Sunday, May 6, 2007

Cylinders, Pistons and Seals

In the introduction, I said that treadle pumps needed cylinders and valves. They also must have pistons,and seals to keep the water from leaking out around the pistons and the cylinder walls. One of the things we are trying to do is to find a way to make these pumps without having to have a full blown machine shop, using materials available in Haiti. Early on it was decided to try to fabricate as much of the pump from PVC and standard plumbing parts as possible.

What you see is an experiment with using leather washers to make a piston seal. Two washers were used, one to seal against vacuum, the other to seal against pressure. The cylinder is a piece of 2" PVC pipe, with a cap on the end. The cap was drilled and tapped for a ball valve so that I could vary the resistance to flow. Long story short, the seals worked, surprisingly well. The addition of vegetable shortening to the leather improved the seal, and greatly reduced resistance to motion.

The results of the piston seal experiment was encouraging enough that a search for check valves was begun.

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