AnsweredAssumed Answered

Solve Free-Body Diagram Using Reactions

Question asked by Martin Taylor on Dec 31, 2016
Latest reply on Jan 9, 2017 by Bill McEachern

Like • Show 0 Likes0
Comment • 6

Hello!

First time posting, hope you find my project interesting

I'm designing a custom playground for a local park and need to calculate the tensions in some support cables.

The internal structure consists of a rigid half-torus held in position by chains and cables in tension:

The half-torus will be covered in netting, allowing children to climb on it. Depending on the location of the children, the center of gravity of the basket system will move, causing the tensions in the cables to change:

I have been struggling to solve the free-body-diagram system of equations in Matlab.

I saw this video where springs were used to model tension cables. Would it be possible to set the basket to "rigid-body mode," add the springs, and view the reaction fores in Solidworks simulation?

I can't even find the option for "rigid-body" in SW15. How to I begin setting up this problem?

Thanks,

No one else has this question

Outcomes

Alessandro Tornincasa Jan 6, 2017 12:43 PM

Martin,

very interesting problem.

If you modeled your frame as structural members everything will be considered as beam elements. I'm not sure you can treat beams as rigid bodies.

It works for sure with solid mesh: all you need to do is right clicking on a body in the simulation tree and choose " make rigid".

In your case, if you want to use beams and considerazione them ad rigid you might define a custom material with very high young modulus (2 e+6 or 2e+7 MPa).

Spring connectors can't be user with beams but only with solids and shells, you will have to find a workaround with mixed meshing.

Another option would be modeling the cables as beams and make them trusses. Problem is that trusses resist compression: you would need to go non linear then and define a custom material that would resist only tension.

Alex

Actions

Martin Taylor @ Alessandro Tornincasa on Jan 9, 2017 6:47 AM

Alex,

Thank you very much for the help!

I took your advice and re-modeled the system with structural members and found the resultant forces. I accounted for the tension/compression issue by not fixing the cables that I assumed would be in compression.

Here is the simulation for the "centered load" scenario, where the basket load is centered:

This is an easy problem to solve by hand, so I was able to compare my results. The results were very close! 520lbs vs 620lbs.

The problem is that the fixed ends of the cables create moments on the structure. I tried setting the cable materials to very flexible (elastic modulus 10000) but this crashes solidworks so I had to settle for 5000 series aluminum (modulus 7e+10).

Is there some way to constrain the fixed joints in a single direction? So they produce no moment?

Thanks again!

Actions

Bill McEachern @ Martin Taylor on Jan 9, 2017 10:31 AM

in you RMB up in the parts area you will be able to set the end DOF for the member.

Actions

Martin Taylor @ Bill McEachern on Jan 9, 2017 2:35 PM

Bill,

Do you mean setting the joint type to a hinge? Not sure what you mean by "RMB" ?

I tried setting the joints to hinges (which I believe eliminates all moments) and I used ABS plastic for the cables, which has an even lower elastic modulus than aluminium.

Still getting some small moments:

Actions

Bill McEachern @ Martin Taylor on Jan 9, 2017 2:40 PM

RMB=right mouse button

Yes I was suggesting releasing the degree of freedom. You could try changing them to truss elements but in my experience it just makes the program crash. A truss is a essentially two force member - no moments.

Actions

Bill McEachern Jan 6, 2017 2:24 PM

You could do this easy enough in GEOSTAR if you have the Premium simulation license but it is a pretty old school sort of program so figuring it out is a bit of a challenge. You would not find a lot of help but there are a few souls out there that know some of how it works. I am not one of them as the last time i used it was in say the early 2000's. But I know it could do this problem with fidelity with beams and springs.

Actions

6 Replies

Alessandro Tornincasa Jan 6, 2017 12:43 PM
Mark Correct
Correct Answer
Martin,
very interesting problem.
If you modeled your frame as structural members everything will be considered as beam elements. I'm not sure you can treat beams as rigid bodies.
It works for sure with solid mesh: all you need to do is right clicking on a body in the simulation tree and choose " make rigid".
In your case, if you want to use beams and considerazione them ad rigid you might define a custom material with very high young modulus (2 e+6 or 2e+7 MPa).
Spring connectors can't be user with beams but only with solids and shells, you will have to find a workaround with mixed meshing.
Another option would be modeling the cables as beams and make them trusses. Problem is that trusses resist compression: you would need to go non linear then and define a custom material that would resist only tension.

Alex
Like • Show 1 Like1
Actions
- Martin Taylor @ Alessandro Tornincasa on Jan 9, 2017 6:47 AM
  Mark Correct
  Correct Answer
  Alex,
  
  Thank you very much for the help!
  
  I took your advice and re-modeled the system with structural members and found the resultant forces. I accounted for the tension/compression issue by not fixing the cables that I assumed would be in compression.
  
  Here is the simulation for the "centered load" scenario, where the basket load is centered:
  This is an easy problem to solve by hand, so I was able to compare my results. The results were very close! 520lbs vs 620lbs.
  
  The problem is that the fixed ends of the cables create moments on the structure. I tried setting the cable materials to very flexible (elastic modulus 10000) but this crashes solidworks so I had to settle for 5000 series aluminum (modulus 7e+10).
  
  Is there some way to constrain the fixed joints in a single direction? So they produce no moment?
  
  Thanks again!
  Like • Show 0 Likes0
  Actions
  - Bill McEachern @ Martin Taylor on Jan 9, 2017 10:31 AM
    Mark Correct
    Correct Answer
    in you RMB up in the parts area you will be able to set the end DOF for the member.
    Like • Show 1 Like1
    Actions
    - Martin Taylor @ Bill McEachern on Jan 9, 2017 2:35 PM
      Mark Correct
      Correct Answer
      Bill,
      
      Do you mean setting the joint type to a hinge? Not sure what you mean by "RMB" ?
      
      I tried setting the joints to hinges (which I believe eliminates all moments) and I used ABS plastic for the cables, which has an even lower elastic modulus than aluminium.
      
      Still getting some small moments:
      Like • Show 0 Likes0
      Actions
      - Bill McEachern @ Martin Taylor on Jan 9, 2017 2:40 PM
        Mark Correct
        Correct Answer
        RMB=right mouse button
        Yes I was suggesting releasing the degree of freedom. You could try changing them to truss elements but in my experience it just makes the program crash. A truss is a essentially two force member - no moments.
        Like • Show 0 Likes0
        Actions
Bill McEachern Jan 6, 2017 2:24 PM
Mark Correct
Correct Answer
You could do this easy enough in GEOSTAR if you have the Premium simulation license but it is a pretty old school sort of program so figuring it out is a bit of a challenge. You would not find a lot of help but there are a few souls out there that know some of how it works. I am not one of them as the last time i used it was in say the early 2000's. But I know it could do this problem with fidelity with beams and springs.
Like • Show 1 Like1
Actions

Solve Free-Body Diagram Using Reactions

Outcomes

Related Content

Recommended Content