Neil's News

Air Compressor

8 December 2008

My database course at Stanford is finally over! I learned a lot and I think I did well. To celebrate I built an air compressor.

One piston on a crank shaft would produce intermittent air flow since most cylinders are single-acting. Two pistons offset 180° would be better, but there would still be 100% pressure fluctuations twice per revolution. Three pistons offset at 120° would fill in these dips and smooth out the airflow. Presumably the more pistons one has, the better. Wrong. Somewhat surprisingly three pistons provides significantly smoother airflow than four pistons

Two pistons:
[Two pumps at 180 degrees]
Three pistons:
[Three pumps at 120 degrees]
Four pistons:
[Four pumps at 90 degrees]
(Black is the sum of the functions. Graphs computed at

With the math complete, it was time to build it. I bought three bike pumps from a sporting goods store and some brass plumbing from a hardware store (freaky moment: the clerk didn't know how to give me 2 meters of tubing, had to order it in yards). Bearings arrived from VXB (their linear motion and large inner diameter bearings are so sexy).

Rather than use Meccano, I chose the challenge of using wood for everything except the camshaft. The primary motivation was to reduce the running noise of the compressor. Definitely a learning experience; in future I need to optimize wooden structures for simplicity rather than for minimal usage of material. Never the less, the result looks good and works well. Enjoy the movie:

(The music is "Morning Nightcap" from Lúnasa.)

The geek is back.

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