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Distributing hinge force laterally?

  1. Oct 7, 2014 #1
    Hi guys, this is my first post on here and my first real delve into physics beyond simply having a keen interest in mechanics and car-related physics, so apologies if my terminologies and such are a bit off.

    Basically, I was having a debate with my father and I want to know if there is a way to effectively distribute a force on a hinge/levered object across the whole width evenly. That sounds complicated, so I'll give an example.

    Say you have a door with a simple beam horizontally across the inside, hooked/nailed to the wall either side of the door. If you then applied force to the door, the majority of the energy would be on the door-handle side of the beam as that's the way the door wants to hinge, correct? So how could you go about spreading that force as evenly across the width as possible?

    EDIT: Here's a diagram to try and clear things up.

    o5x3rk.jpg

    My thought is that applying any force to the door would provide more force on point A than point B. I hope I'm correct in thinking that? I was thinking, what if the distance to the wall was altered at points C and D? So that angle of the beam was no longer parallel with the door. Would that have any effect? I was thinking if the beam is further away from the wall at point C than it is at point D, would that affect the force distribution?
     
    Last edited: Oct 7, 2014
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  3. Oct 7, 2014 #2

    A.T.

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    Apply the force with an object as big as the door, instead of your hand.
     
  4. Oct 7, 2014 #3
    Ah, that's not quite what I meant, sorry. Assuming the force is unchanged.
     
  5. Oct 7, 2014 #4

    sophiecentaur

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    Is this question basically about levers? When you have a door and a hinge, you are in the realms of the mechanics of levers.
    Your model assumes some ideal qualities about the situation that would not actually apply in practice - this makes the question difficult to answer. There couldn't be uniform pressure on the beam if there were any displacement at all or if the contact with the beam is not totally uniform.
    If the beam is not flat against the plane of the door jamb then the only force on the beam will be at the handle end and a similar force will act on the hinges.
    Perhaps you could give a description of the context of this question. It could make it easier to answer. Are we talking about 'security'?
     
  6. Oct 7, 2014 #5
    You could use it for security I guess. The analogy i've given with the door seemed like the easiest to understand in my head. It's like when you see in medieval style films they barricade doors with wooden beams. I was wondering if there was any way of spreading the force more evenly. I guess if you could spread some of the force from the handle end to the hinge end, you could exploit the strength of those hinges?

    EDIT: I just found this image showing a good example. http://i.ytimg.com/vi/To_o7Yq2wgY/maxresdefault.jpg

    In that example, surely because of the lever action of the door, all the force will be on the hook nearest the camera and probably just pull it right off the wall? What I was suggesting to send some of the force to the other side and therefore balance it out (ratio-wise) was to bring that hook nearest the camera away from the wall slightly. In my head, with the limited knowledge of physics I have, that would spread the weight better than having that beam parallel with the door. Mainly to do with the gap between the door and beam I think. By the time the door has opened slightly and is pressing against the beam, it's only going to be pressing on the near side.
     
  7. Oct 7, 2014 #6

    sophiecentaur

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    OK.
    Basically, the 'share' of the applied force on each side of the door jamb will depend upon where it is applied on the beam. By supporting the beam equally on both ends, you are doing the best you can (Unless you know where the invaders will be hitting the door!). The hinge and the door bolts would normally be chosen to have roughly equal strengths but they could be very weak if the beam and fixings were very strong.
    I don't think that you can 'spread the force evenly' except by having a very strong beam that will not flex so that, whichever bit of the door is bent by the applied force, it will be supported by the beam before it distorts too much.

    I'm thinking 'Errol Flynn' here!
     
  8. Oct 7, 2014 #7
    Interesting. Although in some cases, would some flex be beneficial? I'm thinking along the lines of energy absorption now. If the barricade has a bit of flex in it, could that be better at absorbing the energy of a kick than a solid beam? As in, if you covered the door in something relatively soft like foam, that would surely be more effective than covering the door in something harder like wood? To resist a kick/barge/ram.

    Another thing I was thinking came from my car knowledge - anti-roll bars. They use torsion to sort of transmit weight-transfer to the opposite side of a car, so when centrifugal force makes one side of the car dip, it pulls the other side down with it using torsion. Could a similar thing be applied here so that when a force is applied to the handle end, some sort of torsion bar transfers the force to the other side of the door? Would that put extra unnecessary force on the other side or simply spread the existing force?
     
  9. Oct 7, 2014 #8

    Imager

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    Just a thought, the beam in the diagram looks to be upright, on its side might be stronger.
     
  10. Oct 7, 2014 #9
    The beam in the diagram is supposed to be the same as the image I found.
     
  11. Oct 8, 2014 #10

    sophiecentaur

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    All structures flex (even concrete beams) and this will affect the distribution of the forces throughout. Good design will avoid large forces being applied where they could do damage. Looking at the way that 'castle' doors - and even garden sheds are constructed, gives an idea of this, A flexible outer layer will spread forces over a stronger inner layer of bars etc etc. But this will rapidly take you into some very hard engineering. You can find something on Google about basic structural engineering, to suit your level.

    You quote the anti roll bar in motor cars as an example of 'transferring forces' and it's an easy one to consider. An anti roll bar allows you to have relative soft springing in the 'common mode' - so the front wheels can go up and down (together) when you hit a bump - but the anti roll bar strongly couples the two front wheels so they cannot easily move up and down independently - suppressing a roll. the ARB doesn't so much transfer a force as alter the stiffness of a structure in one particular mode (difference mode). The reason it improves the cornering is that it stops the centre of mass of the car moving relative to the wheel track which would compromise stability.

    Some of the intuitive ideas one can have about structures can rely on impossible principles - such as sky hooks- and you have to bear in mind the fundamentals. For instance, the principle of moments (basic lever theory) will always apply and you can't summon up extra forces without providing them with extra bracing to somewhere outside the basic structure.
     
  12. Oct 9, 2014 #11

    CWatters

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    Very good point! For centuries people have been putting the beam in the wrong way.
     
  13. Oct 9, 2014 #12

    sophiecentaur

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    HAHA. I couldn't make sense of that post, actually. The beam looks the right way round to me! (Horizontal and not vertical)
    The only way to help protect both the hinges and the bolt (potentially the weakest parts of the door system) with a single added beam is to have it laying so it is behind both of them. Actually, it is not uncommon to put in a temporary extra vertical beam in double doors for extra security, when the enemy is hammering to get in. :mad:
     
  14. Oct 9, 2014 #13

    CWatters

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    I think you misunderstand me.. Traditionally Cowboys have kept the Indians out by installing the bar across the door as shown on the left of this diagram. It would be stronger if installed as shown on the right. (I think).

    door bar.png
     
  15. Oct 9, 2014 #14

    sophiecentaur

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    Gotcha. You were clearly looking at the Engineering aspects of John Wayne's situations, rather than the flying arrows. (Takes all sorts. lol)
    But it may be not so silly really. It could be easier to make a bracket on either side with a narrow slot than a wide one, to hold the beam. But in the end, bigger is better in those circs.
     
  16. Oct 9, 2014 #15
    I guess ideally you would have a beam in a T shape cross section (rotated 90 degrees), with the flat plane against the door and the other bit to prevent flexing. Like angle iron.
     
  17. Oct 9, 2014 #16

    sophiecentaur

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    Chobham armour for the door, too,
     
  18. Oct 11, 2014 #17
    That's just cheating.
     
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