How does a screen door work?

Factual Questions
1

OK, so it’s not exactly the most scintillating subject to ever grace the virtual pages of the SDMB, but if you will bear with me a moment you will see what I am after.

The screen door to my house is probably like most in that it has a cylinder concentric about a rod that connects the door to the frame. This cylinder looks almost exactly like a bicycle pump of the type lots of us used as kids to blow up basketballs and the like. It is easy to see how this would impede the movement of the door and slow it down so that it doesn’t slam itself shut. But what I don’t understand is where the force comes from that makes the door close when you release the handle?

Is there a spring in the cylinder that always pulls it back to the shut position? I believe I have seen other doors with a smaller cylinder at the top of the door and I may have even seen doors with a cylinder at the top and another one about midway up the door. What is the difference between these types? Do they all function the same way? In the two cylinder configuration is one used to slow the closing of the door and the other to provide the force causing the door to close? The ones I have seen at the top of a door seem to be smaller and are mounted much closer to the edge of the door that is closest to the frame.

If anyone has a link to a schematic it would be greatly appreciated.

2

It’s a pneumatic cylinder door closer. This site has some good, easy-to-read diagrams for both single action and double action.

3

Thanks! That is pretty much how I envisioned it in my mind but I didn’t feel like getting Mrs. Voluble up in arms over my diassembly of yet another common household object. :smiley:

Any guesses on how the multi-cylinder doors are different or am I remembering seeing something that just doesn’t exist outside the convoluted corridors of my own mind?

I am also interested in how these cylinders are manufactured and assembled. I will spend some more time Googling and see what I can come up with. If anyone has a good site for me to check out let me know… otherwise I might get to take something apart afterall!

4

Why two?

Any given cylinder unit only has so much force. On heavy doors, you might need two.

As well, the typical screen door is pretty flimsy, and a closer than really yanked on the top corner might bend it out of shape over the course of years. two closers working together at opposite ends of the door would put less bending stress on it.