Q.
1) What are the main types of pickups, indicating the factors that affect the pickup
performance and the methods of pickup attachment? Explain “in details” the velocity
pickup.
The main types of pickups:-
1) Displacement probs.
2) Velocity transducers.
3) Accelerometers.
Factors affecting the pickup performance:-
1) Measurement point. 3) Orientation.
2) Compressive load. 4) Environmental conditions.
Methods of pickup attachment:-
1) Hard mount vibration transducers to the selected measurement points.
2) A well-designed quick-disconnect mounting.
3) A magnetic mount.
4) Handheld transducers.
Velocity pickup:-
Velocity transducers measure the rate of displacement. Velocity data are normally expressed in
terms of ips peak and are perhaps the best method of expressing the energy created by machine
vibration.
Velocity transducers have an effective frequency range of about 10 to 1000 Hz.
The major limitation of velocity transducers is their sensitivity to mechanical and thermal
damage.
Q.2) Write briefly about the following?
1) Vibration and its importance in maintenance:-
Vibration analysis is used to determine the operating and mechanical condition of
equipment. A major advantage is that vibration analysis can identify developing problems
before they become too serious and cause unscheduled downtime.
Regular vibration monitoring can detect deteriorating or defective bearings, mechanical
looseness and worn or broken gears. Vibration analysis can also detect misalignment and
unbalance before these conditions result in bearing or shaft deterioration.
2) Sources of unbalance:-
a) Blow holes.
b) Oversize web.
c) Eccentric hole location.
d) Eccentric machining.
Q.3) Explain how to detect faults in ball and rolling bearing. The vibration spectrum of
a 1000-rpm machine shows peaks at the following frequencies: 17, 95, 114, and 132
Hz. The machine runs on rolling element bearing with ten rollers of 16 mm diameter, 0
contact angle, and 112 mm pitch diameter. Investigate possible causes.
Defective rolling-element bearings will generate vibration frequencies at the rotational speeds
of each bearing component. Each of the frequencies can be calculated and routinely monitored
using vibration-analysis techniques. Rotational frequencies are related to the motion of the
rolling elements, cage, and races. They include ball or roller spin, cage rotation, and ball or roller
passing frequencies.
The ball or roller spin (BSF) frequency is generated by the rotation of each ball or roller around
its own centerline and can be calculated as
Since the ball or roller defect or defects will contact both the inner and outer races each time it
completes one full revolution, the ball-spin defect frequency will be at two times the BSF or
rotational frequency.
The cage rotation frequency, or fundamental train frequency (FTF) can be calculated as
A defect in the outer race of the bearing can be calculated using the ball pass–outer race (BPFO)
formula:
Inner race defect frequency, or ball pass–inner race (BPFI), can be calculated as
Where N_ shaft speed, in hertz or revolutions per second
D_ pitch diameter of bearing, in inches
d _ diameter of balls or roller, in inches
n _ number of balls or rollers
( ) [ ( ) ]=57 Hz
2
0.5∗1000 112 16
BSF= ∗ ∗ 1−
60 16 112
FTF =
0.5∗1000
60 [ 11216 )]=7 Hz
∗ 1−(
BPFO=
0.5∗1000
60 [ 112 )]=71 Hz
∗10∗ 1−(
16
BPFI=
0.5∗1000
60 [
∗10∗ 1+(
112 )]
16
=95 Hz
First peak (17 Hz) is probably caused by unbalance.
Second peak (95 Hz) caused by misalignment, wear in rollers or excessive load on outer
race.
Third peak (114 Hz) is equal to 7 times of machine rpm and may probably cause by
mechanical looseness and poor fixation.
Q.4) The data shown in Fig.1, include a sketch of a machine (a large motor-driven
pump), operating speed and tabular data of vibration amplitude and frequency in the
horizontal, vertical, and axial directions at each of the machine’s bearings. What are
the possible causes for the vibration which occurs at 1800 cpm on bearing A and B?
Pickup location Displacement Frequency
Vertical 1.8 1800
A Horizontal 2.5 1800
Axial 1.3 1800
Vertical 2.1 1800
B Horizontal 3.2 1800
Axial 1.9 1800
Vertical 1.1 1800
C Horizontal 1.3 1800
Axial 1.2 1800
Vertical 1.3 1800
D Horizontal 1.1 1800
Axial 1.5 1800
The possible causes for the vibration on bearing A and B are:-
1) Unbalance
2) Misalignment
3) Wear in rolling element
4) Defects on outer, inner, or cage
5) Excessive axial and thrust forces on bearing
The best way to eliminate the vibration the bearing must be replaced with another
one.
Q.5) “When using vibration measurement for fault diagnosis in machinery and
equipment, it is necessary to analyze the signal measured to determine the type of
fault and its trend”. Explain the above statement in details using sketches
whenever possible. Draw a suitable circuit diagram, showing the necessary
instrumentations used for vibration measurement and analysis. Draw also a
specimen of vibration spectra measured by the above instruments.
Circuit diagram of process of vibration analysis
The necessary instrumentations:-
1) Vibration pickup (Displacement, Velocity, Accelerometer).
2) Combining unite for collecting all signals.
3) Signal Amplifier.
4) FFT Analyzer (Fast Fourier Transformation).
5) Display monitor to analysis the resulting signal.
Steps of analysis:-
1) Full vibration signatures should be acquired to verify the accuracy and determine
the initial operating condition of the machine.
2) Mounting the vibration pickups at several points in the three directions (Vertical,
Horizontal, and Axial).
3) Attaching all pickups to the combing unite to obtain one signal.
4) The signal pass through the amplifier to amplify this signal and using the FFT
analyzer to transform the signal to frequency signal in S domain.
5) Using computer software and a display to analysis the resultant signal.
6) At each peak the frequency must be calculated and compared with the original
frequency signature of the machine to find out the defect and fixed it.
7) Measurements are acquired every 2 weeks to prevent failure during operation of
machine.
