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Rotating drum being stopped dead by a pin

  1. May 17, 2012 #1
    I have a drum of radius of 1.375" with a wall thickness of .3025". It is rotating about its central axis at 2000 rpm. There are 6 holes drilled into the drum radially. I have a pin that slides in and out of those holes in order to release and stop the drum. I am trying to figure out the force of the impact on the when the drum is rotating and the pin slides into one of the holes.

    Most problems I have seen don't involve something stopping completely, normally there is a before and after velocity. Now, I realize that there will be a bit of bounce when the pin hits the wall of the hole but I have no way of measuring or estimating the velocity after the impact. I want to assume it is 0 (worst case scenario).

    I apologize that I don't have any attempted solution. I do not remember this stuff and it has been a while since I was in university.

    Thanks,
    A
     
  2. jcsd
  3. May 17, 2012 #2

    tiny-tim

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    welcome to pf!

    hi agordon07! welcome to pf! :smile:
    why?? :confused:

    anyway, force is a rate of change (of momentum), so you need to know how long the drum takes to stop :redface:
     
  4. May 17, 2012 #3
    Why?

    It's because I am using the pin to stop the drum from rotating and I need to make sure that the pin I have chosen will be strong enough to withstand the impact.

    What if I want the drum to stop instantaneously. I know this isn't realistically possible, so do I need to estimate the time duration of the impact. Can anyone help me with where to start with that value?
     
  5. May 17, 2012 #4

    DaveC426913

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    Aside: are the holes larger than the pin? They'll have to be. If the holes are the same size as the pin, then the holes will rotate out of the way before the pin has had any time to start to fall.

    From the time the pin first clears the leading edge of a hole, how much time elapses before the pin will hit the trailing edge of the hole? That is how much time your pin will have to begin its movement. If too short, the pin will simply skip over the hole. You could mitigate this by shaping the pin to have a sloped face, thus:
    Code (Text):

    pin (with beveled tip)
     | |
     \ |
      \|
    __ .     ___
      |     |
       hole

    <--- motion of drum
     
     
    Last edited: May 17, 2012
  6. May 17, 2012 #5
    Dave:

    You are right about the pin skipping out of the hole. I have the hole slightly over-sized. I also have a lead in machined into drum so that as the pin meets the hole it has already started to extend. Finally the pin does in fact have a beveled or round edge.

    I am confident that the pin will have enough time to extend and stay in the hole.
     
  7. May 18, 2012 #6

    tiny-tim

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    yes, if you want to calculate the strength needed

    but i know of no way to measure the duration
     
  8. May 18, 2012 #7

    haruspex

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    The mechanism you describe sounds like an effective device for chopping pins in two.
    It's a question of Shear Impact. Googling for that past all the hairdressing salon ads I find http://en.wikipedia.org/wiki/Shear_stress#Impact_shear.
    Assuming you want the pin undamaged, this needs to stay within the elastic range of the pin.
    Damage to the hole edges will also be a concern.
     
  9. May 21, 2012 #8
    I heard of saw blades that use a similar mechanism to prevent chopping off peoples' limbs. The mechanism is apparently fast/ strong enough to fully stop a blade before it manages to do anything more than scratch your finger. I think it actually used a non-reusable cartridge with gunpowder though.

    Anyway, I'm not sure how to approach this problem. Probably figure out how much the pin/ hole can elastically deform and the corresponding force that this produces (not sure if this is hand-calculable), and see if if gives you enough time/ distance to stop the drum? Then just keep playing with the variables until you get something that works?

    The variables, of course, are many. Length and thickness of the pin, materials, thickness/ size of of the hole and who knows what else will affect the elasticity, and therefore the force that the mechanisms experience.

    Can you use a spring mechanism to give you more elasticity and stopping distance?
     
    Last edited: May 21, 2012
  10. May 21, 2012 #9
    Found it! http://www.sawstop.com

    Quickly looking at the description of how this thing works, it appears to be a one-time, self-destructive mechanism. Not exactly the pin-hole mechanism I was envisioning, but it does illustrate the magnitude of forces involved when you try to stop something "instantaneously". Something HAS to give. I think you have to use some sort of spring or something. Somewhere. It will also simplify the calculations.

    Let me rephrase that. You WILL use a spring, whether you want to or not, as everything is elastic. Might as well purposely put it in.
     
    Last edited: May 21, 2012
  11. May 21, 2012 #10

    A.T.

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    Interesting. As trigger they use the change in electric charge of the blade. So the blade has to touch the skin and you have a scratch of which the depth depends on how fast your finger was moving. And I guess you have to be careful with conductive materials near the blade that could trigger it.

    https://www.youtube.com/watch?v=LGiYlyo2-eQ
     
  12. May 21, 2012 #11

    tiny-tim

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    awesome! :biggrin:
     
  13. May 21, 2012 #12

    A.T.

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    Here is a slow motion clip of the deformation at 1:50:

    https://www.youtube.com/watch?v=6X-tj3lk0E8

    At 4:30 they try a real finger moving very slow. I guess the discharge happens with a spark, so he doesn’t even have a scratch.
     
  14. May 21, 2012 #13

    DaveC426913

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    Yeah. Those safety saws are an incredible feat of engineering.
     
  15. May 21, 2012 #14

    A.T.

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    I would still like to see some independent testing in lees ideal conditions. When working with wood your hands are covered in saw dust, and thus potentially less conductive than a clean hoddog or a wet finger.
     
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