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Creating friction in a fall arrest device

  1. Feb 19, 2012 #1
    Hey Everyone,

    I'm currently looking at designing a device that would allow people to escape from working at heights quickly and safely in the event of an emergency (up to 33 m)

    I know there are some products on the market already but since I don't have a great deal of experience with them I'm not overly familiar with how they work but what I'm hoping to achieve is to design a device which can be used for a set range of weights (as if the device is specific to a certain weight range, it is easier to create the amount of desired friction) which would allow a working to step off a platform in an emergency and just lower down the rope unassisted, so the device is controlling the friction force.

    the downward force will be equal to m*a and since mass is constant Im going to want acceleration to be 0 after an initial acceleration to get the person to a safe desending velocity.

    Therefor, what I need is an equation that would give the friction on the rope according to the velocity of the rope passing through this. Is there anyway to do this or does velocity have no bearing on the friction between the rope and the device?

    If so, does any one know of a device I could research that would allow me to control the maximum speed at which the rope could pass through a device. At the moment I'm thinking more electric motor with chain but if I can keep it all mechanical (and thus probably cheaper) that would be preferred.

    On closing I would like to say this is ONLY a theoretical project, so Im not going to go jumping off building or anything so no one is any danger:)

    Thanks for your help!
  2. jcsd
  3. Feb 19, 2012 #2


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  4. Feb 19, 2012 #3
    go learn how they work. engineering is 20% research, 10% innovation, and 71% perspiration.
  5. Feb 19, 2012 #4


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    tommy060289, just in case you do come up with a NOVEL idea and you consider applying for a patent to protect your invention, have a look at some basic patent requirements. One requirement is to include in the “Specification” section this:

    “Background of the invention. You must include the field or subject matter of the invention and a description of all relevant prior inventions. Here's where thorough research pays off. When you refer to earlier inventions, point out specific problems that your invention solves.”

  6. Feb 19, 2012 #5
    Haha, that's an awesome saying.

    I like the type of escape chute instead. You know the garbage chutes they use on highrise construction?


    Ropes suck when it comes to safety, having broken many in my childhood I can tell you they dont age well.

    So, we instead use a chute made of a fireproof/tearproof/waterproof material.

    Add in some chambers for descent control with staggered holes. Coil the whole thing up at the top ready to drop down at the touch of a button. Your company will be called "chutes and ladders"- enjoy. :)
    Last edited by a moderator: Feb 20, 2012
  7. Feb 19, 2012 #6


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    Would those chutes saved any lives on the upper floors of the World Trade Center Towers?
  8. Feb 20, 2012 #7
    Assuming any means at least one? Yes, id have to imagine they would have!

    However pulling the detonator out would have saved more.. but alas/
  9. Feb 20, 2012 #8
    33m is the required spec for the project weve been given at university. I havnt posted it in the coursework section as I thought it would be better to post here about generating ideas than answering a specific question.

    I have looked at the designs around atm and one that caught my eye was like an eight loop capable of putting between 1 and 4 loops in depending on the weight of the user. Hwever, what I cant figure out is if any of these devices would essentially allow you to just walk down a wall without having to handle the rope, which would be the ideal scenario. Hence why Im hoping to see if I could mathmatically proof by stepping off a building and having a slight inital acceleration and then when a certain speed is reached then hopefully the friction will increase causing the accerleration to be 0 and the user to fall at a constant speed.

    If my theory is flawed and the sleed will cause no increase in friction then does anyone know of a device(s) that could allow a brake to trigger once the rope reaches a critical speed?
  10. Feb 21, 2012 #9
    We are talking about the same thing right?
    A burning building is no place to be lowering people one by one, with different weights to be accounted for and such.. not if you have any other option that is.

    I think your device would be better suited to rock climbing or something recreational?
  11. Feb 22, 2012 #10
    The devices that I've seen have all been for special purposes. One of the more interesting ones was for a Mel Gibson movie stunt. All have been wire rope and a reel with some sort of braking system. They are very similar to a load control brake on an overhead crane. The mechanical load control brake will prevent the load from driving the hoist down any faster than the operator controls it to lower. In other words, the load cannot transmit a torque back to the motor when lowering. The mechanisim is similar a coaster brake in a child's bicycle.

    Or you could use a centrifical clutch, but it would have to be a special designed clutch. You would want to drag your brake shoes rather than push them. Pushing them will cause them to grab and lock up, rather than controlling the load downward.

    Window washing swing stage platforms also have a similar mechanisim. If they loose power, the operator can let gravity pull him to the ground again, but slowly and with great control.

    For untrained mass evacuation of the general public from a burning building, such friction devices are already on the market from several companies. They put them on very tall hammerhead cranes at constructions sites, ever since a building under construction caught fire and the operator was unable to escape down the tower. The only actual use of one in an emergency that I know of was when a crazy guy with a gun was climbing the tower and the operator elected to take another route down. But it worked great.

    A friend who climbs mountains will triple wrap his figure eight. He has tested it. If he is knocked out, it will let him down slowly enough so that he won't get hurt. The commercial devices have a similar system of friction on a rope, but it is hidden in a case so that the operator can't mess with it.
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