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Post by turboron on Dec 31, 2020 7:29:08 GMT -5
John, very important paper. I was surprised by his suggestion the synthetic oil be used. In the early 1970s I worked for a major industrial compressor manufacturer. We had several major rotor failures in one period. Dr. Gunter worked with us to design O-ring type squeeze film dampers to solve the problem. Another part of the solution was to reduce the distance between the overhung weight(thrust bearing disc) and the compressor bearing. I meet him one time when he was at the plant. He had just started his company. We hired several of his graduate students to do our rotor dynamics.
Thanks, Ron
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Post by turboron on Dec 31, 2020 7:30:08 GMT -5
All, rotating machinery is a small world.
Thanks, Ron
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Post by finiteparts on Dec 31, 2020 18:28:01 GMT -5
All, That is an interesting paper, as are all the others that are offered free on the Dyrobes site...highly recommended by us all I am sure. Just to clarify though, the paper is discussing the behavior of floating journal bearings, not squeeze film dampers. Additionally, in particular to the SFD design, I recommend that you wander over to Texas A&M University's Rotordynamics Laboratory, specifically, to Dr. Luis San Andrés's "Modern Lubrication Theory series, Lecture 13. Here is a perma-link: oaktrust.library.tamu.edu/handle/1969.1/93197Or here is the site location: rotorlab.tamu.edu/TRIBGROUP/default.htmthen navigate down to Lecture 13. It is much more informative for someone learning rotordynamics than the little bits that can get gleaned from the bits and pieces that you typically find in the technical papers. I also highly recommend poking around the TAMU Rotor Lab Tribology Group's site for all kinds of other rotordynamics gold. Patty, what kind of damping value are you trying to achieve? Are you doing any simulations to try to get it to a good spot, or are you just trying to experiment with it? Enjoy, Chris
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Post by racket on Dec 31, 2020 18:48:04 GMT -5
Hi Chris
But isn't the lube in the clearances around the brass bushes, "squeeze film dampers" ??
Cheers John
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Post by finiteparts on Dec 31, 2020 19:39:06 GMT -5
Hi John,
Not here, because they are discussing floating ring bearings with a ring/shaft speed ratio of 0.020. Since there is a velocity shear across the outer film, this would be considered a journal bearing, not a SFD. A squeeze film damper does not include a rotating surface to generate the pressure wedge, the viscous work is done by pushing the fluid out of the way. So if they were discussing semi-floating bearings, then the outer film would be considered a SFD, but this is not correct when talking about floating ring bearings. Floating ring bearings are two journals bearings in series.
I hope that helps.
Chris
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Post by turboron on Dec 31, 2020 21:38:14 GMT -5
Chris, thanks. Everything you post helps. Keep it coming. Rotor dynamics is one of the really interesting aspects of rotating machinery.
Thanks, Ron
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Post by racket on Dec 31, 2020 22:31:51 GMT -5
Hi Chris OK ... So a turbocharger with a non rotating floating bush bearing as per the Rajay design would be considered to have a SFD, but a normal rotating bush/s would only have a FD to reduce the shaft motion ?? Borg Warner ... www.turbos.bwauto.com/en/products/turbochargerBearingSystem.aspxBesides the lubricating function, the oil film in the bearing clearances also has a damping function, which contributes to the stability of the shaft and turbine wheel assembly. The hydrodynamic load-carrying capacity and the bearing damping characteristics are optimised by the clearances. The lubricating oil thickness for the inner clearances is therefore selected with respect to the bearing strength, whereas the outer clearances are designed with regard to the bearing damping. The bearing clearances are only a few hundredths of a millimetre. LOL....clear as mud :-) Cheers John
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Post by finiteparts on Jan 1, 2021 1:03:53 GMT -5
John,
Floating ring bearings, as any fluid journal type bearing, have both stiffness and damping values derived from the rotationally driven fluid in the bearing...with the FRB having essentially two journal bearings in series. Unlike a REB, which has essentially no intrinsic damping, fluid bearings do possess internal damping.
So yes, the stiffness/damping of the bearing system can be tuned by varying the inner and outer clearances of the FRB. You can design one of the clearances to provide more damping, that is common...but both clearances must also support a load. There is always a compromise and in this situation, the inner and outer films are trying to balance stiffness and damping while keeping the ring speed ratio away from regions of whirl instability. As state above, FRBs are required to provide a bearing stiffness, which is provided by the fluid wedge; unlike SFDs, which do not have any stiffness unless there is a whirling of the shaft center...thus they are usually designed with some form of centering spring. Since SFDs are designed to be optimized for provide damping without the need to provide a stiffness, they are usually called into play to fix a bearing system that has no ability to absorb shaft motion, such as stiff REB systems.
The equations of motion for SFD and FRB are very different and trying to glean information about the performance of a SFD from a paper describing FRB will not be very effective. SFDs perform with very low Reynolds number flows and FRBs operate with turbulent, rotating flows, so the fundamental performance regimes are wildly different.
Interestingly, farther down in the Borg Warner article we see this:
"The one-piece bearing system is a special form of a sleeve bearing system. The shaft turns within a stationary bushing, which is oil scavenged from the outside. The outer bearing clearance can be designed specifically for the bearing damping, as no rotation takes place."
So yes, I stand by my statement, that the article was discussing the behavior of FRBs, which are fundamentally different than SFDs. They may both provide some amount of damping capability, but they are functionally different. Their design and performance are also different.
"OK ... So a turbocharger with a non rotating floating bush bearing as per the Rajay design would be considered to have a SFD, but a normal rotating bush/s would only have a FD to reduce the shaft motion ??"
--- Yes and no. A non-rotating semi-floating bearing would be considered a SFD, but no, a FRB would not be considered a fluid damper...it has an outer bearing which may have a larger amount of damping, but it is still a bearing.
Does that help to clear up the difference?
Chris
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Post by finiteparts on Jan 1, 2021 1:15:24 GMT -5
Chris, thanks. Everything you post helps. Keep it coming. Rotor dynamics is one of the really interesting aspects of rotating machinery. Thanks, Ron Thanks Ron. I am glad that I am helping. I agree that rotordynamics is a very interesting part of the turbomachinery puzzle. I wish I could talk about the turbine rig I worked on that involved some mag bearings, but you know...proprietary rules and all. It definitely steps up your rapid learning about rotordynamics when you can tune stuff on the fly!
- Chris
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Post by racket on Jan 1, 2021 4:42:07 GMT -5
Hi Chris
Yes and no :-(
Half way down page 3 of the Paper ................ " The rotation of the outer surface of the floating bush bearing acts as an uncentered squeeze-film damper"
I provided the Link to the Paper mainly because I considered it provided generalised information about turbo rotor dynamics such as the energy distribution , modes , whirl etc , rather than about SFDs.
I acquired a copy of Shaw and Macks classical lubrication textbook on Analysis and Lubrication of Bearings many years ago when I was having my problems , so I'm aware of FRB requirements and used the info in some of my design/construction work at that time.
LOL....thats why I ended up going back to "brass" and standard turbo components to negate the need to workout bearings , too many other problems arise when doing a build.
Yep , keep your contributions coming , hopefully we'll get Patty sorted and can soon see/hear his engine in full flight :-)
Cheers John
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Post by turboron on Jan 1, 2021 8:22:13 GMT -5
All, to add to the pool of information. The industrial compressors I referred to in my note about Dr. Gunter use tilting pad bearings. The rotors that were failing were running above the 1st bending critical. Part of the problem was that the thrust bearing (a tilting pad design) disc was overhung to far from the bearing. It dragged the 2nd critical down into the running range. These very very long rotors with up to 10 impellers mounted on a shaft. The point being that SFD are now commonly used with hydrodynamic bearings in many applications.
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Post by madpatty on Jan 1, 2021 20:49:34 GMT -5
Hi Guys.
We did it-
Regards. Sorry for the bad quality of the video.
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Post by turboron on Jan 1, 2021 21:17:54 GMT -5
Patty, very glad we were able to help in some small way. Taming a rotor is great accomplishment.
Thanks, Ron
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Post by racket on Jan 1, 2021 21:36:38 GMT -5
Hi Patty
Congratulations my friend :-)
More video as development progresses please .
We've all learned something from the process
Cheers John
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Post by madpatty on Jan 1, 2021 22:35:00 GMT -5
Hi Guys. I was able to run the engine up to 32000 rpm. There's very perceivable sound change once engine gets past the critical speeds. So here's what I think may be happening with this rotor and a bit of history on the testing that I did with the new squeeze film damper- - NEW SFD without any OD increasing spacer between the bearings.
1. Resonance at critical speeds of around 21k (turbine side) and 23k (compressor side) rpm. 2. Max force acting on turbine side bearing at 21k rpm and on compressor side bearing at 23k rpm.
I ran the engine with oil supplied to the SFD at about 1.5-2 bar. The compressor bearing failed yet again at exactly the model predicted value of ~23k rpm.
- NEW SFD WITH 26mm OD increasing spacer between the bearings.
1. Resonance at critical speeds of around 21k (turbine side) and 24k (compressor side) rpm (NOTE- The compressor side critical speed increased to 24k rpm) 2. Max force acting on turbine side bearing at 21k rpm and on compressor side bearing at 25k rpm (NOTE- The compressor side max force speed increased to 25k rpm and YES the max force is not exactly at critical speed but slightly afterwards)
I ran the engine with oil supplied to the SFD at about 1.5 bar and the engine ran successfully.
- Then again I repeated the same test with SFD and 26mm OD spacer between the bearings.
The compressor side bearing failed again at the model predicted value of ~25k rpm.
The reason, the SFD is not working reliably as I would expect. The oil is flooding inside the bearing tunnel without proper drainage. the compressor side O-ring is leaking and compressor side bearing is only partially located in the damping region by design limitations. I think the damper is not holding proper fluid pressure on its OD due to improper supply. Also the damper holding nut is leaking too much oil inside the damper therefore not enough oil is getting transferred to the "damping lands". Regards.
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