Help identifying the source of vibration 1

[kwisatz_haderach]

Where i work, there are two pumps(one standby)(30 kw,2955 rpm) that circulates thermic fluid. Our fitters observed heavy noise and vibration from one of them. After checking they found that the bearing balls were damaged. They did a trial check on empty motor and found it was also vibrating.(They didn't measure it. Just based on experience). So they replaced the motor, bearing housing and seal with new ones. After aligning it i checked the motor and pump for vibration and it was found that the motor has vibration of about 7.51 mm/s rms at its front feet,bearing housing has about 3, while the standby motor(Undamaged one) has about 2 mm/s at motor feet. Initially i thought it was misaligned. After i tested with decoupling the motor, i found the vibration reduced to about 4.5mm/s at motor feet(still very high compared to standby motor) and about 1 at base frame attached to ground. So i concluded the problem should be with the motor, but the electrical department is saying that it is new motor and there is no problem with it. They're saying the base was weak, but it worked fine up until that point. What do you think is the problem? Has anyone experience this before? Please share your insights. This is my first job and i only have a basic understanding of vibration analysis. Please point me in the right direction.
Thanks in advance.

 

[Maslov]

Steve Mould did an interesting video on motion amplification. You can run footage through an algorithm to reveal subtle motion. It might be worth a shot.

https://www.youtube.com/watch?v=rEoc0YoALt0

 

[Stick.]

When the motor was installed and aligned, was soft foot checked for and eliminated if found?

 

[LittleInch]

What has the electrical dept got to do with this?

It is probably fine electrically, but its the mechanical aspects which are the issue here. the motor clearly isn't "new" if you've had to replace a bearing

you need to test the motor for static movement and compare to the manufacturers specification for run out, any slack or shaft movement under force.

Is the frame of the motor all there / not cracked or broken?

you probably need the vendor to see what is the issue, but at 30kW a proper new one might be needed.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[kwisatz_haderach]

Little Inch, we fixed the issue by tightening the alignment bolts horizontally. Now the vibration at motor feet reduced to about 2.5 mm/s. But what i don't understand is what is causing the vibration here? Like if i have a new motor shouldn't it be vibrating less whether it is fixed in place or not?

 

[IRstuff]

While even a new motor might have some vibration, yours is no longer new, since it's been repaired, and not by the manufacturer
> They replaced the bearing housing and bearings, so there could be misalignments, wrong bearings, poor installation, etc.
> You say "but it[the base] worked fine up until that point." but it's possible the base was damaged or weakened
> There might be something else, like the motor was out of balance, somehow.

The bottom line is that there was supposedly vibration bad enough to compel a bearing replacement and there's vibration after. So, a big question is whether the vibration actually improved, or not. If not, then there's something else involved, like the base.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[spciesla]

I interpret "we fixed the issue by tightening the alignment bolts horizontally. Now the vibration at motor feet reduced to about 2.5 mm/s." as using the jacking bolts to restrain the horizontal motion of the motor. Jacking bolts are used to move the component (motor in this case) while it is being aligned. Once aligned, the hold down bolts are tightened and the jacking bolts are backed off. Engaging the jacking bolts is not solving the problem, it is just providing extra stiffness to decrease the vibration. However, the underlying force(s) causing the vibration are still there.

Vibration is a symptom, resulting from the interaction of the forces acting on the machine and it's stiffness and damping characteristics. First you need to verify that the alignment is good and then you need to collect(or have someone competent collect) vibration measurements on this motor and pump. For this size of motor with rolling element bearings, you should be looking at velocity measurements in three orthogonal directions (horizontal, vertical, and axial) if possible on the bearing housings. It can be difficult/impossible to get a good axial reading on the motor outboard bearing. You need to understand the frequency content of the vibration signals, is it running speed (1X), harmonics of running speed (2X, 3X, etc.), a bearing defect frequency (BPFO, BPFI, FTF, BSF), 2xLF (Line Frequency)?

Good luck.

 

[NotecA]

You mentioned that the vibration on the motor mounts is higher than on the bearing housing. The vibration on the motor casing is almost within tolerance (2.8 mm/sec), but it's over 7 on the mount.
In which direction did you measure the vibration ? Vertical, horizontal or axial ?

The first suspicion would be poor tightening of bolt connections, both of the motor to the frame and the frame to the foundation.

The second thought would be to inspect the motor mount and frame for any cracks.

Next, I would take a large crowbar and shake the motor rotor. New doesn't mean good; there can be play. Turn the motor rotor, does it rotate smoothly?

"How can a new motor vibrate, whether it's fixed in place or not?"
An unfixed or poorly fixed motor can fall into resonance, and even the smallest vibration can be greatly amplified.

It's quite likely that tightening the motor alignment bolts added rigidity to the structure, and the resonance shifted to a higher frequency.

If there's a variable frequency drive, reduce the motor's speed by 100-300 RPM and see how the vibration level changes, or you could turn off the motor and measure the vibration level as it coasts down.

If there's a significant change in vibration level with a slight change in speed, it's definitely resonance.

Why one motor has this problem and another, exactly the same, does not - it's necessary to investigate why there's a difference in the rigidity of the construction. Cracks, a broken foundation, damaged vibration isolators...


[link vibromera.eu]Portable balancer & vibration analyzer Balanset-1a[/url]

 

[kwisatz_haderach]

IR stuff, we never did any repair on motor/motor bearing, we found that the bearing balls were damaged in pump side bearing housing, so we replaced the entire bearing housing, seal and installed a new motor after finding the old motor also vibrating more.

 

[kwisatz_haderach]

NotecA, Thanks for your response.
I didn't measure vibration on motor casing, that 3 mm/s vibration is at pump side bearing housing, after replacing the old motor with new one.
I measured in all three directions, but horizontal one was higher at pump feet.

We tightened the hold down bolts and checked but it remained same(in fact, it increased a little bit) and only resolved after we tightened the jacking bolts and frame to foundation bolts.

I checked both base frames and found that the low vibration one got a thick beam below the motor rear feet, and also its feet are bolted to a channel and the channel is then welded to the base frame, while the high vibration one got a rectangular frame(with no beam in the middle like the first one) and the motor feet sits on a plate which is then welded to the frame.

I'll once check for resonance and the motor for play, but we're using this pump and motor setup for so many years. If it's resonance, why show it now?

 

[electricpete]

There's a lot going on here. I vote spciesla a little purple start for hitting some important points:

1 - WHAT IS THE FREQUENCY OF THE VIBRATION?

2 - yes tightening jacking bolts is not an accepted approach for resolving vibration.

You have a number of things that might be pointing towards resonance (vibration at the feet higher than the housing, reducing vibration by tightening jacking bolts).

I'll once check for resonance and the motor for play, but we're using this pump and motor setup for so many years. If it's resonance, why show it now?

Well, it could be the supporting structure has degraded. Maybe a weld cracked or some other hardware loosened. It could also be you have higher excitation now than before. There's lot of unknowns, we have no way of knowing. I'd suggest not to loet the unknowns get in the way of investigating what needs to be investigated.