Gene Vogel
EASA Pump & Vibration Specialist
There are many common causes of high vibration on rotating machinery; too many to list here. But often, what would otherwise be an acceptable level of vibration is amplified by resonance. All machines are susceptible to resonance. Resonance occurs when the natural frequency of some machine component coincides with an exciting force. When resonance occurs it is the combination of exciting force and a natural frequency that results in high vibration; both must be present at the same frequency for resonance to occur. When resonance does cause excessive vibration, it is important to identify the natural frequency and the mode shape of the vibration. A simple bump test, conducted with the machine not running, is a good first step in identifying the natural frequency (Figure 1).
The object of a bump test is to identify a natural frequency. Imagine an electric motor sitting with springs under the feet. If the motor was bumped from the top, it would bounce up and down (vibrate) on the springs at some natural frequency in the vertical mode. Likewise, if the motor were bumped from the side or from the end, it would rock back and forth in the plane in which it was bumped. The natural frequency at which it would vibrate back and forth in the rocking mode would likely be different than the vertical mode. A spectrum analyzer is used to determine the frequency of the vibration when the motor is bumped. For a successful bump test, the machine must be bumped appropriately to excite the natural frequency, and the spectrum analyzer must be set up properly to capture the resulting vibration. It is often necessary to conduct bump tests in several different directions (modes) to identify natural frequencies near the frequency of the excessive vibration.
The motor sitting on springs would have relatively low natural frequencies. If the motor was bumped with a ball peen hammer, there would be a loud “whack”, but very little low frequency energy would be imparted to the motor and the resulting natural frequency vibration would be minimal. To impart lower frequency energy, a softer impact object is needed; perhaps a rubber mallet or a block of wood. There is a direct relationship between the hardness of the object striking the target and frequency of the energy imparted. Softer striking objects impart lower frequency energy; harder striking objects impart higher frequency energy. Common machine rotating speeds are considered lower frequency, so softer striking objects are generally recommended.
To capture the vibration resulting from the bump, the spectrum analyzer must be set up properly. Of course, the vibration transducer should be placed to sense the vibration in the same direction the machine is bumped. However, the transducer should not be placed in the area where the machine is bumped. For instance, if the bump is applied on the left side of the machine, the transducer should be in the same plane as the bump but on the right side of the machine. The setup of the spectrum analyzer for the bump test varies depending on the specific instrument. Some instruments have dedicated setups (apps) for conducting bump tests. Some general procedures and setup parameters that can be used with most spectrum analyzers are presented here.
For a bump test, it is usually necessary to manually set the amplitude range on the spectrum analyzer. The default for most instruments is for the amplitude range to auto-range. So, the range will be at a very low level before the bump, and when the bump occurs the instrument will take some time to auto-range up and will miss the event. It may take several test bumps to get the amplitude range set correctly to capture the event with good resolution but without clipping the signal. (Note that the amplitude range and the display range are different settings on most instruments.)
Many spectrum analyzers (but not all) have a trigger feature that is useful for bump tests. A trigger level is set just above the background vibration amplitude and the instrument waits until that trigger level is exceeded before it initiates data collection. The bump, of course, will exceed the trigger level, and the data sample will contain the results of the energy imparted by the bump. An added feature that a few spectrum analyzers provide is a pre-trigger interval. The example data in Figure 2 illustrates the effect of a pre-trigger interval. The result of the bump does not begin at the start of the data; there is a pre-trigger interval before the resulting vibration occurs. When a trigger function is used, a single spectrum (1 average) can be collected.
If a spectrum analyzer does not have a trigger feature, the best approach is to strike the machine repeatedly while pressing the button to initiate data collection. It may be helpful to set the instrument to take three or four averages and continue to strike the machine repeatedly until the data has been collected.
The spectrum-waveform parameters should be set to capture the frequency range of interest. Most machine rotating speeds, and most troublesome natural frequencies occur below 200 Hz. So, an fMax of 200 Hz is a good starting frequency range. If the lines-of-resolution is set to 200 lines, the data interval will be one second, a good interval to capture the results of the bump. Setting higher resolution results in proportionally longer data collection intervals and the results of the bump will have dissipated in most cases. If the suspected natural frequency is above 200 Hz, the instrument can be set for 400 Hz and 400 lines-of-resolution to maintain the one second data collection interval.
The example of the motor setting on springs is a good description of many common resonance situations. Natural frequencies are the result of the stiffness, mass and damping characteristics of the structure. When a machine is mounted on a weak base, the machine is the mass, the base is the low stiffness (spring). But natural frequencies are present on all machine components. A common component with natural frequencies near exciting forces (rotating speeds) is a motor end bracket. If a motor has high axial vibration at one frequency, end bracket resonance should be considered as a possible cause. Conducting a bump test on the end bracket of the assembled motor is the first step in diagnosing the problem. Tip: If it is a sleeve bearing motor, conduct the bump test with the shaft coasting; when the shaft is stopped, the oil film is lost and stiffness characteristics change.
Another unique resonant condition that has been reported is the junction box on a rectangular frame motor. A heavy junction box on a flat side panel of the motor frame makes a good spring-mass system susceptible to natural frequencies near rotating speeds.
The most common frequencies at which resonance occurs are 1xRPM, 2xRPM and 2x line frequency (2xlf). For electric motors and related machines, these frequencies are within the recommended 200 Hz fMax. The key characteristics of resonance are very high amplitude vibration at one specific frequency, usually in one plane. A simple bump test to identify the natural frequencies is the first step in diagnosing resonance problems. More extensive information on resonance and conducting bump tests is available in the Vibration Analysis for Service Centers manual and EASA Webinars at easa.com/training/vibration-analysis and easa.com/training/webinar-recordings.
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