignment and vibration will remain high.
Types of Misalignment
There are four basic components to check in any method:
- Vertical offset (parallel): absolute height difference between shaft centers when viewed from the side.
- Vertical angular: angular difference between shaft centerlines when viewed from the side.
- Horizontal offset (parallel): absolute side-to-side difference between shaft centers when viewed from above.
- Horizontal angular: angular difference between shaft centerlines when viewed from above.
Required Accuracy and the Role of Coupling Tolerances
Alignment accuracy depends on machine speed and inertia. The higher the speed, the tighter the tolerances that should be
achieved at the coupling. Do not confuse the coupling manufacturer’s allowable misalignment with what is acceptable
for the machine. A flexible coupling may survive large angular or offset values, but the connected shafts and bearings
often will not. Use coupling limits as secondary information only and align the machine to appropriate precision tolerances.
General Alignment Tolerances by Speed
The following table summarizes typical maximum values for residual misalignment at the coupling after alignment.
Values are given for overall parallel offset and angular misalignment. For higher speeds, target the “Excellent” column.
| RPM |
Maximum Acceptable (Good) |
Excellent |
|
Parallel [mm] |
Angular [mm/100] |
Parallel [mm] |
Angular [mm/100] |
| 0–1000 | 0.13 | 0.10 | 0.07 | 0.06 |
| 1001–2000 | 0.10 | 0.08 | 0.05 | 0.05 |
| 2001–3000 | 0.07 | 0.07 | 0.03 | 0.04 |
| 3001–4000 | 0.05 | 0.06 | 0.02 | 0.03 |
| 4001–5000 | 0.03 | 0.05 | 0.01 | 0.02 |
| 5001–6000 | <0.03 | 0.04 | <0.01 | 0.01 |
Important note: Coupling misalignment limits relate to coupling reliability only.
They do not define safe loading for shafts and bearings. Always align the machine to the precision
targets above, regardless of the coupling’s nominal capability.
Alignment Methods
-
Straightedge and feeler gauges: the least accurate method, suitable only for coarse checks.
-
Dial indicators: Rim and Face, or Reverse Dial (rim-to-rim) techniques to measure relative shaft position.
-
Laser alignment systems: implement Reverse Dial principles with higher accuracy and faster execution.
Why Laser Alignment Delivers Better Results
Laser systems reduce measurement uncertainty and speed up the job. They are less sensitive to glare and dirt,
avoid dial-bridge sag, and provide guided moves to reach tolerance quickly.
Electronic systems also enable capabilities not practical with other methods: saving data and generating reports,
entering thermal growth targets in advance, performing accurate soft foot checks,
aligning multi-coupling trains to determine the correct sequence, and handling cardan shaft alignment.
The Role of Precision Stainless-Steel Shims
Use precision stainless-steel shims to set and preserve the final alignment. Quality shims help maintain accuracy
over time because they resist corrosion and deformation.