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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

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