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Old 01-02-2012, 06:56 PM   #11
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Default Re: Using 80/20

Dave,

Could you give me your thoughts about using 1x2 instead of 1.5x1.5 product?

I have some chair-bed structures in mind with the longest span only 30".

I figure the verticals don't matter much since compressive strength is strong but the horizontals would benefit from the 2" depth to minimize deflection. About a third less weight/foot than the 1.5" stuff. (Haven't looked at any deflection tables yet.)

No one would build a house out of square wood stock since it would be too wasteful for the horizontal components. Furniture has some different loads but the principle is the same. If I had some experience I would be better able to minimize weight/enhance grace.

Do you think the connection simplicity of using all 1.5" sq stock outweighs the weight savings and other benefits?

My Reese atv ramps that I used in my design impressed me with their high 650lb capacity rating and low 12.5lb weight per section. Aluminum ain't cheap so it was interesting to see all the hollow boxes and copious welds. I'm guessing robots don't care too much about how many welds they have to make.
Dan
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Old 01-02-2012, 07:24 PM   #12
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Default Re: Using 80/20

I have not looked at the deflection tables but would suspect the 1" x 2" has less deflection than the sq. 1 1/2" with the same load if the 2" is in the correct direction. Obviously the amout of load determines what will work. If it is for a chair for people, I suspect the 1" sq. would also work. Even if the 30" span deflects, what would that hurt?
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Old 01-02-2012, 08:11 PM   #13
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Default Re: Using 80/20

Quote:
Originally Posted by Graphite Dave View Post
I have not looked at the deflection tables but would suspect the 1" x 2" has less deflection than the sq. 1 1/2" with the same load if the 2" is in the correct direction. Obviously the amout of load determines what will work. If it is for a chair for people, I suspect the 1" sq. would also work. Even if the 30" span deflects, what would that hurt?
Thanks for the reply.

I thought I'd do my homework after my post but we are an Apple-only household and the 80/20 deflection calculator is apparently a Windows-only tool. Those cross-sections are way too complex to compute otherwise.

Am I missing anything subtle about connecting rectangles in 3 dimensions rather than square sections? I did figure on making many of the simple connectors myself since 80/20 is quite pricey there.

Dan
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Old 01-02-2012, 10:20 PM   #14
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Default Re: Using 80/20

I looked in the Futura Industries catalog where they have graphs for maximum loads. For 200 lbs load on a simply supported beam (load in center with supports at ends of beam) the graphs show:

1" x 2" in one direction maximum length = 20"
1" x 2" in other direction = 40"
1 1/2" sq. light = 30"

Maybe a real engineer out there can explain the relationship between deflection and the moment of inertia of an extrusion. If I remember right the deflection is related to the moment of inertia. The moment of inertia's are listed for each extrusion in the catalog. You should be able to use the MI to compare the relative deflections of each extrusion. It was 50 years ago when I last did a calculation.

The connections are up to you. Can not help unless I know what you are building.
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Old 01-03-2012, 04:53 AM   #15
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Default Re: Using 80/20

The Futura pdf was very helpful.

If I use the TS1020 1x2 in the strong orientation and support my 30" span at both ends it will support a concentrated load in the center of around 500 lbs for a .1" deflection in the center.

I'll note that the span length gets cubed in the calculation so a 65" span would deflect about 1" under that same load.

(Derived from the Tslots catalog deflection page at Futura if others need to plug in some numbers.)

Just what I needed. Sounds like the 1x2 will support Santa with a pretty big kid in his lap.

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Old 01-03-2012, 01:44 PM   #16
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Default Re: Using 80/20

I'm finding the weak link in my 80/20's strength is rotational twist.

I'm having to re-enforce my bench horizontal lengths as the bed bridge I'm spanning across the middle is actually twisting the 80/20 at the attachment points due to the leverage.

currently one side of my platform rests on verticals and there is no issue, but the hinges I'm using for the swing panel bolt into the horizontal. The hinges are super strong.. the actual 15 series 80/20 is twisting.

Trying to make it clearer.. it is twisting at the arrows:


Easy solution.. I'll just run a brace on the backside opposite each hinge.

In this photo you can see the hinges I have.. and behind them the re-enforcement plates I'm going to use to stop the twisting:


Hope this helps
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Old 01-03-2012, 01:56 PM   #17
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Default Re: Using 80/20

Quote:
Originally Posted by d_bertko View Post
Could you give me your thoughts about using 1x2 instead of 1.5x1.5 product?
Dan
Are you referring to this?



I'm using it as my wall mounts. It is super strong along the 2" span but not so much on the 1" .. if there is any chance it could have longitudinal flex I'd be careful. I don't find it is any lighter than the 1.5 x 1.5 ..it might actually be a tad heavier per foot because the "back wall" of it is so thick.

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Old 01-03-2012, 03:24 PM   #18
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Default Re: Using 80/20

Tools required to build 80/20 structures:

1. Chop saw designed for wood with aluminum cutting blade.

2. Drill press.

3. Sander for deburring parts.

4. Hand tapping machine. I had trouble tapping plates straight so bought machine from Grizzly.com for $82.95. Works great.
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Old 01-03-2012, 05:05 PM   #19
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Default Re: Using 80/20

Quote:
Originally Posted by Geek View Post
Are you referring to this?



I'm using it as my wall mounts. It is super strong along the 2" span but not so much on the 1" .. if there is any chance it could have longitudinal flex I'd be careful. I don't find it is any lighter than the 1.5 x 1.5 ..it might actually be a tad heavier per foot because the "back wall" of it is so thick.

Geek,

I'm not referring to the wall-attachment extrusions. I think that stuff need only be .5" deep in many cases. Heavy loads should probably get transferred to the floor instead. Otherwise you might put a lot of torsional stress on that skinny sheet metal Sprinter wall. Not to mention that anything deeper than you need wastes space, weight, and $$$.

I could not tell from your pix what extrusion series you are using. Is the depth really a full inch? It looked like the .75" would do the job.

My application was for general use instead of wall-aided. So using 1x2 or 1x3 or even 1x4 extrusions make more sense than square 1x1, 1.5x1.5 or 2x2 series if you know that the greatest expected loads will be on one axis. Think skyscraper I-beams or 2x10 residential joists.

The choice of extrusions is certainly complicated by their attachment utility. That Erector-set flexibility costs a lot of extra metal for the same engineered strength of simpler box or other shapes.

The 80/20 is a lot of fun to prototype with but no mfr would use it for quantity production. Lots of weight could be saved using dedicated engineered and probably welded extrusions.

I can appreciate that the 80/20 folks serves an audience that are not production-optimizers. A niche market for their product and they charge accordingly.

Dan
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Old 01-03-2012, 10:31 PM   #20
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Default Re: Using 80/20

Quote:
Originally Posted by Geek View Post
I'm finding the weak link in my 80/20's strength is rotational twist.
...

Hope this helps
Unless there are attached components which will be damaged by the twisting or deflection as a practical matter it isn't generally a concern.

I've used structural grade aluminum similar to Unistrut in shape/design for boat dollies and multiple boat trailer racks among other things. When I first noticed the twisting of the pieces in some applications I thought I might need to add structure. Turns out the structural grade aluminum stuff is very resilient so it wasn't ever an issue. I should think the material of construction for the stuff y'all are using is of similar grade. The 80/20 looks very nice by the way.

Unless you are loading up the furniture with really heavy possessions and bouncing down the road, a bit of deflection is not a concern. Then again, I don't generally do calculations on my designs as some people do. Have fun. vic
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