I love mechanisms, I guess it's a geek thing, but I really like the idea of making things that move, be it simple latch mechanisms in oak or put-put boats from coke cans. In the past (pre blog) I put together a pan'n'tilt mechanism for a webcam, made from a printer stepper motor and a worm drive from an old cd laser mechanism. Recently I really got into the idea of propellers & windmills, basically either moving wind through a mechanism or having a mechanism moved by wind. This was further driven by a colleague at work lending me a book full of woodwork projects to make mechanisms. And so I bought a scrollsaw (most new project ventures have this way of convincing me I need a new tool,...) and started making a couple of cogs. I really wanted to not just get a windmill spinning, but to translate that spinning into another plane by using cogs.
the hard bit about cogs is getting the right number of teeth for a given size of wheel to interact properly with another cog of a different size. I haven't looked into how to figure this out yet, I just copied one of the examples from the book I was lent. After some careful marking out, and 20 mins on the scrollsaw, I had 2 cogs.
I went and bought some dowel from my local b&q, as it seemed easier and less wastefull than me attempting to turn dowels from any wood I had laying around. To fix the cogs on the dowel I drilled a 3mm hole from the edge, though the centre, through the dowel and a little past. Then shoved a toothpick in the hole to fix it in position. Then I just cut the toothpick flush with the edge of the cog to avoid it interfering as the teeth mesh. The advantage of this is that I can actually use pliers to pull the tooth pick out again, and I can easily disassemble should I need to.
Having got some cogs I set to making some blades for the windmill. I found some information that suggested that you want a roughly 15 degree pitch on the blades, and the whole assembly should be tilted 15 degrees up. To make the blades I got some scrap wood, roughly 20mm x 10mm and cut 1 length of 300mm. and 2 lengths of 140mm (so that when attached either side of the centre of the long piece the length from centre to tip of each of the 4 'blades' would be the same. Having cut them I marked 120mm from the end of each blade, and using a protractor marked what a 15 degree slope on the blade would be, turned out to be about 5mm drop on one edge. I marked out each piece so that the slopes would all go the same way, and used a tenon saw to make sloping cuts down to my marked 15 degree line, all along the length. Then used a chisel to cut away the waste and leave me with effectively 4 lengths of wood with the last 12cm of their length with a slope. These would form the 'spines' of the full blades, basically just a surface to attach the blades to that would set the angle.
Initially I used some hardboard, cut into 120mm lengths, and staple gunned to the spines. To reduce weight, I drilled out lots of holes in these, then glued paper to one side. After a little sawing and setting of angles in a simple frame I finally got the basic windmill together. Sadly it wasn't until this point that I thought to take photos...
Here you can see it atop a quick makeshift base, it's not attached to that, just the top section with the cogs held in place is firmly fixed. Lucky for me there was enough clearance from the main spindle to allow the larger cog to both mesh with the small cog and turn without catching. This was very much more luck than judgement.
I left this outside for a while and watched whilst it failed to spin at all, the final nail in the coffin of this first version was being blown over, enough wind to topple the structure but not a hint of turning the blades....hmmm
A little more internet investigation and I found a suggestion that blades should be roughly 4 times longer than they are wide at their widest point. I also wanted to make the blades even lighter, so I hit on the idea of just using corrugated cardboard. It would be stiff enough, but also very light. So I marked out some blades, put an arbitrary taper on them, and replaced the first set. (the staples pulled out easily which was a bonus)
Once again I set up the windmill on the table on my decking, and waited for wind. I went in doors to clean up and start thinking about dinner, when out of the corner of my eye I saw movement....success! the blades where turning, the cogs where turning. Everything worked! and it was turning pretty fast. I caught a fairly brief video:
It wasn't all that windy, so I was super pleased to get this confirmation that the system worked.
Next was the question of what to translate this power into...
I toyed with the idea of moving water somehow, maybe an Archimedes screw, or just buckets on a wheel. However, I settled on the relatively easy option of hooking up an electric motor. Not to power the windmill, but to see what electricity could be generated. To do this I drilled a hole through a piece of wood, then snapped it in half to allow me to clamp it around the small cog on the electric motor, this then gave me space to insert a dowel. So the dowel running from the larger cog was coupled to the motor, using the toothpick method described above. And finally here is the finished piece:
And if I spin it really fast, with a multimeter attached to the motor output... I get a whole 0.5v. Ok so not exactly rocking the wind turbine free energy thing. But still I'm quite happy with how the project went. It was fun, and good experience to build on with a future project. If only I lived somewhere with some land and any kind of consistent wind....
Update: I tried the rig with real wind, rather than just me spinning the blades by hand. I reached 0.84v before the whole thing fell over, so next step is to create a secure base and see how fast it can go in real wind conditions.