This page added 09/27/96 and changed 02/20/97
"Bump-less" bump tests application note
In diagnosing machine vibration problems, it is generally useful to have some knowledge of the machine's mobility, it's frequency response to driving forces. This information can help explain high vibrations at particular frequencies. It is also useful knowledge for monitoring machine structural condition. The normal method for testing resonances usually involves impact/bump tests or plotting frequency response to signals from mechanical exciters, methods that are often not convenient or practical to use, especially on a routine basis. This application note will describe how to estimate resonances from on-line data with conventional data collectors and spectrum analyzers and also illustrate an enhanced method of doing this using the techniques in DCFIL.
Data used in this example is from a 1500 hp. motor used in the ventilation system of the Moffat Tunnel, a railroad tunnel in north central Colorado.
Conventional data collectors and spectrum analyzers
Resonance estimation can be done to some degree with conventional spectrum analyzers and data collectors by inspecting the noise floor of the displayed frequency spectrum either by displaying the data on a logarithmic amplitude scale having sufficient range to display both the maximum and minimum points of the of the spectrum noise floor. The premise for this approach is that non-linear excitation sources in a machine such as bearing flaws, looseness impacts, flow turbulence, and so fourth provide excitation similar to hammer blows or bumps. "Eyeball" filtering is used to separate the normal machine harmonic series produced by shaft rotation from the noise signal response. Of course, this method is by no means exact; like most vibration problems, the ability to think your way through the diagnosis is necessary!
For best results:
Data for the following example was collected using a Vibration Specialties Corporation four channel, 16 bit, analog to digital converter. The displayed spectrum has roughly 25,600 lines but 6400 or 3200 lines would work almost as well in this case. A hanning window was used.
The first plot is the normal spectrum plotted on a linear amplitude scale and the second plot is the same data plotted on a logarithmic amplitude scale.
Figure 1 (above) Linear amplitude, linear frequency normal spectrum.
Figure 2 (above) Log amplitude, linear frequency normal spectrum
Prior to modifying the machine base in 1995 by adding a small stiffener, this machine had a problem of high radial vibrations at twice the shaft of frequency of 15 Hertz or 900 RPM. Vibration velocities at twice shaft rotating frequency typically varied from .05 IPS to .35 IPS. As you can see in the second plot, there is a broad peak in the response curve at roughly 29 Hertz. There is also a sharper peak at roughly 32 Hertz which was previously located around 29.5 Hertz before I applied the slight stiffener to the machine base. The customer had previously tried a strong stiffener to shift the resonance, but it also shifted an axial direction resonance from 11 Hertz to 15 Hertz, changing the shaft frequency axial displacement from .25 mils to 5 mils!
The following plots are linear, log amplitude, and log-log plots of the same raw data processed with DCFIL. For bearing diagnostics, DCFIL works by extracting the machine synchronous signals form the non-synchronous signals that bearings generate. Applied at lower frequencies, this process can make the non-synchronous machine signal spectrum much easier to view. The spectrums processed by DCFIL are actually a 128 average, 200 line spectrum of the same time domain data in the first two plots. Currently, this process is commercially available with the data collection drivers for the VSC version of DREAM®.
Figure 3 (above) Linear amplitude, linear frequency normal spectrum processed with DCFIL.
Figure 4 (above) Log amplitude, linear frequency normal spectrum processed with DCFIL.
Figure 5 (above) Log amplitude, log frequency spectrum processed with DCFIL.
Using a log-log display allows easy "eyeball" comparisons of the damping factors of the resonances.
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