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Hi All,

I'm new to the forums and this is my first post. I have quite a fundamental question about vibration. I have read many descriptions about the various measurement units, Velocity, Acceleration, Displacement. I understand the phase difference and how they are related. Everything I read states that Velocity is the best indicator of the destructive force of the vibration and acceleration is an indication of bearing or gear mesh fault. However, I can’t find a good explanation of why this is the case, in my mind any high amplitude vibration would be destructive?

Thanks.

Tags: acceleration, vibration, Velocity, destructive

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James

You can't analyze what you can't see.  Let's say you take data from a machine supported with rolling element bearings using an accelerometer. Almost all analyzers allow you to change the Y-axis so that you can display the spectrum in acceleration, velocity and displacement units.  If you have the beginnings of a bearing fault you "may" be able to detect it with acceleration units; but maybe not with velocity or displacement units. The integration process (i.e. accel to vel and again, vel to disp) will drive the small (e.g. 0.04 g's) "but important" details of this problem into the noise floor.  By the time you can detect the problem in displacement units, the damage may be severe enough that you've lost valuable time to take early corrective action.

I recall a Nelson Baxter paper where he uses actual data from a gearbox as an example. I will try to find it and reference it here. But you can do this yourself by examining your own data to see how the Y-axis amplitudes are greatly affected

Hope this helps answer your question

Regards

Jim P 

RM
Last edited by Registered Member

Quote: "The data and the machine will let you know what is best."

Absolutely!

and for JamesK I should clarify in my 1st post I was simply trying to give you an example of how a signal of interest (bearing fault) may not reveal itself by not selecting the ideal units "for that particular case".  

FWIW - When I first review data (in my case, typ. seismic accels on bearing caps) for analysis, I default to velocity terms as explained by fburgos. HOWEVER, depending on what I find or suspect, I may change the units to assist in revealing the clues that the machine is trying to provide me.

Good luck...and keep reading on this topic

Jim P

RM
Last edited by Registered Member

Thanks for the responses, I think it is becoming clearer. It may be that I am trying to make a direct correlation between the theory of vibration and translating it to my plant and getting a bit lost. We have a range of machinery at our plant that depending on criticality is monitored differently. We have fixed Bently Nevada System 1 Evo using displacement transducers on the shaft of our high-speed compressor. We also have an outside contractor who takes vibration readings with a portable monitor monthly and gives a report on the slightly less critical machines. And we have a portable instrument in house which give reading in mm/s and g(Enveloping) I can’t get a spectrum from this instrument, just those two values.

It is this hand held unit that I am trying to understand better because I can’t see all of the individual frequencies.

Does the following seem plausible?

It is because the velocity will remain constant over the 0-1000Hz range that it gives the overall vibration in the normal run speed and multiples of run speed of the majority of machines? And gE filters out all of those (0-1000Hz) frequencies so that you can look specifically at a fault that will be generated by a rolling element bearing. That fault may not be severe enough to cause the overall vibration velocity to rise but chances are that it will eventually. At which point you will see both high velocity and acceleration.

But it does make me think of another question, what happens to Velocity readings above 1000Hz?

RM

Don't make it too simple.  

Displacement is closer to strain.  Thus, displacement is fatigue.  Believe this?

Since the material is a spring, K, displacement times K is force.  Therefore displacement is force.  Believe this?

Energy is 1/2 K disp^2.  Therefore displacement represents energy.  
Things are not this simple.

Deflection shape enters into all this.  

RM
@Registered Member posted:

Don't make it too simple.  

Bill's statement should be well taken but I'd like to add one thing and that is that the user should not forget that these devices at their very basic level measure movement of a structure (I'm excluding non-contacting probes at present).

So if we have a gearbox with mesh frequency of 3 kHz and a rather high mesh component of 20 g's peak, the movement of the casing at that frequency is 0.043 mils p/p (0.001 mm p/p), a very small amount of movement.

Using that same example, the velocity would be 0.43 in/sec pk (10.922 mm/sec pk).  Can you use a device that natively measures velocity?  Maybe; the older moving coil velocity transducers had a upper limit on frequency response of about 1 kHz.  More recent piezo based velocity sensors can got out to 5 kHz.  However, it may pay to look at acceptance criteria or guidelines for the machine.  API 613 for Special Purpose gearboxes specs velocity over the range of 10 Hz to 2500 Hz and acceleration from 2.5 kHz to 10 kHz.

So, know your machine, know your acceptance criteria and know your transducer (which is why you started this thread ).

RM
@Registered Member posted:

in my mind any high amplitude vibration would be destructive?

In general and theory, that is true. The practical question associated with your statement that practitioners need to answer is that "is this particular amplitude (vibration) destructive to this machine?" The primary factor in determining the severity of the vibration is amplitude.

Different machines have different sensitivities to vibration. Some machines break faster than others, if all are exposed to the same vibration. Keep in mind that the same physical amount of vibration can be expressed in displacement, velocity and acceleration but all these are only mathematical conversions for the same real-world movement.

Because most vibration analysts use existing standards for condition evaluation, they tend to have their measurements on the same units of these standards so they can make a comparison between the measured value and the limits. Many standards are offered in velocity, so many users use that. They could use acceleration with limits expressed in acceleration (the results would be very similar to those obtained using velocity).

Some analysts insist to take more than one measurement for the same point on the machine in order to use them as velocity and acceleration, for example.

 

Regards- Ali M. Al-Shurafa 

RM

Dear Mr JAMESK,

Wellcome to this forum.you have plenty of explanation for your question and how you take it all.?

when you collect vibration data using any vibration data collector you will get many unwanted noise along with useful data what is required for fault analysis.The problem is that the identifying signal (useful data) is usually drowned out in other noise emanating from the machine.

Hope you know about FFT spectrum analysis,where in rotating machine faults are identified for example shaft unbalance,shaft misalignment are best detected using velocity spectrum where as bearing defect is best detected in acceleration spectrum (acceleration enveloping) It works by progressively filtering out unwanted parts of the vibration spectrum until the exact signal of the bearing defect can clearly be identified, thereby separating low-level, repetitive vibrations from the noise around it.

""Everything I read states that Velocity is the best indicator of the destructive force of the vibration and acceleration is an indication of bearing or gear mesh fault.But it does make me think of another question, what happens to Velocity readings above 1000Hz?""

so, you can still use velocity spectrum to detect bearing defect but it will be a long way to reach the destination and you are most likely to end up with problems rather than solution to problem
RM

It's misfortounate the poor guy got faced with all the turbo machine PhD type guys in looking for an answer.

the instrument you have sounds like either a skf quick collect or a Xmas machine advisor Vibe pen, both instruments mainly just tar an overall reading of velocity and enveloped acceleration, you regally need an instrument with a fft display to be able to put in context what you are looking at. The skf quick collect sensor if you have it has a facility to generate a very simplistic fft, drop me a mail on here if you want plant with that.

RM

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