OBJECTIVES

For the experiment of mechanical vibration machine fault diagnosis ; mass unbalance, the purpose of the researches is to study the phenomena of vibration response to faults and also to simulate mass unbalance in rotating machinery system.

THEORY

Rotating machineries are commonly found in industries. The machines that employ rotating parts can suffer from wear or defect as well as imbalance of mass. At the same time, an excessive vibration can be produced if each part of the machine is not aligned correctly since they are connected to one another by coupling.

Usually, before the rotating machines endure a failing they give some warning sign or alert which can be changes of vibration level and pattern. These faults can be tracked or can be detected by examining and recognizing the vibration signal. Thus, an early attempt could be made to save these machines from severing an entire failure.

The most common cause of excessive vibration is mass unbalance. This happens if the centre of mass does not lie on the axis of rotation which creates something that can be called as ‘heavy spot’ at the rotor and producing a centrifugal force when it rotates. The force then transmits the resulting vibration to other parts of machine or structure. From the equation below the centrifugal force is given by

F = meω² (1)

Where

m: Unbalance mass(heavy spot)

e: Distance of the mass to the centre of gravity

ω: Angular velocity of the operation speed (in radian)

If the machine left out of balanced for long duration of time, this will cause damage to seals and

bearings.

This experiment used the simulation using Machinery Fault SimulatorTM (MFS) (see Figure 2)

and analysing the recorded vibration signal from the sensors.

Figure 1: Illustration of force unbalance causing vibration.

MATERIALS AND EQUIPMENT

1. Vibra Quest Machinery Fault Simulator (MFS) 2. Two balance rotors (gold) 3. Seven accelerometers 4. Two ¼-20 socket head cap screws

Figure 1.1: Machinery Fault Simulator TM (MFS)

PROCEDURES

1. For Baseline measurement, the equipment were all set up as shown in Figure 1.1 where two balance rotors (gold) installed on a straight 3/4” diameter shaft in a centre hung condition with distance in between is roughly 20 cm.

2. Channel 1 (Ch1) was connected for Tachometer to measure the operational speed of motor.

3. The accelerometers were located in the following directions: Ch2-motor horizontal, Ch3-motor vertical, Ch4-motor axial, Ch5-inboard horizontal, Ch6-inboard vertical, Ch7-outboard horizontal and Ch8-outboard vertical.

4. The MFS is properly aligned.

5. In the DAQ control parameters frequency limit is selected to be 2000 Hz, spectral lines 3200 and number of blocks 5.

6. The baseline reading was recorded at 15Hz, 30 Hz and 45 Hz.

7. For rotor unbalanced measurement , an equal weight (screw) was installed on each rotor

in the same plane on the inner diameter.

8. The spectrum was recorded at 15Hz, 30Hz and 45Hz.

DATA AND RESULT

1. Sketch the spectrum of the baseline measurement and rotor unbalance for each operation frequency (15, 30 and 45 Hz) for the first 4 harmonics, i.e. 1X, 2X, 3X and 4X where X is the speed or frequency of operation. Which harmonic has the largest amplitude? (Use different sheet) .

Figure 1: Spectrum of baseline measurement for 15Hz.

Figure 2: Spectrum baseline for 30Hz.

Figure 3: Spectrum baseline for 45Hz.

Figure 4: Spectrum of rotor unbalanced for 15Hz.

Figure 5: Spectrum of rotor unbalanced for 30Hz.

Figure 6: Spectrum of rotor unbalanced for 45Hz.

For spectrum of the baseline measurement for each operation frequency 15Hz, 30Hz and 45Hz

for the first 4 harmonics the operation that has largest amplitude is baseline measurement for

30Hz operation frequency. While, for the rotor unbalanced measurement the operation frequency

that has largest amplitude is unbalanced measurement at 30Hz operation frequency.

ANALYSIS AND DISCUSSION

1. To analyse the data gained, we use need to overlay both graphs and amplitude display by using overlay amplitude – spectrum (refer to Figure 7). This graph was drawn in order to identify the difference of the first four harmonics for the baseline measurement and rotor unbalance for operation frequency of 30Hz.

If we look at the graph (Figure 7), we can see the difference between all those four peak value or amplitude. This difference of peak value will be used to identify the sign of unbalance from the signal.

The graph of frequency vs peak value

2. For the sample calculation for the graph above:

Let X = me

F = meω2

= Xω2

We know that ω = 2πf

Therefore F = (2πf )2X

X = F

(2 πf )2

F = A ; the force is trans missed.

When f = 15Hz and A = 1.6557 × 10−4

X1 = 1.6557×10−4

¿¿ = 1.8640 × 10−8

when f = 30Hz and A = 1.5790 × 10−3

X2 = 1.5790×10−4

¿¿ = 4.4441 × 10−8

when f = 45Hz and A = 5.9086 × 10−3

X3 = 5.9086×10−4

¿¿ = 7.3909 × 10−8

Therefore based on the graph, the operation frequency is directly proportional to the change of peaks.

3. We can correct unbalance by:

Mass addition, Mass removal, Mass shifting , Mass centring.

The unbalance mass can be fixed or corrected by either we adding the equal mass for both rotating bodies and reducing the mass to them. In case of weighing the rotating bodies with mass, we must add the mass about the same distance from the body’s centre and same angle from the normal in the opposite direction. Meanwhile for other case –reducing mass- we just need to remove the added mass from the bodies. For the mass adding method, refer to the diagram below.

CONCLUSION

Based on the experiment, we can stimulate the unbalance rotating machinery using Vibra Quest

Machinery Fault SimulatorTM (MFS). The unbalance occurs in this experiment while placed only

one screw on the half side of the rotor, the rotor will faced unbalance rotation. To overcome this

problem, we need to balance the amount of screw used to put in the rotor equally, so that the

rotor will have equilibrium mass on both side. On this experiment, we have found fault response

to mass unbalance phenomena and cause the unbalance the vibration. Overall for this

experiment, we can say that the objective of this experiment was achieved.

REFERENCES

1 Http://www.pearsonhighered.com/assets/hip/us/hip_us_pearsonhighered/preface/0132128195.pdf

2 Courier Dover Publications, Jan 1, 1985 Technology & Engineering, Jacob Pieter Den

Hartog . 1956 edition ( 436 pages )

3 Universiti Teknikal Malaysia Melaka Mass Unbalance full lab sheet .

4 Thesis.library.caltech.edu/3558/1/Hallanger_l_1967.pdf

FORMAL LAB

REPORT

-INDIVIDUAL-

( MASS UNBALANCE )

NAME : WAN MOHD FARIZZUAN BIN

W.SAMAT

ID NUMBER : B041110208

CLASS : 3BMCS / 2

LECTURER : PROF MADYA AHMAD RIVAI