I have a bogie of train which is mirrored two planes. Which means every corner of the bogie is the same. When i create the four contacts and i run the simulation, i plot different forces in Vertical axis. Why tho? They should be the same. May anyone
Answers
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Hi Christos,
What product are you using for this? Can you provide any images or videos that demonstrate your issue? Can you show the plots?
Thanks,
Adam Reid
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Hi Adam,
First of them thank you for the response. I can provide you more details about my model. I am using motionview for a MBD Analysis of a bogie where all the bodies are rigid. The model has the elements as you see in the picture. Also there two main springs for the translational joint of main frame and side frame. The two springs are the same. I will provide also details for the joints. There are 14 joints:
1) 2 Translational on vertical axis of main frame and side frames.
2) 4 Revolute of axles with side frames.
3) 4 fixed of each wheel with the axle they are on.
4) 4 translational of each snubber with main frame on a vector defined by the snubber.
I provide you with some pictures of model where I explain the problem. 
I also use a force on main frame CG with a control SISO that I use it to accelerate the bogie at 30 m/s and then keep it constant.
The last plot is the contact forces of wheels on rails. As you see the model has different forces on every wheel, why it is a mirrored CAD.
Why is it like this. Is it the calculation of contact force.
If you need anymore information I can provide you. Also I am available on meetings.
Thank you in advance.Best regards,
Christos P.0 -
Christos Papagiannis said:
Hi Adam,
First of them thank you for the response. I can provide you more details about my model. I am using motionview for a MBD Analysis of a bogie where all the bodies are rigid. The model has the elements as you see in the picture. Also there two main springs for the translational joint of main frame and side frame. The two springs are the same. I will provide also details for the joints. There are 14 joints:
1) 2 Translational on vertical axis of main frame and side frames.
2) 4 Revolute of axles with side frames.
3) 4 fixed of each wheel with the axle they are on.
4) 4 translational of each snubber with main frame on a vector defined by the snubber. I provide you with some pictures of model where I explain the problem. I also use a force on main frame CG with a control SISO that I use it to accelerate the bogie at 30 m/s and then keep it constant.
The last plot is the contact forces of wheels on rails. As you see the model has different forces on every wheel, why it is a mirrored CAD.
Why is it like this. Is it the calculation of contact force.
If you need anymore information I can provide you. Also I am available on meetings.
Thank you in advance.Best regards,
Christos P.Hi Christos,
Thank you for the thorough explanation. This amount of detail is much more helpful for diagnosing your issue.
If this model is meant to be completely symmetrical, then each corner weight should be equal (as you've assumed correctly). The first thing to check would be that the center of mass and the mass of each body is correct. Please, check the CG/Inertia summary (shown below) to make sure that each component actually has the correct mass and COG assigned to it. Note: If you are determining the physical properties of the bodies purely from the graphics, the CAD may be imperfect and thus generate a bad mass or COG.
Next I would simulate the bogie while stationary using a STATIC simulation. Again, if the bogie is designed correctly, then the contact forces for each wheel should be identical and steady, not oscillating or noisy. This would also determine if your springs are identical and if the contacts are all defined properly. Finally, this would determine if the location of all of your joints are symmetrical. From your image, it looks like one of you translational joints on the LH side is out of place:
Lastly, if the only difference comes from the TRANSIENT simulation while the bogie is moving, then I would double check that the SISO force you are applying is in the correct location.
Hope this helps!
Adam Reid
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Hello Adam,
First of all excuse me for late answering. I would like to show you some results. Next is the sum of bodies cg and mass.
Then I did an analysis with no force input and just gravity as excitation and i had these results.
I also checked the symmetry of joints and it is okey.
The main problem I face us is that I have really different values for the four contacts of snubbers which are:
like this.
I am not sure if there is any problem because the only not symmetric things that have are:
1) The points of fixed joints of wheels with axle. (because the joint is fixed)
2) The first and second body of springs. Sometimes i have first the snubber and second the side frame or the opposite and i don't know if this affects the model, because this four springs of snubbers have preload also.
3)The first and second body of contacts.Thank you in advance.
Christos Papagiannis
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Christos Papagiannis said:
Hello Adam,
First of all excuse me for late answering. I would like to show you some results. Next is the sum of bodies cg and mass.
Then I did an analysis with no force input and just gravity as excitation and i had these results.
I also checked the symmetry of joints and it is okey.
The main problem I face us is that I have really different values for the four contacts of snubbers which are:
like this.
I am not sure if there is any problem because the only not symmetric things that have are:
1) The points of fixed joints of wheels with axle. (because the joint is fixed)
2) The first and second body of springs. Sometimes i have first the snubber and second the side frame or the opposite and i don't know if this affects the model, because this four springs of snubbers have preload also.
3)The first and second body of contacts.Thank you in advance.
Christos Papagiannis
Hi Christos,
Your mass and CG summary looks to include the rails as well. That is skewing the summary row. You should also place a point at the midpoint of the axles and measure the centers of gravity from that point and not the origin to remove any translational errors as well. By measuring from this new point, you can see how far off your current COG is from the ideal center.
The contact force plot for the snubber shows significant oscillation. Can you attach a GIF of this simulation to get a better understanding of the plot?
The main frame looks to be attached to side frames with 2 pairs of 3 translational joints. If there are no other bodies involved, then that connection is significantly over-constrained. Can you better detail how those translational joints are used?
Hope this helps!
Adam Reid
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Altair Employee