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  • Checking for overturn of chair?

    Discussion in 'Industrial design' started by stgeorge, Sep 10, 2012.

    1. stgeorge

      stgeorge New Member

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      This question is not strictly related to Mechanical Engineering, it's more Industrial Design I guess, but maybe some of you know the answer.



      I got puzzled for the last few days with an overturn check, and decided to ask for help online.



      I used to do an analysis for for overturning effect of chairs and tables, by hand based on 2d support walls overturn check from the structural engineering. This is an example of bench:



      [​IMG]
      ("A" point represents the reference point for overturning)
      Overturn condition:

      F1 * l1 ≤ 1.5 * F2 *l2





      But what should I do when the objects are not planar (planar but extruded in third dimension like this bench)?

      Here is an example of a 3d irregular shaped object:




      [​IMG]




      It is not possible to check for overturn this kind of irregular shaped object by hand. Because forces no longer lie in the same plane:





      [​IMG]


      Some other combinations:

      [​IMG]



      here is .3ds file of this object:
      http://www.mediafire.com/?qxx7gqp1ndt9s6x


      Can anyone help me with this issue?
      How to check objects for overturn, when their overturn moments do not lie in the same plane?



      Thank you for the reply.
       
    2.  
    3. AndrewNew

      AndrewNew Well-Known Member

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      Fairly straightforward to do this with the motion analysis tools built in to many CAD programs or even with FEA. Fix the points of attachment (bottom of the chair legs) to the ground, apply your over-turning loads and check for tensile forces at the attachment points.
       
    4. stgeorge

      stgeorge New Member

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      Thank you for the reply Andrew.
      This tensile (uplift) of the supports you mentioned - their value is crucial for determination whether an overturn check is within dangerous area, or not?
      I thought moments around the overturn axis(line) are crucial?
       
    5. AndrewNew

      AndrewNew Well-Known Member

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      Yes, that sounds plausible. You'd need to determine the sum of the moments of the forces at the points of attachment about the overturn axis to decide whether the chair/bench would actually overturn. I guess with a complex structure you'd need to consider the axis formed by each of the adjacent pairs of points (so 4 axes if you have 4 points). The forces at the 2 points forming the axis would have zero moment about the axis of course. Does that sound sensible? I'm thinking outloud here!
       
    6. stgeorge

      stgeorge New Member

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      Hm, it might be a problem with my English, as it is my second language.

      What do you mean by "attachment"? Support?

      How to determine an overturning axis, when the object is a bit complex, like in this case?
       

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