Vacuum

[Evan]

Right after I finish making the single arm drive for astrophotography that I promised to my daughter I'm going to make a vacuum pump rig. Y'all probably thunk I forgot about the vacuum chuck. Not so. I was looking for a dropped screw and found a can just under the edge of a shelf on the bottom of a bench. In the can was a vacuum guage. I'm good to go!

[wierdscience]

Doesn't that just suck?

[Evan]

Not yet

[darryl]

Evan, my vacuum pump works fine, but I have a piece of 'microbore' that I want to use as a cylinder for my next model. I need to discover a way to run the piston up and down about 16 inches or so in this 3 inch i.d. tubing. I have thought about a 'con rod' of sorts with an enlongated hole in it, with teeth machined around the inner surface of it. The hole would be about 18 in long. A pinion on a motor shaft would engage the teeth, and the rod would be guided as it is alternately pulled, then pushed, by the pinion, giving a large stroke without interfering with the walls of the cylinder. Never tried this idea, though it seems to me that the vacuum attainable would be higher with the larger stroke to bore ratio. Just thought I'd throw this idea at you, or anyone interested, to maybe get some feedback.

[Evan]

Darryl,

You are going to have to show me a picture. I am having trouble visualizing what you have in mind.

[ibewgypsie]

I see pneumatic actuators that work with the rack-pinion principle. Turn a cylinder into a rotary force. I don't have any laying around.

I use a old vacuum pump off a 84 gm diesel car. It was installed into the rear (distributor) hole to drive the accessories. I run it with the lathe or a drill. It is a diaphram, but it will boil water in a canister.

David..

[franco]

Darryl,

Do you have room to use a cross head guide as is used on steam engines? This would let you use any stroke you wanted without any problem with the connecting rod contacting the cylinder wall.

franco

[This message has been edited by franco (edited 11-17-2003).]

[Evan]

Darryl,

Having slept I have a better idea of what you are proposing. A long stroke isn't going to increase the vacuum much. The best mechanical vacuum pumps are vane pumps. There was a thread on this some time ago, I think in the Third Hand forum.

[darryl]

You might be right, Evan, about the long stroke. My logic is that whatever residual air is above the piston at tdc will limit the ultimate vacuum that can be had, and that increasing the stroke will thin this remaining air to a greater extent, so attaining a higher vacuum. This is nitpicking I suppose, as anyone needing an ultimate vacuum will have to go to a different, and more expensive, type of pump anyway.

[wierdscience]

Two double acting air cylinders coupled rod to rod,one running on air with valving the other producing vacuum using opposing check valves.

Trick is using a 4" bore cylinder for power and a 1.5" bore for vacuum.

The other trick would be modifying the vacuum cylinder for metallic rings,no end gap,but rather a scarf or lap end ring would work,probibly would need three sets with a glass filled teflon ring with an o-ring energizer for the final stage.

[This message has been edited by wierdscience (edited 11-17-2003).]