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  1. #1
    Join Date
    Nov 2008
    Location
    Perth
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    Question Swimming Pool Pool Blanket roller

    I have just forked out $550.00 for a pool blanket , to cut down evaporation and to save on pool chemicals, ( doing the right thing for global warming) the only thing is to make things easy to be able to roll up the blanket one should have also purchased a roller. The cost of a roller can run into more than the cost of the blanket. My question is has any member made a roller for this purpose, my idea was to construct one out of PVC sewer pipe for the main shaft with wood frame support ends. The pipe would have block ends with a centre stub axle of 25mm galv pipe. I think one should be able to construct something for less than $ 100.00. Any Ideas would be appreciated.
    Mac
    PS the length of the roller would be around 5metres.
    Last edited by Malcolm Eaton; 28th December 2008 at 10:55 PM. Reason: size of roller

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  3. #2
    Join Date
    Jun 2006
    Location
    East Warburton, Vic
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    Default

    If the roller was to stay on the end of the pool, you could probably get away with the PVC pipe, but if you were wanting to move it away from the pool after you taken the blanket off the pool, the PVC would probably flex too much.

    I used to move the one on my Uncle's old pool which was Aluminium or Stainless and it was heavy when the blanket was on it.

    I'd be checking out if the PVC was up to it before you start getting to far with it.
    Cheers

    DJ


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  4. #3
    Join Date
    Oct 2006
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    Tallahassee FL USA
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    Default

    I saved some money, or thought I had, by purchasing a used one - without the roller. Folks at the pool supply shop informed me that 6" deflection was normal for a 20' x 40' blanket spanning 22 ft (6.7m) on the standard roller. I figured I could design less deflection using large (about 4" nominal) PVC pipe. To increase the stiffness and strength, I post-tensioned it with several steel rods. Didn't work worth a damn, even though the calculations suggested it "should." The post-tensioning introduced too much eccentricity and local curvature, even at final assembly without load.

    For the next trial, I got a bunch of 6" galvanized stove pipe sections, and pop-riveted them together at the bell-and-spigot joints. I hadn't made the end trucks yet. While rolling the blanket, with some wrenching about, the pipe buckled somewhere within the span. It turns out, I used too low a value for the actual weight, and didn't include an impact allowance.

    The next, and probably final, experiment uses 8" steel pipe. I think I've considered everything that could go wrong, but time will tell. Final assembly is pending at the moment.

    FWIW, I'm a Structural Engineer. Luckily, most of the anomalies have been sorted out for heavy civil and military construction, including primitive locations, and I've never had a significant failure in those departments, thank Goodness. "Fault-tolerant" is the watchword for design.

    Joe
    Of course truth is stranger than fiction.
    Fiction has to make sense. - Mark Twain

  5. #4
    Join Date
    Nov 2008
    Location
    Perth
    Posts
    330

    Default Pool Blanket roller

    Thanks Jo for that information, I was thinking of using 150mm ( 6inch) pvc pipe.
    Your attempt to post tension the pipe is quite intriguing and I would of thought that you would have had success , 150 steel pipe sounds to be on the heavy side ( weight).
    How did you actually go about post tensioning the pipe ?
    My thoughts on the stand was to make them out of scrap 19mm form ply and to provide castors to allow to move the total unit.
    Mac

  6. #5
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    There were two problems associated with post-tensioning the PVC pipe, Mac:

    First, the pipe tends to creep under compressive load. Concrete does too, but it's easier to control.

    Second, probably worse, was trying to maintain pure compression with several steel rods. Each rod seems to have experienced varying drag along the pipe, sufficient to throw things out of whack. The small geometry didn't help either.

    By itself, I don't think the PVC pipe is stiff enough to match the commercial product.

    Both sizes of steel pipe I mentioned are galvanized sheet metal - only about 0.030" wall thickness, and very light weight; I can carry the whole 25-foot length under one arm (length before trimming). The strength derives from the large diameter. One would think that a cylindrical pipe functioning as a beam would be a straightforward analysis problem; one would be wrong. A fairly recent text I consulted for theory, contained something like, "Until this discrepancy is resolved, ..." The bottom line is that the pipe is safely designed if the compression stress on the top is less than the buckling strength of the same pipe functioning as a vertical column. Local buckling is a separate problem, and seems to be under control.

    I haven't yet sorted out the end trucks. 19mm is too narrow in any event. I expect to fashion end trucks from some log slabs about 4" (100mm) thick for adequate caster support. I'll probably use some HDPE bearing material (cut from bleach bottles) in holes for the pipe, with a removable cap. And some kind of endwise locators too.

    I hope this helps a little. I may, after the smoke clears, just bite the bullet and purchase a commercial one, and put better casters on it. Assuming, of course, that I don't fill the pool and build a miniature golf course instead.

    Joe
    Of course truth is stranger than fiction.
    Fiction has to make sense. - Mark Twain

  7. #6
    Join Date
    Oct 2003
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    Kentucky NSW near Tamworth, Australia
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    I'm no engineer but how about making up a pipe truss like this to slip inside the PVC pipe. You would need a pipe with a bit of wall thickness so you could weld it ok.

    You could have three or four of the web sections to keep it concentric in the PVC pipe. You could probably use 1/4 or 3/8 rod for the web section.

    You could probably run the figures Joe and see if it would work.

  8. #7
    Join Date
    Oct 2007
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    Alexandra Vic
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    69
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    2,810

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    I am thinking along the lines of a full length spindle of something like 2 in pipe, with bobbins every 300mm or so along the length to support he PVC pipe.

    We had a power operated, wall mounted one supporting a blanket about 60ft x 30ft at the school where I used to work. The roller was built this way but all 316 SS, has about 1 inch of sag when the blanket was fully wound onto it.

  9. #8
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    Jan 2005
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    Sydney
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    i ll take a photo and post tommorrow, mine is not plastic,

  10. #9
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    SS was probably key to the success there, malb. SS is a lot stronger and stiffer than PVC. The bobbins (diaphragms) help to maintain the circular shape against local buckling.

    Bazza, my objective was mainly to reduce the sag. The apparatus you suggest would likely work, but going to thin wall stove pipe was simpler. I considered adding diaphragms of plywood, but construction had me stumped. In either case, making everything straight, AND keeping reasonably intimate contact between the parts, could be difficult. Now that I've used the proper weight, and added an impact allowance of 100%, the numbers work for me with the larger 8" stove pipe, and diaphragms don't seem necessary.

    We're in winter in the upover, so my timetable is somewhat relaxed. I've included this thread number in my WWF index file, and if time escapes for an extended interlude, I'll add more later (when/if, of course).

    Joe
    Of course truth is stranger than fiction.
    Fiction has to make sense. - Mark Twain

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