×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Torsional analysis-Model Setup

Torsional analysis-Model Setup

Torsional analysis-Model Setup

(OP)
Hello All,

I am trying to set up a torsional model for a drive train. In the attached picture i am stock at the area that i have circled in red. How do i account for Nodes 10, 11, 12, 15 and 16 in this model? Can setting up an equivalent system with the loads at the nodes be done for such a system? The N denote nodes and G denote gears. The drive train is for a screw compressor.

Thanks in advance for your help.
Jimmy

RE: Torsional analysis-Model Setup

(OP)
Hello Greg,

I need to set up a torsional model for the drive train shown to be able to predict the torsional frequencies. In the previous cases I have considered, the synchronization gears (G6 & G7) were on the driven end (opposite end of where they are located now) and so the drive shaft with nodes 11 and 12 was connected ONLY to the male rotor and then to the synchronization gears and then to the female rotor. In that case i was able to treat the system as a reduced system (equivalent system, where the gears and other mass points were put onto a single shaft) and get the torsional model. But from the literature what i have now after node 9 is more like a branched system since i have two shafts sharing the input torque from the drive. It is my first time encountering this type of a system and so i was wondering about how i could set up the torsional model in such a case.

I hope that I have been able to explain myself?

Thanks!
Jimmy

RE: Torsional analysis-Model Setup

You have two options:
Use a program that can cope with a branched system in which you just model every mass and stiffness at eacht respective speed.
or, if you can only model straight lines, sum masses 15-18 with node 7 (remember to account for the speed difference), or, since a gear is of near infinite stiffness, sum nodes 7+15-18 to node 6.
That is an approximation, which is valid only if shafts 7-15 and 16-17 are stiff (no elastic coupling or long, slender shafts)
It will give information on the natural frequency of the main branch (6+11-14), but of course not of the lunped branch. You can repeat this and summ the masses of the main branch to node.
Like I said, this is an approximation. Better is to use a branched model.

RE: Torsional analysis-Model Setup

(OP)
Hello Rob768,
Thanks for your input and suggestion. I have been thinking along the same line of using the branch system approach.

I have two additional questions:
1. About the branch system approach. Is there any other method other than using the transfer matrix approach?

2. This is a more general question. When i run some of the torsional models I get imaginary values for some frequencies. What is the exact interpretation if a torsional frequency is imaginary?

Thanks!
Jimmy

RE: Torsional analysis-Model Setup

Hello Jim

if you want only the natural frequencies, you might use a finite element program. Remmember to reduces all masses and stiffnesses to a common speed.

I don't know what you mean by the imaginery values. It may be related to the damping generated, but the program i use only gives real values.

RE: Torsional analysis-Model Setup

imaginary frequency implies that either your stiffness or mass term has gone negative. This is unlikely.

By combining the two paralel rotors into one you eliminate an important degree of freedom, there is a very simple mode shape in which the main rotor is nodal and the two rotrs are in antiphase.

As such it seems to me you need a more complex model. What program are you using?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Torsional analysis-Model Setup

to get the mode Greg refers to, it is also similar to lump the main branch. I agree it may be an important one, especially if the installation is symmetrical and you might get beating effects.

RE: Torsional analysis-Model Setup

(OP)
Hello Greg and thanks for your input. Same to rob768. I am actually setting up the model manually and using MATHCAD for solving for the frequencies and the mode shapes.

I sure want to consider the branched approach as you (rob768) suggested in one of your previous post.

Thanks!

RE: Torsional analysis-Model Setup

(OP)
Hello rob768 and Greg,

I just wanted to know if you could recommend any program can do such a calculation (with the branching) for me? I have some limitations in Mathcad with regards to the size of the matrix it can handle.

Thanks!

RE: Torsional analysis-Model Setup

We use an in-house developed program, as well as the DNV-Pilot program. For just natural frequency calculation, any finite element package will do I suppose. I am not familiar with Mathcad, but a model can't be that extensive?

RE: Torsional analysis-Model Setup

It can't be more than 20x20 can it? mathcad should knock that over in a flash. If it is bigger than that, matlab, octave (free), or perhaps you could use dymola, adams, simulink or similar to build a component based model.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Torsional analysis-Model Setup

(OP)
Hello rob768,

I am having some issues with getting the residual torque for the branched system attached in this post @ I3. When I do it for another case where the Inertia of the gears (Ig) is neglected i am getting it right. However, when it is taken into account my residue torque plot versus omega for using the Holzer method is giving me only the first frequency. I am not getting the second. I am not sure what i am doing wrong.I have been also able to compute the angular deformations correctly at end Inertia.

Any input would be appreciated.

Thanks!
Jimmy

RE: Torsional analysis-Model Setup

It's been some time since i last did it by hand, but with a branched system, you need to add the inertia torque of a secondary branch to that of the main branch. So, considering 1-3-2 as main bracnh, the torque of branch 4 is added to 3.
In the book of Den Hartog you'll find a good example of an iteration scheme for a branched model.
The number of modes is equal to the number of masses and will include a trivial mode.

RE: Torsional analysis-Model Setup

Been playing with the numbers a bit. I noticed that the natural frequencies of each of the "free" masses is 62.5, when considering the central node as fixed. This kind of symmetry may cause the iteration to cause errors or lead to "missed" frequencies.

RE: Torsional analysis-Model Setup

Agreed, most modern programs solve the problem by matrix calculation and will not miss a mode. However, older programs using the Holzer iteration method may go seriously wrong if two (or, as in this case) more modes are within one step of the iteration process. You can see it in the modes, but only if you are very aware of the possibility.
Besides, there is no reason to solve this problem by a holzer tabulation.

RE: Torsional analysis-Model Setup

(OP)
Hello rob768,

I had been busy finding the values like the stiffness, Inertias, of the system attached in the NodesDriveTrain.jpg file and so i have not been able to set up the model. I am now at the stage where i can set the model up and I was wondering if you can suggest how the idle shaft between gears G1 and G3 or on the middle idle gear should be included in the drive train network for the torsional model. Also, I would be grateful if you can throw more light on the way you would set the model up in case you wanted to use the branched network approach. I would also want to use the input shaft as the reference shaft as this shaft is connected to the electric drive, which has a large moment of Inertia. I would be glad if a sketch is included.

Thanks for the suggesting the book by Den Hartog. It helped me to understand the way the junction should be treated in torsional systems.

Thanks!
Jimmy

RE: Torsional analysis-Model Setup

Hello Jim

i don't see the attached file you mention. Forgotten?

RE: Torsional analysis-Model Setup

(OP)
Hello rob768,

I actually sent that file in the first post i sent to the group. It is the post with the drive train line diagram. It should still be attached to this post. In case it is not kinldly let me know.

Thanks!
Jimmy

RE: Torsional analysis-Model Setup

Ah, that one..
Basically a gear connection in a TVC can be seen as near-rigid. I use a high value stiffness (1E8 kNm/rad) for a gear and in the example that would lead to:
J(G1) -> C(1e8) -> J(G2) -> C(1e8) -> J(G3). Then shaft n7 and onwards.
But, as a gear wheel usually has relatively low mass, you can also consider summing all gears to a single value. You will not loose much accuracy there (don't forget to crrect for actual speed)

RE: Torsional analysis-Model Setup

(OP)
Hello rob768,

I wanted you to see if you can help me with some questions. We have done the torsional anaylsis of the drive train for various machines and compared their relative deflections. For four of them, the trend of the relative deflection we're getting from the models agrees with torque measurments we have done. The best of them in terms of torque peak has the lowest relative deflection from the torsional model. But for one of them, the model is predicting high relative angular deflection while in measurement this machine is giving the lowest torque peak at resonance. The question is why? Could it be that we have more damping in this machine than in the others and damping has not been accounted for in the torsional model?

Plus in this machine we have one stage less than in the others and as a result the step up ratio is very large and hence the wheel is very large. Does this flywheel play any role in damping the torsional vibration? I am thinking because it is upstream it may be damping the excitation in the input torque to the machine??

In case you have experience with screw compressor design, i would like to know what elements in the drive train you think are most critical when it comes to torsional resonance? I have done some sensitivity analysis for stiffness and inertia and seen that k within the machine is not affecting the angular deflection much,except for the k value of the input shaft and prop-shaft combined. For the inertia, i have realized that increasing the male rotor inertia has the largest effect on the angular deflection. So what we did was to try to add an additional mass to the male rotor (the drive is through the female rotor) which led to 27% increase in the inertia of the male rorot. The torque peak we got in the measurement remained about the same, but the model predicted that angular deflection showed a larger drop. The question is why? And would increasing the added mass further bring anything good? Does it matter where the mass is put in the drive train?

The other school of thought is that in the first stage of the XK machine we have a larger wheel and we are thinking that the larger angular kinetic energy of this wheel is playing a role in making it respond less to the input torque than in other machines. We also suspect that the oil sump that this wheel is running through is having some damping effect?

It would be great if you can share your thoughts on the issues I have just raised.

Thanks!
Jimmy

RE: Torsional analysis-Model Setup

hello Jimmy,

Difficult to answer all questions. Note that the angular deflection is a relative one only. it tells you little on srtess, because a long nut relatively large diameter shaft will still deflect more that a short, thinner shaft, whereas stress in the long shaft may be lower.
A flywheel does not dampen torsional vibration, but it will have an isolating effect. Vibration does not pass a flywheel easy. A large flywheel on for instance an engine keeps the vibrations "in" the engine, leading to increased torsional stress in the crankshaft, and less vibration in the rest, while a lighter flywheel results in lower crankshaft stress, but more vibration in the rest.
An increase in inertia generally leads to increased deflections. That is not necessarily a bad thing, as long as resonance is omitted.

hope this helps.
regards
rob

RE: Torsional analysis-Model Setup

(OP)
Hello rob,

Thanks for the info. It is helpful. I wanted to know whether you have a background in electric motors? I want to consider forced torsional vibration and for this i need the excitation torque from the motor. I have limited info and upon some work i was able to formulate the following relation: T=To+Tosin(4*pi*n*t/60) for n<1500 rpm and T=60Po/(2*pi*n) for n>1500 rpm. Our resonance is mostly occuring below 1500 rpm and close to 1300 rpm. I need to verify whether this expression is right or not. The To is the rated torque of the motor and Po is the rated power. n is the rotational speed of the motor in rpm and it is a 50 Hz 4 pole motor. In case you have experience with this it would be nice for me to know what you think.

Thanks!
Jimmy

RE: Torsional analysis-Model Setup

Sorry Jim,

can't help you there. We usually use an estimated value of 2-3% of nominal torque, and related to the number of poles. But If anyone knows better, I would gladly hear so i'll use that.
you also need to use a percentage for the number of lobse (usually 3 or 4 lobes?)
We are more into diesels and ships propulsion insallations. Sorry.

gr
rob

RE: Torsional analysis-Model Setup

(OP)
Hi rob,

Always nice of you writing back. I am still looking and would let you know in case i find something interesting. I am also trying my hands on examples to try to understand how to formulate my problem and to apply the theory to solving it when there is forcing and the system is branched. The first question that comes to mind is where am or how am I auppose to apply this external torque? The compressor is being driven by an electric motor that is the source of this torque signal. So do i apply this at the motor? or i have to apply it as well to the other nodes in the system?

Thanks as usual for sharing your thoughts.
Jimmy

RE: Torsional analysis-Model Setup

This is complex matter anf there is no easy answer. you have to determine the forcing functions, taking into account damping and magnifyers. Once you got that right, the calculated mode shapes can be used to extrapolate this torque to other positions per mode of vibration. This is way we use a program: it handles such things automatically :)
I suggest you study Den Hartog on this subject, because, as said, this is complicated matter. Good luck.

RE: Torsional analysis-Model Setup

(OP)
Hello Rob,

And thanks for your last reply. I am still trying to do the forced torsional analysis and we have been able to obtain the time series signal of the motor torque and with this information it was possible for me to determine the forcing or excitation function using curve fitting. The next thing now would be for me to implement it into the torsional model.I will let u know how it goes.

I have this other question. Do you know any good literature on screw and lobe compressors? I want to learn how i can determine the PV diagram of such machines?

Thanks!
Jimmy

RE: Torsional analysis-Model Setup

Hello Jimmy,

can't help you there. We only see them onccasionally, either during measurements or, by change a few a couple of weeks ago, during calculations.

good luck
rob

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources