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Block and Tackle

  1. Jul 16, 2005 #1
    What would the best configuration to open and close a large metal door (wheels are on top) using a simple block and tackle system.
     
  2. jcsd
  3. Jul 16, 2005 #2

    Danger

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    Since you don't specify the door or track configuration, my best advice would be to see how a commercial automatic opener for that particular configuration is set up, then replace the arm and chain/lead-screw/cable/whatever with the line from your pulleys.
     
  4. Jul 17, 2005 #3
    Im just asking a simple physics question (well not that simple since Im asking) about what type of system would give the best mechanical advantage in both ease of opening and closing.
     
  5. Jul 17, 2005 #4

    brewnog

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    Is the door sliding, hinging, or rotating?

    Is the door being opened vertically or horizontally?

    Large furnace doors are often opened with chains attached to the top of the doors, which thread over a pulley and are pulled down by linear hydraulic actuators, raising the door. I've seen some large barn doors which were slid open sideways by a rack and pinion system.

    Please realise that your question is very, very vague!
     
  6. Jul 17, 2005 #5
    The door opens horizontally. It seperates a stock room from a shop. It is several hundred pounds, about 8 feet wide, and slides on wheels on top of the door. Right now we have a rope and weight on the side of the door to make it easier to open and close (employees dont want to shut it due to the wieght). But it still isnt easy enough.

    The questions are: would a block and tackle system work better, should they be on the side or the top, and would be easy to open AND close. What configuration would be the best?

    I was trying to recall my high school physics on pulleys, block and tackle, etc.

    I hope this is enough info.
     
  7. Jul 17, 2005 #6

    brewnog

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    I don't think this is so much a physics problem as an engineering problem, which is probably why you posted it here!

    While a block and tackle system could be used to reduce the force needed to open the door, it's likely to be pretty clumsy and cumbersome, especially since you want to open and close the door.

    For now, I will join Danger in suggesting the research of some commercial systems, or perhaps someone else might have a bright plan.
     
  8. Jul 18, 2005 #7

    Danger

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    This seems, my good man, to be a situation where what we Canuks call a 'chainfalls' might be appropriate. I don't have a clue what the technical name for the thing is. It's essentially a differential chain pulley system (wherein two side-by-side chain pulleys are one or two links apart in size). Depending upon the diameters, you can get up to about 500:1 ratio. Most of our overhead commercial doors have very heavy coaxial springs wrapped around an axle to act as a counter-weight, and use chainfalls for opening and closing. They have an advantage in that the chain being set into a wall-mounted bracket serves as a totally irreversible locking device. A 300kg door takes about 1.5 kg's of pull on the chain to open or close, but more than a tonne of upforce from the outside to open if the chain is tucked in. The same set-up can very well be used in a horizontal situation.
     
  9. Jul 18, 2005 #8

    FredGarvin

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    Chainfalls are OK if you don't have to open the door too many times during the day. They can take forever to open. Closing can, on vertical doors, take your hand off if you're not careful.

    I'd say to look into small motor sets that are used on large doors like hangar doors. I don't see how you'd be able to do both directions without some counterweights and getting more complicated. I guess if you're really wanting to go that route, why not look at the way they used to do old home windows with weighted sashes?
     
  10. Jul 18, 2005 #9

    Danger

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    That's definitely an aspect of their charm that can be done without. When I was at one of my previous places of employment, I spent a good portion of my days running the damned door up and down. Despite the lack of force required, the repetition resulted in a blister or two.
     
  11. Jul 19, 2005 #10

    Cliff_J

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    Davepls - couple more ideas for you.

    If you reduce the force, you increase the distance you are required to move something proportionally since the same amount of work is done. So if you have 3x less force needed, you need to pull 3x as much rope, and that could require 3x more time if pulled at the same speed (as mentioned above).

    I don't think counter-weights are appropriate, its not just the force needed to overcome the rolling friction but also accelerating the sheer mass of the door that is a factor. And if its a few hundred pounds, there's not much of an easy answer for a manual solution unless less force but more distance/time is acceptable.
     
  12. Jul 19, 2005 #11
    Cliff

    Thank you for your response. What about closing versus opening? and does side versus top of door make a difference (in the location of the pulleys)?
     
  13. Jul 19, 2005 #12

    brewnog

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    There shouldn't be a difference between opening and closing, unless there's something you've not mentioned.

    In terms of force/work/energy needed to open the door, the vertical positioning of a device won't make a difference. However, practical considerations might dictate that a system would be better located at the top (where the runners are located) to keep it out of the way, and to reduce any turning moment that might occur from any slight misalignment in the arrangement.
     
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