Title

Machine Fault Diagnosis: Mass Imbalance

Objectives

1. To plan an experiment that compares the vibration in a balanced (baseline) condition with the vibration in an imbalance condition due to uneven rotor mass, condition.

2. To carry out the experiment (as in Objective 1) towards studying the effect of mass imbalance in rotating machinery system.

Introduction and Theory

Machine nowadays play an important role in human life, its help doing a lot of job faster and efficiently. Most of the machines contain several parts that rotate for example the flywheel, gear, pulley, and rotor and so on. The continuously rotation experienced by this part produce a phenomena of vibration. Other than that, imbalances in rotating machine also affect the level of vibration of the machine. Imbalance in rotating machine is unavoidable phenomenon that occurs intentionally or unintentionally. It cannot be eliminated but several precautions can be done to minimize this phenomenon. Disk imbalance is a condition in which the center of mass of a rotating disk is not coincident with the center of rotation. This condition occurs when the mass is added or removed from the rotating disk. It is also occur because of different location or radius of the unbalanced part of the disk.

Formula of unbalanced force: F=mr ω2

Where F = Force due to unbalance

m = Mass of the imbalance

r = Radius of the imbalance

ω = Speed of the rotor disk

By conducting this experiment, several factor can be analyze that is the effect of mass, radius and speed of rotor disk to the unbalanced force produced. The results obtained are to be compared with the baseline to observe which factor affect most to the vibration of the machine.

Procedure

A) Determine the baseline reading.1. Run the program VibraQuest Pro data analysis software and create the project

under root.2. Determine the connection of accelerometer to the respective channels (contain 8

channel including tachometer at channel 1)3. Set the frequency to 15 Hz and run the motor. (For precaution, the lid need to be

close first before running the motor)4. After conforming the signal waveform for each channel is ready, log the data and

wait for acquiring process to finish.5. Then, quickly turn off the motor. ( The motor should not running exceeds 2

minutes for precaution)6. Save the project data into folder.7. Repeat step 3 until 6 by using 30 Hz and 45 Hz. (Maximum speed that allow is

until 45 Hz)

B) Determine reading for different imbalance radius.1. Set the frequency to 15 Hz. (For this part, the frequency in constant)2. Take one of the screws that are given and lock it at the outer radius of the rotor.3. To lock the screw, use the Allen key size 3/16.4. Close the lid and start the motor.5. After conforming the signal waveform for each channel is ready, log the data and

wait for acquiring process to finish.6. Then, quickly turn off the motor. ( The motor should not running exceeds 2

minutes for precaution)7. Save the project data into folder.8. Repeat step 2 until 7 by locking the screw at the inner radius of the rotor.

C) Determine reading for different imbalance mass.1. Set the frequency to 15 Hz. (For this part, the frequency in constant)2. Take one of the screws that are given and lock it at the outer radius of the rotor.3. To lock the screw, use the Allen key size 3/16.4. Close the lid and start the motor.5. After conforming the signal waveform for each channel is ready, log the data and

wait for acquiring process to finish.6. Then, quickly turn off the motor. ( The motor should not running exceeds 2 minutes

for precaution)7. Save the project data into folder.8. Repeat step 2 until 7 but this time, add another screw at the outer radius of the

motor which mean this time using two screw.

Apparatus

1. One set Machinery Fault SimulatorTM (MFS)2. One ¾’’ diameter shaft attached to two bearing housings3. One helical beam coupling4. Two balance rotors5. Seven accelerometers6. One optical tachometer7. Eight-channel DAQ and analysis system8. Two ¼-20 socket head cap screws9. One set of MFS hex wrenches10.VibraQuest Pro data analysis software

Figure 1: Set of Machinery Fault SimulatorTM (MFS)

Data and Results

i. Data show amplitude of vibration obtained from different rotor speed 15Hz, 30Hz and 45Hz. A graph was plotted from this data.

Accelerometer

Speed

Motor AxialM.A.

Motor Horizontal

M.H.

Motor Vertical

M.V.

Inboard Horizontal

I.H.

Inboard Vertical

I.V.

Outboard Horizontal

O.H.

Outboard Vertical

O.V.

15 Hz 7.77E-04 8.95E-04 1.41E-04 2.55E-04 2.46E-03 1.75E-03 1.13E-03

30 Hz 4.64E-03 2.42E-03 2.32E-03 1.60E-03 3.81E-03 5.13E-03 1.24E-02

45 Hz 1.64E-02 7.58E-03 2.23E-03 2.63E-03 7.17E-03 5.68E-03 1.19E-02

Graph of amplitude from different accelerometer for different rotor speed

ii. Data show amplitude of vibration obtained due to different radius of imbalance by using rotor speed of 15Hz.

Accelerometer

RadiusOf Imbalance

Motor AxialM.A.

Motor Horizontal

M.H.

Motor Vertical

M.V.

Inboard Horizontal

I.H.

Inboard Vertical

I.V.

Outboard Horizontal

O.H.

Outboard Vertical

O.V.

Baseline 7.766e-4 8.951e-4 2.687e-3 2.553e-4 2.458e-3 1.749e-3 1.132e-3

Radius Inner 1.106e-3 1.043e-3 3.942e-3 3.649e-4 3.639e-3 2.148e-3 1.787e-3

Radius Outer 1.216e-3 9.683e-4 4.222e-3 4.026e-4 3.921e-3 1.866e-3 1.935e-3

Graph of amplitude from different accelerometer for different radius of imbalance

iii. Data show amplitude of vibration due to different mass of imbalance by using rotor speed of 15Hz.

Accelerometer

Mass of Imbalance

Motor AxialM.A.

Motor Horizontal

M.H.

Motor Vertical

M.V.

Inboard Horizontal

I.H.

Inboard Vertical

I.V.

Outboard Horizonta

lO.H.

Outboard Vertical

O.V.

Baseline 7.766e-4 8.951e-4 2.687e-3 2.553e-4 2.458e-3 1.749e-3 1.132e-3

1 Mass 1.106e-3 1.043e-3 3.942e-3 3.649e-4 3.639e-3 2.148e-3 1.787e-3

2 Mass 1.524e-3 1.245e-3 5.672e-3 5.911e-4 5.286e-3 2.406e-3 2.704e-3

Graph of amplitude from different accelerometer for different mass of imbalance

Analysis and Discussion

Seven accelerometer was used and labeled to observe the amplitude of imbalance at the following angle that is motor axial (MA), motor horizontal (MH), motor vertical (MV), inboard rotor horizontal (IH), inboard rotor vertical (IV), outboard rotor horizontal (OH), and lastly outboard rotor vertical (OV).

To study the effect of speed to the amplitude of imbalance, the motor was run for three different speed, that is the lowest speed was 30 Hz, then 15 Hz and the highest speed ran was 45 Hz. The motor was not allowed to run over 45 Hz to preserve the motor parts from failure. The graph of amplitude for different accelerometer reading then was plotted for these three different speeds. By observing the graph, for the rotor speed of 15 Hz, the value of amplitude indicate was not exceed 0.00246 for all accelerometer reading. This means that the value of imbalance is quite small. For the rotor speed of 30 Hz, the amplitude reading was increase for all channel of accelerometer. But there is significant increase at channel of vertical outboard rotor (O.V) that is an increase of 0.0113rms of the amplitude value. This means that the imbalance force was highly

affected at that side of view (O.V). Lastly for the rotor speed of 45 Hz, there was also an increase of amplitude value for all channels. But there is significant increase of amplitude value at the channel MA, MH, IV and OV. The highest imbalance recorded was at the axial of the motor (MA) with the amplitude of 0.0164rms. This means that with the maximum rotor speed of 45 Hz, it will highly affect the imbalance of the axial motor. In conclusion, an increase of speed of the motor will obviously affect the imbalance of the outboard rotor and the motor itself.

To study the effect of radius to the amplitude of imbalance, the motor was running at constant speed of 15 Hz. The radius was manipulated and the imbalance was compared to the baseline with the same speed of motor. By observing the graph, the imbalance was obviously affected at the channel MV, IV and OV. By comparing with the amplitude of the baseline at MV, IV and OV, the larger the radius of additional mass placed, the larger will be the value of amplitude. This means that the position of imbalance mass will affect the vibration at the vertical channel of the machine. By comparing to the baseline, channel MA, MH, IH and OH was not severely affected by the imbalance. In conclusion, the larger the radius of imbalance mass placed at the rotor, the higher will be the amplitude of vibration at the vertical channel and the machine generally.

To study the effect of mass to the amplitude of imbalance, the speed of the motors was set constantly at 15 Hz. The mass was manipulated by adding two different mass to the inboard rotor. This imbalance then was compared to the baseline result with the same speed of motor. By observing the graph plotted, in general, the value of amplitude was gradually increased at all of the accelerometer channels due to the increase of mass to the rotor. By comparing to the baseline result, the channel that was obviously affected was at MV, IV, and OV. This mean that present of additional mass at the inboard rotor will affect most at the vertical side of the machine. All other channel, MA, MH, IH and OH was just slightly affected by the addition of mass and that value can be ignored due to very small different. For example at channel MH, the change of amplitude was just 0.148×10-4 for 1 additional mass and 3.499×10-4 for 2 additional mass by comparing to the baseline. In conclusion, the larger the mass added to the rotor, the larger will be the amplitude of vibration of the machine.

By comparing all the experimental result with the theoretical formula,F=ml r ω2

the force due to unbalance was directly affected by the change of mass of imbalance, radius of imbalance and the speed of the motor. That is, if the mass, radius or the speed was increased, the force due to unbalance will also increase. Thus, the result obtained from the experiment can be accepted.

The result obtained from this experiment may slightly affected by the error occurred when conducting the experiment. One of the factors that may affect this

experiment is laboratory room condition. The laboratory was equipped with the wall fan that produces vibration when operating. Besides, the natural vibration of the building may also slightly affect the reading of the accelerometer.

Conclusion

After completing this experiment, we had explored the characteristic of unbalanced force and what cause the machine to vibrate at different condition. Begin with the test to determine the effect of speed to amplitude of vibration, and then proceed with adding the imbalance to the machine by manipulating the radius and mass to the machine. The result indicates the larger the speed, mass and the radius of imbalance, the large will be the unbalanced force of the system. This experimental result was compared with

theoretical formula F=ml r ω2 and the result was approved to be same. In conclusion,

the vibration in imbalances condition is higher compared to the vibration in a balanced (baseline) condition. The present of imbalance clearly affect the machine by increase the vibration and thus can cause failure at certain part and reduce the performance of the machine.

References

i. Palm, W. J. (2007). Mechanical vibration. Hoboken, NJ: John Wiley.

ii. Singh, P. K., & Kumar, R. (2013). Study of effect of imbalance in rotor-bearing system due change in radius of gyration. International Journal of Scientific & Engineering Research, Volume 4(3), pp.1-5.

iii. Jalan, Arun Kr., and Mohanty, A. R., (2009). Model based fault diagnosis of a rotor–bearing system for misalignment and unbalance under steady-state condition, Journal of Sound and Vibration, Volume 327, pp. 604-622.

Appendix

Figure 2: Result of amplitude versus frequency for balance case

Figure 3: Result of amplitude versus frequency for unbalance case