OK guys, I'm thinking of making an upside-down tripod rig out of PVC tubing. I've seen them made out of galvanized gas pipe and I always thought it was overkill.

But PVC alone isn't strong enough, so I want to stiffen the PVC with large wood dowel inside.

Ideas welcome on how to shape the wood, what type of wood is best, how to attach the wood to the inside of the PVC, etc. Keep in mind I've got no access to a workshop right now.

I'd buy cheap hardwood dowels from the hardware store. Get the largest diameter dowel that will slip inside the PVC pipes. Scuff up the inside of the PVC with coarse sandpaper or a file. If the dowels are a loose fit, shim up the dowels with wraps of masking tape until the dowels fit snuggly inside the PVC. Once you get a nice fit, epoxy the dowels in place.

Ross

How much weight are we talking about supporting here? And is it dead or live weight?

The main reason gas pipe is used is because it's cheap and easy to work with- and really strong. The only reason I can think of to not use it is the weight- it's heavy, making it more difficult to move.

Instead of supporting PVC with dowel- why not just use the dowel? It's a lot less work, and a lot cheaper than adding epoxy to the inside of the PVC, etc. Depending on the thickness of the dowel needed, there's several places you can get it cheaply. I got a dozen hardwood dowels for $10 once from a fabric store that was going out of business.

-Erik

When you take all the details and add-ons into account (restraining chains, adjustable feet, pully & rope, etc.), gas pipe rigs can weigh in at 300lbs or more. I've seen them up close, and the work required to move them and set them up on a daily basis on the street (I'm a busker.) I always thought I could do it better myself one day.

The rig is meant to hold a person upside down while hanging by their ankles, tied up in a strait jacket & chains (at least in my case.) Total weight would be under 220lbs.

Minimum PVC dia. should be 2". 2-1/2" is probably opitimal. So that means 2" dowel for the inside. I'm not clueless as to how it should all work together, but I am definitely open to suggestions and tricks that you guys might come up with before I get started building.

Chance, look into extruded aluminum. With the right hardware you could make something collapsible as well.

Forget the doweling inside, assuming you can find decent doweling that large (I've never seen it at 2"). Over the height needed I don't think it will be sufficiently rigid to support the live load you're asking it to. Even if you're only 5' tall you'll want at least 9' to the top junction, to allow for pulley, ankle hookup, and room under your head for your arms to extend. This means you'll be buying 10' sticks, and 2" rounds aren't going to be enough for your needs, IMO.

I'd suggest finding some square tubing to fit inside your PVC pipe. The square cross section will be far less likely to flex the way round does. Because of the slightly rounded corners that square metal tubing has (from the factory), a 1 1/2" square fits almost perfectly into a 2" inside diameter PVC pipe (yes, I've done this before). I would suggest aluminum tubing, for its strength-to-weight ratio. Last time I bought square aluminum tubing (it was 2") I paid around $100 for a 20' stick, and you can probably find a supplier that will cut it to 10' and sell you three 10' sticks for a very small upcharge from the going rate.

TW

High-Dollar Stuff

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Yikes, forget what I said. I was thinking you were building something far smaller, and supporting a static prop. "Upside down tripod rig..." Ok, now I get it...

As one of my favorite SNL characters would say, "Nevermind..."

Ross

Okay, outside the box and probably off the wall, but...

How about using larger diameter bamboo poles? They are used for construction scaffolding in Asia. Advantages: lightweight, rigid. Disadvantages: I dunno... maybe cost and availability (tho my local garden center has a good selection), joining perhaps, possibly looks depending on one's aesthetics.

Guys, workable or not-- what do you think?

Now that's a great idea. There might be some tubing the right size. You've got it made if there is. This is cool sounding. Good luck with it.

-=tabman

Is "almost perfectly" a snug fit or a little loose? What schedule PVC pipe are we talking?

On 2009-04-10 19:07, Chance wrote:

“Almost perfectly” means (in this case) it’s a little tight, but doable. In fact, I personally think tight is what you’ll want.

As you probably know, a “2-inch” designation refers to the inside diameter of pipe, but this can change somewhat with the pipe’s schedule. This is because the higher schedule rating is for pipe that can withstand greater internal pressure. Since you are only using the pipe for a decorative shell (I would assume), a higher schedule rating won’t gain you anything.

In practice, among the pipe I keep in stock, 2” PVC pipe (white plumbing grade SCH-40) runs about 2.055” inside diameter, and the 2” ABS pipe we have (black plumbing grade SCH-40) is right at 2.060” inside diameter.

With the 1 1/2" square aluminum tubing I have in my own stockpile the corner radius is specified at 0.02” radius - resulting in a .040” diameter corner – with a 0.125 wall thickness. The exact measure across two diagonal corners is 2.105”. To test the fit I rounded the corners on the leading edge of a piece of the aluminum tubing and I chamfered the mouth of a piece of a piece of 2” PVC pipe, then I shoved the square tube into the round pipe. It went in with only a small amount of force required, and the distortion to the pipe was minimal. I’ve included a CAD drawing below to give you an idea of what we’re talking about here.

If you’re going to have to remove the inner tubing from the pipe you might not want this tight of a fit. In that case I would take a router with a 1/8” round over bit and knock down the corners of the square tubing. The resulting shape should slip into the pipe with virtually no slop, but will not be so tight as to bind up. And yes, you can use a wood router to shape aluminum, if you use a carbide bit and go slow. Of course, you must wear proper eye and hearing protection and watch out for hot metal shavings, but 1/8” is really a pretty light cut – and that’s how they’d do it in most commercial fabricating shops (not to mention the Boeing plant).

So, to summarize, use the 1 ½” square tubing stock for a jam fit, or ease the corners to 1/8” (1/16” radius) for a removable fit. Use whatever type (PVC or ABS) schedule-40 2” pipe is readily available, meets your esthetic needs, and fits your budget.

TW

Click here to view attached image.

That seems to only come in 30" lengths. Unless Chance's avatar is life-size I doubt he'd be able to hang inverted under the resulting pyramid

TW

You might check this out, and if it's not just what you want for the job, look for more within the catalog:

http://www.northerntool.com/webapp/wcs/s......00349918

TW, do your specs include weight? How much is 50' of the 40 and the tubing all together (100' total)?

1 1/2" square aluminum tubing with a 1/8" wall is specified at just under 20 lbs for a 24 1/2 ft length - don't ask me why they list it that way. My calculator tells me that's around 13 ounces per foot, but I always rough estimate it at 1 lb per foot.

The SCH-40 plumbing pipe is something you'll have to weigh yourself, as that varies according to what kind you buy. Some plastic pipe, particularly ABS, is available in lighter weight or heavier weight - depending on whether or not it will be submitted to internal pressure (as opposed to drain pipe). The light-weight stuff, which might be what you want, is manufactured with a smooth outer and inner skin, but a foam-like core - which is to say it has thousands of tiny air bubbles in it that function to reduce its weight. So you'll have to get that weight factor from your pipe supplier.

TW

Thanks TW, good tips all around. I'll just figure they weight the same and call it 90-100lbs total.

As to how they might fit together, I would want a snug fit with minimal distortion of the PVC. So I might just go the way of shaving the corners like you said.

Since you're so quick on the CAD, here's a puzzle for you: At the top of the rig I envision a disk approximately 10" in dia. that the 3 legs will attach to in a perfect 8"x8"x8" triangle (spaced out, not touching). At ground level the 3 feet will be fixed at 8' apart. The legs are 12' long. So what's the angle at the top where they attach to the disk, and what will the final height be taking the angled legs into consideration?

Can someone educate me?

If aluminum tubing would do the job alone, why use PVC? Seems like a lot of work to fit the aluminum inside the PVC.

Dave

Hi Dave,

It began as PVC stuffed with wood. If I could locate the right type of wood I'd still go that route. Personally I don't think aluminum alone will do the job well. "Well" being the optimal word. I think for aluminum alone it would have to be 2-1/2" and 1/4" thick walls at a minimum. What I'm trying for is a rig weighing less than 100lbs regardless of the materials used.

The best aspect of using PVC is the easy joints you can make. With aluminum you still need to add on complicated joinery; with PVC it's easy as can be with standard plumbing fittings. But again, I wouldn't want to trust JUST those fittings alone.

If you have any ideas how to stiffen PVC pipe while keeping the weight down I'm all ears!

Actually, CAD is not needed for that "puzzle"; a scientific calculator is sufficient. Given your exact parameters, and assuming that the measurements are to the outside face of the square tubing (the easiest way to cut the angled ends), the angles of the legs would be 69 degrees (a good number in anyone's book) off perpendicular, resulting in end cuts of 21 degrees. The height of this configuration would be 133 7/8" to the bottom side of the round disk.

If you were to change your angle cuts to 22 1/2 degrees - making a very easy angle to find since it's a detent point on most good chop saws - your base distance between legs would increase to 8'5" and your height would be reduced to 132 7/16". But the angle would be a bit easier to cut.

All the above measurements are rounded off to make measuring easier. Also, I would suggest some sort of non-stretching links between the legs at ground level - perhaps cable or chain - to prevent the possibility of the "feet" splaying out while you're swinging around upside down without a helmet (no doubt).

TW
(PS: I guess I assumed the purpose of the pipe covering was for appearance's sake; I don't think it will add much in the way of rigidity, but maybe...)