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Assignment 1:
Swingin' Spike

"Design and model a solution to a problem from ThinkCycle"

I tackled the "SWING FOR TRANSPORT OF MATERIALS" challenge:

As a project for the students of a village, our group is experimenting with a simple swing for transport of materials (say stones, sand-bags,etc.). A tripod of appropriate height is erected and a coir swing with a basket at the bottom is attached. One person loads the materials and swings the basket, which is caught by the other man at the other end and the basket is emptied and sent back to the first man. Thus the material transfer is completed and continues..

We would like to add an automatic mechanical locking system at either end to stop the basket till filling / emptying..

The village group is enthusiastic about this crude idea and set up. This devise helps the villagers in their day to day work..reducing physical strain. Can any one help ?


My first thought was to use ropes to tie off the basket at each end of its swing. If you wanted to be fancy, you could use Prusik knots at the ends to avoid hitching and unhitching: a prusik knot grabs tightly to the rope it is tied to, but if you hold it correctly in your hand, it will slide freely. Thus you could let the knot slide when the basket is coming, and let go to have it hold tight once the basket has arrived.


But if you were to design a mechanical device...

Here's my first page of brainstorming. I've got the tripod swing at the top, and below that you can see the first idea that popped into my head: some sort of one-way gate. My second idea was some sort of spike with a cleat of some type to hold the rope.

On the second page I've settled on some sort of ascender scheme, and have started to figure out how it would work. I found a guide to designing eccentric-cam ascenders on the web. The key part is an equiangular spiral.

Here's the first whole-system drawing.

And here I've taken it into parts to work out how it could be put together out of two-dimensional cutouts.

I wrote a Java program to compute the proper equiangular spiral; it output an xfig file which I exported to postscript and imported into CorelDraw.

My sketch, with the computed cam, translated into CorelDraw.

The CorelDraw diagram was imported into Rhino3D and the parts were extruded to make 3-d shapes. Then they were assembled, problems found, the diagrams tweaked, rinse, lather, repeat.

A view from the "business end" shows a bit of the cam.

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