Vibration Due To Shaft Misalignment
Unlike imbalance, misalignment does not produce forces that are similar in the radius of the machine. Vibration in one radial direction may be very different in amplitude when compared to vibration in another radial direction. This is one instance where amplitude readings are not only helpful in detecting a problem but also helpful in diagnosing a problem. Misalignment can result in a machine having high vertical vibration on one end and high horizontal vibration on the other with other radial readings remaining low. Imbalance is not likely to cause this amplitude pattern.
Vibration amplitude directly correlates with the amount of imbalance and equipment speed. If machine speed remains constant and the imbalance is doubled, then vibration amplitude should also double. If the speed of the equipment doubles and the imbalance remains the same, then the centrifugal forces will increase by a factor of four and the vibration amplitudes would increase accordingly.
However, correlating the effects of misalignment with vibration amplitude can be difficult. Misalignment effects on vibration are much more complex than with unbalance. In certain cases, improving alignment may actually cause vibration radial amplitudes to increase. The reason for this is that misalignment may restrain movement in one or more directions.
In one instance, the author was called to a plywood plant to help balance a knife hammer mill. Before his arrival, the site vibration analyst collected vibration data and determined that one of the problems with the hammer mill was rotor imbalance. When the machine was opened, it was observed that several balance weights were attached at what appeared to be random locations around the rotor. This is a red flag indicating that problems other than imbalance may be present. Since balancing problems should be corrected only after all other problems have been addressed, the team decided to determine if other problems existed and correct them first.
A review of the vibration data clearly indicated that rotor imbalance was a problem. However, the data also indicated shaft misalignment and looseness. A PRÜFTECHNIK laser alignment system was on-site, so the alignment problem was addressed first. A shaft angular misalignment greater than 7 mils per inch with over 30 mils of offset between the motor and hammer mill rotor was found and corrected. After the machine was started and new vibration data was collected, the vibration amplitudes were found to be much higher. The vibration analyst was perplexed and asked if we had done the wrong thing by aligning the machine. The author’s answer was, “No, the precision alignment helped us to know that the looseness is most likely in the bearings.” The gross misalignment existing previously had the shafts in such a bind that shaft movement was restricted. In all likelihood, this misalignment had greatly contributed to the severe bearing wear, and if it had been corrected sooner, bearing life would have been extended significantly.
When the shafts were put into alignment, the binding no longer prevented shaft movement in the badly worn bearings, permitting vibration amplitudes to increase. The bearings were replaced, the shafts were re-aligned yet again, and finally the rotor was balanced. New vibration data revealed the machine to be within plant vibration standards.
Angular misalignment is indicated by an approximate phase difference of 180 degrees across the coupling when measured in the axial direction. Be aware of mistakes made in measuring axial phase by not allowing for transducer orientation.
Offset misalignment produces approximately a 180 degree phase difference when comparing a radial phase on one machine to the same radial direction on the other machine across the coupling. Phase differences may be anywhere from 150 to 210 degrees. The more severe the misalignment, the closer the phase differences will be to 180 degrees.
When the vibration is at a frequency equal to shaft speed (1×), phase is the best tool for distinguishing between misalignment and unbalance. Take several phase readings at each point to ensure accurate phase data that may vary due to speed changes.
When axial vibration at 1× or 2× is 50 percent or more of radial vibration, suspect misalignment. Be careful here because shaft misalignment isn’t the only problem that can cause high axial vibration.
A. Bent shaft.
B. Resonant whirl.
C. Bearing cocked on shaft (bearing misalignment).
D. Resonance in the axial direction.
E. Worn thrust bearings.
F. Worn helical or bevel gears.
G. Sleeve bearing motor hunting for magnetic center.
H. Couple component of unbalance.
Suspect misalignment when radial vibration at 2× shaft speed is 50 percent or more of shaft speed vibration.
Misalignment can also produce several harmonics of running speed. These harmonics are produced when shaft movement is restricted and the waveform is clipped. Unlike the harmonics produced by looseness, the harmonic spikes in the spectrum should not have a raised noise floor.
Time waveform plots of misalignment should be somewhat similar during each shaft revolution. Looseness will appear much more complicated in the time waveform.
Coupling problems may look like alignment problems. 1× is usually most affected. However, different coupling types produce different vibration indications.
Too long or too short coupling spacers may cause high 3× which may be confused with misalignment. The author’s experience has shown this problem to produce a high 1× vibration also.
A bent shaft produces high axial vibration. Phase shift will be 180 degrees when comparing top to bottom of bearing and 180 degrees from one side of bearing to the other.
In the past, the author performed a test known as the running soft foot test when vibration tests indicated an alignment problem or a possible resonance due to soft foot. This test involves loosening machine mounting bolts one at a time while the machine is running. If done improperly, this test can be dangerous. Modern shaft alignment tools have made it so easy to check soft foot that there is no longer a need for performing the risky running soft foot test. When vibration testing indicates shaft misalignment, simply use a laser tool to check the alignment.