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1.INTRODCUTION TO VIBRATION___________________________________________________________
Maintenance Costs or profits?
Maintenance of machinery in industry has undergone a sea change. In the
last two and half decades there have been drastic changes in the outlook.
The globalisation that is taking place has brought in keen competition in
every field. The only cost left out after all the overheads is only the cost
of maintaining machinery. Maintenance department, which has
traditionally been a cost center, is now being exhorted to operate as profit
center. This has led to introduction of new techniques. There are many
industries where the maintenance policy is being overhauled. The
evolution of maintenance philosophy is discussed in this chapter,
Maintenance philosophy
Ever since industrialization took place, maintenance philosophy has
undergone change. The three basic types of maintenance practices are,
1. Break down or Run To Failure Maintenance
2. Preventive or Time Based Maintenance
3. Predictive or Condition Based Maintenance
There have been introduction of newer concepts like Proactive
Maintenance, Reliability Centered Maintenance, Autonomous
Maintenance etc. in the recent years. The evolution of such concepts over
a period of time has been show in Fig. 1.
Breakdown Maintenance or Run To Failure Maintenance
As the name suggests this maintenance philosophy implies that the
machine is allowed to run till it fails or breaks down. The demerits of
such a maintenance philosophy is listed below;
- Failure at the most inappropriate time leads to production losses
- Machine failure could lead to quality losses
- Skilled manpower not available to rectify the machine defect
- Spares not available to rectify the machine defect
- Machine damaged totally and therefore requiring replacement
- Machine damaged partially requiring replacement of
components
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1920 - Breakdown Maintenance
1940 - Preventive Maintenance
1960 - Predictive Maintenance
1990 - Proactive Maintenance
Reliability Centered Maintenance
2000 - Autonomous Maintenance
2000
1990
1920 1940 1960
YEAR
Fig. 1
The only merit for preferring this type of a maintenance practice will be
when there is a stand by and the cost of maintenance is more than the cost
of replacements. This practice is prevalent in many industries even today
where non-critical applications are involved.
Preventive Maintenance or Time Based Maintenance
This maintenance practice came into being particularly during the Second
World War. When the war machinery failed, causing defeats, efforts were
made in the direction of reduction of failures. Preventive Maintenance or
Time Based Maintenance is the result of these efforts
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VIBRATION ANALYSIS SYSTEMATIC
APPROACH
There have been indeed great strides in analysis techniques for solvingvibration problems. The key to solving the vibration related problems is
by using appropriate technique. It has to be remembered that the means
cannot become the ends. In this Chapter, the problem solving approach
has been outlined.
What Causes vibration?
This is a fundamental question and an answer to this provides the basis
for understanding and solving vibration problems. Rotating machinesalways have some vibration even if it is within limits. Rotors however
well balanced will always have residual unbalance. This residual
unbalance produces a centrifugal FORCE, resulting in vibration.
Therefore cause for vibration in any machine is the forces present in
the machine.
Types of Forces
To simplify analysis, identifying the type of force that causes vibration is
a significant step. The various forces that cause vibration are,
- Steady Force
1. Forces that change direction with time
2. Forces of Friction
3. Forces that increase/decrease in intensity with conditions
like load, temperature etc.4. Forces that cause steady impacts
- Unsteady Force
1. Forces that cause random impacts
2. Random forces caused due to change/reversal in the fluid
flow in fluid handling machines
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Examples of some of the forces
The fig.1 Shows a representation of mass unbalance m of a rotor
rotating with an angular velocity of and the radius being r. This
mass unbalance produces a centrifugal force F = mr2. When the rotorbegins to rotate, this force changes direction with time. This is illustrated
in Fig.2.
Fig.2 Force Changing in direction
Similarly in Fig.3 is shown the impacts generated in a press.
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VIBRATION ANALYSIS
In this chapter, diagnosis of problems causing vibration has been
discussed in detail. The causes for vibration have been listed below;
1. Unbalance
2. Eccentricity
3. Bent Shaft
4. Misalignment
5. Bearing problems (Sleeve and anti friction bearings)
6. Hydraulic and Aerodynamic problems
7. Electrical problems
8. Gear related problems9. Reciprocating forces
1. VIBRATION DUE TO UNBALANCE
1.1 Definition of Unbalance
The ISO definition for unbalance states Unbalance is that condition
which exists in rotor, when the vibratory force in the machine is
centrifugal in nature. There are other simpler definitions. Unbalance is
defined as non-uniform distribution of mass with respect to rotational
axis. It is also defined as Mass Center Displacement or simply MCD.
1.2 Types of unbalance
Identifying type of unbalance is important in determining the balancing
method. Unbalance can be classified into 4 types of unbalance viz.
a. Static unbalance
b. Couple unbalancec. Quasi static unbalance
d. Dynamic unbalance
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1.2.1 Static Unbalance
Fig.1 Static unbalance
It can be seen from the illustration in Fig.1 that in the case of static
unbalance, first the mass centerline and rotating centerline do not
coincide and secondly the shift is parallel. The balancing is done either by
placing a single correction weight in the plane of C.G or by dividing the
correction weight and placing correction weight in two planes.
1.2.2 Couple Unbalance
Fig.2 Couple Unbalance
By definition a rotor is said to have couple unbalance if there are two
equal and opposite masses acting on the rotor separated by a
perpendicular distance. The rotating centerline and mass centerline
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SHAFT AXIS
BALANCE AXIS
SHAFT
AXIS
BALANCE
AXIS
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intersect at the center of the rotor. This type of unbalance when corrected
by balancing in one plane leaves an internal bending moment in the rotor
causing axial vibrations.
1.2.3 Quasi-static Unbalance
Fig.3 Quasi-Static Unbalance
Quasi-Static unbalance is a combination of static and couple unbalance
and is illustrated in Fig.3. This type of unbalance has been found to be
present particularly in multi-stage rotors. The rotating centerline and mass
centerline need not intersect at the center of the rotor. The type ofdominant unbalance namely static or couple determines the amplitude
and phase angles obtained from the supporting bearings. In multi-stage
rotors like turbines and compressors, identifying the dominant unbalance
has been found to simplify the balancing process, when balanced in-situ.
1.2.4 Dynamic Unbalance
In all the types of unbalance that have been discussed so far, there is a
definite relationship between the amplitude and phase readings obtainedfrom the support bearings. This is absent in the case of dynamic
unbalance. Correction of this type of unbalance has to be done at the least
in two planes. This type of unbalance is encountered in normal practice
wherever the length to diameter ratio is generally more than 2.
The Tables 1 and 2 will outline the amplitude and phase relationships for
different types of unbalance and the balancing methods to be adopted on
length to diameter ratio respectively.
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SHAFT AXIS
BALANCE AXIS
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1.3 Diagnosing unbalance
In the previous chapter, it was mentioned that unbalance results in
centrifugal force, which changes in direction with time. It can be seenfrom the Fig.4 below that the sensor experiences maximum force when
the unbalance orients in an axis parallel to that of the sensor.
Fig.4 Unbalance amplitude
It can also be seen from Fig.4 that unbalance will orient in an axis parallelto the sensor only once in a revolution. Based on this the decision rules
for diagnosing unbalance are listed below;
- The vibration amplitudes are high in the radial direction except
in overhung rotors, which experience high axial vibration too.
- The vibration amplitude is proportional to the distance between
the center of rotation and center of the unbalance mass.
- The vibration amplitude varies as square of the speed. If the
speed is doubled vibration amplitude increases to 4 times.
- The dominant vibration amplitude will occur at 1XRPM
vibration frequency.
- The phase angle shifts by 90 degrees when the sensor is shifted
from horizontal to vertical direction. This is in ideal conditions.
A tolerance of +/- 30 degrees is allowed to take care of inherent
misalignment and looseness problems in machines.
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- The phase angle readings will be steady. Watch out for unstable
phase angle readings. Attempting to balance rotor with
unstable phase readings can be frustrating.
Above all to suspect unbalance as the cause for vibration, the 1xRPMvibration will have to be minimum 70% of the overall vibration level.
It is also important to observe the percentage increase in the
vibration level in both the radial directions in relation to the baseline
measurement. Uniform increase of vibration in both the radial
directions will suggest that the vibration problem is due to
unbalance.
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Table 1. Phase and type of unbalance
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L/DL/D
RATIORATIO
BALANCE CORRECTIONBALANCE CORRECTION
GT.2PGT.2P
LESSLESS
THANTHAN
O.5O.5
0-10000-1000
RPMRPM
ABOVEABOVE
10001000
RPMRPMN.AN.A
GT. 0.5GT. 0.5
UPTOUPTO220-1500-150
RPMRPM
150 -150 -
20002000
RPM *RPM *
ABOVEABOVE
20002000
RPM *RPM *
MOREMORE
THANTHAN
22
0-1000-100
RPMRPM
ABOVEABOVE
100100
RPM *RPM ***
ABOVE 70% OF 1STABOVE 70% OF 1STCRITICACRITICATableLTableL
TYPE OF BALANCE PROBLEMSTYPE OF BALANCE PROBLEMS