results and discussion (vibration s)

8/10/2019 Results and Discussion (Vibration s)

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7.0 RESULTS AND DISCUSSION

The graphs of Peak-to-Peak value, RMS value and crest factor were plotted against with the

rotational speed of motor as below. We can observe the vibration waveform varies with time and

the effect of adding bearings with and without unbalanced load by monitoring the change in the

vibration response as to compare with the baseline data

Experiment A: Baseline Vibration

Graph 1- Experiment A : Baseline Vibration (Peak-to-Peak vs Speed of Motor)

Graph 2- Experiment A : Baseline Vibration (RMS vs Speed of Motor)

0

0.0005

0.001

0.0015

0.002

630 810 1180 1320 1490

Peak-t

o-Peak(m)

(rpm)

Experiment A : Baseline Vibration

(Peak-to-Peak vs Speed of Motor)

0.00026

0.00027

0.00028

0.00029

0.0003

0.00031

630 810 1180 1320 1490

RMS(m)

(rpm)


(RMS vs Speed of Motor)


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Graph 3- Experiment A : Baseline Vibration (Crest Factor vs Speed of Motor)

Experiment B1: Bearing Fault- without unbalanced mass

Graph 4- Experiment B1: Bearing Fault- without unbalanced mass


2

2.2

2.4

2.6

2.8

3

630 810 1180 1320 1490

CrestFac

tor

(rpm)


(Crest Factor vs Speed of Motor)

0

0.001

0.002

0.003

0.004

630 810 1180 1320 1490Pe

ak-to-Peak(m)

(rpm)

Experiment B: Bearing Fault -without unbalance weight



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0

0.0001

0.0002

0.0003

0.0004

630 810 1180 1320 1490

RMS(m)

(rpm)



0

2

4

6

630 810 1180 1320 1490

CrestFactor

(rpm)




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Exercise B2: Bearing Fault -with unbalanced mass

Graph 7- Experiment B2: Bearing Fault- with unbalanced mass






0

0.001

0.002

0.003

0.004

630 810 1180 1320 1490Peak-to-Peak(m)

(rpm)

Experiment B: Bearing Fault - with unbalanced mass


0

0.0002

0.0004

0.0006

630 810 1180 1320 1490

RMS(m)

(rpm)

Experiment B: Bearing Fault - with unbalanced mass(RMS vs Speed of Motor)

0

1

2

3

4

5

630 810 1180 1320 1490

CrestFactor

(rpm)

Experiment B: Bearing Fault - with unbalanced mass



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Comparison between Experiment A with Experiment B1 and Experiment B2

(a)Peak-to-Peak value

By observing the results from Graph 1 for baseline vibration, we can conclude that with

increasing speed, the peak-to-peak value increases and it reached a maximum amplitude of

0.001695 m where the speed of motors, =1490 rpm. By observing the results from Graph 4 for

bearing fault without unbalanced mass, the peak-to-peak value increases and reached a maximum

amplitude of 0.002865 m where the speed of motors, =1320rpm and then it decreased. By

observing the results from Graph 7 for bearing fault with unbalanced mass, the peak-to-peak

value increases and reached maximum amplitude of 0.003074 m where the speed of motors,

=1320rpm and decreases.

(b)RMS value


increasing speed, the RMS value increases but decreased at the speed of motors, =1320rpm andincreased and reached a maximum amplitude of 0.000307348 m at the speed of motors, =1490

rpm. By observing the results from Graph 5 for bearing fault without unbalanced mass, the RMS

value increased and reached a maximum 0.000359 m at the speed of motors, =1320rpm before

it decreased. By observing the results from Graph 8 for bearing fault with unbalanced mass, the

RMS value increased and reached a maximum 0.000383 m at the speed of motors, =1320rpm

before it decreased.

(c) Crest Factor

By observing the results from Graph 3 for baseline vibration, with increasing speed, the

crest factor increases and reached a maximum of 2.811149 at the speed of motors, =1490rpm .

By observing the results from Graph 6 for bearing fault without unbalanced mass, the crest factor

increased but decreased at the speed of motor, =810rpm but it was then increased to maximum

of 4.31621 at the speed of motor, =1320rpm before it decreased again. By observing the results

from Graph 9 for bearing fault with unbalanced mass, the crest factor increased and reached a

maximum 3.982582 at the speed of motors, =1320rpm before it decreased.

Effects of Motor Speed on Experiment A


the speed of motor increases, peak-to-peak value also increases as until =1490rpm where themaximum peak-to-peak value is observed. This represent that at this speed, the amount of lateral

movement of machine is the highest and vibrates the most. For RMS value as it has a directly

proportional relationship between peak-to-peak and RMS values, where highest RMS is found at

about =1320 rpm which tell us that the vibration energy is at the maximum at this speed.

Meanwhile for the crest factor, at =1320rpm, maximum impact is exerted on the machine

because of the highest crest factor obtained.


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Effects of Bearing Fault Without and With Unbalance Mass on Vibration Response

When the bearing fault is mounted, the overall peak-to-peak value seems to be higher for

all the motor speed compare to the baseline vibration of Experiment A. The vibration is

intensified because of the unsuitable alignment between the abnormal bearing and the shaft when

the faulty bearing is mounted on the shaft. Hence, it results in more lateral movement of themachine. However, for bearing fault with unbalanced mass, this only occurs at small speed.

When the bearing fault is mounted, the overall RMS value is increased for all the motor

speed compare to the baseline vibration of Experiment A because of the unsuitable alignment

between bearing and shaft and caused it to vibrate with higher energy and intensity. It generates

noisier operating sounds as a result of the vibration during the experiment. But when a screw is

mounted to the rotor, the RMS value drops to a lower speed but it becomes even higher at higher

speed.

When the bearing fault is mounted, the overall crest factor is smaller compared to the

baseline vibration at low motor speed. However, the crest factor is increased when the speed of

the motor is increased as the impact wear is intensified at high speed for the vibration of bearing

fault without the unbalanced mass. When an unbalanced mass is mounted, the crest factor

decreased for almost all motor speed this means that the screw is effective in minimizing the

impact of wear on the bearing due to the oscillating waveform.

Exercise C: Resonance Test

Effect of Rotor Location on Critical Speed

The critical motor speed for the occurrence of resonance increased to =2335 rpm whenthe rotors were shifted to the left side of the shaft. There is a decrease in the speed required to

result in resonance compared to =2160 rpm if the rotor were to be placed in the middle because

of the imbalance position of the rotor increases the equivalent mass of the system. We can also

observe that with lower natural frequency will lead to lower critical speed for resonance.

Effect of Unbalanced Mass on Resonance

The critical speed for the occurrence of resonance decreased to =2120 rpm when the

unbalanced mass (a screw) was added to the rotor. There is a decrease in the speed required to

result in resonance when compared to =2160rpm without the adding of unbalanced mass. This

is because the unbalanced force induced on the rotor, acting as resistance to the shaft rotation.

Hence, higher operating speed was applied in order to have resonance effect since some of the

speed had been converted to cancel out the unbalanced force.


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Damaging Effect of Resonance and Possible Remedial Actions

Resonance has a great impact on the amplitude of oscillation of a system. Therefore it produces

more vibration when the system is subjected to a speed equivalent to its natural frequency.

Among the undesirable effects of resonance include:

Machines failure due to unaccounted fatigue load

Undesirable wear of machines parts

Undesirable noises generated

In order to cut down the effect of resonance, we can introduce a damper system to dissipate the

energy generated from a system such as springy material or absorber. The damping system will

be able to dissipate the amplitude of vibration. Since natural frequency, n= , we can alsomodify the natural frequency of the system by increasing the spring constant (k) of the system in

order to increase its natural frequency to avoid disturbance due to external excitation that might

cause to resonance.

results and discussion (vibration s)

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