Hello all!
Is there any particular reason that we would use one unit over the other for vibration readings, for example take velocity readings 0 to peak rather than peak to peak or RMS.What are the advantages /disadvantages if any?

It's a pretty long story, but the general concensus is to use velocity to diagnose low frequency, shaft related problems like imbalance and misalignment and acceleration for high frequency problems like bearings.

You need to get some basic training. Two good sources that don't require travel are the ilearn interactive cd and the Catagory 1 Correspondence course from Vibration Institute. www.vibinst.org

Sorry, but I don't have a link to ilearn, but it isn't hard to find.

Or, you could hire a consultant from Virginia to come to Trinidad and spend the rest of February and March training.

Danny,

I guess that you have never been to Trinidad.

Nope. Never been to Texas either.

It must be something about places that start with a 'T.'

quote:

Originally posted by vibehigh:
Hello all!
Is there any particular reason that we would use one unit over the other for vibration readings, for example take velocity readings 0 to peak rather than peak to peak or RMS.What are the advantages /disadvantages if any?

So, you want to know the difference between taking Peak, Peak-Peak or RMS (Root Mean Squared)?

This is what I know;
Peak= 1.414X RMS
Peak-Peak= 2X Peak
RMS= .707X Peak

RMS is utilized mainly in Europe, North America is mostly Peak.

Most instruments measure in RMS then convert to whatever you select in your collection set-up

I use Peak Velocity spectra to get see what the most relevant peaks are.

I use Peak-Peak acceleration spectra for an indication of the severity of the energy generated by the bearings due to lack of lube or impacting.

However, the time waveform is still my final deciding factor and that is raw data as long as it's in acceleration (using accelerometers)

I don't use RMS because it is .707 of peak so, what's the point in that??

I think an equipment manufacturer would prefer to use RMS as the values are lower so, when you check for vibration in the specifications make sure you match your data to theirs when you check it out.

So there you are in a nutshell.

There is a reason if you don´t look at the FFT´s but at the sum over a freq. range like the old ISO 10-1000Hz RMS value. RMS is simply explained as the area under the TWF curve so a small addition of some peakiness like a early impact of some kind does not add so much area and make a smaller addition to the value. A peak value is more sensitive if it happens to be a impact in sync with the hi value of such as unbalance it can add 10% to the value or more, next reading they may not sync and the value get lo. So RMS values give a steady trend but not so early increase/detection. Peak give earlier detection for changes but a jumpier trend. This also apply on similar methods for bearing values. x0.7 and x1.414 factors do only apply on simple sinus signals as they are when split into a FFT, not on the complex raw signal. Olov

Excuse me all.Peak and RMS no are only 0.707 factor. For any signals the measure in RMS mean the energy that this signal contain. Is more refered with severity in vibration.

Same considerations in sound systems: PMPO or RMS.

Ricardo Góz from Brazil

quote:

It must be something about places that start with a 'T.'

I guess that means we won't be seeing Danny in Tasmania, either.

quote:

I use Peak-Peak acceleration spectra for an indication of the severity of the energy generated by the bearings due to lack of lube or impacting.

I may be alone on this, but I do not use peak-peak when looking at acceleration values to assess bearing problems.... I use peak acceleration. I trend the maximum peak acceleration values, not the peak-peak values. My thinking on this, is that with bearing and lube problems we are primarily interested in the strength of the "impacts" which are not necessarily full-cycle in the waveform. This would be especially true when a bearing is in fairly poor condition with what I call "quasi-random" vibration.

Vibehigh, it is helpful to look at the 'sensitivity' of the various units relative to frequency. Acceleration is insensitive to low frequency events. That is, low frequency mechanical defects (imbalance, looseness, misalignment) produce very little acceleration. But acceleration is very sensitive to high frequency events (lubrication issues, gearmesh, early stage bearing defects). Displacement is very sensitive to low frequency events, but very insensitive to high frequency events. Many of us use displacement only when doing balancing work where all we are interested in is imbalance. Velocity on the other hand, has about the same sensitivity to low and high frequency events, and so is used to get an overall picture of a machine.

Basically, you should use all 3 units, but for different purposes. You might not need to use displacement, but many older folks have been "trained" by experience to discuss "mils of vibration" because 30 years ago the only machines that had vibration monitors where turbines and pumps that used proximity probes that measured vibration in "mils". Of course, for such machines "mils" is still the appropriate unit, but the vibration universe is far, far larger than it was then (or at least our measurement and diagnostic capabilities are).

This message has been edited. Last edited by: rustythevibeguy,

Thanks all !
Discussion here is ALWAYS very educational!

"Or, you could hire a consultant from Virginia to come to Trinidad and spend the rest of February and March training".

Danny If I could I would.. believe me

I am familiar with the frequency range limitations of the different measurements....
In the CSI software under setup - globals - engineering units... you would see velocity readings can be taken in either 0 to peak, peak to peak or RMS, my question is referring to this, why look at velocity readings in.... for example peak to peak.

Ian,

Bill made that rule up. I was born in Tennessee. I have some friends in New Zeland, so I might show up in Tasmania after all.

Danny,

Let me know if you do

Ian

I never studied Tenn. history intensely. Didn't North Carolina get tired of having (part of) it in the late 1700's. Then there was the state of Franklin. So, maybe Tennessee comes by the 'T' with some difficulty

Vibration expressed in peak unit is a vector and has an associated phase whereas RMS is a scalar and has no phase.

An approximate RMS value can be obtained in the spectrum by summing the squares of the RMS component fllowed by taking the square root.

quote:

Vibration expressed in peak unit is a vector and has an associated phase whereas RMS is a scalar and has no phase.

This comment is off base. RMS data can have a phase, if a trigger is available or cross-channel analysis is performed.

Linear vibration values at a specific frequency measured using a fast fourier transform (fft) are always based on rms values because of the way the transform is calculated and the ability to derive phase is an integral part of the transform.

Thirty-eight years ago (is this long ago enough to define "traditionally"?) when I first started working in vibration, displacement was almost always defined in peak to peak units, the distance that the measurement point moved, and in the USA, these units were most commonly in mils, inches/1000. The primary measurement device used to make the measurement was an oscilloscope driven by some transducer, perhaps followed by a signal conditioning device. Fft devices were not in common use in industry because digital instrumentation was extremely rare and the fft was only recently published in 1965. Some rms voltage measuring hardware existed at that time but it was relatively expensive and had significant practical limitations.

The typical vibration analyzer of the 1960s typically used a half wave rectifier as part of a somewhat averaging peak detector detector circuit; in the 1970s, full wave rectifiers as part of the detectors appeared. To get peak to peak displacement, peak displacement values were doubled. Oscilloscopes were still the primary measurement devices. Unless the measured waveform was a sine wave, the primary way to measure velocity and acceleration was to measure it's peak value either using an oscilloscope or a peak detecting meter circuit with an oscilloscope considered to be the more accurate method. Hence, the most commonly used units were in peak to peak displacement, peak velocity, and peak acceleration because that's what was most practical with the measurement instrumentation of the day.

This message has been edited. Last edited by: Duncan Carter,

Ah; Tennessee -- warms the cockels of your heart.

State of Franklin! Succeeded from England in ~1772 (4 yrs prior to US) under the Watauga Assc. Formed a government and at the battle of Kings mtn and atop the mtn you find their head duck redcoat officer burried what got whupped! I think the grave was clearly marked so when you had to go you'd know where to go. Or, I could be wrong.

Since europe has been cursed or blessed by ISO standard influence we have been using RMS as long as it was technically possible to create by electronics but also here in late 40´s and 50´s peak amplitude was dominating, for the same reasons as Duncan put forward, since we like small numbers, we normally stopped at 0-peak :-) (or lazy?). Later velocity was more favoured. I know we have made true RMS vibrometers from mid 70´s with various electronic miracle´s to comply with ISO. So that may be a reason to differences btw. the sides of the lake. Why did ISO select to define it that way? I point back to my previous ranting. Or was it just convinient to adapt to a old German DIN standard? So why did they select that? Maybe not be more technical than that the instrument they had measured like that or it was about as complex the instruments was able to do things at that time if you had a fancy one? I would not be surprised, still I like velocity RMS so you use what you are used to and build your references to that and it´s not easy to change. Olov

ISO uses pk-pk also. Anyone like Smax?