Ideas for Reducing Sag on 25' Tube

Either foam it, or put a tension cable in it.

OK...I have thought about the tension cable. But not sure how to implement it.

Shout I just run a cable throught the middle of the tube and tighten it up with a turnbuckle? or does the cable have to be across the top of the tube?

thanks for the quick response.

Oh and what type of foam? A-B type stuff?

thanks,

Bill

Get yourself some 3 1/2" diameter disks, about 1/4 inch thick. Drill a 1/4 inch hole in them 1 inch from the OD anywhere.

Feed some cable through the pipe and plates with an I-bolt for tension at each end. I'd place the cable plates so that the cable is at the bottom. This will pull the pipe up as you tighten the cable and put the top in tension and the bottom in compression.

Or foam it.

Why not run the cable at the bottom of the disc at the center of the tube (put a notch in the edge of the disc for maximum offset) and anchor at the top of the tube at each end. Now, tensioning the cable tries to lift the center because the initially V-shaped [in side view] cable wants to straighten out.

If there is solid anchorage at both ends, run a flat bar (in vertical plane) in to, say, 1/4 of the span, and have it support the tube only at its cantilevered tip; this moves the supports inboard and lets the ends of the tube sag as a counterbalance. The support bars can have an initial tilt upward toward the center, or be curved, so that they have a final position close to level when loaded.

I still haven't seen an answer to whether the tube carries fluid or other contents, whether it rotates, etc., each of which will influence what is possible. Could you place a flat plate that just fits inside (height) through the entire length, with a pre-formed arch in it? Properly done, its own weight plus that of the tube would cancel out the arch to yield a straight final assembly. Arching the tube itself is still possible with this idea.

Some of the other ideas sound better; but if you use the tension cable, it must be connected to something independent of the tube so that the cable tension doesn't enter the tube as a column load. I suspect it will take substantial tension to do the job!

Hmmm.....I think not.

A tension cable will reduce the max bending tensile stress in the section and increase the maximum compressive stress but since elasticity is essentially the same for compression and tension there will be no change in the elastic sag of the beam.

Foaming it will help just a little, but since its elastic modulus will be much less than for aluminium (though this is offset by a greater area moment) there will be little if any noticeable improvement.

Thanks. Now we know of two things that you think won't work. Any ideas about what might?

25 helium balloons tied to the center? A single cable up to the roof? A single cable up to a parallel tensioned cable strung a foot above?

Self correction to my earlier post and with proper regard to Lyn's good second post that I seemed to have skipped.

A tensioned cable inside the tube in the middle (along the central axis) will not work as I said, but if it is located inside near the lower edge of the sagging beam (the edge in tension) the tensioned cable will effectively apply an upward moment and reduce the sag.

In moment area terms it makes the beam like it is two tubes welded one on top of the other - much stiffer.

If the bending loads are not high (so compression failure in the top region is not imminent) and a cable can be located inside close to the edge the pipe (ends attached to end plates etc.), the effect will be to make the beam bend around a axis along the bottom edge of the beam and not its centre line as before. This is called the neutral axis where the sum of compression forces and tension forces is equal.

Bending stiffness is a function of beam depth squared and so in the perfect world the result would be to increase stiffness by a factor of 4.

Let's say you only get a factor of 3 in practice, it will decrease your deflection by 3 - and that ought to go a long way to meeting your needs. This idea is related to other ideas posted including the "built in ends" concept and the "under truss member" idea, and will work as well (better) if the cable is place just below the under side of the beam - but it will look ugly and I suspect its not what you are looking for.

Of course it might also be an option, depending on information you've not told us, to add extra tension in the cable and "bend" the beam back to straight.

Can you change to "Rectangular hollow section" instead of what I presume is round tube?

A tube with same internal cross section, but height to width ratio of 3:1 will reduce the sag. Basically, the tube sides act like trussus.

Another alternative might be to form the tube as an arch, so that the sag when installed gives you a "straight" result. This would require additional restraint at both ends to stop the tube rotating to have the arch at teh bottom and thus double the apparent sag.

You could put an I beam inside of it, the beam could even be bent with an arch in it to offset the sag.

This is a step in the right direction. In addition, one could introduce a moment through the fixed ands and adjust the center deflection to be zero.

Is the pipe conveying fluid or just acting as a support for something? If it's the former, suggestions about passing cables etc thru it are unlikely to work.

Is it fixed or simply supported at the ends?

If it's any help, as you want deflection as small as possible, I just worked out deflection due to the self-weight of the pipe. Based on wall thickness 1/4", steel pipe. It wil be about the same for aluminium as the ratio of density to elastic modulus is very similar.

Fixed ends ~ 0.15", simply supported ends 0.75".

Cheers.........Codey

Introduce triangles.

Like this?

You could put another tube inside, but have it a foot or two longer at each end. To those ends you could fasten a cable with turnbuckle, and attach other end of cable down toward the ground; straight, or at an angle inward. When you tighten on the turnbuckles, it will arch the inner tube upwards, thus assisting the outer tube to overcome its center droop.

As an alternative, instead of running the inner tube all the way through, you could use two shorter pieces. One at each end, extending into the outer tube for only a few feet. You might want to fasten these ends in place in some way. As far as the size, the fit should be as tight as possible. The rest... same as above.

Rectangular tube might work well for this; long axis vertical. The farther the inner tube extends out of each end... the more effectively this will work.

Bill,

Rather than sieve the multitude of options which may or may not work, can you tell us about the application. There may be a better alternative which doesn't use your 3" tube and/or the space around it, but we don't know if these may be constraints to the overall situation. A 12" I-beam may be the answer but it may not suit.

Bob

As a sailor my suggestion is somewhat like nick name's second idea.

It is used on masts to prevent or induce bend, and on sail boats they are called diamond spreaders. I have also seen them used on long sewer pipe spans across rivers etc. In that case you use two sets of spreaders instead of one, set about 70º apart from each other (ie, each one pointing down at 35º from vertical), this gives the tube upward lift, and sideways stability as well.

Tell us more about your constraints.

To do it inside the tube: you've got to do something like this:-

Get a length of plastic tube and put it inside the ally. tube:-

Fill the ally. tube with mortar; let it set; trim the plastic tube to length.

Thread a steel cable through the plastic tube, and, post tension it.

As with other solutions the ally tube has to be fixed at the ends so that it can't rotate.

One more, but maybe simplest(?).

As suggested by Lyn, fill it with foam but block the ends with a plug and maybe better add a cable to tension and keep the plugs pressed in: NOW, do this with the tube already in place and sagging down!

When the foam gets cured, and the plugs in place with the cable keeping them pressed in at the ends, turn the tube 180 degrees so that it is now arced upward. Secure the tube from rotating, at the ends.

You can slack the cable, allowing the plugs to move out as much as required to keep the tube horizontal or slightly arced upward ...

P.S. The plugs should extend into the tube about 3 to 4 inch to allow their movement outward when setting the sagging.

If there is nothing in the tube a 25' length of flat stock placed inside vertically will work. An alternate option may be to attatch a 25' length of 2" angle iron to the to the top or bottom of your pipe.

Apart from #2, we've had no comeback from Bill, so we have very little idea of the constraints.

Many posters have tried to help, but without some feedback it's impossible to know if any of the suggestions are feasible, let alone practical.

Cheers............Codey

Flatten the tube along its entire length so it becomes oval/elliptical (approximatley), with the long axis of the oval oriented vertically. It will make the tube more flexible side-to-side, but vertically stiffer, which will reduce the sag.

Maybe you could attach a beam so the tube rests on the beam. This wouldn't require any hangers. You would obviously have to support the beam at the ends.

Seeing the interest and the many solutions I wanted to make a computation and show which are valid quantitatively and which are not to be accepted. I would like to have some more details about the pipe.

According to your values the pipe has the sag under own weight.

To verify it is compulsory to know the internal diameter or the wall thickness. I may be wrong but my estimation is for the wall about 5/32 ". I would like to have the right value.

I consider that the most economical solution is the one which can be implemented without taking the pipe off the supports and without modifications of those.

This post is turning out to be very interesting, entertaining and educational, for a variety of reasons. Unlike the majority of questions asked that require very specialized technical knowledge, i.e. electronics, fluid dynamics, etc., this is a subject that could be called "classic" engineering, in a sense; and it's one where people from a wide range of disciplines can offer at least plausible suggestions based on common sense, clever thinking and ingenuity.

The fact that so much critical information is missing, is making the range of possible solutions even wider. And it just goes to show that there are many ways to skin a cat. That's one of the things that makes this entire field so much fun for all of us. We're solving a puzzle.

It will be interesting if Nick Name breaks it down and catagorizes all the ideas. Many of these solutions have been repeated by various members with only slight variations, but there's probably a dozen or so distinct ideas that could possibly work (of course my helium balloons idea was a joke), based on what little info we have.

But the simple fact that this is a puzzle to be solved is the very reason that I am an engineer, that I enjoy going to work each day, doing what I do. Let's hope Bill gets back to us soon with some more pieces of the puzzle!

Take two 20-foot sections of aluminum angle- 3" x 2" and bolt the longer legs together every 5 feet with stainless bolts and nuts.

Mount the assembly to the top of your tube and secure with stainless screw bands every 4 feet, starting from either end.

The T-Beam will be in compression as the tube tries to sag, so very little risk of over stressing it. The aluminum and stainless steel avoid any risk of corrosion or galvanic action.

I would run a cable above the tube. Let it sag to a comfortable catenary. Then drop short cables from the cable to the tube. They will be longer towards the end. Sort of like a suspension bridge.

I did something similar with a hose drain pipe I was installing at a faire I was at once. My installation did not have to be horizontal, but the hose had to be drawn up and over a walkway. This meant a complex curve. Your problem is really simple.

WOW!

I had NO idea this question would receive such attention!

Thank you all for your time with this.

Let me give you some more details. I have no actual numbers for load. I will just spill the beans and tell you what we are doing.

I work for a company that manufactures automatic pool covers. This alum. pipe is what we call the leading edge of the cover. Its supported on both ends with a plastic insert that is about 4 inches long. There is no fluid flowing in it.

The cover is made of a vinyl material and is attached two different ways. 1 is we have a custom extruded tube. on the cover we sew in a rope that slides in to a C-channel that runs the length of the tube. On bigger jobs we just use a "loop hem" at the end and slide a tube into it. Like a curtain rod.

Here is the problem. I have a customer that did not allow enough space for the leading edge to go inside the recess that holds the cover mech. For a 25 foot span we would have sold him a 4" or 6" loop hem and leading edge. But he build the recess to use our standard custom extrusion.

The recess can not be modified it would require basically starting over.

The sag is so bad the the tube actually drags through the water and hits what is call the bond beam when it go's into the recess or vault. Its going to eventually tear the cover or chip the imported Italian tile! (This is like a 2.5 million dollar back yard...Its unbelievable!)

Let me explain how the ends are supported. the tube rides in a track that is in the wall of the pool. There is a plastic slider that go's in the track that is attached to a 90 degree bracket. This bracket at the top has the plastic plug that slides into the tube. The tube can be secured so it will not rotate.

There are lots of good ideas posted. I like the idea of shaping the tube into an oval. But how would you do that? Could you set up a pipe bender to such a thing?

I also like the idea of filling the tube with something while its under load and then turning it 180 Degrees. But What would you use to fill it with that would be light but still be strong enough??

Sorry this took so long to get back with you all. I really did not expect this much of a interest!

Keep the ideas coming we will all be hero's on this. I told the builder I was going to ask you guys because we are pretty much at our wits end.

Thanks again to all!

bill12780

OH!! I see. Amazing what a little extra info can clear up.

I'd recommend, as I and a couple of others have already alluded to, simply sliding in a flat bar or rectangular hollow tube, with as great a H:W ratio as you can find that will securely fit into the round tube. Position it vertically. This should do the trick as cheaply as possible.

Given the additional details, I suggest you obtain a 25 foot long strip of aluminum (maybe 1/4 to 1/2 inch thick) and inset it into the tube as a stiffener. Of course the width of this strip will be in the direction of the sag. You can have the ends TIG welded to keep the strip from rotating, and you may want to install some D shaped spacers internally or inject some urethane foam to lock the stiffening rib in place. Of course, another option is to use a larger diameter aluminum tube. The sag will go down quickly as the diameter increases.

Then K_Fry 's three washers is the best idea so far (post #31).

To get that middle washer in place to weld it you've got to glue it (hot melt) to the end of a 13' rod. Weld delicately to begin with then when you're sure its fairly well fixed, go for it and the hot melt will melt and the rod can be removed.

Now you've got to tie a small weight on a piece of nylon; stand the pipe on its end and jiggle the weight 'til it falls through the hole; use the nylon to pull through a piece of cord; then tape the cord to the cable to that the ends butt up to each other, and, pull through the cable.

Sorry if I'm trying to "teach my grandmother to suck eggs" here: I just had visions of you getting half way through the process, and suddenly thinking "What the heck do I do now?".

More details about the pipe as I asked. Unfortunately engineering is a precise profession and requires quantitative informations. All ideas are good if they can be implemented unfortunately some are qualitatively OK at 1st glance and cannot be used when number are crunched. So that we need numbers = either wall thickness or ID + Weight of the stuff on the pipe in whatever unit of force / unit of length.

My two cents on that comment, Nick Name:

Although what you say, "engineering is a precise profession and requires quantitative informations", is true 90% of the time, there are plenty of occasions that the precise quantitative information that you speak of, is not entirely necessary to get the job done satisfactorily.

For instance, let's say you're trying to determine what size steel tubing is needed to make something like a handle assembly for one single push lawn mower. You could try to determine how much force is needed to push through grass that's 8" tall, plus the force needed to push down on the handle and lift the front two wheels off the ground, etc, and make all the calculations to determine the optimal tubing size needed to make that handle.

On the other hand, you could also look in a book of stock selections of steel tubing, and without too much thought, decide on 1" thin walled chrome plated tubing that is easily available, and safely go with that.

Although, if you're engineering a steel awning over a hotel entrance, obviously you would need full calculations.

What Bill has is a custom tubing that normally works fine, except that this particular customer has a slightly larger pool. The 4" of sag is more than he usually gets, because of this. The calcs you're asking for would require him to figure out how much weight the vinyl tarp, which is supported in various areas, is putting on his tube; among many other specifics. Since there are a lot of variables involved here, this might be beyond the abilities of someone who confesses he's not a mechanical engineer. It might even be beyond his ability to explain all the variables in their entirety, without full photo's, diagrams, etc. To REALLY do the complete job, we would need more than just the wall thickness, and the weight of the entire tarp.

I think, since the goal is to simply upgrade the rigidity of a tube that normally works for standard pools, to something that can handle a slightly larger pool, and diminish that 4" sag... this might be one of those situations like the lawn mower handle.

No disrespect meant toward anyone's abilities, or to the professionalism of a job well done, but a case could be made here for attempting to reduce the need of making it overly complicated; without resulting in shoddy or unprofessional results.

IMHO.

This does go somewhat contrary to what I wrote earlier about my interest in you categorizing all the suggestions, etc. But the new info that Bill supplied drastically narrowed down the solutions, to just a very few, some of which will clearly work. Don't you agree? No offense meant.

Hey, steady on!

Sometimes we Engineers spend a lot of time and effort working out solutions when the problem is not exactly defined. Believe it or not this costs you money in lost time. This one is a prime case, which is why some of the solutions have involved trusses, etc which are not suitable for the problem or guessing whether the tube was carrying liquid or not.

Your fit-for-purpose example is also correct, but you knew the nature and details of the problem from the start. nick name's comment is truly valid.

My point was that, short of one or all of us going to the work site, it's unlikely we'd be able to get all of the conditions and variables that would truly be needed for a full computational analysis.

Being that, as it is, personally, I'm convinced that the last round of info that Bill gave us, combined with what we knew previously from his original 2 posts, is clear enough to give an acceptable solution.

I'd feel confident in using the internal flat or rectangular stiffener, as was described in several posts, after #37. I guess I'd go with the flat bar, snug & tacked in place. KISS applies in this situation. That term can apply to the solution as well as the process of getting to the solution. It's an interesting case study for all of us. I've enjoyed all the creativity. But I'm sure the OP would like a direction sooner rather than later, so he can complete the job.

But as I said earlier, there are plenty of ways to skin a cat. And that, also applies to both solutions and processes of arriving at the solution. I'm not claiming sole ownership of the best idea, Just saying that as far as I'm concerned, I'm comfortable in saying I have enough information in this particular case, to endorse one of the solutions that would work.

I'm kinda liking the triple decker swing though But AussieBob, you forgot the Aussie version of that swing. Wouldn't the ropes hang "up" from the branch?

....

I totally agree with your comment's 1st part. Not totally with the rest of it. There are situations when load has no effect since other factors are predominant. For such situations what you say is correct. As soon as quantitative effects are concerned even a simple (KISS) quantification which indicates the TRENDS not even the values is welcome since the good and old "cut and try" method applied by many today although efficient is VERY time and money consuming. My point is that an estimation is needed to select the family of interesting solutions and avoid to turn earth head over for only a tree. I shall make it short: what influences the sag? - material via the Young modulus, changing from aluminum to SS would apparently reduce sag in the ratio 7.4E4/2E5=0.37. This would be valid ONLY if own weight can be neglected with respect to load. If own weight is an important part of load the effect can be near to nil since sag depends on the ratio specific mass/ Young modulus and in metric units (I apologize but I am more accustomed to those) for al it is 2.85E-6/7.4E4=3.85E-11 and for SS =8 E-6/E= 4E-11. Both values are very near to each other so that reduction effect is very small and thus not justified. - geometry via the area inertia modulus. J= pi/64*D^4*(1-(d/D)^4). This is the reason I asked for the pipe dimensions without any answer. The factor f=(1-(d/D)^4) has following values d/D= 0.1 f= 0.225 d/D=0.2 f= 0.4096. Thus increasing the wall by a factor of 2 reduces the sag by a factor of 1.82. I have to close now but I come back later. Sorry

File #3

The solutions are all based on the generation of a transverse force in different ways most of them are according to sketch 1 either in ternally or on the outside.

Here it is interesting to look at the tension magnitude required for the sag compensation. In one of the suggestions an internal bend tube was proposed, this has the draw back to introduce a friction which requires an even higher tension which will load axially the pipe.

Above equtions show that the internal soultion asks (in the 1 assumption for "e") about 2.5 more tension than the other. Now the second is also not possible to be used since if buckling has to be avoided (and I suppose this is the case) the "e" value has to be very high. For a safety coefficient of only 1 e>81 mm. In general in buckling the safety coefficient has to be >2.5 to 3 so that e>200mm but the sag is only 100 (4") and it disturbs so that even the solution with the lowest tension cannot be used if a "full compensation" is wished.

Conclusion is those solutions which "QUALITATIVELY" were considered as "OK" are "QUANTITATIVELY" NOT OK.

File #4 Be happy it is the LAST

I made the ffort to show how the change of form from circle to elypse will modify the rigidity.

The values in the upper row are the ratios with which the pipe has been deformed, they represent small axis/big axis. It was considered that the wall length was unchanged. The lower row of values show the corresponding increase of the J value with respect to the one the cylindrical pipe has.

Once more it becomes clear that a QUALITATIVELY good idea is not always a good one when numbers come to play.

And again, with the risk to upset some of the participants, I shall say that using numbers is a way to SAVE MONEY and TIME !

Of course NOT for the handle of a refrigerator door but why not for the bolts fastening it ?

I was once confronted at a car manufacturer site with a problem, a bolt had to be tightened with a torque specified by the Engineering Dpt. The plant manager informed me - I acted as consultant - that he had about 10 to 30 bolts broken per week. He was very unhappy the repair was very time consuming and did cost a LOT.

It was not a critical assembly so most probably it was considered as not important and the torque was indicated without the analysis done for critical joints.

When I made the analysis (it took a few minutes) it came out that if the bolt material would have had the standard minimal specified strength ALL bolt would have been BROKEN. In fact only due to the fact that the bolt supplier used a strnger stuff (higher class) only a few bolts failed.

This shows that even in not critical situations a small evaluation can help.

I have to agree with Nick Name.

The stresses on this tube are not simply downward, and when the cover is used as safety device (keeping motorcycles, tractors, and kids from sinking: http://www.youtube.com/watch?v=Ni0yqKfZK3o ) the forces in the horizontal direction could be very high. The horizontal forces would also be high if the pool leaked, and the cover had rain on it, etc.

I think either Bill should use the right tube, not some hacked-together solution to compensate for the wrong tube size having been used.

A question, what is the sag as it slips into the vault? If you bend the tube up so it is straight under the worst case load, it will bend up again as the load reduces as it carries less canopy as it gets near the vault. Is that a problem?

I do think that bending is the only way, it is called cambering, we often do it with long span beams. Just web search for "Cambering" in your area. I don't think filling with foam will work. Prestressing it with tension wires does nothing to help reduce sag unless it is anchored at the bottom of the opening at each end, and it may overstress in compression. It is a complication working out how much tension to put in the wire and how to do it in practice. Cambering is not exact and, since this is aluminum, may counter any heat treatment that the pipe has received.

Sorry if this sounds negative.

Hi Bill,

I think you should use the correctly sized tube.

If you double the size of a tube, (and keep the wall thickness the same) you get 1/8 the deflection. This is because there is twice as much material in compression and tension, that material is twice as far away from the center of the tube (better leverage) and because a given deflection stretches the material twice as much (and therefore takes twice as much force).

So with a 3" tube, you get way too much deflection (4"). With a 6" tube of the same wall thickness you would get barely detectable deflection (1/2"), and you'd have a happy customer.

In your application, the stress is not straight down, and this becomes especially true if you load the cover in an unusual way (rain on top, water low... some guy drives his motorcycle onto it, because he saw it on you tube, --http://www.youtube.com/watch?v=Ni0yqKfZK3o -- etc... a drunk 300lb neighbor falls onto the cover... ). In these cases, the force is mainly horizontal, and is many times higher than the force acting vertically. Because of these potential changes in the direction and magnitude of forces, a round tube is a good choice, and any reinforcement would have to be able to act is various directions as well.

Best bet is to modify the vault so a properly sized tube will fit (or allow the leading edge to be exposed?)

Any unidirectional reinforcement (squashing the tube, internal truss, prebending the tube) will not work well for your application. Doubling wall thickness would help, but would not cure the problem, and would entail a lot of custom work -- your extrusion would no longer be of use.

The issue is that a 3" tube of anything (even carbon fiber) is just too small for such a long span. Support your 3" tube on saw horses 25 feet apart and press down in the middle... you'll immediately see the problem: what seems stiff at 3' length, turns to spaghetti at 25 feet.

There is probably a way to raise the vault ceiling (although it sounds like it could require a lot of tile sawing). Good luck.

Essentially the same solution as indicated in my earlier post, except use 2" x 1" x 1/4" aluminum angles and "nest" them- base 1" legs in same direction, but one short leg is slightly higher than the other. Adjust the bolting location so that the end of the "upper" angle is touching the top of the inside of the 3" tube and the edges of the lower short leg are touching the lower part of the interior of the tube.

Slide it in, then push it in another 30" or so to center it in the tube. Squirt some silicone sealant through a 3/8" flexible tube along each side of the lower short legs for about 1-2 feet on each side to "secure" it in the tube.

End of sag.

Details make all of the difference. To flatten the tube into an oval, it would have to passed between a pair of rollers set a certain distance apart. Doing this will likely result in the tube having a bend.

I would go with the piece of flat stock or square or rectangular tube inserted inside the main tube as a vertical stiffener.

Also, have you looked at the extruded tubes used on roll-up RV awnings (you may already be using the same tubing). Very similar in that the canvas is attached to the tube via a rope hem slid into a channel for the full length of the canvas.

If the existing aluminum tube is (wild guess) 1/8" wall, going to 1/4" wall would almost double the strength and cut the deflection nearly in half.

I don't know who it is who thought that moment of inertia is off-topic. It is very precisely on-topic.

Bend the tube so it has a 4.0 inch bow. Install it so the bow is up. Then when it bows due to its own weight plus the weight of the pool cover, it will be flat. To implement this you must prevent it from rotating.

Um, would it ruin the ascetics if you just attached a couple of floats under the center of the rod, floating on the surface of the water and keeping it from sagging?

They could be pretty small.

I'm pretty darn sure there is no guarantee of strength if a man or motorcycle should drop down on top of the cover. I could be wrong, but I'm assuming it would void the warranty. So I'm not sure where the horizontal strength requirement is coming from. Bill didn't say there were any issues with that kind and direction of force, so I'm not going to add additional requirements that were not asked for by the OP. Pool covers are his business, afterall. We are not designing a new system, we are adjusting an already proven system that has a glitch. The smaller tubes are used regularly on slightly smaller span pools. You wouldn't be able to drop a 300 LB man on an 15' span and expect it to not do damage to the tube, under any conditions. So suggesting that scenario is a bit silly and extreme. And the tube is not pulled from the center point when it is in motion. So all he needs is to reduce the 4" of sag, as he asked for.

I'm also certain that some comfortable amount of clearance was built into the size of the vault, into which the leading edge is supposed to be stored. So it won't be necessary to eliminate the sag 100%. At least not for mechanical reasons. Aesthetic reasons... well that's another matter. So anyway, let's reduce the sag by 50%, and see how that works.

If he slides in a vertical stiffener made of a 3/8" x 2-3/4" aluminum flat bar (perhaps beveling the edges somewhat to achieve a snug fit), this will reduce the sag sufficiently to fit into the vault. If there is still some contact with the base beam as it enters the vault, it will be slight and it will be a very short section in the center. But it will be clear of the water. Perhaps as an added protection to the Italian tile could be made from a strip of thin teflon sheet that could be sewn to the bottom edge of the tube, onto the vinyl cover to help protect both the cover and the tile, if it should still bump it a little bit.

Or perhaps a couple of short sections of flexible 1" teflon tubing, parallel to the direction of travel and squashed flat at one end to form a small ramp, could be sewn onto bottom of the cover, to help it ease over.

But I believe it will be straight enough to work without interference.

This is certainly the least expensive and least labor intensive suggestion that could possibly be done. The cost & investment in going ahead with this suggestion would pale in comparison to the cost of redesigning the entire pool! There is no need to start tearing apart all that work on such an expensive pool before reasonable attempts are made to attain satisfactory professional results by other methods. And an internal flat bar stiffener of the size I mentioned, will go a long way towards minimizing the sag in this system.

This is being made much more complicated than it need be.

What you suggest is to introduce with a snug fit a flat of 68 x 19 mm over 7620 mm length. I do not consider this as an easy solution. From the dimensions J is such that the sag reduction is almost 50%. I would suggest an other approach which seems to me easier but I do not know if it can be implemented for other reasons function of the design and details I do not know.

It is possible to take a pipe with a bigger diameter but shorter and adapt at both ends end of actual pipe t have same dimensions at interface. To give an indication if same wall thickness would be used:

for 3.5" reduction factor 1/1.62

for 4" it is 1/2.45

for 5" it is 1/4.87 and

for 6" it will be 1/8.52.

The length of small pipe in the bigger has to be at least 4*d to bring a good local stiffness in bending. Adapters are cylindrical pieces easy to machine and assembly is not difficult either. The snug-fit can be obtained since length are shorter.

I shall come later with more informations about the limitations of the different solutions and show that the one I suggested has also limitations which cannot be seen without the not wished computations.

I would consider this thread as a "school" example for the necessity to quantify qualitative ideas as I always insisted to be a MUST.

The "snug" aspect is relative. And yes, if it's too snug, it would be difficult to install. What I meant, is a compromise between being fairly secure vs. ease of sliding it in. Fasteners or tack welds or adhesives or foam could assist in securing it, if need be. The fit would be adjusted by incremental beveling of the 4 edges of the flat bar. A small hand grinder, or even a file would suffice. High precision wouldn't be needed for this part of the task.

Again, WOW!

I know not having real numbers is a big problem. The truth is they have never existed.

Let me explain a couple of things so you may better understand.

There is some deflection on the leading edge has the cover is opened and closed. BUT it has NO effect on the operation or safety of the cover. The strength comes from the fact that the cover has about 12" of slack and actually rides on the water. we have put several thousands of pounds on these covers and the leading edge bends a bit but does not fail. Has far as some one falling or jumping think about falling on a water bed. the force is taken by the water not he cover or its attachments. Our covers (by the way www.poolcovers.com is where I work!) are ASTM and UL approved for use as a Safety Cover. We don't recommend you drive your Harley on the cover like our owner did. There are other companies out there that show an elephant walking on their cover. I am sure the don't recommend that either. hahaha.

In a perfect world YES we would use the 6" tube. But in order to accommodate this they would have to tear up all the tile, drain the pool, lower the bond beam of the pool (the section of pool wall that separates the water from the Vault (where the cover resides) remove the mechanism and dig the Vault out deeper, and then redo all the above. (they may even have to lower the track in the wall of the pool)

They are not going to do this. If we can't fix what we have they will remove the cover and go to a pin down manual version, they really don't want to do this.

I know that alot of you are tried and true Engineers and I respect your need for quantitative data here. But it just does not exist. There are Four major cover companies all of them were started in someones garage at one time and there was no "Hey what is the actual load on the leading edge?" It was more like "Hey that is sagging a bit....Get a bigger thicker tube...What the ends don't hold? Put another bolt in it."

The best suggestion is to reduce the sag as much as possible. I think if we did 50% has mentioned it would be fine. The leading edge sag is only noticeable while in operation (actually moving) the rest of the time the LE is in the Vault or butted up against the opposite wall.

So it sounds like a Vertical insert is the easiest, quickest, cheapest way to go. At least to me. If you don't agree please speak up and let me know. I am just trying to help our customer here. I am NOT a mechanical Engineer. I am the Electronics guy!

Next would be implementation of this idea. I don't know who mentioned it but sliding a insert through a 25 foot long pipe and keeping it oriented correctly is not going to be an easy thing to do.

Also, Did I mention that this IS a custom extrusion. I am not even sure what the ID is. I will have to check. But I would bet dollars to donuts its NOT a standard size.

So ideas on how to implement this idea? or rebuttal's?

Thanks again for everyone's hard work on this.

Bill

If this section is your extrusion - have them fin the inside, and/or chord fill top and bottom.

Then extrude it a stronger alloy and T6 temper.

If it's not your extrusion, maybe you could soft plastic coat it, to reduce the risk of tile damage.

Pay attention to UV degradation and the effect of chlorine, on 'soft' plastics/rubbers.

Otherwise do as suggested above with the diameter and resign yourself to cutting tiles.

All what you know is the OD = 3". Above solution which takes ONLY this one into consideration could be OK but you have to decide considering conditions on place.[/p]

It is similar to an other suggestion (the suggested material has a lower Young modulus, in fact many suggestions show that people think more "stress" related than "deformation" or "strain" related) but easier to implement since you can use any standard pipe with an ID > 3" and it has the advantage that the bonds are loaded almost only in compression. For the bonds you may use any resin resistent to water since it is protected against UV by outer pipe. The sag reductions are listed in previous thread.[/p]

You reinforce the existing pipe. The ends are as to day, up to you to see ,if it works.

You point out some of the issues involved in non-engineered solutions.

If this solution were correctly engineered in the first place, we would not be having this discussion. Engineering is quantitative. Without knowing the wall thickness of the tube, we cannot calculate the load to which the tube is subjected to produce 4" sag. If we cannot calculate the load, then we cannot calculate a solution. (The load obviously does not come from the vertical force from the very small amount of fabric not supported by water. It has to be catenary tension, which can be very high, even if the vertical foarce is low.) Then you are back to pure guess work... and you have already shown that guess work does not work.

You apparently misunderstand the nature of the load. If you say that the sag occurs when the device is moving then the sag must be related to the horizontal force applied by the take up mechanism. Gravity does not change every time you actuate a switch. If you watch carefully, you will see that the tube bends in the direction of the tension caused by the cover dragging as it is being moved, and this tension cannot be straight down when the cover is moving: the cover does not hang vertically from the edge of the tube, under that condition. The cover must form a catenary curve, and the load direction is likely to be on the order of 45 degrees from horizontal (and quite possibly, more horizontal yet -- in videos of these covers in motion the catenary appears closer to horizontal). Any micky-moused reinforcement inside the tube, if unidirectional, must take into account the direction of the load (A vertical bar will add very little stiffness if the load is from 45 degrees.)

You say that the deflection has no effect on the operation and safety of the cover. But you say that the deflection will cause the cover to wear out because of the collision with the tile. These statements seem directly contradictory.

Will the customer believe that the deflection is OK? In an area in which homes have 2.5 million dollar back yards, it is likely that there are pool covers that have leading edges that perform correctly, without noticeable sag.

If the pool is drained and the cover is closed (a realistic possibility, even if your instruction manual advises against this) then the loads are entirely different, and the horizontal tension can become very high (from rain load, from a group of neighborhood kids using it as a trampoline when the owners are away, etc.) Pools are notoriously dangerous, and covers are ostensibly safety devices, but they must be engineered for possible likely situations such as these -- otherwise you lose in court, when these kids break legs as the plastic end fittings come off the tracks and the cover falls, with kids, to the bottom of the pool.

Any reinforcement applied to the outside of the tube will be more effective than one applied to the inside. Any reinforcement applied all around the tube will allow it to deal with the changes in the force directions. (This gets to Tornado's insistence that moment of interia is critical.)

A 3" tube with 2" sag takes up as much space as a 7" tube (the effective radius becomes 1.5" + 2"). So if you can eliminate all but 1/2" sag (by using a larger tube) you can make a much larger tube fit.

You can't make a silk purse out of a sow's ear. The futility of using a 3" tube across a 25 foot span should have been obvious to the people who carried it to the site. Such a tube is "whippy" even if you shake it by hand from the center -- let alone apply real loads to it.

If I were the customer I would not accept anything less than a 6" tube: you have already said this is standard. Why foist off a substandard solution on a customer (especially one with the sort of influence that a $2.5 million back yard suggests)? If you (and others in the company) think seriously about replacing the tube, then you should be able to come up with a solution that is not horribly costly.

Why go to a manual cover? -- that seems like a pretty extreme difference.

If you believe that there is very little tension along the length and width of the cover, then disconnect the cover from the sides and ends and repeat the "Float a Harley" experiment.

If you say that the sag occurs when the device is moving then the sag must be related to the horizontal force applied by the take up mechanism.

Hmmm I wonder if just reducing the speed of the motor would provide a reasonable compromise.

During the discussions no data about load was given.

Here is a possibility to estimate it from the available information :

If we assume -as in the OP- the load as uniform sag is:

f= q*L^4/(128*E*J) where: (1)

q= load per length unit (N/mm for instance)

L= distance between supports = 25' = 7620 mm

E= Young modulus of aluminium = 7.4 E4 N/mm²

J= inertia moment of pipe section = PI/64*D^4*(1-alpha^4) with alpha=d/D. D= 3"= 76.2 mm

In the case that the only load would be the own weight

q = qown= PI/4*rho*g*D^2*(1-alpha^2)

with:

rho= specific mass = 2.85 E3 kg/m^3 and g= gravity acceleration = 9.81 m/s^2

We also know that the deformation under load is ≈4"= 100mm.

We do not know "d" but we can estimate for "t" (wall thickness)a range around D/10. We can compute "alpha" for those estimated values and we obtain:

t 1/8" 3/16" 1/4" 5/16"

d 2.75" 2.625" 2.5" 2.375"

alpha 0.9167 0.875 0.833 0.7917

f we consider only the own weight as load equation (1) becomes:

f = rho*g*L^4/(8*E*D^2*(1+alpha^2)) (2)

With above values for "alpha" we obtain:

alpha 0.9167 0.875 0.833 0.7917 f= 0.02742/(1+alpha^2) [m]

=27.42/(1+alpha^2) [mm]

f 14.9 15.53 16.2 16.86 [mm]

We know that the estimated value is 100mm so that the specific load will be own weight + external load = own weight * 100/f or external load = own weight *(100/f-1) --> qe= (100/f-1)*qown

qe/qown

5.71 5.44 5.2 4.93

qown 0.0204 0.0299 0.0390 0.0476 [N/mm]

qe 0.116 0.163 0.203 0.235 [N/mm]

Ge=qe*L

884 1242 1547 1788 [N] The mean value being = 1365 N

I agree that the estimation is related to quite broad uncertainty but at least we know in which range it can be found.

As I said this example can be a "school" one for the way a problem can be solved.

no...I think you misunderstood what I was saying.

we refer to sag being in the vertical plane and deflection in the horizontal. The sag is there no matter what! in the vault moving across the pool or at the end butted up against the end wall.

The deflection only occurs when the ropes start to pull the cover open or close once the cover starts to move the deflection disappears, well almost, but that is of no consequence as long as the cover is moving evenly so the leading edge dose not bind between the tracks.

Your right about using a bigger tube that is we recommend. But like I said, the actual pool builder (who is different from the guy who installed the cover) is just NOT going to rebuild that whole end of the pool.

I never thought of using a bigger tube that was NOT a 6" Like you said the space used by the sag would allow us to accommodate a bigger tube maybe a 4" or 5" which is obviously the best way to go. That right there has been the biggest, best idea yet! I am going to have my customer measure and see what we can do this way. I think its the best idea yet.

As far as redoing the "float a Harely" Test with out the leading edge. Depending on the pool, Undertrack (in the wall) or toptrack ON top of concret surrounding the pool. If it was a TopTrack the you could do it because the leading edge is not over the water so it you drop on the pool or walk on it. The only thing that may happen is some water may spill over out the front. An Undertack would allow water to spill on top of the cover that and the weight of the harely could possibly pull the ropes off the wall or even the track out of the wall.

I am very proud to be a part of this team. Our product saves countless lives, and saves people THOUSANDS of dollars every year! I belive we have a very simple and elegant mechanizm that works well, stands up to the elements very well. Our system have a lifetime warranty on the mechanical parts. In a few places it seem a bit snarky towards me and poolcovers in general. This was the builders fault. I am just trying to help the entire crew (none of the contractors are getting thier final payment tell the owner signs off on the project. My customer (our cover) is the only thing stopping this from being done) by trying to find away to solve the problem.

OH and just so you know. I do know what the inside of our extrusion looks like. It was alluded to that I didn't have clue about it. I just don't, off the top of my head have the wall thickness and the actually ID. I am sure I can find a drawing and get all of that. But, since I don't know what kind of number to tell you about load i figured it was pointless. Maybe I am wrong. I am not a ME. But I am not stupid I don't think. Besides would you re-tool your die to solve ONE customers Problem? I am near positive they are NOT going to do that.

If I can use a 4-5" tube with a stiffener attached to the top and bottom of the out side of the tube I think it will be ok...Might not even need the stiffener! I mean they all sag! Even the 6" tube would sag some. It just can ride of the water like the bow of a boat! :)

If any one has any more ideas, please post them.

Thanks (Sincerely...I mean it) for all your time, hard work, knowledge, thoughts etc..

Best regards to all!

bill12780

In my answer #58 you have indications about the effect of a pipe "over" the actual pipe so that you can make the minimal choice according to available space. This approach allows you to have same ends.