Torsional analysis-Model Setup

[mechanicaljw]

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

 

[mechanicaljw]

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

 

[rob768]

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)

 

[mechanicaljw]

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

 

[rob768]

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

 

[mechanicaljw]

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

 

[rob768]

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

 

[mechanicaljw]

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

 

[rob768]

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.

 

[mechanicaljw]

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

 

[rob768]

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