vib3.doc

7/27/2019 vib3.doc

http://slidepdf.com/reader/full/vib3doc 1/17

5. VIBRATION DUE TO MECHANICAL LOOSENESS

There are three distinct types of Mechanical looseness. They are,

- Looseness of structure/base frame- Looseness related to pedestal cracks

- Looseness related the rotor

Mechanical looseness amplifies the amplitudes due to exciting forces

already present in the machine. Mechanical looseness is due to loss of or

reduction in stiffness.

5.1 Looseness of structure/base frame

Weakness/looseness of the machine feet, base plate and concrete base,

deteriorated grouting, distortion of the structure or base frame and loose

hold down bolts are some of the structural related problems causing

vibration.

5.1.1 Identifying Structure related vibration

- Vibration is characterized by dominant 1XRPM amplitude.

- Phase behaviour is a local phenomenon pertaining to driver or

the driven equipment.

- Highly directional vibration.

- Phase angle difference between horizontal and vertical

directions will be 0 or 180 degrees.

The symptoms are similar to that of vibration due to eccentricity.

Spectrum taken from a machine having soft foot has been shown in

Fig.5.1. Notice the dominant 1XRPM vibration peak in the spectrum. The

problem is confirmed by carrying out phase analysis. The variouselements of a rigidly held structure will move together. In case of any

looseness, there will be relative motion at the interface resulting in

loss of amplitude and phase angle changes. In Fig.5.2 is shown a case

of weak base plate. A phase change of 180 degrees at the interface

between the base plate and the concrete base indicates a weak base plate.

7/27/2019 vib3.doc


Fig.5.1 Spectrum of a machine having weak structure

2.1 mm/s

1.9 mm/s

MACHINE FOOT 90/1.5 mm/s

270/0.5 mm/s

BASE PLATE

CONCRETE BASE 0.45 mm/s

FOUNDATION

Fig.5.2 Amplitude and phase behaviour

7/27/2019 vib3.doc


5.1.2 Cracked Structure/Pedestal

Cracked structure, pedestal, uneven supports, loose bolt on bearing

housing and improper bearing fit come under this type of looseness.

Symptoms

List of symptoms for this problem is given below.

- This type of problem is excited by unbalance or misalignment.

- Characterised by 2XRPM vibration peak. When the heavy spot

is at the bottom dead center, the bearing is forced down on the

pedestal. Similarly when the heavy spot is at the top Dead

center, the bearing is pulled away from the pedestal. Theapplied force acts twice in one revolution. 2XRPM vibration

amplitude is 50% of 1XRPM or more than that.Fig.5.4 is a

typical spectrum seen when type B looseness is present.

Fig.5.3 Different types of looseness

- Due to rocking motion the phase angle readings will be erratic.

Beware of balancing when mechanical looseness is present.

7/27/2019 vib3.doc


Fig.5.4 Spectrum of a machine having loose mounting bolt

5.1.3 Identifying looseness in the rotating system

This type of looseness is associated with the shaft and most common

cause of mechanical looseness. Some of the examples are, bearing loose

in the housing, excessive clearances, loose bearing liner in cap, loose

rotor on shaft and shaft turning with in inner race. This type of looseness

results in following vibration characteristics.

- Multiple harmonics of running speed even up to 10XRPM will be seen in the spectrum. See Fig.5.5

.

- Direction vibration will be noticed. Particularly vibration in the

vertical direction will be higher

.

- Due to possible rubbing events, half time harmonics (1/2XRPM,

3/2XRPM etc.) will also be seen in the vibration spectrum.

Refer to Fig.5.6

7/27/2019 vib3.doc


Fig.5.5 Spectrum of a machine having bearing loose on shaft

Fig.5.6 A spectrum with half time harmonics

Multiple harmonics can excite resonance inducing amplitude

amplification in corresponding to the excited frequency.

7/27/2019 vib3.doc


6. VIBRATION DUE TO FAULTY SLEEVE BEARINGS

Sleeve bearings are usually used in large high-speed machines. Various

designs of sleeve bearings are available and some of them have been

shown in Fig.6.1.

Fig.6.1 Various sleeve bearings

Vibration problems in sleeve bearings can be expected when there is

excessive clearance, loose liners and lubrication problems.

6.1 Bearing clearances

Loss of lubrication properties leads to a wear mechanism setting in. In the

early stages of wear, rubbing takes place. This causes further wear

resulting in larger bearing clearances. With larger clearances, impactingor mechanical pounding takes place.

- In the initial stages of wear, frictional rubbing forces are

generated. During rubbing, the rotor tries to pull the sleeve

along with it and the sleeve has a tendency to stop the rotor

from rotating. The average speed during rubbing is half the

speed of the rotor. Therefore in the early stages of wear

1/2XRPM and its harmonics are seen in the spectrum. See

Fig.6.2.

7/27/2019 vib3.doc


- When the clearances increase, the vibration becomes

directional. Look for increase in vibration levels in the vertical

direction while trending.

- With increased clearances, rotor impacts cause, multipleharmonics to appear in the spectrum. See Fig.6.3.

- Wiped sleeve bearings invariable show very high 2XRPM

amplitude in the vertical direction in addition to other

harmonics.

Fig.6.2. Spectrum with progressively increasing bearing clearance

7/27/2019 vib3.doc


Fig.6.3 Spectrum from a wiped sleeve bearing

6.2 Lubrication related problems – Oil Whirl

This type of problem is seen in machines operating at high speeds and

above second critical speed. Pressure lubricated sleeve bearings exhibittendency for oil whirl.

6.2.1 Mechanism of Oil whirl

The mechanism of oil whirl has been illustrated in Fig.6.4. The

lubricating oil in the sleeve bearing has the following functions.

a. To support the rotor

b. To eliminate metal to metal contactc. To carry away the heat generated due to load acting on the oil

film.

As the shaft rolls, lubricating oil lifts the shaft and forms an oil wedge.

This oil wedge carries out the functions mentioned above. There will be

shaft eccentricity due to shaft centerline and bearing centerline not

coinciding. When this eccentricity increases due to transient conditions,

additional oil is pumped into the space created by the shaft. This results

in increased oil film pressure. This causes the shaft to whirl around the bearing.

7/27/2019 vib3.doc


Fig.6.4 Phenomenon of Oil whirl

6.2.3 Identifying Oil whirl

- Generates vibration amplitudes at 0.42 to 0.48 X RPM

vibration frequency

.

- Oil whirl left unchecked can cause severe damage to the rotor.

- By changing the oil temperature oil whirl can be corrected

temporarily.

- Oil whirl can be intermittent and is excited by external source

vibrating at the bearing’s oil whirl frequency.

- Pulsations in piping when transmitted and if the exciting

frequency coincides with oil whirl frequency, resonant whirl is

induced.

7. VIBRATION DUE TO FAULTY ANTI-FRICTION BEARINGS

Rolling element bearings are usually manufactured with fine tolerances. It

has been found that only 30% of the bearings last full life. The premature

failure of the bearings takes place due to

a. Product defects

b. Operational faults and

c. External sources.

In many cases a good part life of the bearings is lost during installation.

7/27/2019 vib3.doc


Commonly known causes for bearing failure are, excessive load,

misalignment, Defective seating, faulty mounting, improper lubrication,

poor sealing, false brinellling and electric current.

7.1 Nature of rolling element-bearing vibration

- Defects in the rolling element bearings cause high frequency

low amplitude vibrations.

- The bearing vibration defect frequencies are not integral

multiples of Shaft speed.

- The bearing vibration is generally confined to the nearest point.

- The vibration frequencies generated are ultrasonic

frequencies, natural frequencies of the bearing components,

defect frequencies based on the geometry of the bearing and

sum and difference frequencies.

7.2 Bearing Failure stages

Bearings have been known to go through different stages of failure,

before terminal failure occurs. By identifying these stages during

vibration trending in addition to avoiding breakdowns, bearing could be

made to last longer.

7.2.1 Ultrasonic frequencies

This is the initial stage of the bearing failure. During this stage, when the

microscopic flaws are present or due to improper lubrication, spikes or

shock pulses are generated. These spikes and pulses are evaluated by

spike energy™ and Shock Pulse™. When the spike energy reaches a

typical value, it is recommended that the lubrication is checked or re-done. After lubrication if the spikes reduce and remain at a lower level

than the indication is that the problem was only a lubrication problem.

However if the spikes reduce for a short period and then raises again, then

it is clear indication of a flaw having set in. Fig.7.1 is shows a typical

initial stage failure. In this stage approximately 10% of the average

bearing life (60 months) is left out.

7/27/2019 vib3.doc


Spikes/pulses (0.25 gSE)

Fig.7.1 Ultrasonic frequencies

Fig. 7.1a Spike energy spectrum of lubrication problem

7.2.2 Natural frequencies

Even as the machine is allowed to run beyond the first stage, the defect

begins to deteriorate. The impacts or spikes will excite the natural

frequencies of the bearing components. These frequencies are generally

in 30000 CPM to 120000 CPM range. This stage is shown in Fig.7.2. It

has to be noted that the spike energy values have increased. Usually in

this stage 5 to 7% of the average bearing life is still available.

7/27/2019 vib3.doc


Spike/shock pulse (.25 to .5 gSE)

Fig.7.2 Natural frequencies of bearing components

Fig.7.2a Natural frequencies of bearing components

7.2.3 Bearing geometry defect frequencies

As the defect deteriorates further, the spike energy values begin to

increase. The impacts will be clear impacts due to rolling element passing

over inner and outer races. These impacts generate defect frequencies

based on the geometry of the bearing. The method of calculation of these

7/27/2019 vib3.doc


frequencies is shown in Table 7.1. The appearance of the defect

frequencies in the spectrum is shown in Fig.7.3.

Usually the amplitude corresponding to defect frequency of the defective

part grows. Sometimes defects in the cage appear in the spectrum more asa side band frequency than a carrier frequency. In this stage the spike

energy value increases from 0.5 gSE to 1 gSE. 2 to 5% of the bearing life

remains in this stage. It is only during this stage defects in the bearing

could be visible for naked eye.

Table 7.1. Calculation of bearing defect frequencies

The important aspect of bearing frequencies are that they appear in the

spectrum only in the event of a faulty bearing. The amplitudes

corresponding to these defect frequencies may vary depending upon the

operating conditions. However small the amplitudes may be, these defect

frequencies are clear symptoms of a defective bearing. These bearingdefect frequencies are available these days in software.

7/27/2019 vib3.doc


Spikes and pulses (0.5 to 1 gSE)

Fig.7.3 Third stage of bearing failure

Fig.7.3A third stage of failure

7.2.4 Bearing gives way

Towards the end of the third stage of bearing failure, harmonics of the

bearing defect frequencies are noticed. It will be desirable to increase the

number of surveys on the machine and closely monitor the vibration

levels. The rate of failure from the end of third stage to terminal failure is

unpredictable. It is recommended that an indent be made for replacement

of bearing at the end of the third stage.

While monitoring the spikes closely, there will a point when the spikessuddenly drop. The machine should be shutdown and bearing replaced at

this point of time. The typical spectrum is shown in Fig.7.4. Notice the

broadband random noise. If the machine is allowed to run beyond this, it

will cause seizure.

7/27/2019 vib3.doc


Spikes an Pulses (sudden drop)

Fig.7.4 Last stage of bearing failure

7.2.5 Guidelines for determining bearing failures

In machines without previous history, the relationship between the spikes

and bearing failure can be established by experience. The Table 7.3

indicates the measurement patterns for such evaluation.

VIBRATION LEVELS

Disp Vel Accl SpikesMachine condition

Machine Ok Ok Ok Ok OK

Watch M/c

Bearing

defects

OK to run

Ok Ok Ok High

Watch M/c

Bearing

defectsOK to run

Ok Ok High High

Machine

ProblemsOk High High High

Machine

problemsHigh High High High

Table 7.2 Guideline for measurements for bearing defects

7/27/2019 vib3.doc


When a bearing fails, different physical changes are seen in the bearing.

The Table 7.3 indicates the effects of such physical changes and the

causes along with remarks. It is recommended that the bearing removed

from the machine be inspected. This is to serve as guideline for finding

root cause failures.

From the foregoing discussions on vibration due to faulty rolling element

bearings, it could be seen that monitoring and trending the bearing

vibration, maximum life of the bearing could be extracted. Usually

permitted values of spike energy are shown in Table 7.3.

Enveloped spectrum and spike energy spectrums are used these days to to

detect early, rotational frequencies. The method is shown in Fig.7.5

Effect Cause Remarks

Surface Fatigue

Smearing

Scoring

Debris denting

Overheating

Lubrication problem Low viscosity

Roller bearing having

high thrust load

Foreign matter causing

dents

Friction from race

turning on shaft

Spalling Improper fit

Defective seats

Cracking and flaking

of race

Abrasion

Corrosion

Debris denting

Over heating

Poor sealing Dirt entering during

installation.

Moisture carried by air

Damage to raceways Unbalance

Misalignment

Typically damage ininner race

Typically damage in

outer race

Grooves in raceways

Fractures

Retainer failure

Misalignment Due to overloading

fractures result

Brinelling Background vibration

Shock loads

Poor mounting

High static impact load

Severe vibration of a

machine at rest

7/27/2019 vib3.doc


Table 7.3 Effect and Cause of bearing defects

Initial Value Maximum Value

Ball bearings

Fig.7.5 Enveloping process

TIME BANDPASS

FILTER

TIME

ENVELOPER

TIME FAST

FOURIER

TRANSFORM

SPECTRUM

BDF

vib3.doc

Documents