Asymmetrical stiffness shaft

Question:

Will a horizontal shaft having asymmetrical stiffness in two orthogonal directions perpendicular to the axial and supported by rolling element bearings, produce 2xRPM if vibration measurement is taken on bearing housing ?

Now, same question under the same conditions except for the bearings - this time of journal type.

PS
The answer may sound obvious to many but overall opinions may differ.

DGluzman

Original Post
If I was to compare two "identical" machines (same balance, same alignment condition, same support stiffness, etc.) my gut feel says I should not expect any difference at the seismic accel.
The only thing I can think that an asymmetric rotor would affect is perhaps a split critical. Depending on the relative stiffness of the rotor to support stiffness (hence, different bearing types), I may not even see that at the seismic probe.
Just thinking out loud....and, because its 3:30 am and I'm stuck here at work with little sleep, I could be WAY off base...LOL

Jim P
Asymmetric rotors do not affect split criticals other than overall shaft stiffness. They do produce 2X on both type of bearings.

They can even cause an instability (at least in lightly damped rotors, perhaps experimental type rotors). Rolling element bearings provide little damping - oh well draw your own conclusions from that.

Some large 2-pole generators with fluid film bearings have a lot of 2X. Most manufacturers learned to cut slots to make them more symmetric long ago.
I agree with Bill that split critical is typically associated with asymmetric stiffness of the stationary component (stator and support) as opposed to asymmetric stiffness of the rotating component (rotor).

I would say it is pretty easy to imagine that the assymetric rotor can result in 2x vibration IF there is a constant-direction force or moment present.

The most straightfoward example of constant direction force would be belt load.

It is certainly possible that in presence of shaft misalignment, some couplings will exhibit constant direction moment in a direction to straighten the shaft.

Gravity is also a constant direction force, although relatively low magnitude.

I notice that section 3.5 of "Rotordynamics" by Muszynska addresses dynamics of an anisotropic rotor in isotropic supports. I haven't studied it to understand what she's saying.
In the old days (Ok, some will tune out here.) when one watched startups with and oscilloscope, one would see 2X at 1/2 of the first critical speed. The wise analysist would take note of this and be prepared at the critical in case it was critical.

If you have recorded a shutdown or startup you may be able to see the 2X at 1/2 the critical. It even works on a Bently Nevada rotor kit, where the asymmetry comes from the attachment screws on the masses - line up 2 masses with screws in same plane for better effect.
I went back to some old VI notes and there in vibration 101, was a table showing normal 2X for large 2 pole generators. But I just couldnt visualize the source. Even with a bend, I could only resolve a rotating 1X vector.
With static loading on an asymmetric rotor, even a small eccentricity will add a twice per rev force due to different stiffness.


Thanks for the clarification guys

Jim P
quote:
But I just couldnt visualize the source


The manner in which we teach this so that people can visualize what is going on is to take a yard stick and rotate it. Having a rectangular cross section it has significantly different stiffness in two planes. In one plane it will deflect readily with minimal midspan load applied. In the other plane deflection is significantly less. Rotate the yard stick one revolution and you will see it deflect twice, hence 2X.

Simplistic, but it gets the idea across.

John from PA
quote:
Originally posted by John from PA:

In one plane it will deflect readily with minimal midspan load applied. In the other plane deflection is significantly less. Rotate the yard stick one revolution and you will see it deflect twice, hence 2X.


The above is simple and conceptually correct explanation, and if one will measure frequency of stick's relative movement it will indeed be at 2x.

But my question was about measuring seismic vibration on the bearing housing. Why would the housing move at 2x ? There must be a dynamic force acting upon it!

The explanation can be as follows:

Under the influence of a static gravity force and due to difference in shaft stiffness, the shaft ( its midspan or overhung portion for rigid bearings )is moving ( deflecting ) radially at 2x, thus, creating a dynamic force as a result of accelerated motion of mass. This dynamic force is then transmitted to the housing.

Dave

2X vibration in GT was also discussed at this forum in the past, I would like to drag ur attention towards that problem. That vibration was of transient nature, on gas turbine with split shafts and measured on housings with seismic probes found only at #2 bearing which is supporting compressor aft. Rotors of axial compressor (center hung) is coupled with gas generator (GG) or gas producer (GP) which is overhung thus makeing one shaft of GG or GP. The second shaft, mechanically separate is of Power Turbine. Air compressor rotor and GG rotor both are balanced separately and then assembled/coupled during overhauling (assembly) of gas turbine to make first shaft of GG.

Can we assume asymmetric stiffness of air compressor rotor and GG rotor is the cause of transient 2X vibration that is observed at 86% to 89% speed of GG? This 2X peak remains negligible before closing of air bleed valve that is designed to close with PCD pressure at 84% GG. As bleed valve closes, 2X peak starts rising and it crosses the allowable limit, within speed span of 86%-89%. If gas turbine allowed to stay at speed where 2X has highest amplitude, it starts decreasing and gets back within allowable limit in a minute or so. But if speed is kept increasing and goes higher than 89%, amplitude of 2X peak drops rapidly and comes within allowable limit instantly.

Air compressor rotor is supported by #1 and #2 bearings, while GG rotor is overhung rotor and supported by #3 bearing. No 2X is found at #3 bearing housing. Where we measure these vibrations on housings is not exactly in line with these tilt pad bearings. For #3 bearing housing, distance where #3 bearing is installed and where we capture vibration is quite long, may be because of this we don't get 2X in spectrum at this point.

Can we assume asymmetric stiffness of machine and its supports or in coupling of air compressor with GG rotor a possible cause of this transient 2X vibration in GT?

If this may b the reason, from which component (supports i.e. housings or rotors) should we investigate further to reach the root cause?

Can we assume asymmetric stiffness of air compressor rotor and GG rotor is the cause of transient 2X vibration that is observed at 86% to 89% speed of GG?

Where would the asymmetry come?  Sometimes people talk about cracked shafts when bringing up asymmetry in turbine rotors.  Otherwise, you probably don't have an asymmetry.

Walt Strong posted:

Is the GT a heavy industrial frame or a light aircraft derivative? Have you fully evaluated shaft alignment and accounted for thermal/load/thrust change when bleed bypass air closes?

Walt

Since the OP posted the question with respect to both rolling element bearings and fluid film bearings, I suspect his question is broad based and thus applicable to both "industrial frame" and "light aircraft derivative".

"I suspect his question is broad based and thus applicable to both "industrial frame" and "light aircraft derivative" Perhaps you are correct, but I would/did not assume that. I was trying to see how the new current discussion fits with the 9-year old discussion, so I will eagerly wait for comments to see where this discussion goes.

Walt

William_C._Foiles posted:

Can we assume asymmetric stiffness of air compressor rotor and GG rotor is the cause of transient 2X vibration that is observed at 86% to 89% speed of GG?

Where would the asymmetry come?  Sometimes people talk about cracked shafts when bringing up asymmetry in turbine rotors.  Otherwise, you probably don't have an asymmetry.

In Gas Turbine, can this asymmetry be caused by housings or stators?

Walt Strong posted:

Is the GT a heavy industrial frame or a light aircraft derivative? Have you fully evaluated shaft alignment and accounted for thermal/load/thrust change when bleed bypass air closes?

Walt

Industrial gas turbine from Solar Turbines, Centaur 40 Model

Walt Strong posted:

Is the GT a heavy industrial frame or a light aircraft derivative? Have you fully evaluated shaft alignment and accounted for thermal/load/thrust change when bleed bypass air closes?

Walt

For shaft alignment, it is OK, but how to evaluate or account for thermal/load/thrust change when bleed valve closes? As I said earlier it is a transient nature of vibration that does not persist for all speeds of Gas Turbine. It exist during a specific speed span. It appears during acceleration of GT from Idle Speed (72%) to Loading Speed (92%) and similarly during shut down when engine coasts down from Normal operating speed (92% ~ 100%) to 84% where GT stays until cool down timer times out before final shutting down. In both cases, bleed valve closes (during acceleration) and opens (during shut down). There is a strong possibility of generation of this peak due to thermal/load/thrust changes. HOW TO EVALUATE THESE?

I am talking about this problem during factory testing of GT where it is started and stopped 4 times, every time, speed span during which we observe abnormal 2X peak is little different. One reason may be different applied load from Dynmometer (Load Cell for measurement of torque and hence horse power) during each start/stop. CAN OTHER REASON BE CHANGE IN THRUST/THERMAL PROPERTIES?

Sometimes when we give fast acceleration to GT during 86% to 89%, 2X peak abnormality is found at a higher speed span e.g. 88% to 91%. This again points out some thing problematic with load/thrust changes.

Stator asymmetry should not cause this.  However, rotor-stator rubs can cause 2X.  

Is there any sub-synchronous with the 2X?  I ask, because you mention a relationship with a bleed valve.  If the bleed valve is not timed correctly or opening correctly, one can get incipient (or worse) surge, which would result in sub-sync with harmonics possible.

Is this exactly 2X?  How did you determine this if it is exact or not?  A rub would not be that unusual in a transient condition.

I believe most would classify this turbine as a light industrial.

Pakhtoon posted:
Walt Strong posted:

Is the GT a heavy industrial frame or a light aircraft derivative? Have you fully evaluated shaft alignment and accounted for thermal/load/thrust change when bleed bypass air closes?

Walt

Industrial gas turbine from Solar Turbines, Centaur 40 Model

The Centaur 40 has tilt pad bearings and usually is equipped with casing transducers.  Is that what you are using for the data that is of concern?

Hi John from PA! Let me answer you first. As i said, this problem is being faced during factory test. Bearings are tilt pads, with five pads, and load between pads. Transducers are velocity transducers but processor is from OROS that displays a continuous or real time spectrum. During speed span 86% to 89% 2X is predominant and its amplitude is more than allowable marking.

 

William_C._Foiles posted:

Stator asymmetry should not cause this.  However, rotor-stator rubs can cause 2X.  

Is there any sub-synchronous with the 2X?  I ask, because you mention a relationship with a bleed valve.  If the bleed valve is not timed correctly or opening correctly, one can get incipient (or worse) surge, which would result in sub-sync with harmonics possible.

Is this exactly 2X?  How did you determine this if it is exact or not?  A rub would not be that unusual in a transient condition.

I believe most would classify this turbine as a light industrial.

Rotor rub is not the case often until and unless GT is allowed to operate at this high 2X and that high is not more than 0.28 IPS. Only once we captured orbit during high 2X with CSI 2140 which showed rotor rub. Mostly its not the case.

Function of bleed valve is perfect and smooth, no abnormality. There are normally no sub-sync components and again no harmonics. Only sub-sync frequency found in spectra is from bearings of Dyno (driven equipment in our case) but that is persistent through out the operation. Its amplitude that we observe at this location of GT (2nd Bearing or Comp Aft) is normally too low to consider.

2X is exactly 2X, no doubt. it is determined in relation with the speed of rotor that is being continuously measured with speed sensor.

When vibration data was captured on new GT with CSI 2120 and accelerometer at same speed span and on same housing, 2X was not as high as we find in our overhauled GT engines, rather 3X component was also present with dominant amplitude in the spectrum that we don't even observe in case of our overhauled GT engine.

As I discussed earlier, axial air compressor and GG or GP rotors are separately balanced and then coupled during assembly. Can there be a problem with asymmetrical stiffness of this coupling? GG rotor is overhung on # 3 bearing. Compressor rotor is center loaded on # 1 & # 2 bearings. Compressor has a taper with 3 solid keys which is coupled and hydraulically stretched with hollow section of internal taper and key ways of GG or GP Shaft.

When assessing whether one frequency is 2 times another, one of the best ways is to look at the time signal (or an orbit of two signals).  Using a speed sensor is not a great way to determine the exactness of a multiple.  

I am somewhat puzzled by the statement below.

Rotor rub is not the case often until and unless GT is allowed to operate at this high 2X and that high is not more than 0.28 IPS. Only once we captured orbit during high 2X with CSI 2140 which showed rotor rub. Mostly its not the case.

Rotor rubs are often found on turbines.  Rotor rubs are not what is found in a signal analysis, but they are a physical issue that occurs.  Signal analysis may be consistent or inconsistent with a diagnosis of a rub, but a rub is a rub if it actually occurs and not one if it does not occur.  So, I do not understand the meaning of this statement.

Your response indicates that the 2X from the velocity transducers is above some limit on factory test.  Is this a factory test at the OEM with OEM limits?  Are there any 2X indications on shaft relative probes?

I had signals captured with CSI 2140 with me, I checked Waveform and found again that it is exactly 2X.

Rotor rub is not the case often until and unless GT is allowed to operate at this very high 2X. (Very high means amplitude of 2X approaching to 0.28 IPS.)

Only once we captured orbit during high 2X with CSI 2140 which showed rotor rub. Mostly its not the case.

As we disassembled GT having this problem a number of times, sometimes we find rubbing of compressor blades tips, sometimes we do not.

Rubbing of GG or GP turbine section blades is very obvious and is always there. But it do not have an effect on this 2X.

I was talking about rub found in signal, meaning that we confirmed that rubbing was actually being happening. When we disassembled that GT we found compressor blades tip rubbing. On an other time, when GT was being tested at our facility (same facility as of OEM) amplitude of 2X was a bit lower this time (but still higher than limit 0.17 IPS {this is the limit on factory test}). we did not find any signal of rubbing in orbit. When GT disassembled we did not find blades tip rubbing of compressor.

This is factory testing of GT although at our facility but exactly the same as at OEM facility.

If you mean proximity probes by shaft relative probes, This is Centaur 40 model, not equipped with proximity probes. Data of vibration coming only from casings mounted velocity transducers.

Rubbing of GG or GP turbine section blades is very obvious and is always there. But it do not have an effect on this 2X.

Rotor blades rubbing may be unlikely to produce much 2X.  However, GTs can have stator vanes rub the rotor, also.

On a machine with fluid film bearings 0.28 IPS (even 0.17 ips - 0-p?)can be rather signification as far as a symptom.  Natural frequencies can enhance this in a speed range, potentially.

It may not be to difficult to add some temporary prox probes for a better look.

Time synchronous averaging is another way that might be useful to determine an exact 2X relationship.

There has been mention of fluid film bearings, but there was only confirmation of tilt-pad thrust bearing (cold end?). So what are the radial bearings and PT thrust bearing types? Both rotors are rated for about 15,000 rpm. The second critical of the power turbine is specified at 12,690 rpm. 87% of 15,000 rpm is 13,050 rpm that is remarkably close to the PT 2nd critical!

Perhaps the OP has a lot of data and information that was not presented. My focus would be on basic rotor and casing structure dynamics before trying to evaluate whether asymmetric shaft stiffness is the cause or a contributor to the high vibrations.

Walt

Some background information:

http://s7d2.scene7.com/is/cont...aterpillar/C10550270 Solar Centaur 40 Data Sheet (15,000 rpm)

https://www.solarturbines.com/...ages/centaur-40.html cut-away view with link to data sheet same as above

Eng-Tip Forum

dgallagher (Mechanical) 30 Sep 03 10:37

Give the Technical Support group at Solar a call in San Diego, CA at Tel: 1 858 715-2060. Solar Web Site: http://esolar.cat.com/

 Yats (Mechanical)

(OP) 7 Jan 04 09:17

Thanks everyone. We eventually got the manual. For everyone's learning the gas producer design speed is 15 015 rpm. The 1st critical occurs at 6000 rpm. The 2nd at 8200 rpm. The power turbine design speed is 15 700. The 1st critical is at 5140 and the 2nd. at 12690.

From <https://www.eng-tips.com/viewthread.cfm?qid=73062>

EthosEnergy Light Turbines has the capability to support the Centaur 40 range or engines from the legacy CS3000 to the latest 4700 DLN and low BTU units.
Rebuild company that describes the 40 as an aged GT with many models or changes over the years

From <http://www.ethosenergygroup.co...-40--50-Engines.aspx>

Combustion Turbine Models Supported Within This Users Group
Requires Membership to Log-in
From <http://catsolar.users-groups.com/

"Can we assume asymmetric stiffness of air compressor rotor and GG rotor is the cause of transient 2X vibration that is observed at 86% to 89% speed of GG?"
From <https://www.maintenance.org/to...y=458713470396251135>

0.87 x 15000 = 13050.0 rpm that matches 2nd critical at 12690 rpm
2.0 x 13050 = 26100 / 60 = 435 Hz frequency of vibration at 87% shaft speed

Walt

Hi Walt! you quoted "There has been mention of fluid film bearings, but there was only confirmation of tilt-pad thrust bearing (cold end?). So what are the radial bearings and PT thrust bearing types?" My words: "Walt! Centaur 40 with all different ratings i.e. T-40, T-45, T-47 have radial fluid film tilt pad bearings. But thrust bearings of engine and PT are both fixed taper land thrust washers, and no tilt pad thrust bearings." Your quoted words: "My focus would be on basic rotor and casing structure dynamics before trying to evaluate whether asymmetric shaft stiffness is the cause or a contributor to the high vibrations." My Words: "I will feel good if you can help in this dimension. We have disassembled engines to see what physically happened to cause this vibration but we found nothing wrong with assembly or build up. What I felt is that some triggering of natural frequency at any critical speed that I do not know." Your quoted words: "0.87 x 15000 = 13050.0 rpm that matches 2nd critical at 12690 rpm 2.0 x 13050 = 26100 / 60 = 435 Hz frequency of vibration at 87% shaft speed" My Questions: 1. If engine speed (0.87 x 15000 = 13050.0 rpm) coincides the 2nd critical of PT (12690 rpm) then won't there be an effect on vibration of PT instead of Engine (GP)? Actually, contrary to this, Engine Vibration starts increasing because of 2X component while PT vibration is not even changed? 2. If any rotor passes through 2nd critical speed, which component frequency will show more significant effect of resonance 1X or 2X? 3. Shafts of GP and PT are mechanically separate, (split shaft engine), can critical speed of PT has an effect on GP side?

PAKHTOON,

You started a new discussion piggyback onto a nearly 10-year-old general discussion "horizontal shaft having asymmetrical stiffness" that you apparently believe is in some way related to the cause of your specific problem. It would’ve been far better to read your discussion as separate posting. If I understand correctly, your hypothesis is that the shaft coupling between the compressor rotor and the turbine rotor has asymmetric stiffness, and that is the cause of the 2x vibration response.

The coupling is a rigid joint between two shafts of symmetrical stiffness; as an assumption. The coupling is of symmetrical design, but it may be assembled in a way that causes asymmetric stiffness; as an assumption. The hypothesis of asymmetric stiffness can be tested with the rotors removed from the case and hanging from slings or resting on soft supports at the bearing journals. Impulse-response tests should be conducted on each rotor while uncoupled. This would identify rotor bending natural frequencies; that would be different from the balance resonance frequencies with the rotor on the bearings and in operation. The rotors would then be coupled so that the composite natural frequencies can be measured with the shaft in several index/angular positions while keeping the impulse-response measurements in the horizontal plane. If the coupling is acting as an asymmetric stiffness joint between the two shafts, then there should be a significant difference between the natural frequencies in orthogonal directions. If this is the case, then try reassembling the coupling a few times to see how consistent the results are. This test is obviously limited to evaluating coupling stiffness under cold nonoperating conditions. The test does not indicate what would happen to the coupling stiffness as the shaft thrust load and temperature change. It would be difficult, perhaps not impossible, to apply an axial static load and measure the change in rotor bending natural frequencies. I would be considering other possible causes to the high vibration before pursuing this hypothesis and tests.

My hypothesis for the information that you provided is that the dynamic force is from shaft misalignment at that either a shaft critical speed or stationary structural resonance (most likely) is excited at a very narrow speed range. My hypothesis assumes the opening of the bypass valve to be coincidental with the onset of the high vibration. I realize it was stated that the shaft alignment was okay, but this does not necessarily indicate that the tolerance was adequate (by procedure or adequacy of results) or that thermal distortion of the turbine casing could change shaft alignment during operation. It should be easy to conduct an impulse-response test on the stationary structure including bearing housings, compressor, turbine, and exhaust casings to look for a natural frequency near 435 Hz.

If a thorough test of the stationary structure does not reveal a natural frequency, then it should be assumed that it is a rotor natural frequency that is being excited. There is some evidence that this is possible, since compressor blade tip rubbing was reported.

I missed or misread your bearing description, so sorry for that. I did find it interesting and coincidental that the power turbine critical speed was nearly identically to the frequency of high vibration. The power turbine shaft speed is not synchronous with the gas turbine shaft speed, so it would be a big coincidence if the speeds were the same at 87% speed. I did not see mention of the PT rotor speed, but I assume it is being measured.

You have with reached one clear result from disassembling and reassembling the machine; the same parts and same assembly methods do not change vibration nor lead to an understanding of what is causing it!

Walt

Thanks Walt for your valuable feed back. Based on your comments I have performed Impulse-response tests on compressor and turbine rotors. When test performed on compressor rotor only I found natural frequencies 600 Hz, 750 and 993 Hz. When test performed on turbine rotor only I found natural frequencies 319 Hz, 860 and 1469 Hz. Then I coupled (joined) them both to form a single rotor and performed impulse-response test on various positions of coupled rotor, I found natural frequencies 50 Hz and 220 Hz. For rotors of an other gas turbine I found natural frequencies 48 Hz and 210 Hz. As I have earlier shared that stud that is used to make joint of compressor rotor and turbine rotor is stretched by hydraulic force to a certain elongation (within elastic range). This parameter has upper and lower limit of elongation. Natural frequencies were found to be almost same for both upper limit and lower limit of stretch elongation. I checked contact on the joint between compressor rotor and turbine rotor. I found it 100 % for rotors of these 2 gas turbine engines. Now I am not suspecting asymmetrical-stiffness of compressor rotor and turbine rotor to be the reason of this high 2X vibration. If we roll out this possibility, then we are left with natural frequencies of rotors, stators, static housings or casings. You have also suspected excitation of natural frequency of rotor, based on the fact that blades tip rubbing was observed. Here I add that 2X is almost same (very little difference due to difference in stiffness on both directions) on both Horizontal and Vertical directions. If this would have been found different on both sides then most probability of resonance of casings is to be suspected or not? I will perform impulse-response test on casings and see if any frequency close to 435Hz is found. Suppose we find a natural frequency of 435 Hz, and a resonance is confirmed but the forcing frequency causing resonance will still be unknown. I am sure about one thing that high vibration is exactly 2X of gas turbine operating speed at that time. Since this vibration is found often and not all the time and in all engines. Therefore this is a case of resonance but for only those engines where forcing frequency itself is of such an amplitude that it is resonated to a value to go beyond limits imposed by OEM. Previously I asked you a question "If any rotor passes through 2nd critical speed, which component frequency will show more significant effect of resonance 1X or 2X?" Please provide your comments on this.

Add Reply

Likes (0)
×
×
×
×