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Force needed to open 1000 lb swing arm

  1. Jul 28, 2014 #1
    Hello: I am working on a machine for a non-profit to build homes in developing countries. I have laid out the basic machine concept and then became stuck trying to figure out how to open the 1000 lbs machine with only manual labor. The machine sits generally vertical with the opening side arms sitting at 15 degree off of vertical. The arms needs to travel through o degrees vertical then to -15 degrees for a total travel of 30 degrees. the pivot point is at the bottom of the arm, and the arm is 5' long.

    I have an opportunity to use several levers to gain mechanical advantage but am not able to determine which lever provides the greatest advantage.

    As can be seen by the diagram there are 3 points of interest.

    a.) Simply pushing the arm open from the top; located 5' from the pivot point.
    b.) A vertical lever arm that passes through the center of gravity, Called Lever 02.
    c.) A lever are 4" above the pivot point, Called Lever 01.

    May I ask for you help in determining what the forces need to be to open the machine arm at these three points?

    Richhttp://sheltertheworld.org/pic/PBS.jpg [Broken]
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jul 29, 2014 #2

    Simon Bridge

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    Homework Helper

    Force produces acceleration - so, in principle, any force will open the arm.
    Some forces will just be slower than others.

    In practice, there will be stiffness/friction resisting the movement - so the minimum force needed must be equal to that.

    You need to reduce your question into something more basic - like how much work is needed.
    You can figure out how much power to get from human muscles from bio-mechanics - there are rules of thumb and tables and things.

    The minimum work is that needed to lift the center of mass against gravity - after that it should, if well lubricated, fall the rest of the way. IF the energy-rate needed is greater than that supplied by the body, you will need to add some additional lever or pulley into the design.

    That should help you think about the problem better.
  4. Jul 29, 2014 #3


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    Welcome to PF.
    You have given too much redundant detail and too many possibilities for application of a force.

    It will depend on the direction that you apply the force.
    You have a 5 foot long pivot arm that weighs 1000 pounds.
    The centre of mass is at about 2.5 foot.
    It must swing from +15°, through vertical at 0°, to –15°.

    To start the movement you must push horizontally to move the centre of mass up a 15° slope.
    1000 * Sin(15°) = 260. pound horizontal force.
    If the horizontal force is applied to the top of the arm it will require only 260 / 2 = 130 pounds force.

    You can reduce the force by making it an “inverted pendulum”. Attach a tension spring between the arm and a higher point on the fixed vertical axis. That can reduce or even reverse the force at the +/– 15° extremes.
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