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RustyCas

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Reply with quote  #16 
Hey Jim, is that an “adjustable” motor sheave? Was this fan purchased as a constant speed fan, and then converted or used as variable speed with a VFD? What is the design speed of the fan, and what is the speed at 47 Hz?
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Danny Harvey

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Reply with quote  #17 
I noticed the same thing as Rusty but haven't have time this week to answer. If those sheaves get grooved, it makes all sorts of racket when the belt passes. I believe that it induces a sort of twist in the belt that excites the strum frequency when it happens at just the right speed. 

If it is a variable pitch sheave, you might consider replacing it with a fixed pitch that gives you the same fan speed. I have had clients do that on several AHU's that exhibited similar problems but not as precisely defined as yours.  If it stops as quickly as it starts, you can probably block those speeds out in the inverter if that's an acceptable work-around for the client.

To me the fact that both sides of the belt react at the same frequency indicate that it is the static tension that is creating the resonance, rather than the tension from the belt pull.  If it were the load tension, there would be a higher natural frequency on the tight side and lower on the load side. You should be able to effect it by adjusting the belt tension but you've got to have enough to transfer the torque.

I would also expect that hidden in the mounds in the spectrum would be sidebands of belt speed which appears to be about 190 cpm.

You might be able to freeze it with a strobe while running at 47 hz to find the strum frequency. You might be able to get it with a guitar tuner app on a cell phone. I used it on some pipes in the wall of a church choir room to determine that the chiller downstairs was off key. The choir wanted to sing in B-flat and the pipes were singing in D. If you've got a good enough phone, you can do slo-mo film and get it from that.
electricpete

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Reply with quote  #18 

I watched the video in slow motion (You can adjust the video playback speed in windows media player by right-clicking on the video during playback and selecting enhancements / play speed settings.)

Even though the left (slack) side has a lot more motion than the right (tight) side, what I see is that the right side starts and stops vibrating just before the left side starts vibrating.  The same pattern repeats several times (right vibrating briefly just before left starts vibrating).  It's as if some kind of transient starts on the right side and then propogates to the left.  What does that tell us? Not sure exactly, but to me it fits a lot better with a transient involving belt slippage 

One possibile troubleshooting avenue would be a load changing experiment to see if this is slip related. For squirrel cage fan, increase load by increasing flow (axial flow fan is the opposite you decrease flow to increase load). I'm going to assume squirrel cage fan:

  • Experiment 1 - Adjust speed just below the problem speed, then increase flow by opening a damper further or opening a once-through flowpath like discharge to atmosphere. If that increase in loading stops the problem, you know it's related to gross slippage(*). Or...
  • Experiment 2 - With system at low speed or shutdown, close a damper (to reduce load at any speed). Then repeat your experiement slowly increasing speed to see where the problem begins. If it gets to higher frequency before vibrating when throttled, then you know it's related to gross slippage (*).
  • * if its gross slippage,  that would probably be either lack of adequate initial static tensioning, or excess load torque drawn by the fan, or maybe belt/sheave condition. 
just my thoughts on something I've never seen before fwiw.
ivibr8

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Reply with quote  #19 
Gentlemen
Thanks for all the feedback.  Lots to think about.
When I was working full time, I could visit a problem machine again most anytime and do some additional tests to confirm or eliminate root-cause. Now that I do this part time at remote sites, I don't have that luxury and rely on the client to perform a COA and get back to me.

Rusty, as far as I can tell, this ventilation return fan was always powered by VFD controller.
The pulleys are definitely fixed. The motor pulley is ~ 4" dia and the fan pulley is ~ 6-3/4" dia. According to the maintenance staff, the fan ran OK at full speed prior to the damage it occurred to the fan cage.

I wish I had more time to spend on this machine. I grabbed what data I could get hoping that when I got back home, everything would make sense.   Uhhhh    not [confused] 

So at this time, I am now waiting for the KISS results mentioned earlier. Hopefully, simple alignment and proper tension will fix the issue. If not, I will ask to return and investigate further with some of the ideas you've all given me in response.

Thanks again
Jim P

DFullerPrebal

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Reply with quote  #20 
We ran across this same problem on a job site Thursday. The fans in question are brand new 3 months old they are DWDI forward curve wheel 10 hp fan speed 723 at 57 HZ this unit is a package air handler built by Trane they are using a single BX106 belt. Fan speed limited to 57 HZ due to cfm and static requirements.

The contractor we were there for had said the fan sheave had come loose from the shaft and came off so after replacing the fixed bore with a taper lock sheave they wanted to be sure the balance etc was ok.

What we found was a significant belt resonance starting at 53 up to 57 Hz, we checked alignment and tension adjusting both minimally and no real change. Changed to a regular v-belt from the cogged belt with no change. Since there was another unit on this roof identical to this we went and looked at it only to find the fan sheave on it was also on the verge of coming off the shaft. Readjusted and found the exact same vibration issues as the first unit.

So we decided to change from a single belt to a 2 belt unit. In going to the bearing house we found the single belt with drive selection was rated to handle right at 10HP. Switching to a 2 belt drive eliminated the issue all together. So thinking these drive set ups from manufactures are sometimes most likely right at the limit of what they can handle and when under load etc you see in some cases the belt resonance.


DFullerPrebal

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Reply with quote  #21 
Sorry forgot to add we did a bump test attaching a probe to the motor and fan sheaves both in the axial direction and tapped the belt it showed a 1725 cpm vibration which works out to the 57 HZ vibration.

So the belt resonance was matching motor speed
ivibr8

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Reply with quote  #22 
Thanks for the response !

I wish I could update the status of this old post; unfortunately, I never heard back from the client.....good or bad.     This is a sore point with me.    [mad] 

I never had this problem when I worked full-time since I worked at one site where I controlled the PdM program and could always check on whether my recommendations were correct or not.  How else do we learn or get better?

In any case, I just contacted the client again with this information in the hopes I get some kind of feedback. If I do, i would consider it to be a minor miracle and close this thread with something useful.  <----Post Edit....well that didn't read well since the information DBFuller just provided IS very useful. I was referring to my own situation with the client.

Regards
Jim P
Alex

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Reply with quote  #23 
I am surprised how often the machine producers forget to consider / overlook such things as dynamic forces, vibration, resonance etc...
Danny Harvey

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Reply with quote  #24 
I worked at a power transmission supply house for years and in the literally thousands of v-belt drives I selected, I never put a single one-belt system into use. The additional cost is so small that it was pretty much the industry standard. Things are more competitive now, though and the slightest bit of savings might now be a consideration where it wasn't 25 years ago.

The most common cause of QD bushings (which are frequently referred to as taper lock) failing is the use of Neverseez on the three bolts that are there to draw the tapered surfaces together and clamp down on the shaft.  When Neverseez is applied to the taper these three little bolts become torque transmitting devices and fail quickly. 

I don't think this applies to genuine Taperlok bushings because of their hubless mounting system.
electricpete

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Reply with quote  #25 
Danny, those sound like good things to remember.   

I had posted somewhere along the way about a QD hub that had a crack above the keyway.  I can't find that thread, but the powerpoint is attached.  I'm pretty sure we would never use anti-seize on rotating equipment except maybe on a high temperature pumps (and I'm not sure about that).  But maybe the problem with this qd hub started with bolt loosening ?  I'm not sure.   What do you think?

 
Attached Files
pdf CrackedSheaveHub.pdf (325.04 KB, 18 views)

DFullerPrebal

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Reply with quote  #26 
The sheaves in question on the trane unit were fixed bore with just a setscrew over the key
The second fan he pulled the setscrew and saw the mark in the top of the key so thinking at one point it was tight
Danny Harvey

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Reply with quote  #27 
Pete,

I would suspect that one was cracked by and oversized shaft or key, by uneven tightening of the three bolts, or during removal if an impact wrench was used to push the sheave off.

For a short while, they made some of the smaller bushings our of sintered steel and they would crack trough the setscrew hole so regularly that they discontinued them.

AT second glance, I would say that this crack might have been caused by driving a screwdriver into the split opposite the keyway.  It's a very common practice and it breaks the bushings just like what you have shown. It's also possible to crack it and not know it. I'll leave the other guesses out there because they are real but yours is probably a screwdriver.

Take a look at the split in the hub and see if there are marks from a flathead screwdriver being driven into the OD.
electricpete

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Reply with quote  #28 
Thanks Danny.  I don't have access to the part any more.  From my other photos I don't see any prying marks in the gap opposite the keyway, but not positive.  I can see why you suggest it though - the crack initiated on the inside of the keyway as if there was something trying to bend things in a similar way to prying open that gap.

Danny Harvey

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Reply with quote  #29 
I've seen it happen from really stubborn grease/fine dust stuck to the shaft and not cleaned off sufficiently. Then force is applied to try to position the bushing axially (frequently with a pipe over the shaft and a hammer) and the same thing happens as when you wedge a screwdriver into the slot.  The ID of the bushing opens up and it cracks at the setscrew.
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