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Fulcrum Base

  1. Mar 22, 2013 #1
    I am building a prop for Aladdin's floating carpet. It will be a see-saw with the 2 characters on the carpet 300 lbs, 5 ft from fulcrum. 1500 ft lbs pressure.
    My question is how do I build a fulcrum 3 ft high, must have small footprint and movable.
    What is the torque on the pivot point,? how much weight is needed at base of fulcrum?
  2. jcsd
  3. May 1, 2015 #2


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    Hi and welcome to PF
    We should get some definitions right here, first. The fulcrum is the pivot and, as long as it is strong enough to support the total weight on it, it can be any (stable)* design you want. You have a lever here and the longer the 'other end' (control arm) is, the less force you will need on it. To decide on the length of the lever, you use the principle of moments, which says that, at equilibrium (balance):
    (length of carpet arm) times total weight = (length of control arm) times force from stage hand plus counter weight.
    The vertical distances travelled will also scale with the arm lengths and you would need to choose a control arm length that's not too long and not too short. For 1500 lb ft of torque, the control arm needs to be scaled according to how heavy your balance weight is. The total downward force on the fulcrum is the total of all the downwards forces (weights of everything plus the stage hand's efforts). But that's only a fraction of the forces on the pillar when there is movement and people getting on and off.
    IF the carpet is to remain horizontal whilst going up and down, you would need some pantograph arrangement. - or using vertical guides. This could be a very untrivial project.
    *You won't get away with a very small footprint, for safety reasons. This has to be more chunky than you think, I feel. A wide base (15mm ply and quite a bit wider than the carpet) with a chunky timber upright with several brackets will be needed. If you want 3D movement then your universal joint needs to be very strong and not allow the carpet to tip.
    The more initial time spent with paper and pencil, the less, last minute, time spent with saws and screwdrivers!
    Good luck.
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