Shaft Alignment: Why Even Perfect Balancing Cannot Replace It
Shaft alignment (coaxiality) and rotor balancing are two distinct but interrelated procedures. Many people wrongly assume that balancing will cure every vibration problem. Yet if the shafts are misaligned, no amount of balancing will help.
Key point: Balancing removes only the mass imbalance of the rotor. Shaft misalignment creates an entirely different type of vibration that balancing cannot remove. Trying to "balance out" misalignment is doomed to fail and simply wastes time and money.
In this article we explain what misalignment is, how it manifests, and why the correct order is — alignment first, then balancing.
What shaft misalignment is
Misalignment occurs when the axes of rotation of two coupled shafts (for example, the shaft of an electric motor and that of a pump or fan) do not coincide. This places additional load on the bearings and coupling and produces vibration.
Typical cases:
- Motor + pump
- Motor + fan
- Motor + compressor
- Gearbox + driven machine
Types of misalignment
1. Parallel misalignment
The shaft axes are parallel but offset (they do not lie on the same line).
2. Angular misalignment
The shaft axes intersect at an angle.
3. Combined misalignment
A combination of both types — the most common case in practice.
Signs of misalignment in the vibration spectrum
Characteristic signs:
- High amplitude at the second harmonic (2×): this is the principal sign of misalignment
- A peak at 1× is often present as well
- High axial vibration (along the shaft): the key distinguishing sign
| Fault | Dominant frequency | Axial vibration | What to do |
|---|---|---|---|
| Imbalance | 1× | Low | Balancing |
| Misalignment | 2× | High | Shaft alignment |
| Looseness | Many harmonics | Medium | Tighten the bolts |
How to tell misalignment apart from imbalance
A practical diagnostic example:
A pumping unit, motor at 3000 rpm (1× = 50 Hz):
Measurement 1: sensor fitted radially (horizontally) on the bearing on the coupling side
- Peak at 50 Hz (1×): 4 mm/s
- Peak at 100 Hz (2×): 5 mm/s
Measurement 2: same bearing, but sensor fitted axially (along the shaft)
- Peak at 50 Hz (1×): 1 mm/s
- Peak at 100 Hz (2×): 7 mm/s
Conclusion: a marked dominance of the second harmonic (2×) in the axial direction is the classic sign of shaft misalignment. Balancing will not help here; alignment is required.
Shaft alignment services
We carry out laser alignment of shafts on industrial equipment
Request the serviceLaser alignment as the modern method
Modern laser alignment systems make it possible to align shafts to an accuracy of hundredths of a millimetre. The process usually takes 1–2 hours.
Advantages of laser alignment:
- High accuracy: down to 0.01–0.02 mm
- Speed: 1–2 hours instead of the several hours required by traditional methods
- Clarity: the instrument shows the exact offset values
- Documentation: an alignment report with before-and-after data
When to carry out alignment:
- When installing new equipment
- After a repair that involves replacing the motor, pump or coupling
- When high vibration at 2× is detected
- Periodically (once every 2–3 years for critical equipment)
- Always before balancing when misalignment is suspected
Conclusion
Alignment and balancing are two sides of the same coin called "the fight against vibration". They complement one another but do not replace each other:
- Alignment removes vibration caused by shaft misalignment (the peak at 2×)
- Balancing removes vibration caused by rotor imbalance (the peak at 1×)
The correct order:
- Diagnosis (identify the cause of the vibration)
- If 2× dominates → shaft alignment
- If 1× dominates → balancing
- Often both are needed
Professional specialists always carry out thorough diagnostics and tackle problems in the correct order. That is what guarantees the result — stable, low equipment vibration.
📖 How to identify the cause of vibration: a guide to vibration diagnostics
Alignment and balancing
Instruments and complete services
Balanset-1A instrument
A vibration analyser for diagnosing misalignment and imbalance
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