Rotor Analysis Using SpectraQuest

Acoustical Testing I

Professor: Doug Jones and Dominique Chéenne

Written By John Garretson

Additional Group Members:

Marek Kovacik

Chris Kaiser

Josh Rivkin

Abstract

Tests were performed on a SpectraQuest Machinery Fault

Simulator to determine the effects of various rotor

configurations on the spectrum and waveform of a running

machine in the vertical plane. Configurations included

changes to location, weight distributions and quality of

rotors. Tests showed that amplitude increases as

imbalanced rotors are introduced and a visible envelope

from the rotor unbalance can be seen in the waveforms.

1. Introduction

Marek Kovacik, Chris Kaiser, Josh Rivkin,

and John Garretson used SpectraQuest

software and Machine Fault Simulator to

analyze the vibration in the vertical

plane associated with varying rotor

configurations.

2. Methods

2.1 Test Equipment

SpectraQuest Machine Fault Simulator

Image 1 Machine Fault Simulator

Image 2 MFS Rear view

The Machine Fault Simulator, MFS, is

used in conjunction with a computer

running SpectraQuest VPro data

acquisition software on an

accelerometer array. For the purpose

of our tests, only one accelerometer

was used, measuring the vertical axis.

Additional test equipment required for

the Rotor Unbalance test consisted of a

flexible helical beam coupler, a drive

shaft and two bearings, two golden

balanced rotors, one blue cocked rotor,

two screws used as unbalancing weight,

a .001in. displacement gauge, and a

strobe light.

2.2 Frequency Calculation

Testing required that the MFS be run at

10 Hz, 30 Hz, and 60 Hz. In order to

obtain the proper frequencies a strobe

light was used. Unfortunately the strobe

measured rpm, not Hz, and was not

able to go to a high enough rpm, 3600,

to match 60 Hz. To accurately

calculate the frequency of the spinning

shaft the strobe light was first set to 600

rpm, which is 10 Hz. When the shaft

appeared to stop moving in the strobe

the shaft was moving at 10Hz. Because

the strobe did not go beyond

approximately 1500 rpm this same

method couldn’t be used to calculate

30 Hz and 60 Hz. Instead, two different

marks were made on the coupler 180

degrees apart and the strobe was set to

900 rpm, or 15 Hz. When the marks on

the shaft stop moving and don’t

alternate between each other it means

that the shaft is moving at a frequency

that is an octave 15 Hz. The first octave,

15 Hz, was skipped, and the speed of

the motor was increased until the

second time the shaft “stopped” in the

strobe. This is when the shaft is spinning

at 30 Hz. The third octave, 45 Hz, was

again skipped and the speed increased

until the fourth time the shaft “stopped”

in the strobe, which is when it was

running at 60 Hz. Each time the correct

frequency was attained, the level that

the motor was running at on its digital

readout was recorded and used later to

reproduce each frequency.

2.3 Test Setups

Tests were performed using the

guidelines in Exercise No. 7 in the

Applied Vibration Analysis Laboratory

Exercises manual. All test setups were

examined at 10Hz, 30Hz, and 60Hz. A

diagram showing the relative locations

of the various test equipment is shown

below.

Figure 1 Diagram of various test setups orientations.

2.3.1 Baseline Center

The first test setup required that both

good rotors be placed on the shaft and

tested with no weight, seen in Image 3.

Image 3

2.3.2 Weighted Inline Center Inner

Next, weights are added to each rotor,

on the same plane, on the inner

diameter, shown in Image 4.

Image 4

2.3.3 Weighted Offset Center

Then, one of the weights was removed

and offset ninety degrees from its

original alignment with the other weight,

shown in Image 6.

Image 5

2.3.4 Baseline Cocked Center

Next, all weight was removed and the

cocked rotor was installed between the

outboard rotor and the outboard

bearing, as seen in Image 7 below.

Image 6

2.3.5 Weighted Cocked Center

Then, weight was added to the cocked

rotor, shown here in Image 8.

Image 7

2.3.6 Baseline Over

Then, all the rotors were removed from

their centerhung position and one of

the balanced rotors was place beyond

the outboard bearing, in the overhung

position, shown in Image 9 below.

Image 8

2.3.7 Weighted Over

Next, one of the weights was added to

the outer diameter of the overhung

rotor, shown below in Image 10.

Image 9

2.3.8 Baseline Cocked Over

The balanced rotor was then removed

and replace with the cocked rotor with

no weights in it, shown in Image 11.

Image 10

2.3.9 Weighted Cocked Over

The final test setup consisted of weight

being added to the cocked rotor in the

overhung position.

Image 11

3. Results

3.1 Waveform Analysis

Comparisons of the data from

unbalanced rotor testing to balanced

rotor testing data shows that

unbalanced rotors produce larger

amplitudes when driven at the same

frequency as balanced rotors. This can

be seen in Figure 2 below by the larger

red peaks in the top graph than the

green waveform the red waveform is

compared to.

Figure 2 Comparison of Balanced to Unbalanced Rotors

When the speed of motor was

increased from 10Hz to 30Hz and from

30Hz to 60Hz amplitude increased in the

resulting waveforms. The density of the

spikes in the waveforms also increased

proportionally to the increase in speed.

This can be seen in Figure 3.

Figure 3 Comparison of 60Hz in the upper waveform to 10Hz in the lower waveform.

Below, in Figure 4, it is shown that the

cocked rotor has little change to the

vertical plane’s waveform. What can

be seen is that an unbalanced rotor has

visible envelope from its unbalance.

Figure 4 Comparison of Cocked Rotor to Unbalanced Rotor in the same position. The unbalanced waveform has a visible enveloped created from its unbalance.

3.2 Spectral Analysis

When changing the orientation of the

rotors from the “centerhung” to the

“overhung” position the data shows that

the centerhung position is much noisier

around 1 kHz than the overhung as

shown by Figure 5 below.

Figure 5 Comparison of Overhung vs. Centerhung Balanced Rotors. The top graph, Centerhung, has a lot more energy around 1 kHz than Overhung graph on the bottom.

When the weight was shifted 90 degrees

on one of the unbalanced rotors a

strong peak developed around the 20

Hz range in the spectrum which was not

present when the weights were inline.

This can be seen in Figure 6 on the next

page.

Figure 6 Comparison of 90 degree weight shifted Rotor with inline unbalanced rotor.

When comparing the cocked rotor to

the unbalanced rotor spectrums from

the overhung position the 20 Hz spike is

seen again in both but is much more

prominent in the unbalanced rotor than

in the cocked rotor.

Figure 7 Comparing Cocked vs. Unbalanced Overhung

When the unweighted overhung

cocked rotor’s spectrum was compared

to the weighted overhung cocked

rotor’s spectrum, the weighted cocked

rotor acts very much like an

unbalanced non-cocked rotor as far as

the data of a single accelerometer in

the vertical plane is concerned, again

showing a very strong spike at 20Hz.

Figure 8 Comparing Cocked Rotors in the Overhung position, the upper spectrum has a weight on the rotor and the lower spectrum is unweighted.

4. Discussion

Though most of the data shows that the

cocked rotor acts very similarly to a

regular rotor this is inaccurate. The

reason it appears this way is because

only one accelerometer was used

during the testing, so only one

dimension was begin tested. If an

accelerometer was used in the

horizontal plane the reverse would

probably be true, meaning the cocked

rotor would show much more of an

impact on the results and the

unbalanced rotor would be negligible.

5. Conclusion

In conclusion, the data shows that

amplitude increases as the speed of the

motor increases as well as when an

unbalanced rotor is present. More

peaks are seen in the waveform as the

frequency of the shaft increases, and

the amount of peaks is directly

proportional to the increase in

frequency. The data showed that a

cocked rotor doesn’t have much of an

effect on the plane it is perpendicular

to. The data also shows the trend of a

strong 20 Hz component in unbalanced

rotors in the overhung position and the

envelope created by unbalanced

rotors.