Has anyone done "central tire pressure" on inner tubes?

Justin W · Post by **Justin W** » Sat Jul 26, 2008 2:16 pm

Hello again, respected holographers the world over!

I'm posting another rant in an effort to explain my plans for suspending what will surely be a heavy load of concrete pavers on some small inner tubes without fear of soft or hard spots due to differences in pressure. My central tire pressure plan will also allow me to bleed the air and settle the table on blocks if it's not to be used for a period (another great insight per Colin), then be able to re-inflate the whole rig from one single valve.

My thoughts initially went like this: I'll need some sort of pressure rail - a distribution point. From there I can send air down flexible valve extenders to individual inner tubes. I figure the more inner tubes the better. Say (rough figure) the table with all the pavers weighs in at maybe 200lb... If I can arrange for that 200lb to be suspended by many inner tubes, then my (perhaps faulty) logic says that each individual tube will be supporting a small fraction of said weight, and therefore be actively suspending the weight at much lower pressure than would be the case for just a few tubes...

Right... so I'm visualizing distributing pressurized air... I'm a do-it-myself kinda cat, and a pressure rail strikes me as a simple-enough endevour: perhaps a short length of sch40 PVC pipe capped at the ends and drilled to accept a series of snap-in valve stems - one of them air in, the rest air out... Simple enough - but - ARG! there's no such animal as a female valve stem

That fact left me in a lurch for some time - the flexible valve extenders that were clearly such an integral part of my plan all had a nice threaded female end perfect for coupling firmly to the inner tubes, then of course terminated in a standard schrader valve male end with the valve core and all that.

So how in the holy heck do I couple that male end to my pressure rail? I considered cannibalizing a bunch of tire pumps for their hoses - it was the only place I could think of to find female valve couplings. Still unacceptable; A quick-release coupling strikes me as unreliable for holding pressure over a period of time.

Then inspiration in the form of common sense struck: Why not just remove the valve core from the male ends of my flexible lines and epoxy those mothers right into my pressure rails? YEAUH. My plan now is to fabricate one or two pressure rails from short lengths of sealed PVC pipe with the de-cored male ends of flexible lines glued straight in and a single snap-in valve stem to pressurize from. Chances are I will thread an extender onto the inlet valve to reach toward the floor and make charging the system easier with a single pump.

If I get a "flat", I can just drop the table onto the blocks I'll leave there (between tubes) and slide the blown tube out, replace it...

Yah?

Any comments, my friends?

JohnFP · Post by **JohnFP** » Sat Jul 26, 2008 4:27 pm

"Fix a Flat" has femal screw on ends. I would use that white silicon plumbers tape also.

Justin W · Post by **Justin W** » Sat Jul 26, 2008 4:49 pm

Hey...cool

Thanks, John.
That's an idea. Maybe yah if I have trouble getting my pressure rail to seal well enough, I could find hose fittings to affix into the rail, then hose-clamp fix-a-flat hoses to them. Neat! That would give me a number of additional inches on my whips as well

Thanks. Cool.

Colin Kaminski · Post by **Colin Kaminski** » Sat Jul 26, 2008 10:03 pm

My first table had a manifold that fed 4 tubes. The main problem was when I started adding very heavy equipment to the table. Floating a table on air is much like floating a boat on the water. If you add a lot of weight at one end it sinks. Without the manifold this is much less of a problem. For my 3 x 5" table I would have problems with anything over 50# on the table.

Justin W · Post by **Justin W** » Sun Jul 27, 2008 12:17 pm

Cool, Colin

Yes, I imagine this is a consideration. I very likely have spent too much time considering the mechanics of facilitating central pressure and not enough time considering the mechanics of using it....

I can picture that quite vividly - an off-balance load resulting in a listing table. This thought makes me glad I have so far been working with a plan in mind that involves a very symmetrical arrangement of the heavier working components (mostly clustered toward the center anyway).
With the exception of a few small beam steering mirrors, my table should be balanced.
Oh, and my laser on one corner - is cool - I already had plans to weigh the laser and controller plus the aluminum platter they sit on, and place an equal weight on the opposite corner... No sweat.

Yeah, no sweat, thank you for bringing that up.
Colin, do you have any hot tips or sweet design tips for making a manifold work?

Jeffrey Weil · Post by **Jeffrey Weil** » Sun Jul 27, 2008 1:43 pm

Hello Justin,

Theres really no need for more than 4 tires. Its very counter intuitive but they will not get to a high pressure. Really.

I've made many tables and right now have a 1250 pound Newport. All on 4 tires. And small ones at that. They came from lawn mowers.

They are only about 15 inches in diameter so people are expecting them to be bulging on the edge of bursting. But, if you push your finger into the side of one you quickly realize that its under very very low pressure in fact.

So low that if you want to read the pressure no regular gauge would do. You have to use a syphmomanometer, a blood pressure gauge. You can get one from walmart or wherever for 30 bucks or so. So it has to be well under 4 or 5 pounds or a reg gauge would register some pressure. Thats low. You don't need the gauge though, inflate them so the table is just floating, not much more. You don't want it too bouncy. Don't worry if the pressure is not the same in the tires. Just that the height the table has been raised is about the same.

Colin is right about the distribution system, forget about that. If you put something heavy on one side of the table you want that tire to reach a higher pressure than the others to support that weight. The manifold would mess that action up.

Your right about using the valve extenders. Its a drag to have to lift up the table, remove the tire and fill it. Much easier and more exact to fill it in position. Somewhere in your town is a place that makes up hoses. They will have all the parts and hydraulic crimper's to attach the ends well. Down here in Miami my place is Amazon hose and rubber. I had my 1.5 foot extenders made up for just a few bucks each. Beautiful brass valves and connectors to the existing tire valves. Proper Goodyear rubber hoses. All pro crimped together. Well worth it. Don't go cheap on this, you'll just be leaking some air slowly and it'll be a pain.

Inside the tires center, the donut hole put a small piece of 2x4. That way if you release all the air the table won't crush the tire, its also a nice safety feature to keep anyones hands from getting crushed in case something unexpected happens.

If your ever here in Miami Beach your welcome to come to the lab and check it out if you like.

Jeffrey Weil
NorthBeach Holography Inc.

dave battin · Post by **dave battin** » Sun Jul 27, 2008 2:43 pm

a very neat system ive seen is to use four small innertubes and remove the tire valve on all four tubes and now connect them with clear tubing that will fit over the entire valve stem and secure with a standard hose clamp, now connect all four together in a series. now you need to find a tire valve and stem from a butchrised innertube , connect this to the end of the tube to act as an inflater valve. now if you will be needing to adjust the pressure more on one side than the other, simply put inline hose valve/clamps near each innertube (these valves simply slip over the plastic tubing and by squeesing it,it constricts/stops the air flow) allowing you to "pump up" one or more innertubes at a time. additonal parts would be inline pressure guage and a nice electric compressuer(a bycycle pump works fine)

BobH · Post by **BobH** » Sun Jul 27, 2008 2:49 pm

I agree with everything Jeff just said. If you use a pressure manifold, your table will always be at a different tilt depending on where you lay your tools on the table. It will be very touchy in that respect.

The first holographer I met when I moved to California in '81 was Steve McGrew. I visited him at his home in Santa Clara at the time, where he had done his DCG work as published in SPIE #215. He advised me to leave small gaps under the inner tubes to allow the air to escape from the central hole. Otherwise, the tubes would be very stiff. I installed filler hose extensions as Jeff suggests, and ran them through holes in the plywood supporting the inner tubes, then out through gaps in the cinder block legs. No carpet, no carpet, no carpet!!!

Justin W · Post by **Justin W** » Sun Jul 27, 2008 3:02 pm

Right on.

Manifold idea: Scrubbed.

I'm stubborn, but not too stubborn to listen to good sense

Everything about only tending to four tubes and not going through the hassle of building a manifold sounds fantastic!
According to my preliminary calculations, my proposed 4X8' table with concrete pavers covering the 32ft area will weigh in about 640 lb. If a table twice that weight can ride in comfort on four lawnmower tubes, I have to imagine i'll be OK. Hurray. My next question is regarding the trustworthiness, Mr. Weil, of valve extenders. Some internet browsing brought me across these 3ft whips designed for inflating spare tires. Three feet sounds nice (and is the longest I've encountered as far as a pre-fab valve extender goes.

Reckon these jobbies are worth a hoot? - http://www.campingworld.com/shopping/it ... ator/34430

dave battin · Post by **dave battin** » Sun Jul 27, 2008 3:09 pm

the clear tubing is about 25 cents/per foot thast a lot cheaper than $20 each ! you will really be using quite low pressure here, i think these will be a little over kill, buy the clear tubing and spend te leftover money on film!