Tuesday, May 18, 2010

Parallel motion

(The following material was previously posted on my other blog. It turns out that the parallel motion won't be needed after all, due to a very convenient differential design, but I thought I'd share it here anyway.)

While working on the design for a large Burning Man project, I was faced with a need to have a connecting rod coming off of a rocker arm, but to prevent a very complicated joint it could only pivot in one axis at each end. This was vexing, until I realized that this is not a new problem. No less a figure than James Watt himself faced something similar, when first moving from atmospheric to true steam engines where the piston pushed as well as pulled. The piston rod needed to be kept vertical, while the lever arm it was connected to needed to move in an arc. What to do?

He came up with this, the Watt Linkage. Arms AB and CD pivot about their ends. But on the connecting link CB between them, there is a magic point E (its exact position depends on the ratio of AB to CD) whose motion is something very similar to the analemma-like shape to the right. If you keep the angular displacement of the main arms small compared to their length, E moves almost perfectly vertically. (Technically there is still some horizontal movement, but that's well below anything I'll be caring about. If you want perfection, go with the Peaucellier–Lipkin linkage.)

So, that's awesome, but how do you use it for a pump arm like I'm talking about?

You pantograph it! You pantograph the hell out of it! The same parts are there as before, but we've added another two links so that we can use the far corner instead of E. This is a lot easier to attach a piston rod to, and it keeps the tie rod connection close in to the main frame.

Rather charmed with this solution, I went ahead and prototyped it. Good thing, too, because it turns out I didn't really understand it very well to begin with! But further research and testing cleared things up.

As you can see in the video, the motion of E isn't perfectly vertical, because I didn't get the geometry of the linkages particularly great. But you can plainly see how much less its horizontal motion is compared to the pump rod itself. I'm not the world's biggest James Watt fan (everyone hates on Edison, when there are so many assholes in the history of science and technology to choose from!), but he really did make some amazing leaps of design. Props where props are due.

Sunday, May 9, 2010

George Washington's Grist Mill

Still on vacation, but I couldn't resist making a post about this. On the way out of DC today we took a bit of a detour and ended up passing by George Washington's grist mill and distillery.

Grist Mill 09

Not knowing anything about it we stopped on a lark, and I'm very glad we did. The distillery was interesting, but the mill really blew me away. Fully operational dual millstone system, run from a nice ~3 meter backshot water wheel. It includes two elevators and a very slick set of sifting and drying mechanisms for partial automation of the process, all of which were part of the original 18th century mill on the site.

Grist Mill 11

The whole thing is stunningly beautiful. The building felt alive while the mill was running, in a way that a primarily metal-based system would be hard to match. Drive shafts crossed the entire building, and large belts drove some of the smaller mechanisms. The workings were taken from an 1818 mill, though the wood cogs are of course much newer. There was an old set piled in one corner, with their teeth worn almost clean through. The old wrought iron linkage rods and even the bolt heads had worn designs stamped into them by some bored smith all those years ago.

Grist Mill 08

(Random flickr images; my camera was acting up today. The others in the collection are nice, too!)

Friday, May 7, 2010


Watch this space for details of Attoparsec projects as they are conceived, designed and built. Pictures and technical details should be the standard. You've been warned!