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Vibe-Rater

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Reply with quote  #16 
Hi Nok, your question was to Mark but if I can chime in. Yes there would be a beat frequency. Diff b/n 2x and 100 Hz. rgds
Barry

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Reply with quote  #17 
Big Al posted ( I think that they had aligned it with the bearing sitting at the bottom of the worn housing, but when the motor was run, the shaft lifted to its magnetic centre and misaligned the coupling. )
I do not think the roror will lift and rotate on a magnetic center. If this theory was correct a motor with sleeve bearing would never have bearing problems. In years past motor repair shops would remove the rotor and energize the windings and toss a steel ball in and if the windings were ok (?) the ball would rotate around the iron. Their thoughts were if there was a winding problem the ball would stop. I have also seen photos of large motors that did not have bearing top haves.

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Barry

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Reply with quote  #18 
My understanding of the OP that he was working with a squirrel cage induction motor.
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RustyCas

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Reply with quote  #19 
A 3-lug coupling will produce a dominant 3X vibration if significantly misaligned. In general, misalignment must be pretty severe to be diagnosed with vibration data. Essentially, “misalignment” has been way oversold by vibration and alignment equipment vendors. With any “flexible” element coupling, be sure the hubs are not jammed up against the hubs axially. The element will distort somewhat when loaded, so there needs to be a little axial clearance, to be prevent an axial load from being produced.

Any sleeve bearing rotor is going to “lift”:by the thickness of the oil film, but that’s only going to be .0005 - .0015” so insignificant in terms of alignment, IMO. Something that is often overlooked, is the integrity of the fasteners (weak base, crappy washers, bolts too small, shims too small, mismatched shim sizes, bolts stretched, threads worn or corroded, etc). This becomes more significant as machine size increases.

My general rule for flexible element couplings is, if you can’t see visible signs of element distress (coupling “dust” or particles, worn element, excessive twist, etc), then there’s no significant misalignment.

Misalignment is oversold and over-diagnosed, and will leave egg on your face if you’re not careful.

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Walt Strong

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Reply with quote  #20 
I aligned a small pump with laser system, and the vibration went up quite a bit. I opened the coupling and found "Woods" element to be severely distorted from previous misalignment. Replaced the insert, and vibration was good. Moral of story; always inspect coupling and repair/replace before shaft alignment!

Walt
David Eason

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Reply with quote  #21 
Hi guys,
I'll make my first post on this board by saying I agree with Rusty's post, maybe that'll get me enough points to start me out on the right foot.

I noticed excessive looseness mentioned a couple times, what about a lift check? Wouldn't that give a clue?

David Eason
MarkL

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Reply with quote  #22 
Quote:
Originally Posted by RustyCas
A 3-lug coupling will produce a dominant 3X vibration if significantly misaligned. In general, misalignment must be pretty severe to be diagnosed with vibration data. Essentially, “misalignment” has been way oversold by vibration and alignment equipment vendors. With any “flexible” element coupling, be sure the hubs are not jammed up against the hubs axially. The element will distort somewhat when loaded, so there needs to be a little axial clearance, to be prevent an axial load from being produced.

Any sleeve bearing rotor is going to “lift”:by the thickness of the oil film, but that’s only going to be .0005 - .0015” so insignificant in terms of alignment, IMO. Something that is often overlooked, is the integrity of the fasteners (weak base, crappy washers, bolts too small, shims too small, mismatched shim sizes, bolts stretched, threads worn or corroded, etc). This becomes more significant as machine size increases.

My general rule for flexible element couplings is, if you can’t see visible signs of element distress (coupling “dust” or particles, worn element, excessive twist, etc), then there’s no significant misalignment.

Misalignment is oversold and over-diagnosed, and will leave egg on your face if you’re not careful.


Rusty thanks for that, from what you say Imwould whole heartedly agree, especially the misalignment needing to be real bad to present in such a way.

On inspection there were no obvious axial issues, the space between the hubs was ok, no signs of rubber debris from the coupling.
The insert is fairly new as the coupling element was replaced in October when they laser aligned it, the machine was only taken back out of service prior to this alignment to replace some faulty one way valves in the pipe work, the shims they used were a mess though and they had left a lot to be desired. I used 3mm plate to fill gap on them then shimmed with 2 precision shims on each foot to make tolerance, soft foot was checked and ruled out. No pipe strain issues(water is always same temp (approx 15 degrees Celsius)
I was unable to do a lift check but I suggested it before leaving the site when I saw the Vibe results.



RustyCas

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Reply with quote  #23 
Mark, as for your original question, heads on the shaft is generally the preferred method. Hubs can have a little movement in them unless the hubs are a shrink fit. Depending on the system you are using, using the shafts can present a problem if the alignment is way out (roughing-in, for instance) - you may not be able to get a reading because the beam falls off the receiver as its rotated. . The less distance between the heads, the more out-of-alignment “range” your unit has. So heads on the hubs extends the range since the heads are closer. And I will sometimes use magnetic brackets on larger machines since it makes for a more compact setup and is often faster.

Be aware that when using a laser, unlike when “sweeping” a stationary shaft/hub with a dial indicator, you are aligning the centerlines of the two points your hubs are mounted on. You can still have a bent shaft or cocked hub, and you’ll never know it. So a thorough alignment will include checking shaft and hub run-out. When doing an alignment due to a vibration problem, every thing must be checked - assume nothing.

Learned this the hard way.... had a customer who wrecked a motor (broke the shaft off) when the hold-down bolts came loose (was aligned 2 weeks earlier by someone else who didn’t use the proper washers for the oversized baseplate holes). I told them to be sure and check the run out on the gearbox input shaft, as it was a violent failure and the shaft might be bent. When I arrived I asked about it and was assured the shaft was good. So I mounted my heads on the shafts near the bearings and did my usual alignment. Vibration was horrible on startup. Swept the gearbox hub and TIR was 0.078” - yes, 78 thousandths. So they had the gearbox rebuilt and I aligned it again. But they had the geared coupling hub on the output hub mounted *backwards* (offset teeth), so I had to come back and align it again when that was corrected.

So because they couldn’t originally wait one extra day for me to do the alignment, they destroyed a 500 hp motor, had a gearbox shipped out and rebuilt, and paid me 3 times to travel and do an alignment. (Can’t make this stuff up)

Attention to detail is critical. Getting “good numbers” (or a stupid smiley face) is only about 20% of the job, and maybe not the most important part.

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Danny Harvey

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Reply with quote  #24 
"I aligned a small pump with laser system, and the vibration went up quite a bit. I opened the coupling and found "Woods" element to be severely distorted from previous misalignment. Replaced the insert, and vibration was good. Moral of story; always inspect coupling and repair/replace before shaft alignment!"

Walt, 

I had the exact same experience.  Mine had the added pleasure of occurring after I had warned the client for almost 2 years of misalignment. Then they blamed us when the vibration amplitude went up after we aligned it and they DIDN'T replace the element.

I might have a picture but it had significant bends on opposing halves of the Type E sleeve.  

To finish the story off (I hope), they declined to change the pump bearings at the time and replaced only the mechanical seal.  Another 1 1/2 years and another rebuild. They didn't blame us this time though.
Danny Harvey

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Reply with quote  #25 
"The element will distort somewhat when loaded, so there needs to be a little axial clearance, to be prevent an axial load from being produced."

The gap between the hubs should be in the enclosed instructions if they ever make it to the floor and are ever read. Smart phones make finding this kind of this much easier on the run but the assembly instructions should be given to whoever is doing the assembly. Some important stuff is hidden in all the filler.
Danny Harvey

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Reply with quote  #26 
"Hi guys,
I'll make my first post on this board by saying I agree with Rusty's post, maybe that'll get me enough points to start me out on the right foot.

I noticed excessive looseness mentioned a couple times, what about a lift check? Wouldn't that give a clue?

David Eason"

David,

Welcome.  I'm pretty sure I remember you from forums past.  Everyone here has started on the right foot and stayed on it so far.

You wouldn't believe some of the looks I get when I suggest a lift check.  Then they find out it is really as simple as it sounds and join me in wondering why they haven't been doing this all along.

Anyway, I'm not sure where the looseness came up, but a lift check is always a good idea if you suspect it.
David Eason

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Reply with quote  #27 
Danny,
Yes, I remember you too along with a lot of other names I see here. Thanks for the welcome.

David Eason
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