Misalignment: A major machinery problem
Most experts agree that over half of all machinery problems are caused by misalignment.
Misalignment is created when shafts, couplings and bearings are not properly aligned along their centerlines. The two types of misalignment are angular and parallel, or a combination of both.
Angular misalignment occurs when two shafts are joined at a coupling in such a way as to induce a bending force on the shaft.
Parallel misalignment occurs when the shaft centerlines are parallel but displaced from one another.
Possible causes of misalignment:
- Thermal expansion due to a process working with heat (as with a turbine). Most machines are aligned cold, and then as they operate and heat up, thermal growth causes them to grow misaligned.
- Machine directly coupled not properly aligned
- Forces transmitted to the machine by piping and support members
- Foundation uneven, shifting or settling
Effects on bearings
Misalignment usually causes the bearing to carry a higher load than its design specification. This causes bearing failure due to fatigue. Fatigue is the result of stresses applied immediately below the load carrying surfaces and is observed as spalling of surface metal.
How to diagnose misalignment
Use overall vibration, FFT spectra and phase measurements to diagnose misalignment problems.
- Angular misalignment causes axial vibration at the running speed frequency (1x)
- Parallel misalignment produces radial vibration at twice the running speed frequency (2x)
Since most misalignment is a combination of angular and offset, both the radial and axial measurements’ 1x and 2x frequencies are analyzed.
Also, while imbalance forces are the same in the horizontal and vertical positions, misalignment forces are seldom the same in both positions.
FFT spectrum analysis
Note: 2x amplitude is not always present.
With misalignment, a higher than normal 1x/2x amplitude may occur. A high 2x amplitude can vary from 30% of the 1x amplitude to 100 to 200% of the 1x amplitude.
- Couplings with 2x amplitudes below 50% of 1x are usually acceptable and often operate for a long period of time
- When the vibration amplitude at 2x is 50 to 150% that of 1x, it is probable that coupling damage will occur
- A machine whose vibration at 2x running speed is above 150% of the 1x has severe misalignment, the problem should be fixed as soon as possible
Phase measurements are a very useful tool for diagnosing misalignment.
If possible, measure the phase shift between axial readings on opposite ends of the machine. It’s important to note that all phase values are ±30° because of mechanical variance.
- Angular misalignment – In the axial position, a phase shift of 180° will exist across the coupling or machine
- Parallel misalignment – In the radial direction, a phase shift of
- 180° will exist across the coupling or machine. A 0° or 180° phase shift will occur as the sensor is moved from the horizontal to the vertical position on the same bearing
- Combination angular and parallel misalignment – In the radial and axial positions, a phase shift of 180° will exist across the coupling or machine
- With severe misalignment, the spectrum may contain multiple harmonics from 3x to 10x
- If vibration amplitude in the horizontal plane is increased two or three times, then misalignment is again indicated
Summing it up
If there is an abnormally high 2x/1x amplitude and there is a coupling or belt, then there may be misalignment.
If the radial 2x amplitude is abnormally high and there is a coupling or belt, then there may be misalignment.
If the axial 1x amplitude is abnormally high and there is a coupling or belt, then there may be misalignment.