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Diagnosing frequency response of the actual
system. The examples discussed here
illustratebothapproaches.
The phase with respect to the tacho-
meter on the shaft remains stable and
suggests that the two resonances are
and Solving With either approach, you often need
to consider additional elements as part
associated with the pump's housing or
mountingstructure,nottheshaft.If
Resonance of the mechanical system. For
instance,thesupportstructurefora
thevibrationlevelisunacceptable,we
need to either avoid operating speeds
pump can affect the pump's dynamics around 3600 and 4600 RPM or modify
Problems because it adds its own mass and
stiffnesspropertiestotheequation.
the dynamics to move the resonances
up or down.
by Phillip Garrett and Tom Lantrip, This is the reason why pieces of
Southern Company Services, Inc. equipment that behave well before Stimulus-Response Testing
One of the most common reasons for being installed sometimes misbehave Another common test technique is
high vibration of plant equipment is once they are installed--and why excitinga structurewithan external
some form of resonance of the equipment suppliers need to make stimulus and measuring how the
equipment components or support surethattheirproductsstilloperate structuredeflectsinresponse.Impact
structure. Every mechanical system properlyafterinstallation. testing is one of the easiest methods.
has resonant frequencies (damped You typically excite the system with
naturalfrequencies).Ifthesystemis Run-Up and Coast-Down an instrumented impact hammer and
exposed to vibration at these frequen- Testing use a signal analyzer to measure this
cies,itwillamplifythevibrations.The A common way to identify resonances input force and the response of the
effectsofthisamplifiedvibrationcan empiricallyistooperatetheequip- system simultaneously. The resulting
range from premature wear and ment across its range of operating frequency response function provides
excessive maintenance to material speeds while measuring the vibration both magnitude and phase plots to
fatiguetocompletestructuralcol- it exhibits. Run-up and coast-down helpyoulocatenaturalfrequencies.
lapse. Any time you're dealing with tests, which monitor vibration from
rotating machinery, such as the variety standstill to maximum speed and back A second option uses an electric or
ofpumpsdiscussedinthisarticle, down, are a quick way to see if hydraulic shaker to excite the
vibrationproblemsareapossibility. troublesome resonances are present in structure. The shaker is driven by a
thesystem. source signal from the analyzer. A
Solving vibration problems can random noise signal excites across the
increase uptime and decrease mainte- When you see a discrete peak in the entirefrequencyrange;asinusoidlets
nance, replacement and operating magnitude that is accompanied by a you sweep across and dwell at
costs.It'satwo-partprocess.First, phaseshiftatthesamefrequency,it's particularfrequenciesifneeded
you need to identify the natural a good indication that a resonance (similar to doing run-up and coast-
frequencies and compare these exists there. Figure 1 shows the down tests). The shaker excitation is
frequencies to the operating speeds of magnitude and phase of the vibration transmitted through a stinger rod to a
your equipment. Second, to resolve we measured during a coast-down test load cellattached tothe structure. The
the problem you have to make a on a boiler feed pump. We acquired load cell provides a measure of the
choice between changing the operat- the data with proximity probes on the inputforce.Aswithimpacttesting,
ing speed or changing the resonant shaft and used order tracking to keep your signal analyzer measures the
frequencies(orboth). the measurement stable as we ramped input force and the output response
the speed down.
Two Approaches to Identifying
Resonances
You have two options for identifying
resonant frequencies. With the
analytical approach, you can use finite
element analysis to build a computer
model of the system (the piece of
equipment,itssupportstructure,etc.)
and simulate its dynamics. With the
empirical approach, you measure the
Figure 1: The insignificant phase
shifting at about 3600 rpm sug-
gests that the corresponding
magnitude peak is a structural
resonance, not a shaft reso-
nance. You can see that the
magnitude peaks at around 3600
and 4600 RPM, indicating two
resonances. (Plotted with
Microsoft