Date: Tue May 27, 2008 11:01 pm. By: Neon John
On Tue, 27 May 2008 08:57:01 -0700, "Ulysses" wrote:
Time to change the subject line, I think.
This happens to me faily often. I search and Google and find little or nothing about whatever I'm looking for. I think part of the problem is that many people will ask a question, and as in your case get no response based upon actual specific experience, and go on to figure it out themselves. Then the results are not posted so others can share that experience.
One of the keys to google is thinking outside the box. Try to imagine what terms OTHER than the ones you use might be used by the author of what you're searching for. And try to think of related areas. The following will be a good example and will address your air compressor dilemma.
For example I recently searched for information regarding using a wind turbine to drive an air compressor. All I found was responses saying it would not work but nobody really said why. Well, being the kind of guy I am I had to try it for myself. It didn't work, and I'm not sure why. I would have thought that simply getting the compressor head to turn would result in a cumulation of small amounts of compressed air but it would not build up. I'm guessing the compressor needs to turn at a certain speed in order to work. Once I figure it out I will post my results even if they are negative. Someone else might read it someday and get it to work. Just because something is impossible doesn't mean it can't be done ;-)
As you've found out, conventional air compressors aren't very well suited to low speed operation. There's the cogging problem, the periodic resistance on the compression stroke that at high speed is overcome by flywheel inertia. Then there's the blowby problem. As you know, you can slowly turn either an engine or an air compressor through its compression stroke and accomplish nothing. All the air leaks past the piston and rings. Again, both machines rely on speed to cause the compression to happen fast enough that the blowby has little effect. Plus, most piston rings require a certain amount of backing pressure to properly seat.
All you need for low speed compression is a suitable design. One that doesn't rely on inertia or conventional piston rings.
Recently I became interested in big bore air rifles. Think .50 caliber with enough energy to kill big game. Like this, for instance
http://www.quackenbushairguns.com/
The gun is pre-pressurized to from 2500 to 4500 psi. Most folks use diving tanks but believe it or not, there is actually a hand pump available that can generate that kind of pressure.
http://www.airgundepot.com/hand-pump-samyang.html
Of course, in the fine print for one of the rifles, it notes that from 1500 to 2000 strokes are required for the first pressurization. Yikes! Holy exhaustion, Batman!
Not being interested in building my triceps or wearing myself out and not being terribly interested in having to travel 50 miles to the nearest dive shop to beg 'em to fill a tank for a non-certified non-diver, I naturally started looking for alternatives. I momentarily considered building a slow speed crankshaft affair that would drive that manual pump with a motor. Having some doubts about its durability when used like that, I decided to look further.
There are, of course, commercially made high pressure air compressors but they're big $$$$. One of the air gun e-stores sells a ChiCom made unit for $1995 or thereabouts, far too rich for my blood.
So I went googling. I didn't find anything related to air guns. I knew that both divers and emergency responders also use high pressure air. Divers are more likely to homebrew something than are firemen so I included diving terms in my search. There I hit paydirt. Consider this homemade low speed shop-air-drive oxygen compressor.
http://www.brianb.org/images/Scuba/Homemade/Gas%20Booster/gas_booster.htm
From practically zero pressure to 3400 psi in one stage! Pretty zippy. I would have done several things differently. I would not use the lever arm but instead would find the correct diameter air cylinder to generate the necessary force directly. That way the two cylinders are directly coupled and there is no bending moment. I'll probably also go with more than one stage. And since I happen to have an electro-hydraulic power pack, I'll probably use a hydraulic cylinder instead of air.
Anyway, here's a design of a pump that can be directly driven from an old-fashioned farmer's wind mill. You can drive it directly from the reciprocating motion that these windmills deliver to the ground. Actually, the air cylinder that he used to drive his pump would probably be more suitable. You'd want the inlet and outlet check valves to be as close to the cylinder as possible to minimize dead space.
So how long will it take you to get one running? :-)
John -- John De Armond See my website for my current email address http://www.neon-john.com http://www.johndearmond.com <-- best little blog on the net! Tellico Plains, Occupied TN There is much pleasure in useless knowledge. —Bertrand Russell