relative shaft displacement vs bearing housing vibration
relative shaft displacement vs bearing housing vibration
(OP)
may anyone pls explain for me about the different between these methods for early fault detection in rolling element bearings.
what is the advantages of these methods?
thanks
what is the advantages of these methods?
thanks





RE: relative shaft displacement vs bearing housing vibration
Prox probe measurements are generally only performed on fluid-film bearings. In that case shaft vibration is not as easily transferred to the housing and prox probe may give more info about shaft movement within the bearing.
RE: relative shaft displacement vs bearing housing vibration
The outcome of the experiment showed that positive or negative spikes or both were detected on the plot depend on the fault location relative to probe tip. It seems that this method can be used to evaluate the bearing condition because spikes interval and its direction (positive or negative)indicated the specific bearing fault.
Can any explain for me why this method only used for journal bearing and not used to detect rolling element bearings fault.
RE: relative shaft displacement vs bearing housing vibration
In practicallity, I would say that it is a very expensive, less reliabile method than using an Accelerometer externally on the bearing cap. Not many end users are willing to drill their bearings to the outer ring due to associated cost and possible fatigue of the bearing. Field experience has shown that traditional vibration analysis and demodulation techniques are preferred on rolling element bearings.
As Pete mentioned above, Prox probes are valid sensors in a journal bearing application, but even then, an Accelerometer may be a sufficient cost effective solution - For a heavy rotor in a light casing like an electric motor, a case mounted accelerometer will provide sufficient information. For a light rotor in a heavy case, a case mounted accelerometer would be deficient.
Cheers,
Dave G.
www.reliabilitydirect.com
RE: relative shaft displacement vs bearing housing vibration
RE: relative shaft displacement vs bearing housing vibration
Regards,
MICJK
RE: relative shaft displacement vs bearing housing vibration
The outcome of the experiment proved that the measurement of the relative shaft displacement can indicated specific bearing faults at their early stage. My quention is whether this technique has been investigated by other researches and how is their outcome or conclusion?
RE: relative shaft displacement vs bearing housing vibration
I doubt you can improve on the sensitivity (as Mick mentioned frequency limitations would be wokring against you), but even if you can that really adds no benefit since as I mentioned those defects already are visible long before we call them bad.
What benefit is there assuming you could somehow improve sensitivity unless you can do it at reduced cost or complexity (which is doubtful). What type of application do you have in mind?
RE: relative shaft displacement vs bearing housing vibration
The measurement of relative shaft displacement is direct and not influence by other vibration sources. As the experimental data analyzed in time domain (timebase plot), only the displacement of the shaft due to bearing faults are identified. If there is other vibration sources, the direction (positive or negative) and interval of the spikes are different.
I also have conducted experiment to detect REB fault throught the bearing housing vibration with accelerometer. The data analyze with envelope analysis, which showed that the fault can be identified easily. But this method also easily influence by other vibration sources which lead to misleading the result.
So the relative shaft displacement method may be can be used with other fault detection methods together to identify the bearing fault.This is because the more information we have will provided us to do a good decision.
RE: relative shaft displacement vs bearing housing vibration
RE: relative shaft displacement vs bearing housing vibration
"I also have conducted experiment to detect REB fault throught the bearing housing vibration with accelerometer. The data analyze with envelope analysis, which showed that the fault can be identified easily. But this method also easily influence by other vibration sources which lead to misleading the result."
In a typical industrial machine condition monitoring program with route-based data collection, a high demod magnitude would prompt the analyst to look at the frequency content of the demod spectrum and/or regular spectrum. There will be a distinctive pattern of bearing fault frequency harmonics which certainly cannot be confused with any non-bearing fault. There may be some false alarms on demod magnitude from other non-bearing impacting (for example impacting at rotational speed or looseness). These can likewise be identified by their frequency signature, and they will not contain bearing fault frequency harmonics.
It sounds like interesting research. But as was mentioned, it seems like a cheap, easy, sensitive means for bearing fault detection is already available in housing accelerometer measurements.
RE: relative shaft displacement vs bearing housing vibration
If anyone has information about the relative shaft displacement and bearing housing vibration measurement advantages or their limitation, pls let me know. Thanks.
RE: relative shaft displacement vs bearing housing vibration
An accelerometer based on-line monitoring system does not have these difficulties. It is pretty easy to look at the data from an accelerometer and tell if the accelerometer is good, or visually inspect the externally mounted accelerometer and tell if it is properly mounted, etc. This examination is not so easy with a proximity probe.
For me this reason alone would be enough to steer me away from a shaft displacement based system for rolling element bearings, even if it were an on-line system. Don't mistake what I am saying as meaning that I think prox probes are never helpful. I think there are many good applications for them.
Skip Hartman
http://www.machinerywatch.com
RE: relative shaft displacement vs bearing housing vibration
If the shaft is damaged during operation, this will caused an abnormal reading to the prox. probe, which can indicated this damage as a problem in the other machine components but not the bearing.I think this will provide us the information about the machine condition, where a corrective action can be taken. A damage shaft may be caused unbalance to the machine system.
To detect the bearing fault in time domain, only the properties of the bearing and the shaft speed are need to determine the spikes interval and their direction (positive or negative).The prox. probe only measure directly the relative displacement of the shaft, which is caused by the faulty bearing.
RE: relative shaft displacement vs bearing housing vibration
Regards,
MICJK
RE: relative shaft displacement vs bearing housing vibration
These problems are not detected when the machine is restarted because most users do not go completely through a thorough calibration procedure designed to detect and correct all these problems. The reason they do not go through the procedures is because either they do not know about the procedures, or they decide it is not worth the time and expense to do so. Runout (electrical or mechanical) can cause a proximity probe to read either higher or lower than the actual shaft displacement due to phase cancelation or phase addition. In either case it defeats the purpose of the installation by causing the machine protection system to trip too early (nuisance) or too late (catastrophic failure).
The frequency limitations of the proximity probe have already been pointed out and are significant. I have seen high speed centrifugal compressors with speed increasers run to failure because of worn gears. The machine was equipped with a PROPERLY CALIBRATED prox. probe system that never registered an abnormal level because gear mesh frequency was a higher frequency than the highest frequency the probes could detect. We watched the G levels go up for a couple months and told the machine owner that the machine had to be shut down. Two days before failure we read 42 g's acceleration at gear mesh frequency (normal was about 4 g's) yet they said they had to take a chance with it and continue to run it as long as possible. The displacement probes never even shut the machine down before failure!
The point is, why try to force the use of these probes into an area where we already have adequate technology? Accelerometers are very successful at detecting rolling element bearing problems in most applications even without any fancy signal processing. Not only do they do a good job, but they do not have a lot of the inherent problems of proximity probe systems. For the more challenging applications enveloping technology seems to adequately enhance the raw signal to provide early warning of developing problems. As many have said these problems can be detected often before the damage is visible when the bearing is inspected.
Apparently there are some applications of rolling element bearings you are involved with where the use of rugged, lightweight, inexpensive accelerometers cannot provide the proper detection of bearing wear. Perhaps if you shared with us the specific rolling element bearing application you had in mind we would understand better the need to adapt a different technology. For myself (and I am sure many others), the detection of rolling element bearing problems has been one of the least challenging problems in vibration analysis, except for a couple machinery types that present some specific challenges that require a little additional caution when acquiring data and care when analyzing the data.
Skip Hartman
http://www.machinerywatch.com