I recently had a discussion with a very (many years experience) knowledgeable vib. tech. I myself have some experience in VA also.The questioned I asked was when taking a baseline reading on a pump(just installed) if it was mis-aligned or had softfoot,pipe strain etc. you wouldn't see it because of coupling and bearing support.But down the road you would see it because of rapid wear from the bearings. He said that the baseline would let him know if it was aligned or not. Can you really detect misalignment of let's say offset of .006 - .008 compared to .001 -.002????? I have found pumps of offset as much as .015 and when tested, the tech. said it was smooth as he ever had. Has anyone experimented in this??

That's an interesting topic. There was some previous discussion on a similar topic here:

http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/4571054772/p/1

There are also some older discussion on this from reliability-magazine.com now available on Oli's board.

I have also seen machines found to have quite a bit of misalignment that never showed on vibration.

As a first approximation, the forces from misalignment are static (don't change with time) and therefore don't cause vibration (but these static forces can still cause stress on bearings and other machine components). It is only secondary effects that might allow misalignment to show up as vibration. These may include stiffness that varies with angle (for example due to shaft keyway or due to 3-jaw coupling).

As many have said, the only way to know for sure if you have misalignment is to measure it (and account for OL2R).

I feel a lot of it would have to do with what type of coupling it is and what kind of condition is it in. Take for instance, if it is a grid or gear type and it is lubed properly with some misalignment, I don't think misalignment would show anything like an identical machine with the same misalignment next to it with a dry, locked up coupling. The lubed coupling would have the lube to let the grid or teeth "slide" as the shafts rotated. Now take for instance a shimpack type. I feel this would show misalignment easier since there will be the push and pull in the shafts no matter what due to the shimpack being "more ridgid. Just thinking out loud.

Good answers: brother Woods rubber third member vs Thomas - Thomas will show the 5 mils while Woods will tolerate 35 and show almost nothing.

I believe you would see the misalighment pretty readily though i don't know of any studies done on how much misalignment it would take to see in VA.

Realistically, it depends upon the system response. Different machines, mounted in different ways and with different couplings will show higher or lower vibration levels.

You definitely will not have to wait to see bearing degradation to see higher vibration.

I made a statement in error: tolerate 35 mils above.

You'll see the effect of misalignment with rubber grindings on the base from the third member but in the vibration spectra you'll basically see nothing as it'll almost have no affect on the 1X, 2X and/or 3X or 4X component/s. It has a huge damping factor whereas the Thomas with metal third members readily transfers tit-for-tat.

Depends on the size and type coupling and the system mass and stiffness. Sometimes you'd see it, usually you wouldn't.

I have been around Rusty and Sam enough to learn to pay attention to what they say.

On the other thread, a few other experienced folks also chimed in that they have seen serious misalignment which does not show up on vibration. Myself I have seen two machines found severely misaligned that did not show up on vibration. (One was a rod drive mg set with a gear coupling. The other was a larger vertical condensate pump/motor with a rigid coupling.) I have seen one misaligned machine that showed up unmistakably on vibration (a 3-jaw coupling with unmistakable 3x/6x/9x pattern). Several others I haven’t paid close enough attention to draw a conclusion. A small data set, but it fits in with what others say.

Here is an excerpt from “Shaft Alignment Handbook”, Third Edition by John Piotrowski, CRC Press 2007 ISBN 10: 1-57444-721-1

quote:

2.2.5 RELATIONSHIP BETWEEN VIBRATION AMPLITUDE AND MISALIGNMENT SEVERITY
If two pieces of rotating machinery are coupled together and misaligned by 5 mils=in. and
vibration readings are taken on the bearing caps and then the unit is shutdown and misaligned
to 10 mils=in. (double the initial amount), started back up and vibration readings taken on the
bearing caps again, the overall vibration amplitudes measured will not be twice the amount
compared to the first set of data collected at misalignment conditions of 5 mils=in.
Increasing misalignment may actually decrease vibration levels. Conversely, if a drive
train that has been running misaligned for a period of time was shutdown and realigned
more accurately, the vibration levelsmay increase after it is restarted. Many drive trains may
be slightly to moderately misaligned and vibration analysis will not be able to detect the
misalignment condition. Surprising facts, but true! To verify these statements, let us review
some experimental tests and actual field data taken on misaligned rotating machinery....

What follows after that in Piatrowski's excellent book is about 20 pages of spectra from various machines and coupling types as misalignment is varied. I haven’t fully digested it all those graphs, but I’m sure it supports the conclusions.

I think that most vibration folks are already very familiar with the fact that the vibration response to a given force depends on mass and stiffness... mostly in terms of how far you are from resonance. That variability or uncertainty applies to vibration from misalignment as well as to unbalance and other sources.

But the unique thing about misalignment IMO that may not be fully appreciated is that there will be large static forces that generate no vibration. If you are lucky with the right type of coupling or conditions, there will also be dynamic forces which result in vibration. Even in that case, we would likely underestimate the force if we have mentally “calibrated” ourselves to think in terms of the force/vibration relationship associated with unbalance (even though that itself may be somewhat unknown). In the thread linked above, there is a link to a paper by Gerald D’Ans which shows the static forces from shaft misalignment in general are often 10 times more than the dynamic forces from misalignment (he investigated the effects of coupling type, load torque level, speed etc which affected the ratio). So if you had 0.1 ips due to misalignment on a given machine, the associated total force (static plus dynamic) might be equivalent to what that machine would see if it were shaking at 1.0 ips from unbalance. And even though the forces are static in the sense that they are constant magnitude and direction when viewed from a stationary reference frame, they are rotating with respect to the rotor and will accumulate fatigue cycles on couplings, shafts. They will also contribute to loading/fatigue of rolling bearings.

I have attached a figure to try to demonstrate how shaft misalignment can create static forces (forces which do not change as the shaft rotates and therefore do not produce vibration). Again I think the most straightfoward and logical result of misalignment is a static force as shown. As was mentioned before, there are a number of mechanisms where misalignment can create dynamic forces as well. One is if the stiffness in a given direction (for example horizontal) changes as the rotor rotates... for example shaft keyway as it rotates past the H direction or as a couping bolt rotates past the H direction. I think there are some complicated rotodynamic explanations as well.

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

ShaftMisalignmentStaticForce1.ppt (38 Kb, 43 downloads)

quote:

I have seen one misaligned machine that showed up unmistakably on vibration (a 3-jaw coupling with unmistakable 3x/6x/9x pattern)

It's really hard to generalize about what misalignment spectra will look like because of all the variables, but the small 3-Jaw couplings used on a lot of lube and hydraulic pumps have spectra that look a lot alike. Following are some examples.

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

Here's another...

And a third...

Those are a little different than what we saw. With a Lovejoy 3-jaw coupling misaligned and the insert degraded, we saw harmonics of 1x with the higest harmonics at 3x/6x/9x (more details and spectra attached).

This makes sense to me based on the physical construction of the coupling where we expect the stiffness varies abruptly three times per revolution.

I’m sure as you point out each case is different. In our case the clear 3x/6x/9x left no doubt the vibration was associated with the coupling response. If it had been like your first or third plots... a series of harmonics peaking at 3x without the accompanying high 6x and 9x, the diagnosis might not have been as clear (that pattern could be looseness almost anywhere on the machine).

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

ThreeJawMisalignment.ppt (164 Kb, 22 downloads)

Thanks! I knew I could depend on alot of experience to explain this better. By the way the coupling was a geared grease type coupling.(Koplex)Again thanks!

It was only a few years ago that training and charts from the big name instrument suppliers emphatically stated things like "parallel misalignment causes vibration at frequency A, and angular misalignment causes vibration at frequency B."

Some big manufacturing companies and power plants in the NorthEast made some pretty significant maintenance decisions using those rules.

Fortunately (?) Now that more folks have tasted first hand the rich cornucopia of coupling and machinery types the rules have properly evolved to be much less rigid. Although it WAS comforting in a way to have the strict rules to follow.

That wonderful symptoms chart from Sohre showed "misalignment" effects as likely to appear at percentages varying from 5% to 60% of the time at frequencies from 0.4X to "very high frequencies."

quote:

Originally posted by electricpete:
In our case the clear 3x/6x/9x left no doubt the vibration was associated with the coupling response.

Besides 3x reduction, 1x has also been reduced after elimination of misalignment by a factor of ~2.5. Is it an indication (and why) that before alignment vibration at 1x was due to 3-jaw coupling stiffness variation at 1x?