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A kinked shaft causes twisting of the supporting bearing as the rotor

rotates. Therefore when the phase readings are taken in the axial

direction, at four 90-degree intervals, the phase angle relationship

will be as shown in the Fig.3.1. There will be 180-degree phase

difference between top to bottom and side-to-side phase readings.Another way to check a bent shaft is to use a shaft stick on either side

of the supporting bearing alternatively a dial gauge could also be

used to check this.

Fig.3.2 Using Shaft stick

3.1.2 Spectrum and Phase analysis for bow in the shaft

An illustration has been shown in Fig.3.3 to explain the uniform bow in

the shaft and also the phase angle behaviour.

Fig.3.3 Uniform bow in the shaft

Copy right of gVibration Busters Pvt. Ltd


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A look at the typical spectrum will reveal that the spectral characteristics

do not differ between a bow in the shaft and a kinked shaft. But the

uniform bow in the shaft, instead of twisting bearings, will cause tilting of

the bearings. Therefore axial phase readings taken on one supporting

bearing will remain the same. The axial phase readings taken on thesupporting bearing will differ by 180 degrees.

1. 3.00 1. 11.00

2. 3.00 2. 11.00

3. 3.00 3. 11.004. 3.00 4. 11.00

4. VIBRATION DUE TO MISALIGNMENT

When driving and driven equipment is coupled together, alignment of the

driving and the driven shaft is carried out. Misalignment therefore is a

major problem during the installation of the machine or after overhaul.

4.1 Common causes for misalignment

Some of the common causes for misalignment are,

- Settling of the foundation or base.

- Corrosion of base or shims.

- Deterioration of the grouting or shrinking.

- Differential expansion between the bases of the driver and

driven equipment.

4.2 Types of misalignment

There are three types of misalignment viz.

a. Angular misalignment

b. Parallel or offset misalignment

c. Combination misalignment



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Fig.4.1 different types of misalignment

4.3 Identifying vibration due to Misalignment

The driver and driven equipment share the forces . Depending upon

the stiffness characteristics, the vibration amplitudes will differ in the

bearings across the coupling. The vibration frequencies however will be

common for both the driving and driven equipment. Severity of the

misalignment is determined by the ability of the coupling to withstandmisalignment.

In the event of coupling being stronger than bearings, the forces

resultingdue to misalignment will damage the bearings. Since the axis

of rotation of the driver and the driven equipment are not in the same line,

an increase in the energy demand will be noticed whenever

misalignment sets in. The current drawn in a motor always goes up when

misalignment is present in the machine. The severity of misalignment is

determined by the ability of the machine to withstand the misalignment.

The diagnosis rules are listed in the Table 4.1

Type of misalignment Symptom Direction

Angular 1XRPM Axial

Parallel 1,2 X RPM Radial

Combination 1,2,3 X RPM Radial and axial

Table 4.1 Diagnosis rules



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Phase analysis is essential for confirming the type of misalignment.

4.4 Angular misalignment Phase behaviour

The mechanism of action of the axial force is shown in Fig.4.2 below.

Fig 4.2 Angular misalignment

The phase behaviour is marked by 180-degree phase difference between

the phase readings taken on the bearings across the coupling. It can be

seen from the Fig.4.2 that the axial forces varies from maximum to

minimum once in a revolution. Radial forces remain uniform throughout

the revolution.

4.5 Parallel misalignment Phase behaviour

The mechanism of forces when there is parallel misalignment is shown in

Fig.4.3.

Fig.4.3 Parallel misalignment



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- Produces a predominant 2xRPM peak in the vibration spectrum

in the radial direction.

- 180-degree phase shift will be noticed in the phase readings

across the coupling in the radial direction.

- Axial forces remain uniform throughout the revolution.

- Radial forces vary from minimum to maximum twice in a

revolution.

- Since parallel misalignment causes gyroscopic motion,

horizontal misalignment can reflect as vertical vibration and

vice versa.

Excessive misalignment can over a period of time generate a series of

running speed harmonics.

4.6 Other types of misalignment

In addition to the above , other types of misalignment have been shown in

Fig.4.4

Fig.4.4 Other types of misalignment


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