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Why do my resultant forces change with a material change?

Question asked by Matt Owen on Nov 23, 2018
Latest reply on Nov 28, 2018 by James Riddell

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Hi,

I am modelling a frame with three masses on. The two smaller ones are approximately 90Kg and the larger one is 60Kg. When modelled with 6 fixed supports the reaction forces in centre are much larger than at the edges (which makes sense to me). I then modelled some blocks to represent the mounting feet under these same 6 locations. When modelled as a fairly stiff material such as steel the reaction forces are similar to the fixed case. However, when I model these as a rubber material which is still within it's elastic limit the reaction forces are completely changed with the forces fairly evenly distributed across all 6 mounts. I have attached screenshots of the reaction forces when fixed at the 6 points, with steel feet and with rubber feet (the material properties for the rubber are also attached). Could anyone shed any light on what's going on here?

Thanks for you're help, Matt.

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Rubber properties.JPG59.9 KB
Steel.JPG108.3 KB
Fixed.JPG119.6 KB
Rubber.JPG105.3 KB

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Erik Kostson Nov 26, 2018 9:17 AM

If you look on the total reaction (~2600 N) it is all the same for all 3 models (fix, steel, rubber), and that is the most important thing. That is, applied loads are balanced by the total reaction at the supports (thus equilibrium). Now obviously because the rubber is softer, the spatial distribution of the support reactions could be different from the steel feet case (which is stiffer and close to fixed support).

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James Riddell Nov 26, 2018 10:37 AM

That is exactly what you should expect to see with different Young's Modulus and Poisson's Ratios. Deflections and stress would not be exactly the same either although they should be in approximately the same locations. The fully fixed case would be producing additional stresses by not allowing the frame to flex.

Erik Kostson has it in a nutshell.

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Matt Owen Nov 28, 2018 6:47 AM

Okay, that makes sense. I assume that that is one limitation of hand calcs then as the deflection wouldn't be taken into account...? Also, how would people then be expected to size up mounts depending on the force they're under? Would it be sized on the fixed case or would people have to produce an accurate model with the actual stiffness of the mounts in question being chosen?

Thanks for your help guys, Matt.

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James Riddell @ Matt Owen on Nov 28, 2018 7:01 AM

Matt Owen wrote:

Okay, that makes sense. I assume that that is one limitation of hand calcs then as the deflection wouldn't be taken into account...? Also, how would people then be expected to size up mounts depending on the force they're under? Would it be sized on the fixed case or would people have to produce an accurate model with the actual stiffness of the mounts in question being chosen?

Thanks for your help guys, Matt.

To whom are your referring?

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Erik Kostson Nov 26, 2018 9:17 AM
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If you look on the total reaction (~2600 N) it is all the same for all 3 models (fix, steel, rubber), and that is the most important thing. That is, applied loads are balanced by the total reaction at the supports (thus equilibrium). Now obviously because the rubber is softer, the spatial distribution of the support reactions could be different from the steel feet case (which is stiffer and close to fixed support).
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James Riddell Nov 26, 2018 10:37 AM
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That is exactly what you should expect to see with different Young's Modulus and Poisson's Ratios. Deflections and stress would not be exactly the same either although they should be in approximately the same locations. The fully fixed case would be producing additional stresses by not allowing the frame to flex.
Erik Kostson has it in a nutshell.
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Matt Owen Nov 28, 2018 6:47 AM
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Okay, that makes sense. I assume that that is one limitation of hand calcs then as the deflection wouldn't be taken into account...? Also, how would people then be expected to size up mounts depending on the force they're under? Would it be sized on the fixed case or would people have to produce an accurate model with the actual stiffness of the mounts in question being chosen?

Thanks for your help guys, Matt.
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- James Riddell @ Matt Owen on Nov 28, 2018 7:01 AM
  Mark Correct
  Correct Answer
  Matt Owen wrote:
  
  Okay, that makes sense. I assume that that is one limitation of hand calcs then as the deflection wouldn't be taken into account...? Also, how would people then be expected to size up mounts depending on the force they're under? Would it be sized on the fixed case or would people have to produce an accurate model with the actual stiffness of the mounts in question being chosen?
  
  Thanks for your help guys, Matt.
  
  To whom are your referring?
  Like • Show 1 Like1
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Why do my resultant forces change with a material change?

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