To all,

Have you ever experienced high sensitivity during a balancing procedure due to more then normal bearing clearance?

The spectrum did not exhibit any signs of looseness showing just a nice 1x and it took a 0.06 lb weight to bring vibration from 12 mils p-p down to almost zero for this 500 lb rotor. And it also did not take too much time for this hammer mill to go back to 16 mils as hammers wore out.

The frame and the bearing housing tightness was confirmed. The hammer mill has a floating OB bearing (cylindrical roller bearing). The IB spherical roller bearing does not show an increase in vibration as does the OB one. Phase is stable.

Will be a clearance check warranted?

Thanks,
David

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

David - yes. Excess clearance can affect balancing in sometimes very peculiar ways. I was given the task of balancing a fan a a paper mill that would not go below 3 mils displacement. We tried walking the unbalance down in small increments and, when the displacement got to roughly three mils, the phase would change by 180 degrees and the displacement would increase; after the bearings were replaced, it balancanced in a normal, linear, manner. I've also seen several instances of the converse where the apparent unbalance would totally disappear when the unbalance droppped below some level that should have still given a residual displlacement. It all depends on the rotor dynamics. I've also seen fans that had worn bearings that were hyper-sensitive to any changes in operating conditions. One that I especially remeber was a fan in a carbon black plant that I had balanced with the plant manager watching. I got it to a very low level which he was happy with and so I left. Before I could leave the parking lot, he ran out and told me that I needed to come back and redo it.

This message has been edited. Last edited by: Duncan Carter,

David,
Alot of times what happens when you have a lot of looseness in the bearing is that hte phase becomes unstable. If you are using the CSI Fastbal II program you can watch this graphically. I don't know but if this is exactly what is happening to you but if it is you will have a devil of a time trying to balance under these conditions.

Kelly

David,
I see you have a new job.
I have limited experience with hammer mills but what I do have is similar to yours. We would balance a hammer mill the best we could and soon after it seemed we were doing it again.
When the unbalance was corrected did the spectrum then show looseness indications at all?

I have seen when clearance were very excesssive the only real peak was 1x and not alot of harmonics. This was a severe case though.

James

Did you check for resonance? If you are running near resonance rotor will be very sensitive small wt change.

Kelly, James, DK

As I mentioned before phase was reasonably stable before balancing. You will be surprised but I can't check for resonance with the old firmware version, but based on phase stability it does not look like this is the case. No harmonics to speak of, just high 1x amplitude of about 0.9ips. There is 10:1 ratio between hor. and vert. vibration but I attribute it rather to difference in stiffness then directional resonance. After balancing, also no harmonics.

The fact that it goes out of balance is normal as hammers wear out but this hammer mill started shaking again too quickly. The sensitivity to unbalance and what affects is the issue.

David

David,
I should have read your original post better, however after reading through the posts again it sounds like you have a clasic case of resonance. Generally speaking a ration of 3:1 vert:horz is a good indication that you may have a resonance problem. With a ratio of 10:1 I would say that you most likely have a resonance problem.
A resonance problem will make the machine very sensitive to weight. It will also make it unpredictable as far as its sensitivity to weight. Be very careful trying to balance until you have found and corrected the source of the resonance because you can cause a catastrophic event if you put the wrong amount of weight in the right place

Call your local CSI Saleman and ask him if you could test drive a new 2130 for a week. They usally have no problem with it and if he knows you may just send it to you. That way you can do your Res test and know for sure.

In my limited experience, when I have a high V:H ration I usually always find a resonance problem.

Hope this helped

quote:

phase was reasonably stable

.... Why exactly does everyone think that phase is NOT stable running near resonance?

Rusty,
In my limited experience I have found that when I tried to balance a machine that was near or in resonance condition that the phase was not very stable and that weight changes were erratic. What I mean is that the sensitivity would change from one trail run to the next and it was generally alot of change too not just a minor amount. Made for a long day of balancing

I have found it much easier to first correct the resonance condition and then balance as necessary. In alot of cases correcting the resonance condition brought the vibration into acceptable tolerances thus not requiring balancing at all.

Now you may like to fight the good fight and spend frustrating hours balancing in a resonance condition. Me, I'd rather spend the time I saved drinking beer and riding steel horses.
.

quote:

it took a 0.06 lb weight to bring vibration from 12 mils p-p down to almost zero for this 500 lb rotor.

27 grams (0.06 lbs) seems like an awful small weight for a 500 lb. rotor running at 12 mils. What is its speed, somewhere around 500 cpm?

Looks like it would not take a long time to wear away 27 grams from a hammer mill's hammers, huh?

Would be nice to see what changes, if any, take place during a coastdown. Can you take a coastdown plot of the amplitude and phase ,in the direction you use for balancing, with the "old firmware"? If you can, you will have to print directly from the meter and then scan it, in order to post the plots here. One thing, do not use the "averaging" function for the amplitude and phase during the coastdown.

My guess it is running almost on top of its critical speed.

I am not familiar with a hammer mill but we had a similar problem with a hammer shredder.
Ten hammers opposite each other and eleven on
the 90/270 plane.All varying weights.
When we first started running these machines
to shred bark and sticks it ran good till the hammers wore down,We replaced the hammers and
the thing shook like a dog coming in from the rain.Information availiable told us to weigh
the hammers and even out the sets but what we were not told was to put the heaviest in the middle and work out to the lightest at each end.
Once they were set properly the machine ran smooth and has never had to be dynamically balanced that I know of.

quote:

Why exactly does everyone think that phase is NOT stable running near resonance?

I have always thought phase may not be stable near resonance in some cases.

My reasoning is that the phase changes rapidly with speed in the area of resonance. And if resonance should change by a very small amount due to change in stiffness (possibly temperature related), phase would change.

I don't have any experience to back it up regarding PHASE. I do know on our big vertical motors operating near resonance (before we installed braces), the MAGNITUDE seemed to vary fairly widely between measurements (we don't measure phase often). I have suspected it was due to change in stiffness and resonant frequency but not positive

I am with Rusty, if speed is stable and repeatable, then both amplitude and phase should be stable and repeatable as well, assuming that resonance is the only issue. If you are taking readings at different speeds each time, and there is a resonance in the vicinity of those speeds, then yea there will be some difference in amplitude and phase.

The amplitude change with a small speed change would depend on the damping. If the resonance is very lightly damped and has a narrow, sharp peak a very small speed change could make a big difference. If it has a decent amount of damping, a broad peak, a small speed change shouldn't affect phase much.

"The phase changes rapidly with speed in the area of resonance.".... just referring to the shape of a typical coastdown curve. I hope you agree phase changes fastest in the area of resonance. Whether fastest is fast .... another question.

Going back to my post... note I said phase may not be stable near resonance in some cases.

I don't think it is true as a rule but as I mentioned we have some vertical low-speed circ water pumps with resonance at operating speed and we saw very erratic vibration magnitudes. After we added braces we reduced the magnitudes and also reduced the variability. My attempt to explain this was that there are variations in the stiffness of pump head bolted connections and anchor bolts with temperature... resulting in shifts in resonance frequency, which affected where we were operating compared to resonance. If this explanation were true, it would stand to reason that phase would vary in a similar manner as magnitude. I admit it's just a theory... welcome to other ideas.

On the question whether phase is unstable at resonance in general, I defer to Rusty and Steve.

Most should know that phase changes most rapidly at (near) a natural frequency. Why? Physics and Mathematics.

The rate of change of phase around a natural frequency relates to the damping present. R.E.D. Bishop showed how to use the slope (rate of change) of the phase to determine damping around 45 years ago.

In the U.S. we would probably call professor Bishop Red. Only a joke!

article on phase change
http://www.update-intl.com/VibrationBook3k.htm

more articles
http://www.update-intl.com/VibrationBook.htm

Bishop and others documented this on real machines. Anyone who has done many startup and coastdown plots has 'documented' this. Personally, I've seen this on hundreds of machines.

By the time something like this makes into a bunch of text books, it should be in most case accepted fact.

Hey all,

I'm with Bill - thought everyone was on the same page with this one. Although I may not have seen as many as Bill Foiles, I have seen the same phenomenon (maybe a hundred or so) and accepted it as a way of life as it is repeatable.

Hey Rusty you lost me on the trial weight goes to 250. I have always followed the old board and I am still trying to get a handle on this one. Please expand on this I would really appreciate it.

Thanks