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How to remove mechanical advantage from a lever

  1. Aug 12, 2008 #1
    I have a simple lever with two points of equal force, each at a different distance from the fulcrum.

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    What I want: As the arm turns, one point of force moves at a faster rate than the other.

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    What I DO NOT want: One point of force having a mechanical advantage over the other.

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    If I were to introduce two tracks in parallel, with each point of force on a track, but able to freely slide up and down the same lever arm as they moved across the tracks, would this remove the mechanical advantage from the force point farthest from the fulcrum?

    If not, is there another solution such as some lattice configuration, or even gears or something other than a lever?

    Again, all I want is two points of force moving together with one point of force moving at a faster rate than the other. Thanks.
     
  2. jcsd
  3. Aug 12, 2008 #2
    What I think you are describing violates the laws of thermodynamics. If you have two equal forces then they must have equal displacement. That is:

    F1 x D1 = F2 x D2

    where:
    F = force
    D = displacement

    The only way this relationship can not hold true is if there are losses in the lever or mechanical system you are using. There are two ways of accomplishing these losses.

    1. A mechanical device with multiple degrees of freedom where one of those degrees is subjected to a damper, removing work from the system.

    2. A hydraulic/pneumatic system where a working fluid is subject to some type of transformation that it absorbs external work in the working fluid itself, ie compression.

    Sorry if I misunderstood your question. If I have, can you draw a picture?
     
  4. Aug 12, 2008 #3

    mgb_phys

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    Exactly - because if you turn it around, you are asking.
    I want a lever where I can press down a short distance and have the other end move a large distance with the same force.

    It would be useful, but would have to create energy from nothing!
     
  5. Aug 12, 2008 #4

    Dale

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    It should be possible to build such a device as an active device. It certainly wouldn't be a simple lever. Probably the easiest way would be to build an electronic device to monitor the force and displacement on the control and then use something similar (in reverse) to produce the same force but a different displacement.
     
  6. Aug 12, 2008 #5

    russ_watters

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    Btw, you said you want to remove the mechanical advantage, but the mechanical advantage is simply a ratio and it manifests with both a change in displacement and a change in force. So really you are saying you want to keep the mechanical advantae but remove the mechanical advantage - an oxymoron.
     
  7. Aug 12, 2008 #6
    Thank you for your replies. You have in essence, answered my question. I was wanting to know if there was a sort of 'commonly known' way of removing the leverage 'gained' to the force position farthest from the fulcrum. I see now that there is not. Thank you.

    Because of my limited knowledge of physics terminology (and principles unfortunately) it's hard to explain my dilemma.

    I am now thinking that the only way to achieve my goal of two points of force acting on the same arm at different distances from the fulcrum but without the farther position having a leverage advantage is to somehow change the angle of force on the farther position, so that it is not directed at a 90 degree angle to the arm, thus weakening the 'active' force.

    I've been reading the web about angular momentum. Times like this I really wish that I had a head for this sort of thing. If you have any thoughts on this 'angle of force' approach, please post them. Thanks again for your time and attention.

    FURTHER EXPLANATION: Just FYI, another way to explain what I am needing is this... I need a lever-arm with say 1 lb of force acting on the arm at 6 inches, and a second 1 lb force acting on the arm at 12 inches, but in the opposite direction. My problem is finding an arrangement in which these forces are balanced so that they cancel each other out and the arm doesn't move due to either force.

    Am I correct in thinking that I could achieve the scenario above if the force acting on the arm at 12 inches was arranged so as to point in a 45 degree angle to the arm, thus halving the 'active' force acting on the arm?
     
  8. Aug 12, 2008 #7

    DaveC426913

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    I suspected this was what he was asking about.

    He's looking for a practical solution, not a principle, so he has a braid range of ways he can get the dsired effect without violating any laws.

    There are lots of ways of balancing the forces. We'd need to have a better understanding though. How are the two levers connected? Where are they connected at the other end?
     
  9. Aug 12, 2008 #8
    Zeek, try doing some research on mechanical CCPM (continuously collective pitch mixing) for model helicopters. Some of the geometry gets a little complex but I believe accomplishes something similar to what you are after.
     
  10. Aug 12, 2008 #9

    russ_watters

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    Angling the forces doesn't matter - the vertical component is the only component that is used on the lever.

    No, I suspect what you are looking to do isn't possible.
     
  11. Aug 13, 2008 #10

    DaveC426913

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    Right but ... wouldn't angling the components distribute the force between vertical and horizontal? Then you'd just ignore the horizontal component (say, by using some sort of sliding attachment), leaving you with a weakened vertical component?
     
  12. Aug 13, 2008 #11

    DaveC426913

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    I have whipped up a really simple diagram to ensure I've captured what's going on.

    Have I missed anything? Input welcome.

    It does NOT contain the solution yet, it is merely the starting point. I'll modify it if anyone has any ideas.
     

    Attached Files:

  13. Aug 13, 2008 #12

    LURCH

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    What if you split the difference between the two points of contact, like this?

    fulcrum.gif

    (hope that shows; I've never tried adding an attatchment before).

    Anyhow, the two things that look like pneumatic cylinders could be repalced by springs. In theory, the mechanical advantage of each point of force would also be a mechanical advantage for the spring resisting that force. The point of force with the greater mechanical advantage would also have a resistance that has greater mechanical advantage. Wouldn't the two cancel each other out?
     
  14. Aug 13, 2008 #13

    russ_watters

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    My point is that the horizontal component is pushing against the fulcrum. It isn't doing anything, so it can just be ignored completely.
     
  15. Aug 13, 2008 #14

    russ_watters

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    Clever-looking, but no. The blue points are still where the torques are applied and the device still moves.
     
  16. Aug 13, 2008 #15

    DaveC426913

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    Zeek, could we understand a little more about what's applying the force and the relation of the forces to the arm?

    I get the feeling we're overthinking it.
     
  17. Aug 13, 2008 #16

    DaveC426913

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    I would be useful to establish whether this is true or not, before everyone offers a zillion hare-brained modifications.

    It seems to me there's no reason why it couldn't work. As simple a solution as a spring to dampen the force of F2 accomplishes what he wants in principle, would you agree?
     

    Attached Files:

  18. Aug 13, 2008 #17

    stewartcs

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    How exactly would this not move? If F1 = F2 then the torque at the fulcrum from one direction would be double that from the other direction since the distance from the fulcrum and the points of applied force is double. The rotation would be counter-clockwise.

    Perhaps I'm missing something.

    CS
     
  19. Aug 13, 2008 #18

    DaveC426913

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    The spring on F2 would absorb half of the force, balancing it with the force from F1. It's not meant to be the solution, merely to demonstrate that, in principle, there are ways to modify the forces.
     
  20. Aug 14, 2008 #19

    russ_watters

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    But that's the thing: in principle the internal forces are irrelevant. It is the extrenal forces on the system that make the lever arm move or not move. You cannot absorb/redirect forces like this, it's a violation of conservation law. This is how perpetual motion crackpots are made - they look for ways to do exactly what is being looked at in this thread.
     
  21. Aug 14, 2008 #20

    Dale

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    OK, if you want to keep a lever arm from rotating you need to balance torques (r x f = |r| |f| sin(a)). As you said, if you angle the force with the longer r you can get a smaller torque, but you need a sharp 30º angle, not just 45º. The other thing you could do is add a third torque, e.g. add an electric motor at the fulcrum to provide the extra torque you need. Since you are keeping everything stationary there are no concerns about violating conservation laws, no work is being done.
     
    Last edited: Aug 14, 2008
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