analisis bajas frecuencias 237267.pdf

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Summary

This case study from a UK Paperboard manufacturer clearly

illustrates the ability of bearing enveloping techniques to

successfully diagnose faults in bearings that operate at lowspeeds. It also dispels the myth that study of any problem on low

speed machinery requires use of specialized “low frequency”

sensors and equipment.

Low Speed Bearing

MonitoringA Case Study of Low Speed Bearing

Monitoring in a Paperboard Plant

MB01001Mel Barratt8 pagesMay 2002

SKF Reliability Systems@ptitudeXchange5271 Viewridge CourtSan Diego, CA 92123United Statestel. +1 858 496 3554fax +1 858 496 3555email: [email protected]: www.aptitudexchange.com


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Low Speed Bearing Monitoring

© 2004 SKF Reliability Systems All Rights Reserved

Introduction

In late 1997 the in-house condition monitoring program at Iggesund’s plant in Workington

(UK) detected a problem associated with a

cylinder support bearing on their Number 2

Board Machine. Iggesund staff estimated thatcomplete failure of the suspect bearing could

result in machine shut down for up to six

months, due to the size and weight of thecylinder in question. This is the Machine

Glaze (MG) cylinder, which is more than 6

meters diameter, and weighs approximately165 tons (Figure 1). The Number 2 Board

Machine is over 200 meters long and

manufactures coated carton-board for the

packaging and pharmaceutical industries.

Figure 1. MG Cylinder.

The MG cylinder typically operates at around

12 RPM. It is a commonly held misconception

that studying any problem at such a low speedrequires specialized equipment. It is true that

resulting vibration from basic mechanical

problems, such as unbalance or misalignment

occurs at low frequencies that fall outside therange of most “general purpose”

accelerometer sensors. However, the vibrationfrequency generated by a fault in a rollingelement bearing is still relatively high, even at

low rotational speeds. Therefore, they may be

studied using “standard” equipment, provideddue allowance is made in the configuration of

measurement parameters.

Users of modern vibration instrumentation are

accustomed to fast data collection times. Oncommon electric motors running at around

1500 – 3000 RPM it is necessary to sample

only a few seconds of data to enable faultdetection at an early stage of development. Itshould be remembered that in 2 seconds, a

motor doing 1500 RPM completes 50

revolutions of movement. The same length ofdata sample applied to a bearing at 12 RPM

means that less than half of one shaft

revolution is studied. Therefore, study ofrolling element bearings operating at low

speeds does not necessarily require special

equipment, but does require special

consideration. Using a technique known as“Enveloped Acceleration Measurement” may

further enhance the effectiveness of vibration

analysis on low speed bearings.

Although a bearing fault can transmit asignificant force through the bearing housing,

the response of the supporting structure is

usually very small (as measured by anaccelerometer mounted near the bearing load

zone).

Figure 2. Time Domain Data From Accelerometer.

Figure 2 shows a time domain plot of such an

accelerometer signal. It depicts a bearingdefect impulse signal summed with low

frequency vibration, due to imbalance or

misalignment. The measurement difficultyhere is to accurately separate and sense these

small bearing signal excitations in the


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presence of generally larger vibration

components. In the very early stages ofsurface distress, transducer signals are buried

in noise. Measurements of these early-stage

signals require instrumentation thatincorporates wide dynamic range, lowinherent amplifier noise, and circuitry to

enhance these negligible bearing response

signals. In the early stages of bearingdeterioration, defect frequency components

are very small and are usually not discernible

in the transducer signal’s normal amplitudespectrum plot. It is during these early stages of

bearing wear that enveloping methods are

useful to enhance the response signals of small

repetitive defect impacts.

This incident serves to illustrate a number of points:

• The cost effectiveness of a disciplined

vibration monitoring program

• The value of the “enveloping” techniques

in bearing fault diagnosis

• The manner in which low speed bearing problems may be studied without

specialized (i.e. low-frequency) sensors

and equipment.

Detection of the Problem

A study of vibration velocity measurements

taken from the MG cylinder’s front side bearing cap over a three-year period (Figure 3)

shows a mean value of 0.74 mm/sec RMS

(root mean squared value). As monitoringcontinued during late 1997 is was noted that

this level rose to a new record of 1.21 mm/sec

RMS. The change was clearly visible despite

the fluctuations that occurred in the value as a

result of different machine operatingconditions.

A velocity level of 1.21 mm/sec RMS on

many machines is not significant. Indeed,many other machines on the Iggesund site

typically operate at higher levels. However,

the MG cylinder operates at low speed,

typically around 12 RPM. It was the change inthe level that was considered significant

enough to warrant further study.

Figure 3. Vibration Velocity Trend Data.


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Figure 4. Velocity Spectrum.

Study of the vibration velocity spectrum(Figure 4) indicates a harmonic family with

spacing of approximately 3 Hz. The peak

levels were very low, as was the total spectralenergy. Defective bearings usually display

higher levels of vibration with more clearly

defined peaks.

The bearing was identified as an SKF 230/630

CAK-C4-W33 (double spherical roller). The bearing defect frequencies at this speed were

calculated:

• Inner-race defect frequency = 3. 05 Hz

• Outer-race defect frequency = 2. 55 Hz

• Rolling element defect frequency = 0. 2

Hz

• Cage rotational speed = 0. 9 Hz

• Rolling element rotational speed = 1. 08Hz

After applying this information to a further

study of the velocity spectrum, it was

concluded that the peaks belonged to either aninner-race or outer-race frequency, but it was

not possible to be more specific. (All bearingsare prone to a degree of “slippage” and

“sliding”).

Typically, the signal from a bearing defect is

attenuated as it travels through the machine

from its source to the sensor. It may bereduced by as much as 50% when it crosses

the interface between two surfaces. The signal

from an inner-race defect crosses moreinterfaces before the vibration transducer

mounted on the bearing housing senses it.

Therefore, an inner-race fault may be moreserious than the vibration levels suggest.

Using the Enveloping Techniquefor Further Fault Analysis.

Analysis of faults in rolling element bearingsinvolves the study of vibration generated by

impacts occurring between flawed rolling-

contact surfaces. Typically, this vibration iswithin the range of a good, general-purpose

industrial accelerometer.

The signal is passed through a band-pass filter

to separate the high frequency components


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from the low frequency machinery vibration.

The bursts of high frequency vibration fromthe bearing are very repetitive, due to the

bearing’s geometry and speed. The signal is

passed though a “peak detection circuit,”which emphasizes the repetitive components,and de-emphasizing random noise.

Applying this technique to the vibrations from

the MG cylinder resulted in the enveloped

acceleration spectrum shown in Figure 5. Asyou can see, the resulting spectral peaks

strongly suggest the problem is with the inner-

race. Studying the side bands around the fault

frequency peaks further supported this

diagnosis.

Figure 6 shows a zoomed view of the

enveloped spectrum. The spacing between thesidebands is 0. 2 Hz, which corresponds to the

rotational speed of the inner-race. This effect

in the FFT display is caused by themodulation of the inner-race defect signal asthe rotation of the raceway carries the defects

in and out of the bearing’s load zone.

Thus, use of the bearing enveloping technique

positively identified the defect as an inner-race problem – most probably a raceway

crack.

Figure 5. Enveloped Acceleration Spectrum.


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Figure 6. Spectral Sidebands.

Subsequent Action

The bearing was closely monitored on a daily

basis. The resulting trend data is given in

Figure 7. Levels continued to be higher than

previously recorded amplitudes, and variedwith machine speed. The highest recorded

level on the bearing was 1. 98 mm/sec RMS.

The offending bearing was removed during a

planned shutdown. Examination of the bearingrevealed at least two raceway cracks.

Raceways had a mirror-like surface with

discoloration. These can indicate adeterioration in lubricant film thickness,

possibly caused by the presence of water.

There were also shallow craters withcrystalline surfaces and gray / black streaks on

the raceways. Engineers came to the

conclusion that the failure stemmed mainly

from a lubrication problem, and that waterwas present in the bearing at some time.

SKF believes that the Iggesund condition

monitoring team picked up the second crack,

which was caused by the stresses imposed bythe original crack. Cracks in bearings are

generally seen as secondary damage caused by

primary defects such as wear and distress.

As was expected, there was an improvement

in readings taken after bearing replacement.Figure 8 shows the enveloped acceleration

spectrum taken from the new bearing.

The trend of subsequent velocity readings is

given in Figure 9. Velocity levels returned tothe previous low values, with some fluctuationfrom varying machine speed.


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Figure 7. Velocity Trend Prior to Shutdown.

Figure 8. Enveloped Acceleration Spectrum Following Repair.


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Figure 9. Velocity Trend After Repair.

Conclusions

Note that, even at the time of bearing

replacement, there were no other indications

of the developing problem. There was nodiscernable increase in the bearing’s operating

temperature, nor any noticeable difference in

machinery noise. The incident provides agraphic example of the improved maintenance

lead-time provided by an organized approach

to vibration monitoring.

Use of the enveloping technique to attain

accurate and specific fault diagnosis in this bearing demonstrates the possibilities of

employing standard vibration equipment to

study low-speed machinery faults.

People question the need for a specific

diagnostic system for use on rolling element bearings. In many situations, it is accepted that

the engineer only needs to know whether or

not the bearing is fit for further duty; thenature of the fault within the bearing becomes

irrelevant. Whether the fault is an inner / outer

ring, cage or balls makes little difference. Thisis particularly true when the bearing is part of

a small machine or assembly, and

maintenance consists of replacing that

assembly. However, even in such cases thedecision to take a plant off-line for the

purpose of carrying out that work can havesignificant financial and operational

implications. The ability to base such a

decision on very specific information providesthe engineer with more confidence when

making such recommendations.

References

Early Warning Fault Detection in Rolling

Element Bearings Using MicrologEnveloping, SKF Condition Monitoring Inc,

Application Note CM3021.

Monitoring of Slow Speed Bearings Using the

Microlog CMVA 60 ULS (Ultra Low speed),

by Dr Bob Jones, SKF Condition MonitoringInc, Application Note CM3052.

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