Troubleshooter’s Guide to Vibration (April 01, 2007)

There are several common vibration frequencies, and we’ll begin to deal with them in Part 2 of the Troubleshooter’s Guide to Vibration (Part 1 appeared in MRO’s Feb. 2007 issue). These include (1) rotor frequencies, (2) rolling element bearing fault frequencies and (3) sleeve bearing fault frequencies.

Future issues will cover frequencies involving (4) gear drive problems, (5) belt drives, (6) fan blades and pump impellers, (7) electrical equipment, (8) chain pass, and finally, there will be a section on (9) resonant frequencies.

1. Rotor Frequencies: The primary causes of vibration at what is referred to as rotor frequency are unbalance, such as when a weight shifts or material has unevenly collected on a blade or impeller; and misalignment, such as what might occur with a worn coupling or cocked bearing, bent or bowed shaft, or an eccentric rotor.

Any or all of these conditions can cause a 1 X shaft speed vibration or a harmonic of that frequency.

These conditions cause approximately 75% of all vibrations in industrial plant rotating equipment. The following illustrations provide examples of these conditions (see figures 3, 4, 5 and 6).

2. Rolling Element Bearing Fault Frequencies: Anti-friction bearings inherently have low starting friction but high running friction. To ensure relatively long life, grease-lubricated bearings should have a shaft frequency (in rpm) that does not exceed 7,200 divided by the shaft diameter in inches. The shaft frequency for oil-lubricated bearings should not exceed 9,600 divided by the shaft diameter.

For example, a 6-in.-diameter shaft supported by grease lubricated anti-friction bearings should not be run at a frequency higher than 1,200 rpm (or 20 Hz).

Rolling element fault frequencies are caused by fatigue or running wear, incorrect or insufficient lubrication, misalignment, or manufacturing flaws within the bearing components themselves. There are four fundamental frequencies in anti-friction bearings. They are:

* Fundamental train (cage) frequency (FTF)

* Ball pass frequency of the inner race (BPFI)

* Ball pass frequency of the outer race (BPFO)

* Ball spin frequency (BSF).

These defect frequencies depend upon the shaft speed and bearing geometry. If the type of bearings used in the machine are known and recorded in maintenance files, manufacturer’s data can be obtained to provide the roller or ball diameter, pitch diameter, number of rolling elements and the contact angle (see figure 7).

Knowing these values is essential to accurately determine the bearing fundamental frequencies. If the bearing frequencies are not known, or cannot be obtained from the bearing manufacturer, the following calculations can be used to obtain the approximate frequencies of the bearing’s inner and outer raceways. (Note: On double row bearings, it is only necessary to count the number of balls or rollers on one side to calculate bearing frequency).

Inner Race Frequency: # of rolling elements (balls or rollers) X rpm (of the shaft) X 65%.

Outer Race Frequency: # of rolling elements (balls or rollers) X rpm (of the shaft) X 45%.

It is also important to be aware of which race or bearing ring is stationary and which rotates. Most industrial applications use bearings in which the outer race is held stationary and the inner race rotates. Outer race fault frequencies are usually the most common, since the outer race is always under load.

Rolling element bearing failures generally occur in four stages. Tiny fatigue cracks and initial bearing spalling appear in ultrasonic frequencies ranging from about 250,000-350,000 Hz. This is precisely why ultrasonic testing is recommended as a condition monitoring technique and will frequently indicate Stage One of a bearing failure before vibration analysis may detect a problem.

A Stage Two bearing defect is usually accompanied by component natural frequency ‘ringing’. These frequencies may be bearing support resonant frequencies and side bands begin to appear on the spectrum.

At Stage Three, bearing defect frequencies and harmonics appear. As wear or fatigue spalls progress, more defect frequency harmonics appear and the number of side bands increases. The bearing should be replaced immediately when these conditions appear (see Figure 8).

Frequently, as a bearing reaches catastrophic failure, Stage Four, vibration amplitude at 1 X rpm increases, while bearing defect and component natural frequencies actually begin to disappear. These frequencies are replaced by a random, high-frequency ‘noise floor.’ The wise analyst will not wait this long to recommend bearing replacement.

When determining bearing fault severity, it is suggested that the spectral data be viewed in velocity and the wave form in acceleration. Depending upon the severity of the bearing fault, it is possible to see 20-40 g swings in amplitude. This indicates that the bearing is in its final stages of catastrophic failure.

The bearing fault frequencies that appear in the spectrum may appear with harmonics and/or side bands around the frequencies.

Harmonics are integer multiples of a given frequency and may indicate bearing looseness, which could be caused by excessive internal clearances, loose bearing mounts, a bearing loose in its housing, a bearing slipping on its shaft, or a cocked or misaligned bearing. Each harmonic indicates either a new defect or enlarged wear around a defect site on the same bearing.

Cocked or misaligned bearings can be determined by taking phase readings in the axial position on all four compass positions of the bearing. These conditions will show readings from one side to the other that are 180 out of phase.

Side bands appear in vibration spectra as the result of rotating modulation of amplitude or phase motion of the component being tested. In the case of bearings, any looseness caused by wear or related conditions will show up as pulses at multiples of rotational speed.

Side bands often appear around the bearing’s inner race frequency and always indicate a problem, although it is not always related to the bearing. Possible problems may be related to a loose coupling, gear problems (in gear drives), electrical problems (in motors), or to the severity of a potential bearing problem. Since the coupling is the easiest component to inspect, it is suggested that the problem investigation begin with the coupling.

As the bearing failure progression increases, the spectrum may show a non-synchronous peak in the high-frequency range as high as the number of balls or rollers X the race frequency, which may be a harmonic of that frequency.

As failure progresses further, the spikes may actually become shorter and a broadband of energy may appear, indicating that the bearing geometry has changed.

In addition, the appearance of multiple defect frequencies with side bands indicates that the bearing may only have 2-3% of its life remaining and a catastrophic failure is imminent.

Keep in mind that if shaft speed is 300 rpm or less, a bearing may last several months after a defect is detected. On the other hand, when shaft speeds are in the range of 3,000 rpm or higher, a damaged bearing may fail very quickly, so an immediate speed reduction may allow the scheduling of a repair without too much interference with production.

With regard to excessive bearing load and its potential for premature failure, consider the problem of a one-ounce unbalance on the blade of a fan, which is in turn supported by a bearing that is designed for an L10 life span of 60,000 hours. If the unbalance is not corrected, this bearing’s life expectancy will be reduced by approximately 42% or 25,000 hours.

Therefore, when a spectral analysis indicates a bearing defect, reduce the load immediately to extend bearing life until repairs can be scheduled.

One final recommendation with regard to recognizing bearing de
fects is to keep in mind that it really doesn’t matter if a mistake is made in recognizing the various bearing fault frequencies, since the entire bearing will be replaced anyway.

The key to recognizing a bearing fault is to learn to recognize spectral patterns. Once any bearing frequency is recognized, steps can be taken to replace the bearing and then analyze the cause of the failure if it was premature.

3. Sleeve Bearing Fault Frequencies: Journal or sleeve bearings have high starting friction and low running friction. At low rpm, the friction is high due to boundary lubrication conditions. The friction decreases as the shaft moves into the rotating position where there is a full film of lubricant between the shaft and the bearing’s inner surface.

Journal bearing clearance should not be less than 2 mils and should be at least 1 mil to 1.5 mils for every inch of shaft diameter. This clearance is important as it affects system vibration, because damping increases as clearance decreases.

A vibration problem associated with plain sleeve or journal bearings is the possibility of hydraulic instability of the rotating shaft inside the bearing. This vibration is caused by oil whirl or oil whip. Oil whirl is caused when a wedge of lubricant is forced to rotate as the shaft rotates. This uneven oil film moves the shaft in an eccentric motion as the shaft rotates, which if severe enough will cause a vibration.

The frequency will appear somewhere between 35-49% of the shaft’s rotational frequency due primarily to the internal friction of the lubricant (see Figure 9).

Solutions to oil whirl and whip include reducing the load, reducing bearing clearance, reducing speed, reducing lubricant viscosity, increasing operating temperature (which will reduce lubricant viscosity), changing the bearing design or applying the use of tilting pad bearings, if applicable.

Technical editor Lloyd (Tex) Leugner is the principal of Maintenance Technology International Inc. of Cochrane, Alta., a company that specializes in the resolution of maintenance problems and provides training for industry. He can be reached at 403-932-7620 or texleug@shaw.ca.

Part 3 of this report will be published in the June 2007 issue, and will continue the discussion of common vibration frequencies and how to deal with them.

References

The Simplified Handbook of Vibration Analysis, Arthur R. Crawford.

Rotating Machinery Vibration, Maurice L. Adams, Jr.

Introduction To Machinery Analysis and Troubleshooting, 2nd Edition, John S. Mitchell.

Vibration Spectrum Analysis, 2nd Edition, Steve Goldman, P.E.