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P=F/A Load Cell

  1. Sep 25, 2015 #1
    We are applying a force with a machine that has a dial that measures lbs.
    P=F/A
    We have 5 blocks with a combined area of 20 in^2 and we want to apply a pressure of 35 PSI to them. Therefore F(total) for the entire Area is ~700 lbs. This is what the machine reads.
    If we put the load cell on top of the blocks it should read 700 lbs (the total force applied) correct?
    However, then each block should relieve the force of 140 lb (700/5), correct? Or does each block receive 700 lb?
    If we put the load cell next to our blocks and therefore adding support to the entire load. will it read 700 lb or will it read 700 lb/(5+load cell)= ~140lb? do all the blocks receive 700 lbs or is it divided evenly among each block?

    My boss with a pHD says the load cell should read 700 lb (the force) but it's divided among the blocks right?
     
  2. jcsd
  3. Sep 25, 2015 #2

    SteamKing

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    It's not clear how these blocks are arranged. Are the 5 blocks stacked one on top of another, laid out flat in some array, what?

    Pictures will tell a better story.
     
  4. Sep 25, 2015 #3
    they are not stacked. they are spread out evenly, sandwiched between 2 heavy metal plates and then pressed. i probably shouldn't post pics of our work but here is a little example.
    thanks
     

    Attached Files:

  5. Sep 26, 2015 #4

    Dale

    Staff: Mentor

    The structure that you have is statically indeterminate. It means that you cannot predict the force on each block without considering its position and its stiffness. On average each will have about 117 lbs, but the stiffer ones will have more and the placement matters too.
     
  6. Sep 26, 2015 #5
    That's what I thought. The blocks are nearly identical and (nearly) evenly separated and the force (nearly) evenly applied. Thus the force (nearly) divided amongst each block.
    The load cell (next to/ in line with the blocks) measures pounds, so if 700 lb is applied it should read something close to 117-140 lb (give or take depending on the size of the load itself) but certainly not 700 correct?
     
  7. Sep 26, 2015 #6

    SteamKing

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    Not unless something is keeping the other blocks from loading up.

    This arrangement doesn't seem to make a lot of sense, at least, from the standpoint of calculating what load each block supports.

    If you want to test what happens if a block is loaded to 35 psi, design the test so that you know for certain that block is carrying a load which produces 35 psi.

    The thickness of the plates, the arrangement of the 5 blocks, and probably a couple other factors which are not apparent, all factor in to how a force applied to the plate is shared among the blocks.
     
  8. Sep 26, 2015 #7

    Dale

    Staff: Mentor

    if the load cell is stiffer or larger than the blocks then it could be supporting most of the load. Consider the extreme cases.

    If the load cell is very stiff and slightly larger than the blocks then it would take the full 700 lb load and the blocks would be unloaded.

    On the opposite extreme the load cell could be enough shorter than the blocks that it doesn't even touch the plate and so would be unloaded.

    Basically, this isn't a good way to measure anything useful. Small variations in the setup could change the reading from 0 to 700 lbs.
     
    Last edited: Sep 26, 2015
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