Vibration level increased after precision laser shaft alignment 1

[Orga78]

Hi,

We recently conducted vibration data collection on of our pump system (positive displacement pump), and the vibration reading at pump side was high, while motor side was within limit.

Coupling is flex elastomeric tyre coupling.

Alignment check was done by using laser precision method. The reading "before" was collected and showed out of tolerance for up/down. left/right was within limit of 0.09 mm (the pump's RPM is 750).

Laser shaft alignment was performed, and we managed to get the alignment tolerance within limit, +0.09 mm.

Pump was later run for testing, and vibration measurement was taken. However, the reading had showed reduced of vibration at pump side, but increased at motor side.

Is this common? Is there any other factor that causing the vibration to increase even after the correct shaft alignment was done?

 

[Tmoose]

Is vibration frequency and phase information available before and after the alignment ? What is the amplitude of 1X vibration in mils ( 0.001s inch) ? Those measurements will be of value compared to the dial indicator measurements I would make, described below.
What make and model of coupling?

"we managed to get the alignment tolerance within limit, +0.09 mm. "
It sounds like you had difficulty finalizing the alignment. Like maybe the equipment moves called for by the laser alignment confuser did not produce the expected results. Problems like that often are the result of inadequate base design stiffness and quality of installation or construction.

On top of that, for all their good qualities, molded elastomeric tire couplings are inherently "bent" and stiff enough to be capable of bossing and tossing around a flimsy base. When running, that shows up as a purely mechanical 1X rotation "cranking" motion, which = 1X vibration. Some styles of elastomeric coupling with clamping rings can be loosened to let the elastomer return to a more neutral position, and reduce the cranking.

I'd start with some mechanical tests on the coupled motor and pump while off line. Mount a dial indicator grounded to the equipment room floor, with the indicator stem bearing oriented vertically on the off-line motor housing near the output shaft. Then gently rotate the pump shaft (to avoid pushing on the motor directly). Observe and record the dial indicator reading, and mark the "high" spot on the motor side coupling. Reposition the dial indicator to measure the motor housing horizontally. Repeat the shaft rotation, and observe the indicator and record the result.
The motion of a well designed, well installed equipment during this static test ought to be essentially zero. Logically any 1X mechanical motion will likely show up as the same amount of 1X vibration (measured in mils, as mentioned above) when running. Or more, as is often the case, resonance is involved.

With the indicator still in place, measuring horizontally, push on the motor sideways by hand with 10 or 20 lbs force while observing the indicator.

If any of the readings are over 0.0005", I'd roll the pump with the dial indicator mounted to test for motion vertically each of the motor feet, the motor hold down bolts, base plate near the motor feet, and base plate where it bolted to the floor. I expect you will significant, measureable motion at some or all of those locations.

Whenever vibration up to about 20X is judged excessive measured on the equipment bearing housings, I recommend continuing with a detailed vibration survey on the driver and driven right down to, and including the equipment room floor.

 

[drawoh]

Orga78,

Could your coupling be out of balance somehow?

Is the torque into your positive displacement pump constant, or is it pulsed? Maybe you should try something other than an elastomeric coupling.

--
JHG

 

[CouplingGuru]

A very short distance between shaft ends makes alignment harder to achieve. So using cheap elastomeric couplings comes at a cost, in alignment difficulties and are prone to maintenance. With that being said, you have aligned the shafts or the coupling hubs? always align the shafts, sometimes that requires removal of the hubs which can be difficult when cheap short span couplings are installed. Also, check run outs on the hubs themselves, all these low cost couplings are bought overseas imported and re-bored. This can sometimes have a run-out of 0.030" that can be quite a lot of vibration. Also check your insert, you may have worn the insert and if you are re-using it, it will still cause imbalance to to shape irregularity. finally, check hub fit, you may have a real sloppy clearance fit. Most likely you have a little bit of everything which adds up to a problem.
Post what you find out. Good luck.

When it comes to couplings we are always here to help.

http://WWW.PSCCOUPLINGS.COM

 

[BrianE22]

This is not very likely but maybe with the initial misalignment there was a cancellation of forces and moments such that the motor didn't vibrate as much. Then you removed the force/moment caused by the misalignment and the motor now vibrates more because of some other problem.

Did you measure the vibration as a function of frequency? That might tell you something.

 

[Tmoose]

"while motor side was within limit."

What are those limits? Whose specifications?

Until you can complete the mechanical measurements in my first post, How about some picture of the installation, including some of the arrangement's tender nether regions ( motor and pump hold down bolts, base plate construction and grouting details, base plate hold down bolts ) .

thanks,

Dan T

 

[Orga78]

Hi All,

Thank you for the inputs so far. Been travelling to other site lately.

See following for the vibration reading of Motor NDE & DE, BEFORE and AFTER alignment was adjusted:

Motor NDE
unit: mm/s (RMS)
Component Before [After]
Axial 1.3 [5.25]
Tangential 1.3 [0.40]
Radial 0.7 [2.37]

Motor DE
Axial 1.25 [5.04]
Tangential 1.20 [0.27]
Radial 0.69 [1.88]

While for pump side, the vibration has reduced.

Pump DE
axial 6.7 [3.16]
tangential 1.4 [1.19]
radial 1.8 [1.16]

Important findings during alignment work:

1. The flex coupling screw of both sides (motor, pump) are very loose (can easily turn the screw by hand). We tightened all of them after alignment work completed.

2. High axial vibration of motor's NDE & DE have amplified after alignment & coupling screws tightening.


Question:

1. Is it possible the coupling "stiffness" could have caused the high axial vibration at motor side after the shaft alignment was done and corrected?

2. Before this, the coupling's screws were loose, and there was low vibration at motor side. Could this condition, allows the flex coupling to act as good "absorber" for the axial forces from the motor when rotating?

 

[Orga78]

Here are the photos of the motor-pump assy, and the flex coupling for your reference. Perhaps, will give you clearer picture of the issue.

 

[Orga78]

Photo of flex coupling

 

[drawoh]

...

Question:

1. Is it possible the coupling "stiffness" could have caused the high axial vibration at motor side after the shaft alignment was done and corrected?

That was my theory. You mention that your pump is positive displacement. Is the driving torque constant or is it pulsed (piston pump?). Sending force pulses through an elastic coupling at the right frequency should do all sorts of weird things.

--
JHG

 

[electricpete]

Check the frequency of vibrations for a clue.
[ul]
[li]If motor increase is a characteric frequency of the pump, then yes it’s being transmitted from the pump.[/li]
[li]More likely you have increase in 1xRunning Speed vibration, not as useful a clue since can still be a wide range of problems…[/li]
[/ul]
If 1x, possible causes might include:
[ol 1]
[li]Maybe coupling was not assembled correctly, leaving an unbalance on the motor hub.[/li]
[li]Maybe the coupling has been damaged. Those types of couplings can deform and tear.[/li]
[li]Maybe there in an intended vertical offset for vertical to running change, and it was erased when the machine was realigned.[/li]
[li]Maybe the alignment readings were misinterpreted[/li]
[/ol]

Looking at the picture of the machine bases shows some unfamiliar things to me:
The bottom of the nearest motor foot appears to have a machined gap under the center of it (inverted U shape). I haven’t seen that, usually that’s where the shims go. For that matter, I don’t see any shim tabs sticking out anywhere (where the shims at?). Maybe I’m not understanding what I’m looking at.

Those bases look tall and narrow, although maybe some structure in the center gives em some stiffness. I wonder what Tmoose would say about those bases.


=====================================
(2B)+(2B)' ?

 

[JJPellin]

I see split lock washers under the bolts heads. I don't care for that type of lock washer. Are these the coupling manufacturer's original bolts? I would suspect that the bolts were replaced. The bolts could be too long, bridging across the coupling gap and contacting the opposite hub. This could explain the high axial component in the vibration. Make sure that these are the correct bolts of the correct length. A coupling drawing would be nice.

I see full size, full length keys hanging out of the backs of the hubs. This is low speed, so it may not be a big problem. But, that extra key-stock is certainly throwing off the balance. These keys should be cut down flush to the shaft or shortened to fill half of the exposed shaft key-way.

Johnny Pellin

 

[Tmoose]

Hi Orga78,

You got dealt a real bad hand on this one.
Is the equipment still in warrantee? For my money the manufacturer has some pretty serious design deficiencies to correct, until PROVEN otherwise.
You can't balance and align away design problems like that.

That style coupling is very prone to being "bent" and causing the "cranking" I mentioned in my Mar 4 post.
Before doing anything else I would do the mechanical checks with a dial indicator I describe in that Mar 4 post as well.
And add measuring the points on the base plate indicated by red circles in the attached image.

EPete wondered what I have to say about the base design.
Before the results of the mechanical tests and a head-to-toe vibration survey are in, my opinion is the same as something I posted
on this board or another a while back. 0:22 to 0:27 is a nice summary.

https://www.youtube.com/watch?v=hl8H-rm6kt4.

Is that motor foot really welded to the (so-called, in name only) motor base ?
If so, who welded it, and why ?

Dang the towers made of U-channel steel directly under the motor feet are tall, and made of what looks to be shockingly thin material.
The crossmembers probably help some laterally, but bolting the motor feet to a surface that is only about 1/10 the motor foot thickness is just an awful idea. API and ASHRAE guidelines would likely condemn the base design just for the thin-ness. A minimum thickness of .5 inch/3 mm would be more like it.

Could you post some pictures looking at the end of the motor base towers, to show what, if any, reinforcement is in there ?

Then the towers are welded to to the middle of the pump base plate. Perhaps there is reinforcing steel running hither and yon inside, underneath.
With strategic location of ribs and stiffeners a stiff structure can be made of very thin material.
There must be a continuous load path from each motor foot to anchor bolts that secure the assembly to the foundation/floor.

https://www.aisc.org/globalassets/modern-steel/steelwise/steelwise-february-2013.pdf

https://abag.ca.gov/bayarea/eqmaps/fixit/ch2/img020.GIF

But, if what we see is what you got, that base from the motor feet down is simply way to flexible to be around rotating machinery.

Perhaps the OEM optimistically hopes that thorough filling with grout will transform that large flat section from a trampoline to a monolithic structure.
I see no obvious holes for grouting anyhow.

A detailed vibration survey of the points indicated by red circles in the attached image would also help make it clear at what points the support structure is simply inadequate. 1X vibration over about 0.01 ips peak would confirm my opinion this base is woefully inadequate.

 

[Strong]

I do not see any shim stock under the motor in the 1st photo. How can it be aligned? A practical rule of thumb for shaft key length is to add shaft keyway length and coupling hub keyway length and divide total by 2 to get length of key. Do this for both motor and pump shaft keys. I had an instance where vibrations increased after shaft alignment. It was because the Woods-type elastomer coupling element had taken a permanent distorted shape as a result of the significant misalignment. Vibrations were fine after replacing coupling element. I agree with Dan(Tmoose)that motor support is suspect for amplifying vibrations.

Walt

 

[Tmoose]

I'm having trouble editing my 15 March post to include the attachment. Maybe this will work.

thanks,

Dan T

 

[electricpete]

For my own benefit, I'd like to ask about these suggested tests, which seem potentially useful, but I haven't heard of them and I'd have a hard time justifying them in my work environment without some backup or better understanding.

I'd start with some mechanical tests on the coupled motor and pump while off line. Mount a dial indicator grounded to the equipment room floor, with the indicator stem bearing oriented vertically on the off-line motor housing near the output shaft. Then gently rotate the pump shaft (to avoid pushing on the motor directly). Observe and record the dial indicator reading, and mark the "high" spot on the motor side coupling. Reposition the dial indicator to measure the motor housing horizontally. Repeat the shaft rotation, and observe the indicator and record the result.
The motion of a well designed, well installed equipment during this static test ought to be essentially zero. Logically any 1X mechanical motion will likely show up as the same amount of 1X vibration (measured in mils, as mentioned above) when running. Or more, as is often the case, resonance is involved.

This is a check for misalignment (and perhaps a few more conditions that might create cranking)? It makes some sense but I haven't heard of it. Is there any reference that suggests this method? have you used it often?

With the indicator still in place, measuring horizontally, push on the motor sideways by hand with 10 or 20 lbs force while observing the indicator.

To check for weak support? Another logical but nonstandard test. Any reference? Would it be sufficient to measure running vibration on the base (as you suggested below) instead?


=====================================
(2B)+(2B)' ?

 

[Strong]

The OP did not state what the dominant frequency was before and after alignment. The dominant frequency could be the pump pulsation frequency and not from unbalance or shaft misalignment. More data is needed to avoid speculation and guessing. I did work on a similar looking pump that was on a membrane filtration skid, and pulsation was dominant! Lets see some vibration spectra!

Walt

 

[Tmoose]

Hi Epete,

The mechanical tests from my March 4 post did not knowingly come from any published source.
As I recall I started doing tests like that back in the last millenium when a few episodes of aligning and bearing-housing-only vibration measurements failed miserably with coupled pieces of non-standard equip (MIT test rig, and ION implantation generator, ) mounted independently ( no common base) on isolators.

One had a big old rubber tire coupling, for which there is NO hope not to be "bent".

The other used Thompson disk type couplings with a fancy FRP two foot long non-conducting spool piece between the motor and death ray generator. The customer's thoughtful technician pointed out if the initially tightened, way over capacity coupling disk pack bolts were not loosened to let the fat stack of disks neutralize after alignment, the disks could be seen to be "puckered" and they had NO problem cranking driver and driven around many MILS 1X when rotated by hand, and not surprisingly, about the same at 1800 rpm.

After those adventures, when a vintage Ferrari at J Geils' KTR European motorsports with Giubo rubber coupling at the transmission had "excessive vibration" it did not take very long to abandon trim balancing and get out the trusty 1" travel mag base dial indicator. It once again could be counted on to show immediately there was MECHANICAL problem when all the vibration measurements and a wall chart could do was this -

https://media1.tenor.com/images/e4958bce511d6608c646d0e0f7184de4/tenor.gif?itemid=5888148

The thoughtful techs spent some time with shims and their dial indicator, reduced the "bent" coupling induced cranking to some low number, and the problem was pretty much solved.

As for capturing detailed vibration measurements up and down the structure to determine weakness, I would fear It is possible that a WHOPPING amount of unbalance could be causing it. It can be real useful to figure out where the weakness lies.
Whereas, wherefore, when the indicator is setup, if the structure leans over measurably when I give it an old man push, there is trouble a brewing.
A properly done fancy analysis takes support structure stiffness into account. If the bearing pedestal moves over 0.010" in response to my 50 lb push, I'd say the stiffness is a mere 5000lbs/inch. A far cry from the 100s of 1000s of lbs/inch a good machinery base can provide.

best regards,

Dan T

 

[Eng-Ahmed]

Hi Orga78 , It's my first share here , as a designer i use this flex. coupling when no ability to make precision alignment as it can work well with 0.5: 0.9 mm Misalignment which is too large to use laser ,only straight edge and vernier are enough .. BUT the problem in this coupling is to fix well on the shaft (your photo doesn't show fixation on shaft)its inner is split and some types use a taper bush while others use 2 opposite bushes so if motor side bush is not coaxial with motor shaft then you get high axial vibration and the coupling will be destroyed after 1 month . please check that, happy day