vibration sign of over hung unknown initial conds

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Vibration and Force Signatures of Overhung Rotor Rotating Machine with SQi-05A-072008Unknown Initial ConditionsTechnote, SpectraQuest Inc. (July 2008)

Vibration and Force Signatures of Overhung Rotor Rotating

Machine with Unknown Initial Conditions

Eric Li, Suri GaneriwalaSpectraQuest Inc.

8227 Hermitage Road Richmond, VA 23228

Ph: (804)261-3300 Fax: (804)261-3303

July, 2008

Abstract: Rotating machinery with overhung rotors is very common in industry.Unbalanced rotor and misaligned shafts usually causes excessive machine vibration,

generates large forces on bearings and thus reduces the machine life span and may lead to

property loss and even human life loss. Two-plane balancing of overhung rotors is one of

the most challenge problems maintenance engineers may encounter. As a prerequisite,Successful diagnosis of unbalanced overhung rotor system must be performed. The

vibration and force signatures of unbalanced overhung rotors with unknown initial

conditions are different from those of the systems with center hung rotors and have been

studied in this tech note. Experiments were carried out on a Machinery Fault SimulatorTM

(MFS) for both balanced and unbalanced rotor systems. The data were analyzed using the

VibraQuestTM

software and efforts were focused on identifying the system characteristicsignatures.

1. IntroductionUnbalance is one of the most common reasons of excessive machine vibration. It has

been said that 80% of the vibration problems of the rotating machinery can be solved bybalancing and alignments. A small amount of unbalance weight in rotating system mayhave devastating effects as the system is operating with high speed or running near the

critical speed. Therefore, extreme care must be taken in balancing high speed rotatingsystem to avoid any potential damages. Overhung rotors are commonly used in fluidturbo-machinery, such as pumps, propellers and fans. The vibration and force signatures

of the overhung rotors are different from that of the center hung rotors, which have beenwell studied.

The vibration and force signatures of unbalanced overhung rotor can be simply explained

as below: the unbalance forces will cause the bearing to move in a circle in its own plane.

Refer to Figure 1, when the force is upward on the outboard bearing (the one close to theoverhung rotor), the tilting of the bearing will be away from the rotor, so the axial phase

at the top of the bearing will be out of phase with the vertical. At the inboard bearing (the

one away from the overhung rotor), the situation is reversed relative to the outboardbearing at any given time. When the rotor is forcing the inboard bearing upward, it willforce the outboard bearing downward, so the vertical phases will be 180 degrees apart

between the two bearings. Also, the bending of the shaft between the bearings will causes

the inboard bearing to tilt toward the rotor, opposite to the tilt of the outboard bearing.


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Therefore, the axial phase at the top of the inboard bearing will be out of phase with the

vertical.

Figure 1 Configuration of Single Plane Overhung Rotor

2. Test SetupThe running test was carried out on a Machinery Fault Simulator

TMMagnum, as shown

in Figure 2. Two types of overhung configurations are examined. One is the one planeoverhung, with just one rotor cantilevered on the right side of the shaft, exactly as inFigure 1. The other configuration is a two-plane system as shown in Figure 2, with one

rotor centered and another rotor cantilevered. In order to examine the linearity of the

system, different unbalance weights are introduced by applying 1, 4, 5 or 6 grams ofscrew onto the rotor. Three triaxial accelerometers are mounted, one on the top of motor,

and one on the top of each bearing housing. Force transducers are also mounted on the

bottom of each of the bearing housing. The shaft in experiments is 5/8-inch in diameter

and 31 inch in length. Rotors are 6 inch in diameter and weight 570 grams each. Dataacquisition and analysis were carried out using SpectraQuests multi-channel sound and

noise system VQPro. The test rig dimension and configuration are shown in Figure 3.


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Figure 2. Machinery Fault Simulator

TM

Magnum

Figure 3 Dimensions of the Test Rig (a) one rotor overhung (b) one centered and oneoverhung (Blue: triaxial accelerometer; red: force transducer)

3. Test Procedures

Experiments were carefully designed so as to find out the vibration and force signatures

of unbalanced rotating system with overhung rotors. Both baseline and unbalanced

system were tested at four different shaft speeds: 600 rpm, 1200 rpm, 1800 rpm and 2400rpm in order to compare the systemsresponse.

For system with one rotor overhung, the experiments were done by two steps:


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A. The baseline was tested at four different RPMs and signals were stored

B. System was retested with different unbalance weights: 1,4 and 6 grams

(a) (b)

(c) (d) (e)

Figure 4 Different unbalanced configurations by adding screws at different locations

For system with two rotors, the experiments were done by the following steps:

A. Baseline was tested with different shaft speeds: 600, 1200, 1800 and 2400 RPMs

B. System was retested with unbalanced weight only on centered rotor. Addedweights were 1, 4, 5 and 6 grams, as shown in Figure 4(a).

C. System was retested with unbalanced weight only on cantilevered rotor. Added

weights were 1, 4, 5 and 6 grams, as shown in Figure 4(b)D. System was retested with unbalanced weights on both rotors: static unbalance

type, added weight was 4 gram, as shown in Figure 4(c).

E. System wad retested with unbalanced weights on both rotors: coupled unbalance

type, added weight was 4 gram, as shown in Figure 4(d).F. System was retested with unbalanced weights on both rotors: dynamic unbalance

type, added weight was 4 gram, as shown in Figure 4(e)

4. Experimental Results and Discussion4.1. System with only one rotor overhung vibration signature analysis:

4.1.1 Spectrum analysis

The vibration and force signal analysis of the one rotor system was carried with twomethods, namely: spectrum analysis and the phase analysis. As we stated before, the

unbalanced weight will generate centrifugal forces and thus will apply extra forces on

both inboard and outboard bearings. Figure 5 and Figure 6 show spectrum amplitude(1X) of horizontal force component on inboard and outboard bearing housing as function

of shaft speed and unbalanced weight.


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Vibration and Force Signatures of Overhung Rotor Rotating Machine with SQi-05A-072008Unknown Initial Conditions

Technote, SpectraQuest Inc. (July 2008)

SpectrumA

mplitude

SpectrumA

mpli

tude

7.00E-02

6.00E-02

5.00E-02

Baseline_ImB_F_H

1g_InB_F_H

4g_InB_F_H

5g_InB_F_H

6g_InB_F_H

4.00E-02

3.00E-02

2.00E-02

1.00E-02

0.00E+00

500 1000 1500 2000 2500

Shaft Speed (rpm)

Figure 5 Spectrum amplitude (1X) of horizontal force component on the inboard bearinghousing as function of shaft speed and unbalanced weight

3.00E-01

2.50E-01

2.00E-01

Baseline_OutB_F_H

1g_OutB_F_H

4g_OutB_F_H

5g_OutB_F_H6g_OutB_F_H

1.50E-01

1.00E-01

5.00E-02

0.00E+00

500 1000 1500 2000 2500

Shaft Speed (rpm)

Figure 6 Spectrum amplitude (1X) of horizontal force component on the outboard bearinghousing as function of shaft speed and unbalanced weight


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Table 1 Bearing Force Ratio vs. shaft speeds Ratio and unbalanced weights

BaselineForce SpectrumMagnitude ratio(inboard Vertical)

Force SpectrumMagnitude ratio

(outboard Vertical)

Shaft rpmratio

0 gram 1 gram 6 gram 1 gram 6 gram

1 1 1 1 1 1

2 1.41 0.96 0.71 2.58 5.06

3 2.80 0.97 0.84 6.18 12.09

4 5.91 1.32 2.75 14.39 26.97

Table 1 provides a quantity analysis. We can see that the force resulted from unbalance

on inboard bearing did not change too much with respect to shaft speed and unbalanceweight. However, the force on outboard bearing increases sharply as the shaft speed

increases. For instance, with 1 gram unbalance weight (~0.17% of the rotor weight) and

shaft speed at 2400 rpm, the force resulted from unbalance on the outboard bearing was

14 times of the same system that runs at 600 rpm. Also, the force increases as unbalancedweigh increases and the system is also nonlinear with respect to unbalance weight.

Figure 7 Spectrum of inboard bearing housing axial acceleration with shaftspeed at 2400 rpm: (upper) Baseline (bottom) 6 gram unbalanced weight


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Figure 8 Spectrum of outboard bearing housing axial acceleration with shaft

speed at 2400 rpm: (upper) Baseline (bottom) 6 gram unbalanced weightAnother very important signature of the rotating system with unbalanced overhung rotors

is the existence of axial acceleration [1]. Figure 7 and Figure 8 clearly indicate the

situation. Both inboard and outboard bearing housings show a nonlinear large increment

of the 1X spectrum magnitude of axial acceleration.

4.1.2 Waveform analysis

Waveform or time domain analysis is also very important in the diagnosis of rotatingmachinery unbalance. This is due to the fact that when unbalances exist, the phase of

force or acceleration from inboard and outboard bearing housings will be either in phase

or out of phase. By checking the waveform diagram of the collected signals, we may be

able to tell whether or not there exists any unbalance.

Figure 9 shows the waveforms of inboard and outboard baseline bearing forces in thehorizontal direction. One can clearly see that they are in phase. However, by checking

Figure 10, when unbalanced was introduced, the inboard and outboard baseline bearingforces became out of phase, a difference of 180 degree.


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Figure 9. Baseline bearing housing horizontal force waveform at 2400 rpm

In phase: (left) inboard bearing (right) outboard bearing

Figure 10. Unbalanced (6 gram) bearing housing horizontal force waveform at 2400rpmOut of phase: (a) inboard bearing (b) outboard bearing

4.2 Two rotors system vibration signature analysis:In order to analyze the vibration and force signatures of the unbalanced rotating

machinery with overhung rotors, we tested the rotating system with 1) only oneunbalance weight on either centered or cantilevered rotor, 2) one unbalance weight oneach rotor but at different locations. The unbalance type can be actually classified into

three categories:

i) Static unbalance, by definition it means that two unbalance weights are located at the

same place on each rotor, case #3 will fall into this category.

ii) Coupled unbalance, this means that unbalanced weights located at opposite locationon each rotor, case #4 falls into this category.iii) Dynamic unbalance, all the unbalance cases that can not be classified into category I

and II will fall into this category. Dynamic unbalance is the most common type of

unbalance that an engineer will encounter. Case #1, #2 and #5 are typical dynamicunbalances.

Due to the nature of center-overhung rotor system, the diagnosis and balancing ofsuch a system have been proved to be much more difficult than that of the center-loaded


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SpectrumM

agnitude(1X)

rotor systems [1,2]. In this technical note, by both spectrum analysis and waveformanalysis, we actually can not tell which type of unbalance the system has. However, for

most of the cases, we are able to tell that there exist unbalances in the rotating system.

4.2.1 Case I: One unbalance weight on the center rotor

Figure 11 Full spectrum of inboard bearing axial acceleration at 2400 RPM:

(upper) baseline, (bottom) 6 gram unbalanced weight

3.00E-02

2.50E-02

2.00E-02

1.50E-02

baseline-InB-Ax

1g-InB-Ax

4g-InB-Ax

6g-InB-Ax

1.00E-02

5.00E-03

0.00E+00

500 1000 1500 2000 2500

Shaft speed (RPM)


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SpectrumM

agnitude(1X) 1.20E-02

1.00E-02

8.00E-03

6.00E-03

baseline-OutB-Ax

1g-OutB-Ax

4g-OutB-Ax

6g-OutB-Ax

4.00E-03

2.00E-03

0.00E+00

500 1000 1500 2000 2500

Shaft speed (RPM)

Figure 12. 1X spectrum magnitude of the bearing housing axial acceleration as function

of shaft speed and unbalance weight: (upper) inboard (bottom) outboard

Figure 11 shows the full spectrum of the inboard bearing housing axial acceleration at

2400 RPM, the upper figure is the baseline and the bottom one is the system with 6 gram

unbalanced weight. One can hardly tell the differences between these two spectrums.

Further investigation on the axial acceleration spectrum of inboard and outboard bearinghousing with different unbalance weight and operating speed gave interesting results. The

1X baseline spectrum magnitude was close to that of the system with 1 gram, 5 gram and

6 gram unbalanced weight, while the system with 4 gram of unbalanced weight has thelowest 1X magnitude, as all shown in Figure 12. The results suggest two possibilities:one possibility was, due to the nature of such a centered-overhung system, the system

may have initial unbalance and the addition of the 4 gram weight happened to balance theeccentricity. Another possibility is that we are unable to do the diagnosis based on the

information at hands; further works are needed in order to be confidently telling theexistence of unbalance.

4.2.2 Case II: One unbalanced weight on the overhung rotor


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Figure 13. Full spectrum of the inboard axial acceleration at 2400 RPM,

upper: baseline, bottom: 6 gram unbalance

For the rotating system with only one unbalance weight on the cantilevered rotor, the

diagnosis also dependents on two characteristic signatures: 1X radial force (inboard andoutboard, vertical and horizontal) spectrum analysis and 1X axial acceleration (inboardand outboard bearings) spectrum analysis. As shown in Figure 13, the full spectrum ofthe inboard bearing housing axial acceleration at 2400 RPM, there was a significant

increment of 1X axial acceleration increment accompany with the existence of unbalance.Further analysis of the 1X spectrum magnitude of outboard bearing housing and theradial force reveal the same trend. Two results are shown in Figure 14 and Figure 15.


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Spectrum

magnitude(1X)

SpectrumM

agnitu

de(1X)

7.00E-02

6.00E-02

5.00E-02

4.00E-02

Baseline-OutB-Ax

1g-OutB-Ax

4g-OutB-Ax

6g-OutB-Ax

3.00E-02

2.00E-02

1.00E-02

0.00E+00

-1.00E-02500 1000 1500 2000 2500

Shaft speed (RPM)

Figure 14. 1X spectrum magnitude of the outboard bearing

housing axial acceleration as function of shaft speed and unbalance weight

7.00E-01

6.00E-01

5.00E-01

4.00E-01

Baseline-OutB-F-V

1g-OutB-F-V

4g-OutB-F-V

6g-OutB-F-V

3.00E-01

2.00E-01

1.00E-01

0.00E+00

500 1000 1500 2000 2500

Shaft speed (RPM)

Figure 15. 1X spectrum magnitude of the outboard bearing

housing vertical force as function of shaft speed and unbalance weight

4.2.3 Case III: two rotors system: static/coupled/dynamic unbalance type

For the rotating system with one unbalanced weight on both the centered and cantileveredrotors, the unbalance diagnosis also dependents on two vibration and force signatures: 1X

radial force spectrum analysis and 1X axial acceleration spectrum analysis. The

completed results are shown in Figure 16, 17 and 18. A general conclusion is: the 1Xradial force spectrum and the 1X axial acceleration spectrum will increase with the


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Spectrumm

agnitude

(1X)

Spectrumm

agnitude(1X)

increase of unbalance weight and operating speed. The most significant increment lies in

the outboard axial acceleration, as shown in the bottom figure in Figure 16.

The difficulty of the diagnosis is how to classify the results into the three types ofunbalance: static, coupled or dynamic. Unfortunately, we did not see any consistence

trends in the 1X spectrum analysis, which means by just looking at the 1X spectrum of

axial and radial components, we are unable to tell the unbalance type. This is because wehave no information of the system initial conditions. The system may already have thefault like unbalance misalignment etc. However, we can make the conclusion that the

existence of unbalance is closely related to the increment of the 1X axial force increment,and especially, significant increment with respect to 1X outboard axial acceleration.

5.00E-02

4.00E-02

3.00E-02

2.00E-02

Baseline-InB-Ax

4g-Static-InB-Ax

4g-Coupled-InB-Ax4g-Dynamic-InB-Ax

1.00E-02

0.00E+00

-1.00E-02500 1000 1500 2000 2500

Shaft speed (RPM)

5.00E-02

4.00E-02

3.00E-02

2.00E-02

Baseline-OutB-Ax

4g-Static-OutB-Ax

4g-Coupled-OutB-Ax

4g-Dynamics-OutB-Ax

1.00E-02

0.00E+00

500 1000 1500 2000 2500

-1.00E-02

Shaft speed (RPM)

Figure 16. 1X spectrum magnitude of the bearing housing axial acceleration

as function of shaft speed and unbalance weight. Upper: inboard, bottom: outboard


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Spectrumm

agnitude(1X)

Spectrumm

agnitude(1X)

1.20E+00

1.00E+00

8.00E-01

6.00E-01

4.00E-01

Baseline-InB-F-H

4g-Static-InB-F-H

4g-Coupled-InB-F-H

4g-Dynamic-InB-F-H

2.00E-01

0.00E+00

500 1000 1500 2000 2500

Shaft speed (RPM)

1.20E+00

1.00E+00

8.00E-01

6.00E-01

Baseline-OutB-F-H

4g-Static-OutB-F-H

4g-Coupled-OutB-F-H

4g-Dynamic-OutB-F-H

4.00E-01

2.00E-01

0.00E+00

500 1000 1500 2000 2500

Shaft speed (RPM)

Figure 17. 1X spectrum magnitude of the bearing housing horizontal force



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Spectrumm

agnitude(1X)

Spec

trumm

agnitude(1X)

1.20E+00

1.00E+00

8.00E-01

6.00E-01

Baseline-InB-F-V

4g-Static-InB-F-V

4g-Coupled-InB-F-V4g-Dynamic-InB-F-V

4.00E-01

2.00E-01

0.00E+00

500 1000 1500 2000 2500

Shaft speed (RPM)

1.20E+00

1.00E+00

8.00E-01

6.00E-01

Baseline-OutB-F-V

4g-Static-OutB-F-V

4g-Coupled-OutB-F-V

4g-Dynamic-OutB-F-V

4.00E-01

2.00E-01

0.00E+00

500 1000 1500 2000 2500

Shaft speed (RPM)

Figure 18. 1X spectrum magnitude of the bearing housing vertical force



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5. Summary and Future Work

In this work, the vibration and force signatures of unbalance rotating machinery with

overhung rotors was studied by using the multi-functional Machinery Fault SimulatorTM

Magnum. Experiments were carried out with different operation speed, different

unbalance type and different unbalance weight. The data collected during the runningtests were analyzed and several observations can be reported.

1. Rotating system with overhung rotors, the existence of unbalance is closelyrelated to two important signatures: the 1X radial force/acceleration spectrummagnitude, and more significantly and strongly, related to outboard bearing (the

one close to the overhung rotor) 1X axial acceleration spectrum magnitude.

2. Waveform analysis or time domain analysis is also very important and helpful inthe diagnosis of unbalance. Phase analysis can help to make the final decision.

3. Due to the nature complexity of the system, using 1X spectrum information only,

we are unable to classify the unbalance type, especially when there is no

information of the system initial conditions.4. More researches are needed for: classification of unbalance type and the final andreal challenge job: balancing.

6. References:[1] Randy Fox, 1998, Balancing Overhung Rotors, technical paper presented at

ENTERACT 98 Information Through Integration Cincinnati OH April 19-22

[2] Jose A Mendez-Adriani, 2004, Consideration on the field balancing of the overhungrigid rotors, shock and vibration digest, v37 n3 179-187

vibration sign of over hung unknown initial conds

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