Paper Problems

Mon, 8 Mar 2010

In the back of my mind I have ideas for various models that I just can't work out. I know what I want to do but haven't worked out how. To me, these models are the really interesting ones. They're especially satisfying when I can finally work out a solution. The latest model to move from this mythical collection into the realm of the solved was the XOR goat. I still have plenty of mechanisms I can't work out though.

Randomness: I like the idea of a paper decision maker. Ask a question, pull the handle and either the Yes character pops up of the No character. For it to work though the chances of each character popping up needs to be randomly 50/50. Making something repeatable in a mechanism is easy, making it random; not so much.

Power: There are a few different models that would benefit from a power source. The wave machine shown here would be neat if it could be made to wave at you across the room, without the need to crank any handles. There are a few possible approaches to powering a model, sand or salt pouring into a waterwheel type machine. Elastic band power – the problem being stopping the model from collapsing under the force of the elastic. Or how about a clock type mechanism with a counter weight and an escapement.
Powered models are the ones I feel that I could solve with a bit of effort and some research time.


Gears. So I actually have a few models that already use gears and they work fine. A single gear to another gear is okay, especially if you are gearing down. But three or more, or gearing up, or even just trying to actually transmit a bit of power and it all goes pear shaped. I think, based on a few experiments, that the main problem lies in the axle rather than the gear itself. A square axle going through a round hole is, by necessity, going to have issues with tolerances. This lets the gear wheel move, and not in a good, round and round, way. If the gear wheel is not accurately fixed on its axis then as soon as you try and do anything interesting with it, the teeth jump. Small holes and a wooden skewer would probably solve the problem but I'm a purist, I don't like idea of using wooden skewers in a paper model so gears stay on my list – for the moment.

There are more of these tricky mechanisms; things that I have yet to solve. For example, I really like the idea of making a butterfly that sits on the end of a thin, delicate branch and opens and closes its wings, the mechanism fiendishly hidden within the branch.

I'm thinking of setting up a section on the websites where I can list these problem mechanisms then tick them off when they are solved.

Any ideas/solutions to any of these would be gratefully received. Back to work on the catapult!

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Comments (6)

  • Shelley Noble March 9, 2010 at 6:47 am

    OOOOoooo! I want a fiendish

    OOOOoooo! I want a fiendish butterfly! Love watching your wheels turn!

  • tim March 9, 2010 at 3:24 pm

     RE: ‘gearing up’  How about

     RE: ‘gearing up’  How about hexagonal (six-sided) axles? They should provide more points of contact between axle and bearing.  I use these all the time in my kits, but must confess I haven’t yet tried to ‘gear up’, so don’t know if this would work.  I also lubricate bearings with the end of a candle (not lit!) and find that this reduces friction a lot.  

    • Anonymous March 9, 2010 at 8:50 pm

      Silly question, but why not

      Silly question, but why not ROUND axles ? Couldn’t you just roll up a tube of paper several layers thick for an axle ?

      • robives March 10, 2010 at 7:24 pm

        Round axles work reasonably

        Round axles work reasonably well, the problem is that it is dfficult getting the radius of the shaft just right. The shaft needs to be fracionally smaller than the hole. Roll the paper too tight and the axle moves about, too loose and the gear won’t turn.

        I’ve tried hexagonal shafts, they work reasonably well, but not well enough. I need some kind of bearing. Maybe a cone fitting into a hole would work as it cone can be moved back and forth to achieve optimum fit.

        • Anonymous March 14, 2010 at 1:19 pm

          RE: Randomness:  This is a

          RE: Randomness:  This is a very interesting problem. It’s not easy to introduce into a mechanism.  A rapidly spinning wheel would theoreticaly come to a stop in a random way, but in practice any minute irregularity in construction would make it tend to stop in the same place each time.  How about a wheel with several indentations on the edge, which correspond to different yes/no outcomes.  The wheel is spun rapidly (perhaps flicked by a lever) and is then stopped abruptly a second or so later by a falling bar, which engages with one of the indentations. The falling bar provides the random element, especially if it falls fairly slowly and the wheel is spinning fast.  This is all very well, but making the whole darned thing out of paper is another matter all together!

          • robives March 14, 2010 at 10:04 pm

            Interesting ideas on

            Interesting ideas on randomness! It should be possibl;e to use a rotating wheel with different sizes of cut outs but it would have to have a vertical shaft so that differing weights in the wheel aren’t affects by gravity. Some experiments are called for I think

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