Pulley Pet Hummingbird
I have just put out a new automata kit. Now available in my online store www.contraptioncart.com
It is based on a simple string climbing mechanism that I recently discovered. When the strings are put under a little tension the mechanism mysteriously rises upwards. It is a great base for building automata on as it provides linear motion at the same time as a rotating spool to drive finer movements. The wing mechanism is a variation on a genius method I have seen used by Keith Newstead. When a real bird flaps it's wings, it does not just move them up and down. There is also a small amount of forward and back motion as well as some forward and back tilt. By just using a bent shaft we get all 3 of these motions into our flapping pattern. Thanks Keith!
Anyway I'm calling this the Pulley Pet Hummingbird. It is a relatively simple little kit that should be reasonably easy to assemble in under 40 minutes with the instruction video.
Now how does this gravity defying string climbing mechanism work? Here is how I understand it...
We have a small pulley of radius "r" fixed to a large pulley of radius "R".
This assembly is hanging from the string wrapped around the small pulley. The string around the large pulley hangs down to be pulled. Imagine our mechanism as a see-saw suspended at point "A". In order to make the pulley hang in place and not unwind due to gravity, our torques about point "A" must be balanced. ie the weight force "W" applied at distance "r" must equal the pulling force "P" on the string applied at distance "(R-r)".
W * r = P *(R-r)
Pull the string a bit harder and you toques become unbalanced making the pulley wind itself up the string against gravity.
That's all for now.