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Can beating frequency in a srew compressor be at 22 Hz?(2)

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 mechanicaljw (Structural) 13 Aug 12 11:41
 Hello All, We are having a screw compressor that has 3 male lobes and 5 female flutes that runs on a 90kW, 4 pole, 50 Hz motor with synchronization speed of 1480 rpm. However, at a speed of 1350 rpm, we are experiencing a resonance frequency of 22 Hz. We are unable to pin point the source of this frequency and to tell whether it is torsional or structural? Because the driver is the female i computed the difference between 2 times line frequency, which is 45Hz and the excitation frequency and got 22.5 Hz. In our measurements we are getting 22 Hz and error of about +1 as the resonance freuqncy. So at times we are close to 23Hz. Since i know the beating frequency to be result of two frequencies interacting, i am woundering whether this is the cause of our problem? Or am I doing something wrong? Because for beating to occur from what i have read the two interacting frequencies are usually quite close to one another? What is also happening is that at this 1350 rpm where we have this resoance the torque suddenly increases. So we to figure out whether the this is as a result of the resonance? Any thoughts would be appreciated. Thanks! Jimmy
 Strong (Mechanical) 13 Aug 12 13:07
 1350 rpm = 22.5 Hz!! Am I misunderstanding your stated problem? If this is the excitation frequency, then resonance could be lateral and excited by misalignment or unabalance or it could be torsional. I would conduct impact-response test on motor-compressor-piping and also measure torsional vibrations. I use a stain-telemetry system for both static and dynamic torque (torsional vibrations). Walt
 mechanicaljw (Structural) 14 Aug 12 3:19
 Hi Walt, Thanks for your thoughts. I probably was not too clear in my problem statement. The resonance frequency we are getting at 1350 rpm is 22.5 Hz. So the question now is where is it coming from? So i was thinking that it could be due to the difference between two times the line frequency and excitation frequency (ie 2*45-1350*3/60=22.5Hz). The reason being when i compute this I am getting 22.5Hz, which is the resonance frequency. This difference i am thinking could be interpreted as the beating frequency, since it is as a result of the interaction of two close frequencies. This is where i am not sure and would love to know whether having 22.5 Hz as beating frequency is feasible? Thanks for your suggestions. Jimmy
 electricpete (Electrical) 14 Aug 12 6:01
 What Walt strong said makes sense to me. Isn't 22.5 hz the 1x running speed vibration? 1350/60=22.5 ===================================== (2B)+(2B)' ?
 mechanicaljw (Structural) 14 Aug 12 6:17
 Hi Electricpete, You are right that it is the 1x running speed vibration. So does this mean that this is the source of the problem? And if yes how could this be possibly resolved? I am new and so forgive me if my questions are basic. Thanks! Jimmy
 Strong (Mechanical) 14 Aug 12 11:33
 Jimmy, 2x line frequency is 100-Hz (electrical from Magnetostriction with 50-Hz power) 22.5-Hz x 5-lobes is 112.5-Hz (Pulsation frequency, assuming the female-rotor is direct-driven without gears) The possible Beat Frequency is 112.5 - 100 = 12.5-Hz (Beat period is 1/12.5 = 0.08-seconds and probabaly not audible. Have you actually performed vibration analysis on this machine? Walt
 mechanicaljw (Structural) 14 Aug 12 11:48
 Hi Walt, Thanks again for your input and your suggestions. We have done and still doing vibration analysis on the machine. To your comment. The line frequency at where we are having the the resonance is 45 Hz at the speed of 1350 rpm. The motor has a VFD attached and so at this speed the line frquency is 45 Hz. Inserting that into what you did would give us a possible beat frequency of 22.5 HZ? Or are you suggesting that the line frequency for a motor is independent of the rotational speed? I would be grateful if you can refer me to any citation that best describes how the vibration analysis should be performed. Another critical point (question): We did monitor the input torque and did an FFT of this input torque and clearly we have 22.5 Hz as a resonance frequency in this input torque signal to the compressor. But how do find the component(s) that this input signal is setting into resonance using vibration analysis? Your help is appreciated and I'm grateful. Jimmy
 Strong (Mechanical) 14 Aug 12 12:13
 Jimmy, I forgot about the VFD! 2x line frequency (from VFD) is 90-Hz This is not likely to Beat with Pulsation frequency at 112.5-Hz (yes, the difference is 22.5-Hz, but no Beat here) What is the dominant vibration frequency when motor speed is 1350 rpm? Why do you say a Beat is present; audible or in measurements? As stated before, if dominant vibration frequency is at motor speed, then excitation source is probably unbalance or misalignment. A resonant sructure or torsional may provide amplification. A Beat is not a source of excitation, but it can be the result! Walt
 GregLocock (Automotive) 14 Aug 12 19:00
 A beat frequency cannot cause a strucural problem, as it doesn't actually exist. That is, a signal at 100 Hz, and another at 105 Hz will beat at 5 Hz, but there is no physical energy at 5 Hz. Cheers Greg Locock New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
 mechanicaljw (Structural) 22 Aug 12 12:19
 Another problem or question about why are we having an increase in the torque at this resonance frequency? Any idea or theory behind this behavior? We have done modal analysis (FEA) of our compressor and gear housing and we are getting low frequencies that we suspect might be the source of our excitation. Can this be the case? @Walt: Thanks so much for your input and insight. Well appreciated. To your question about the dominant vibration frequency when our motor speed is 1350 rpm. I am actually not on site where we have our machine. So i have asked the guy in charge to feed me this information. I would let you know when he writes back. Thanks! Jimmy
 BrianE22 (Specifier/Regulator) 22 Aug 12 13:55
 Your force (or torque) can go up or down at resonance depending on whethor you are applying it on the spring end of the spring/mass system or on the mass end of the spring/mass system.
 GregLocock (Automotive) 22 Aug 12 15:43
 Sorry I should add "in a linear system" to my previous post. If you add ratchets or stiction or any number of complex systems then you can generate power at the beat frequency Cheers Greg Locock New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
 mechanicaljw (Structural) 23 Aug 12 2:51
 Hi Greg, Thanks for the response to my question. I wanted to know whether you have any citation (paper or a book, etc) that you can refer me to that discusses the subject of my qeustion is some details? If I understood you correctly, you are suggesting we may be applying the torque at the spring end, in this case our prop-shaft or what? This is because we are seeing an increase in our torque at resonance. Thanks! Jimmy
 amanuensis (Automotive) 23 Aug 12 14:09
 The behavior described by Greg is hard to understand (for me). Linear system : No energy at the beat frequency Non-linerar system : Energy at the beat frequency Is it due to the fact that modes are totally uncoupled in a linear system, while it is not the case in a non-linear system ? Thanks, Amanuensis
 mechanicaljw (Structural) 24 Aug 12 3:23
 Hi Amanuensis; Thanks for the contribution. I would like to say that from previous discussions it was said that the 22.5 Hz resonant frequency is not a beat frequency (please see the posts by Walt & Greg above). This particular machine had an original housing (old design) that did not show resonance at this frequency of 22.5 Hz. The current one is a modification of the old one where the gear housing and drive prop-shaft connection have changed. So like Greg said it is likely due to misalignment or balancing. But this is only one side of the question. The other one is the reason we are seeing the torque increase when we run at the 1350 rpm motor speed? I wanted Greg to provide me with explanation or better with some citations that i can read to try to understand where he is coming from. In case you have any reference(s) I would be glad to have them as well. Thanks! Jimmy
 GregLocock (Automotive) 24 Aug 12 7:15
 There's no cites, it is maths. If you have a 22 hz sine and a 23 hz sine driving a system, then absent non linearities, there is only energy at 22 and 23 hz. Cheers Greg Locock New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
 mechanicaljw (Structural) 24 Aug 12 7:22
 Hi Walt, To your last question about the dominant vibration frequency when the motor speed is 1350 rpm in our machine. The value is at the 1st harmonic of the first stage gear frequency (2880 Hz). Thanks! Jimmy
 mechanicaljw (Structural) 24 Aug 12 11:44
 Hello Greg, I have a follow up question about why the torque is increasing at resonance. Can we say that because we are seeing a torque increase at resonance that the resonance is torsional? Thanks again and forgive me if my questions are basic. I am new in this area and still learning. Regards, Jimmy
 GregLocock (Automotive) 24 Aug 12 17:48
 Sorry I look at data and plots, working from verbal descriptions usually drives me bananas. Cheers Greg Locock New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
 amanuensis (Automotive) 25 Aug 12 11:46
 Yes, but non-linearity does not necessarily lead to energy at the beat frequency. The main effect of non-linearity is certainly harmonic distorsion.
 mechanicaljw (Structural) 27 Aug 12 4:47
 Hi Greg, Thanks again for the response.I was wondering what specific data you would love to look at? I have data for torque input measurements and FRFs data from vibration analysis that has been done. Kindly let me know if you need any specific data. Thanks! Jimmy
 GregLocock (Automotive) 27 Aug 12 5:03
 Well OK, let's see your spectra from the running system and some FRFs, in excel or csv format. Cheers Greg Locock New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
 electricpete (Electrical) 27 Aug 12 7:47

Quote:

use a stain-telemetry system for both static and dynamic torque (torsional vibrations).

Quote:

Another critical point (question): We did monitor the input torque and did an FFT of this input torque and clearly we have 22.5 Hz as a resonance frequency in this input torque signal to the compressor.

Quote:

I have a follow up question about why the torque is increasing at resonance. Can we say that because we are seeing a torque increase at resonance that the resonance is torsional?
What is the ratio of the 22.5hz torque to the static torque?

I agree, more data would be good. Tough to follow.

=====================================
(2B)+(2B)' ?

 mechanicaljw (Structural) 31 Aug 12 5:14
 Hello All, I am grateful for all of your assistance. @ electricpete:The compressor torque peak to peak variation is 650 Nm @ resonance compare to 80 Nm @ non-resonance conditions (pls see the attached excel file). @Greg: I have been trying to figure out what would useful for you. I am attaching this file that shows some post-processing of our torque data together with those that were taken in the vibration analysis. The first stage gear seems to be where we are having a problem. The vibration amplitude there at the critical speeds is consistently very high and during tear down a wear on one side of the gear was observed. Please let me know if you have any thoughts. Thanks! Jimmy http://files.engineering.com/getfile.aspx?folder=b2309079-2af1-4ab9-9cde-98
 electricpete (Electrical) 31 Aug 12 12:00
 Aha. Very good data. LPS for that. It appears you have some harmonics of running speed in your torque spectrum regardless of speed. But "vertical" (constant-frequency) line at 22hz in the plot certainly suggests resonance at 22hz as you concluded. And I can't tell from the plots, you stated 650N-m pk-pk variation at 22hz vs 80N-m at other speeds. I was interested in ratio of that to the static power as rough indication of the severity. I don't know your static power or torque but can covert 650Nm at 22hz.. equates to 90kw pk/pk power variation or 45kw each way. Your 90KW motor when operating at reduced speed has thermal limit corresponding to constant torque. So at around half speed, the motor is derated to 45KW. If motor was operating at it's max steady load of 45kw (based on thermal rating), the power would be oscillating 0 to 90KW. I guess that's pretty severe. fwiw I tend to agree with your diagnosis of torsional resonance. ===================================== (2B)+(2B)' ?
 electricpete (Electrical) 31 Aug 12 12:04
 Thinking through in general ways to attack a torsional resonant condition. 1 - It requires resonance and excitation. I'm not sure what is the source of 1x turning speed torque excitation throughout the speed range. Even 80N-m at non-resonant condition sounds like a bit of a swing. What causes it? Misalignment? Beats me. Might be worth investigating as a first easier approach. 2 - Other approaches I think would seek to detune or dampen the resonance. A pretty big step requiring machine modification. ===================================== (2B)+(2B)' ?
 electricpete (Electrical) 31 Aug 12 12:08
 By the way since you have a variable speed machine... don't know where operation requires the speed but of course detuning may just push the same problem to another speed. ===================================== (2B)+(2B)' ?
 mechanicaljw (Structural) 4 Sep 12 6:17
 Hi electricpete, I am grateful for your insights and your thoughts. I have taken them into account and would speak with my colleagues to see if we can look at some of your recommendations. I will also look at the torque ratio that you requested and get back to you. We had concluded that damping might do for a short term fix, just like you suggested. So we are in the process of looking at the appropriate material for doing that. We may want to consider damping the structural vibration and the torsional as well to see which would have the most impart. To your comment: "By the way since you have a variable speed machine... don't know where operation requires the speed but of course detuning may just push the same problem to another speed"-I agree that when you detune you may only shift the energy in the system to another speed. I think if it were to shift to higher frequencies that would be good but we are constrained by the operating range of the machine. And because it is fixed we cannot do anything about this. I'll let you know what happens when the damping is done. Thanks! Jimmy
 electricpete (Electrical) 4 Sep 12 8:35
 Do you have any thoughts about what source of excitation is creating the 80N-m torque oscillation at 1x running speed, even during off-resonant conditions? ===================================== (2B)+(2B)' ?
 mechanicaljw (Structural) 10 Sep 12 4:43
 Hello Can you comment further on your previous question "Do you have any thoughts about what source of excitation is creating the 80N-m torque oscillation at 1x running speed, even during off-resonant conditions?" Do you mean that we still see peaks @ 1x rotational speeds in the torque FFT data even when we are running at non-critical speeds? I was thinking about what you also meant by static power? in one of your previous questions? Thanks! Jimmy
 mechanicaljw (Structural) 10 Sep 12 4:44
 The previous post I sent was intended for Electricpete. Sorry that i forgot to address it to him. Thanks! Jimmy
 mechanicaljw (Structural) 10 Sep 12 8:14
 @ electricpete The major problem with the torsional vibration theory is that we cannot change the 1st mode by varying prop shaft stiffness. Can you give this some thoughts. Why do you think this is happening? Thanks as usual. Jimmy
 mechanicaljw (Structural) 10 Sep 12 10:38
 @ electricpete For the last question i am thinking because we used the static stiffness instead of the dynamic stiffness? Can that be possible explanation for why the natural frequency did not change with change in the stiffness? Thanks! Jimmy
 electricpete (Electrical) 10 Sep 12 14:43

Quote:

Can you comment further on your previous question "Do you have any thoughts about what source of excitation is creating the 80N-m torque oscillation at 1x running speed, even during off-resonant conditions?" Do you mean that we still see peaks @ 1x rotational speeds in the torque FFT data even when we are running at non-critical speeds?
Yes, 1x and several harmonics of 1x are present at all speeds in your torque waterfall/campbell plot labeled “Autospectrum(torque) – Input”. We can’t judge the magnitude from those plots, but the text underneath it states that there is 80 N-m pk/pk of torque oscillation even at non-resonant conditions.

Is it a lot? I’m not sure but I tend to compare it to the “static” = steady = non-varying = dc component of the torque.
The rated torque of the motor is P / w = 45,000W / (2*Pi*25hz) = 286N-m.
I assume you are operating at or below the peak torque rating. So the peak-to-peak torque variation you are seeing even at non-resonant conditions is at least 80/286 ~28% of the steady torque. It seems like a lot to me. Certainly would not be expected for centrifugal machines ... I don’t know about lobe compressors.

Regardless of whether it’s expected or not, if you have this non-insignificant excitation present at non-resonant conditions, it’s not surprising it will increase by a factor of 10 or so when you vary speed to resonant conditions.

Since varying speed is part of the application, seems worthwhile to investigate the source of the 1x torque oscillation and see if it can be reduced.

Quote:

The major problem with the torsional vibration theory is that we cannot change the 1st mode by varying prop shaft stiffness. Can you give this some thoughts. Why do you think this is happening?
Again the torque waterfall plot seems to provide compelling evidence of torsional resonance in 2 respects:
1 – you stated the highest torsional variation occurred when speed was 22.5hz and torsional variation decreased away from that point
2 – there is a white band at 22.5hz for all speeds.

I think buried in your question is a statement that you have attempted to stiffen the shaft, but saw no change in behavior, is that what you’re saying?

It may be that the torsional stiffness of the shaft is much higher than other torsional stiffnesses in the system (like the coupling). In that case, doubling the stiffness of the shaft does nothing if you don’t change the coupling (I’m neglecting any weight changes associated with this stiffening). As a simplistic example: imagine you have a series combination of springs with spring constants 1E6, 1E3, 1E6. The effective spring constant of the series combination is about 1E6. Now double the 1E6 stiffness and leave the 1E3 alone. The effective spring constant of the series combination is still about 1E3.

=====================================
(2B)+(2B)' ?

 electricpete (Electrical) 10 Sep 12 14:46
Sorry, typo correction. Change 1E6 to 1E3 as follows:

Quote (electricpete)

As a simplistic example: imagine you have a series combination of springs with spring constants 1E6, 1E3, 1E6. The effective spring constant of the series combination is about 1E3. Now double the 1E6 stiffness and leave the 1E3 alone. The effective spring constant of the series combination is still about 1E3.

=====================================
(2B)+(2B)' ?

 mechanicaljw (Structural) 14 Sep 12 5:44
 Hello electricpete, Thanks again for your valuable contribution. I in deed saw the typo and was just about to ask when you wrote to correct it. The last point you made about the stiffness not changing because of the possibility of connection to other flexible joints that have lower stiffnesses was spot on. To further investigate this we have a test coming up that would look at a rigid or a solid drive shaft with no flexible joints. We hope that this will raise the first mode frequency away from the lower frequencies we have been having. To look at the effect the prop-shaft torque might be having on the system we have repeated the measurements with a hydraulic drive motor and the compressor sound was smoother when compared to the prop-shaft driven unit. The torque increase at 1350 rpm vanished and from the run up data there was no resonance freq below 100 Hz and the over all vibration level was lower. So it is safe to say that the compressor itself is not the source of the problem. The only other observation that we need explanation for is the fact that there was still large fluctuation in the torque. The question now is why? Could there be some form of coupling between the torque signal and the pressure inside the compressor? Kind of a flow induced problem? Your thoughts would be appreciated as usual. Jimmy.
 electricpete (Electrical) 20 Sep 12 8:30

Quote:

To look at the effect the prop-shaft torque might be having on the system we have repeated the measurements with a hydraulic drive motor and the compressor sound was smoother when compared to the prop-shaft driven unit. The torque increase at 1350 rpm vanished and from the run up data there was no resonance freq below 100 Hz and the over all vibration level was lower. So it is safe to say that the compressor itself is not the source of the problem. The only other observation that we need explanation for is the fact that there was still large fluctuation in the torque. The question now is why?
So, if I understand correctly, when using hydraulic motor, you still had torque oscillations (on the order of tens of N-m) accross a broad range of frequencies? The only difference is that you didn't run into a frequency where this oscillation increased to hundreds of N-m?

If the answer to my two questions above is yes, then I would think the excitation remains and the resonance was removed or damped.

How removed - perhaps moved in frequency above 100hz due to much lower rotating inertia of the hydraulic motor than the electric motor possibly combined with shorter shaft (?).

How damped - perhaps the hydraulic motor provides significant torsional damping (?)

Quote:

Could there be some form of coupling between the torque signal and the pressure inside the compressor? Kind of a flow induced problem?
Beats me.

=====================================
(2B)+(2B)' ?

 electricpete (Electrical) 20 Sep 12 8:32

Quote:

How removed - perhaps moved in frequency above 100hz due to much lower rotating inertia of the hydraulic motor than the electric motor possibly combined with shorter shaft (?).
also of course stiffer coupling?

=====================================
(2B)+(2B)' ?

 GregLocock (Automotive) 24 Sep 12 1:58
 Hi Jimmy Can you restate exactly which problem you are trying to solve? Looking at your torque campbell's plot I have a hard time believing the 22 Hz vertical 'resonance' line is a mechanical resonance. Could you plot a narrowband, say 20-25Hz amplitude vs rpm for that torque data? Cheers Greg Locock New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
 mechanicaljw (Structural) 26 Sep 12 2:54
 Hello Greg, I am working on getting the narrowbank plot and will send it as soon as it is done. To your question about what we want to do. We have a screw compressor that is run by a diesel engine from a tractor. The drive is a propshaft. When the compressor runs, we are getting this strange noise @ 13 Hz. When the drive is changed to an electric motor, the noise we are getting is @ 23 Hz. The campbell's plot that i sent was for the latter. Just recently, we replaced the electric motor with a hydraulic drive and campbell's plot of the torque showed no resonance below 100 Hz. The resonance freq we had was @ 700 Hz. The machine was not noisy and the torque increase was not significant, which is not the case for the others that i have mentioned above. The task we have on our hand is to do away with this noise and to reduce the torque @ the resonance frequency. We first must answer whether the problem is a mechanical resonance or a torsional problem. The line diagram attached may help clarify the problem. In this case the electric motor is the drive and frequency or speed of the motor is identical to the frequency of the noise we are getting in the system. The same is true for the diesel engine drive. As a last resort we are working on using a torsional damper or a mechanical damper to see if this would elimniate the problem. @ electricpete: I agree that in the case of the hydraulic drive the resonant frequency was probably shifted from the 23 Hz to 700 Hz or damped. The peak torque that existed in the case of the electric motor or the diesel engine no longer existed, only that it does fluctuate. Recent test with Solid propshaft: 280 Nm peak torque amplitude (much lower than standard propshaft) 40 Hz Resonance frequency For the standard or original propshaft: 700+ Nm peak torque amplitude Resonance freq (believed to be first mode torsional) is @ 24 Hz Care was taken to ensure the same shaft length and dia. What are your thoughts? Can we say the frequency change of the 1st mode when the standard propshaft is changed to a solid one is an indication that we have torsional resonance? Thanks! Jimmy http://files.engineering.com/getfile.aspx?folder=639c9075-6a06-465a-99cf-45
 jeyaselvan (Mechanical) 11 Oct 12 4:45
 Hi Jimmy, Sorry for commenting a little late on the thread. I have earlier worked quite a lot on a beating noise problem with a screw compressor, for which you can see more information on thread384-276533: Beating / modulating noise and vibration Beating / modulating noise and vibration. My question here is " Are you concerned about the beating noise for frequencies around beat frequency of 22Hz"? The reason I am asking this is that usally beating will be of audible concern only if it is in the range of 0.5 to 5 Hz typically. Beyond this, perceptionally we may not able to identify a beat. Is it that your case is a typcial case of resonance or rather operating closer to a resonance?. I have earlier faced and measured a typically beat like torsional response, while I was operating closer to a torsional natural frequency of the drive train. I remember even in that case, the dynamic torque goes very high is of concern.The strange noise is common when you are operating at or close to the natural frequency. Based on the informtation you have provided so far on the shift in natural frequency of the drive train for engine, electric motor or hydraulic motor are typical behaviour of drive train with varying torsional inertias. My suggestion would be that you do an analytical torsional analysis based on drive train inertias and stiffnesses based on lumped masses approximations and verify they reflect the same as you measured. Normally, the first few torsional modes are of concern. Regards Jeyaselvan
 mechanicaljw (Structural) 11 Oct 12 9:34
 Hi jeyaselvan, Thanks for writing and for your insight. Never mind that you responded late. It is better late than never. I have actually seen your previous posts on this subject and i read it extensively to see if i could find a clue to our problem from it. I have also taken what you suggested in you recent post. I think like you rightly said it cannot be a beating problem and we are focusing more on torsional resonance probably due to the drive train. One of our approaches has now been to numerically predict the torsional modes we are measuring and then we hope to tune the system away from torsional resonance frequency by making some changes to the shaft design. Our problem is that we have not been able to match the FEA stiffness we are predicting with that that would be needed to give us the measured torsional lst mode frequency of 40 Hz. For the point mass model we have we are using FEA to determine the shaft stiffness that we seem to be over-estimating and so we are getting a numerical 1st modal frequency of 60 Hz. Did you have to do something like this and if yes how did you estimate your shaft stiffness? We are using ANSYS STRUCTURAL to do the FEA. The other approach we are trying is to use torsional damper to see if that would help. Thanks! Jimmy
 jeyaselvan (Mechanical) 11 Oct 12 10:07
 electricpete (Electrical) 11 Oct 12 10:27
 Is the original/standard shaft hollow? If so, fwiw I think it reinforces the conclusion of torsional resonance (also strongly supported by the torsional campbell diagram). ===================================== (2B)+(2B)' ?
 electricpete (Electrical) 11 Oct 12 10:34

Quote:

If so, fwiw I think it reinforces the conclusion of torsional resonance (also strongly supported by the torsional campbell diagram).
I should've clarified what I meant by "it".
What I meant was
If so, fwiw I think your experiment of changing hollow to solid shaft and examining effect on torsional oscillation magnitude and frequency reinforces the conclusion of torsional resonance (also strongly supported by the torsional campbell diagram).

=====================================
(2B)+(2B)' ?

 mechanicaljw (Structural) 12 Oct 12 5:00
 Hi Jeyaselvan, Thanks again for sharing and your input. It has been helpful. I will try to answer some of your questions and for those that i did not understand i will appreciate it if you can explain in some details. 1. About whether our drive train is resilient. I would say yes and we have approximated the system as a two point mass system like you said in your post. This was because in previous calculations the mode shape for 1st mode shows all masses rotating relative to prop shaft. Due to rigid body motion the 1st mode freq' was always therefore governed by K more so than I1 and I2. So for example if you doubled the inertia of the 1st stage input gear mass the effect would be negligble So for quick rough calculations we used 2 mass + k 2. The question for us now is how to determine the stiffness. The k we have been using so far is the static k but from your previous post it must be the dynamic k? 3. Do we know the source of the 1x excitation from teh engine with actual end application? I would say the source is the input torque from the engine or the electric motor. The type of engine: we have in the real drive a 9.6L 234kW in line 6 cylinders diesel engine with firing order 1-5-3-6-2-4, firing every 120° of crankshaft rotation. The electric motor for the test stand in the lab is a 90kW 4-pole 50 Hz driven by ABB ACS800 inverter VFD. 4. Quote "In any of the case, it is hard to have a first order excitation, unless u have severe misfiring resulting in half order excitations".. Can you please explain this further. I do not understand what you mean exactly. Are you saying that having 1x excitation like we do is because of misfiring? Not sure what misfiring here is referring to? Thanks! Jimmy
 jeyaselvan (Mechanical) 12 Oct 12 6:30
 mechanicaljw (Structural) 15 Oct 12 9:47
 mechanicaljw (Structural) 16 Oct 12 6:24
 Hello Jeyaselvan; I just realized that in trying to answer questions 1&2, i made a mistake. It should read "strain gauge on slip rings" and not "string gauge on slip rings". Sorry about that. Thanks! Jimmy
 jeyaselvan (Mechanical) 17 Oct 12 9:27
 Hi Jimmy, 3. Since this is a 6 cyliner engine, the thrid torsional order and its harmonics are expected to be dominant apart from the torsional excitations from the airend, which are at the lobe meshing order and its harmonics. The lower half orders and first order excitation comes from the distortion in the Torque - crank diagram, which I have attached, for a 5 cyl engine for illustration. You may refer to text book by Nestorides ( I have a recent edition) / Den Hartog more information on torsionals. 4. Sorry, I meant the cancellation of the primary and secondary forces and moments. Torsionally, still I am still unaware of the source of 1X excitation. Even if you have a natural frequency at 11/12Hz, there need to be an excitation to excite the 1X. For laterals, your unbalance may be good enoguh. By the way, is the whole assembly on mounts?.any check on mounted resonant frequency measurements? Were you able to predict the 11/12Hz and 22Hz natural frequency from your torsional model? Can you explain more than "This is due to the forced frequency of the engine opposed to compressor input frequency which we normally have on electric motor tests" 5. Yeah, I understand. this is will aid in packaging. Since my experienece with oil free pressure compressors (higher pressures), where in power transmission through timing gears are not often desired. That should be fine for supercharger kind of applications. Regards Jeyaselvan http://files.engineering.com/getfile.aspx?folder=954a79c1-7280-4a71-9347-2b
 mechanicaljw (Structural) 19 Oct 12 2:31
 Hello Jeyaselvan, Thanks for writing and for providing me with more information. Appreciated. To the points we've raised: 4. Q. Is the whole assembly on mounts? Ans. No, the compressor is suspended from the side on the chassis of the truck. I am attaching a picture to give you an idea of what i mean. Not sure if the side-mounting in this case can be a source of the problem? Our torsional model has not been able to predict the 11/12 Hz and 22 Hz. When we modified the drive with a solid hydraulic drive, the 22 Hz excitation frequecy in the torque vanished and the noise and torque peak did not exist. The resonance frequency became 40 Hz. The two point mass torsional model in this is predicting 60 Hz instead. And we suspect the problem to be coming from the stiffness determination of the shaft. We determined k using FEA simulation in ANSYS Workbench. Not sure about what we are doing wrong. I know the k should be dynamic and what FEA provides i think is static. So we want to determine the dynamic k and see what that would give us. Quote: "Can you explain more than "This is due to the forced frequency of the engine opposed to compressor input frequency which we normally have on electric motor tests". You are right that we are not sure about this explanation. What i just meant is that the 11 Hz becomes excited when the driving frequencies of the engine excited it. But i am thinking the explanation you provided might be it but i have to try to find the books you recommended to do some more reading. Our approach now is to predict the torsional frequency numerically and then tune the shaft away from that frequency. We are also seeking a torsional damper that we want to integrate with one of our prop shafts for it to be tested. Thanks again for your input. Regards, Jimmy http://files.engineering.com/getfile.aspx?folder=bc9c5b9c-9748-40df-88d6-3c
 mechanicaljw (Structural) 22 Oct 12 2:35
 Hello Jeyaselvan, I forgot to add in my previous email that we do have PTO ratio from the diesel engine to the compressor and it lies between 1.2-1.5. The compressor speed is therefore higher. This i am sure is also playing a role in why we are having resonance at 11/12 Hz. My question is that given that our speed range is 1000-1800 Hz and we are having resonance at 11/12 Hz which is less than the lowest compressor speed, could it be that the 11/12 Hz is not a resonance frequency but an excitation frequency? Thanks! Jimmy
 mechanicaljw (Structural) 25 Jan 13 3:30
 Hello Jeyaselvan, I am looking at your response dated 11th October 2012 to this post, where you said you had a problem with a beating close to a torsional frequency and that your torque magnitude increased. I would be grateful if you can share more light on this and how you were able to resolve it. In the lastest measurement we have done on a MAN Truck Chassis we are experiencing beating around a torsional frequency of 10 Hz that seems to be present at all speeds. The source of this we believe is the engine. The question is we are also experiencing a pulsation that we are thinking is structural @ 11.7 Hz. Could it be that the 10 Hz is exciting this 11.7 Hz in the compressor or chassis and the interaction between these frequencies could be the source of the beating we are having? Thanks in advance for your input. Jimmy
 jeyaselvan (Mechanical) 8 Feb 13 1:45
 Hi Jimmy.. Yeap.. that was almost close to 10yrs back..it was on a test rig with motor(thru VFD) & compressor through a disc pack bibbys coupling on either sides of the torque transducer, wherein we had that isssue. I managed to pull back a snap shot of the measured data..the units are as measured in Volts..you could see the mesaured torque beating at the speed closer to resonance.. In that case, the main concern was not beating, but the abnormal behaviour of torque transducer (strain gauge based) operating at resonance.(we even had mechanical failures of the torque transducer instrumented shaft, a couple of times, until I diagnose the problem). With stiffness increase of the weakest element in the link, we moved it outside our operating speeds range. That was the first time I was exposed to torsional resonance! In your case, just check your beat freq is indicating your assessment. Slips under direct drive combination could end in beating . May be you could some run up / down studies to see any obvious resonances are in your frequency range of concern? If you could instrument for torsionals, that would be great.If not, you may use conventional lateral measurements, since you often expect some indicators due to torsional/lateral coupling, if you are lucky. Regards Jeyaselvan http://files.engineering.com/getfile.aspx?folder=b859b3f1-3fc8-4ece-91bd-18
 mechanicaljw (Structural) 12 Feb 13 6:18
 Hello Jeyaselvan; As usual, thanks for the input and the attachment. What we are experiencing is similar to the plot you sent to me. Does it mean you had a modulation frequency? In our case we are thinking that there is this 10 Hz modulation that we cannot show where it is coming from, but suspecting that it would be torsional. In the lateral measurements, we had a frequency close to it (11.7 Hz) and when that mode was suppressed (by bracing to increase stiffness) the frequency and peaks remained about the same (10 Hz noise was still being heard). Leading us to think that it cannot be a resonant frequency. We do not have instruments for torsional, unfortunately. The other observation is that the noise coming from the compressor itself is at a much higher frequency but for each of the peaks at the higher frequencies we have a side band of 10 Hz. I wanted you to share any thoughts you might have based on my input above. In one of your posts you indicated that if we use conventional lateral measurements we can expect indicators due to torsional/lateral coupling. Can you elaborate more? Thanks! Jimmy
 jeyaselvan (Mechanical) 12 Feb 13 8:48
 Hi Jimmy, That attachment was of a beating. The beating was between 90Hz (torsional excitation frequency) and 97Hz(torsional natural frequency). (the data attached is more illustrative) The beating pattern could be due to the existence of a) two closer torsional excitation frequencies or torsional natural frequencies and b)if you have an excitation frequency closer to a torsional natural frequency frequency. May be if you can change the speed by 10-20%, then you can observe for changes in beat frequency. If this is so, you may have type (b) beat, and if this is not, you may have type (a) beat. Then you can investigate your system with more detail / refinement. May in you case, since lobe meshng is dominat, suggest looking for natural frequencies close to your lobe meshing frequency (atleast two orders)..Hope this helps Reagrding your query on torsional / lateral coupling, since in most drive systems with gears / rotors , there is every chance that torsional resonance can be captured in housing vibration measurements. The torsional dynamic amplifications often induces lateral forces, and hence be captured in housing rup up measurements. Sometime back I was lucky enough to capture a torsional resonance induced gear rattle due to gear mesh excitations around 2000Hz through normal housing vibrations (this case in axial directoin, the gears being helical), which was subsequently verified analytically through a Campell diagram for torsionals and experimentally through angular velocity measurements. Regards Jeyaselvan http://files.engineering.com/getfile.aspx?folder=c7aec889-7288-4f84-8e34-8f
 mechanicaljw (Structural) 13 Feb 13 4:21
 Hello Jeyaselvan, And thanks for the prompt response to my previous post. Appreciated. I wanted to say that we are about 90% certain of the source of our problem. We have a modulation frequency in our torque signal (10 Hz on the MAN truck and about 12/13 Hz) on the test stand (electric or diesel). In all the tests that has been carried out for different screw compressors these frequencies are always present. So we have concluded that this modulation is driving the response of our compressors. In one case, where the synchronization gears are located at the opposite side of the air end (same side as driving gears) the response is less harsh and quieter when compared to the case where the sny gears are located at the air end. Even though on the test on the deisel engine we also see a torque peak fluctuation for the case with syn gears on opposite end of air end, the noise is still lower. From a torsional model we realized that there is a large difference in the rotor deflection modes of these two configurations. The former is about 838 Hz and the latter about 297 Hz. From the mode shapes we are thinking the sny shafts seems to be problematic because in the latter we have to drive through 2 additional shafts and so we are thinking it has more compliance. The sny shafts diameters are also different. Smaller in the latter than in the former configuration, which we think might be compounding the issue. The lower mode of the latter is making us to believe that they are being excited by the lobe passing freuqency? The idea now is to modify the case where we have sny gears on the air end and see what that would do. So the question is whether you have any previous experience with working with these kind of configurations and whether you faced such issues? Thanks! Jimmy

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