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Trapdoor Elevator - with a twist! (no people)

Discussion in 'Scenery, Props, and Rigging' started by daworld, Apr 7, 2011.

  1. daworld

    daworld New Member

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    Hi guys, I have a dilemma that I hope someone can help me with. I would like to construct a non-personnel elevator trapdoor. e.g. something that is flush with the floor in the "closed" position and has a plug so the "open" position is safe as well. Total vertical distance is approx 2 feet.

    The purpose is to raise a bench from beneath the stage to on stage - without someone on a rope to either hoist the bench up or down. The plug also needs to be secured with positive latches or something to secure it incase somebody steps on it / sits on the bench.

    I was toying with the idea of a closed pulley system that uses different mechanical advantages to lift and lower the bench with the same counterweight... but I'm not sure if that is the most elegant solution (or even possible). I would prefer not to use any electronics. My current "base design" is the elevator trapdoor on metalworking.com

    Whattya think? Can you help me?
  2. MPowers

    MPowers Well-Known Member

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    I have designed and built just such a unit. The primary trap door was a drop and slide and the "floor" the chair sat on filled the hole and was flush to the stage floor when in the raised position. The rise mechanism was a 4 corner chain drive which effectively locked the unit in the up position. There were also two two pneumatic pins that clamped the lift in the up position as two people were indeed on the lift during the action. The same two clamp pins locked the drop and slide door in place when the door was closed. The Drop and slide and the locking mechanism was pneumatic and the chain drive was a variable frequency 2hp motor, all controlled by a small PLC. There were several other safety mechanisms incorporated such as electric eye sensors to prevent closing if anything/one was blocking the hole. It's been a few years but I think that's the gist of it.

    How much time and budget do you have?
  3. daworld

    daworld New Member

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    Hi Mpowers, thanks for the reply! Your solution sounds quite robust. Since no people will be on the lift, I assume I can get away with a smaller motor, say around 1 - 1.5hp?

    I am a little confused with the pnuematic clamp pins as I have not seen a pair. I assume when the trap was "up" that the pins locked it in place by clamping them to something??

    I have lots of time as I won't need it until summer but my budget is quite small, under $200. If needed I can press-gang a welder and maybe an electrician for a day or so.

    Do you have any pics of your trap unit?
  4. DuckJordan

    DuckJordan Well-Known Member

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    Doesnt matter
    Pins mean it locks into a pin hole of some sort. Is this walked on at all? My biggest concern with this is we had a trap fail in our space causing our choreographer 6 weeks on crutches and a broken ankle. If anyone walks on this thing it had better me strong enough to hold a car. All steel construction for bracing and no less than 10" on centers for anything less than 1/2" ply.

    Maybe I'm over engineering but two things in this world scare me more than anything else. rigging and traps.
  5. venuetech

    venuetech Active Member

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    does your stage have traps that this will fit into?
    can it be a simple bench with no back?

    asume this will be danced on and walked on in both the up and down position. so you need to provide very positive safety devices to secure it.
  6. MPowers

    MPowers Well-Known Member

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    It was quite "robust" as it had to lift a 4x8 steel frame deck with an actual dentists chair on it for the Dentist scene in "Little Shop..." The size of your motor has nothing to do with the number of people on it except while it is moving. If all you are moving is a bench and a small wood deck under it, perhaps a 1/2 hp would do. The motor size is dependent on the total weight to be lifted, the distance and the time. In fact, that is how a "Horse Power" is defined. In an effort to determine how much force early steam engines could produce, ( for pumping out mine water in the early 1700's, steam boats and locomotives were still 100 years in the future) they were compared to draft horses. Later, James Watt determined that a horse could turn a mill wheel 144 times in an hour (or 2.4 times a minute). The wheel was 12 feet in radius; therefore, the horse traveled 2.4 × 2 pi × 12 feet in one minute. Watt judged that the horse could pull with a force of 180 pounds. So:

    power = work/time = force x distance/time = (180 ft-lbs)(2.4 x 2 pi x 12ft) / (1 minute) = 32,572 ft-lbs per min. This was rounded to an even 33,000 ft·lbf/min.
    One HP will lift 33,000 pounds, one foot in one minute.

    Today we have electricity to produce precisely measured units of power and Horsepower (HP) is the name of several units of measurement of power. The most common definitions equal between 735.5 and 750 watts.[1] Horsepower was originally defined to compare the output of steam engines with the power of draft horses. The unit was widely adopted to measure the output of piston engines, turbines, electric motors, and other machinery. The definition of the unit varied between geographical regions. Most countries now use the SI unit watt for measurement of power. With the implementation of the EU Directive 80/181/EEC on January 1, 2010, the use of horsepower in the EU is only permitted as supplementary unit. The mechanical horsepower, also known as imperial horsepower, of exactly 550 foot-pounds per second is approximately equivalent to 745.7 watts.

    So, (now that I've bored you out of your mind) the size of your motor depends on how much, how far, how fast.

    The Pneumatic "pins were actually two types, both De Staco brand. The first was a cam-over straight line piston clamp.
    Item No: TC-850 [De-Sta-Co Pneumatic Straight-Line Action Clamp, Large, 16,000 lbs. holding cap.]
    Two of these raised and lowered the frame the drop and slide door rode in. In the up position they "camed over". Like a fail safe brake or similar device, they relied on the presence of power (air pressure) to release. They would hold up to 16,000 pounds each, in the locked position, they would lift or push 2,000 pounds each in motion.

    The locking pins that engaged when the lift was in the up position were De Staco
    Item No: TC-8031 [De-Sta-Co Pneumatic Straight-Line Action Clamp, Fully Enclosed, 2,000 lbs. holding cap.]
    3/4" diameter cold rolled pins were pushed through mating holes in the lift frame and the deck frame.

    The operation was controlled by a PLC so things could only happen in a specific order. When the door frame had dropped it allowed the door to slide, when the door finished sliding the motor could lift the deck, and so on.

    If your school has a science class that works on robot wars or similar, you have the engineering available for the control. Getting the mechanical part designed might be a bit more of a problem.

    I'm afraid I do not have drawings or pictures as this was all a few years and a couple of jobs ago. There are simpler ways of engineering the gag. The drop and slide door has been known since Roman times and they didn't have pneumatics or motors, course if a gladiator or lion got pinched or caught, they really didn't care that much.... so I've heard? :)
    Last edited: Apr 9, 2011

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