Determining steel sizes based on load?

[Greg Di]

Are there some sort of tables that tell you how much weight a given piece or shape of steel will carry without deflection?

I want to build a 10' long by 22" wide rollout bed for the back of my utility van so that I don't have to walk in to get my larger tools out (table saw, miter saw, compressor, etc). The platform would be on wheels, glides or rollers and would extend out about 8 feet from back. Sort of like this: http://www.extendobed.com/

I'm thinking of building a simple frame with expanded metal as the bottom. I haven't figured out the track system yet, but I'm brainstorming.

Obviously, I want to keep weight down, but I want the tray to be strong enough not to deflect. I will probably install a pivoting leg on the end to make ground contact at full extension.

[boykjo]

I am not aware of any tables.........

For strength to weight ratio, your best bet is to make it out of aluminum and use I beams to reduce any flex. How deep are your pockets.........

[whateg0]

I have a chart that has some numbers on it. But it's either at home on the laptop, or on the HDD that was in the computer whose MB died. I don't remember all the specs that are in it, but it was somewhat useful. It described strength of a single piece of channel, 2 pieces of channel back-back, 2 pieces of channel 3" apart, or something like that. I downloaded it off the internet 'somewhere', but I'd have to search to find it now.

Dave

[FarmallMan]

You can determine this information from the tables in a Strength of Materials engineering text or the Machinery's Handbook (check your local library)....if you cant find them on the net. They should also walk you though how to use them.

Nick :cool:

[poolie_p]

go find a wrecked U-haul truck, they have awsome ramps

[Gaze]

Yeah, Greg, the stress and deflection can be calculated with a formula. Figure out your equipment weight and that will be your dead load. If each piece is similar in weight, it can be figured as distributed load or if a piece is particularly heavy, it can be figured in as a point load.

It sounds like it's a cantilever problem. That is until you swing out your support leg at the end, then it switches to a end supported beam. Different formulas for each.

1st step: Weigh your stuff and a load diagram can be produced. If there are large variations, it is important to know which piece of equipment is to be located at which point along the slide out "tray", according to your desired layout.

It might be best to design it where the fold out leg can be opened into place, maybe on wheels, at the closest point possible beyond the van, then use that support (with wheels) to roll out the rest of the tray to full extension. That way the length of the cantilever is minimized, allowing for the smallest (lightest) support material.

I bet if you post up a sketch, myself or someone here can tell you what sizes you'll need. No implied liability, of course :D

[usmcpop]

Here is the patent with the drawings for that slide-out device.
http://www.google.com/patents?id=EMgHAAAAEBAJ&pg=PA1&dq=6328364&source=gbs_selected_pages&cad=1_1#PPA1,M1

Maybe you can glean a bit of info from the specs and descriptions of these various brands of slide-outs: http://www.truckcargobed.com/

[whateg0]

I would just plan to have several of the heaviest, or perhaps densest items on the platform. But that would dictate heavier material. The nice thing about it is it does not force you to put items of X pounds here, and only items of Y pounds here. You can rearrange as needed without having to worry about the weight.

The drop leg is a good idea, but the OP idea of having it extend down when the platform is fully extended is probably the easiest to work out. Otherwise you have to deal with potentially uneven surfaces getting to that point.

Putting the heaviest items toward the front of the vehicle will minimize the amount of stress on the platform when it is being extended. At full extension, until the drop leg is down, the platform is under the highest stress, and if you are putting a lot of weight on it, there will be a lot of stress there. If you can find a dealer nearby for the Extendobed, you might go have a look at one of theirs to get an idea of what is needed to support whatever weight they spec. From the pics on the website, it looks like they're using about 6" channel for the platform frame and the 'anchor' frame. The 'transfer' frame looks to be either a HR I-beam or perhaps a fabricated I-beam. The channel may be fabricated too to give a better load bearing surface for the bearings. All just guesses, though, without seeing it in person.

Dave

[Greg Di]

Here is the patent with the drawings for that slide-out device.
http://www.google.com/patents?id=EMgHAAAAEBAJ&pg=PA1&dq=6328364&source=gbs_selected_pages&cad=1_1#PPA1,M1

Maybe you can glean a bit of info from the specs and descriptions of these various brands of slide-outs: http://www.truckcargobed.com/

Wow...that's a cool Google feature. Never knew it existed!

[Greg Di]

I'm looking around the net for the wheels or roller guides. MSC seems to have a lot to choose from, but I'm not quite sure of a good way to attach the rollers to the frame. Maybe a thru-bolt with a sleeve or something?

[whateg0]

If you use a standard bearing, you can just bolt through it and tighten it down. As long as the outer race can spin freely, it oughta work. That's what I did on my (in the works) CNC torch.

Dave

[Greg Di]

Maybe this will help you help me. I really don't know too much about what kind of stock is available to use. I'm sure there are some metal profiles that will make great tracks for the rollers...I don't even know what to ask for.

I usually use this software to show customers their decks and cabinets...

[whateg0]

How heavy is too heavy? I know that's kind of a hard question to answer, but if you do a compound slide, which would also make full extension possible, I think it would reduce the stress to the frames as the contact points would be more spread out. That's just a thought.

Dave

[whateg0]

I don't know about the capacity, but could you use some sliding door track? I know it's pretty stout, and the rollers are already made to fit. I know it eliminates the fun of designing from scratch, but it might be easier.

Dave

[usmcpop]

Hint, hint, hint: if you go to that truck cargo bed thingy site and sift through all the links and pictures and stuff, they'll mention things like the 11 gauge rails and stuff. You can occasionally glimpse a picture of the guts that are maybe good enough to scale and get in the ballpark. How do you think the Chinese reverse engineer things in 30 days without ever getting their hands on the real thing? There's no substitute for structural engineering, but we mere mortals can come close with a bit of observation and experimentation. Have fun!

[SundownIII]

See.:rolleyes:

There really is a reason people go to school for engineering. Answers to all your questions are covered in a Strength of Materials class (level 200/300 classes) at any reputable engineer school.

You haven't come close to providing enough information (length, weights, moment (where loading occurs), dead load, live load, etc) to come close to a solution.

Without proper engineering, just build it as strong as you "think" it needs to be. If it fails, build it again with heavier material.

[enlpck]

Two books you should have for this kind of work: Machinery's Handbook and a basic strength of materials text (Try _Strength_Of_Materials_ by Den Hartog, in reprint from Dover. Inexpensive, pretty good, and, back in the day, it was my text when I took the class. Even Barnes and Nobel usually has it. Moderate use of calculus, but clear explanations)

As to why more data is needed, the deflection of a beam is linear with load (double the load, double the deflection) but is proportional to the cube of the length of the beam. Failure can occur several ways, the two most relevant are shear and yield in bending. Shear isn't likely to be a major issue for you (other than shear failure of fasteners), so lets look at bending. Small changes in beam length cause large changes in deflection due to the cubic relation. Surprisingly, the load capacity for yield failure drops off linearly with length (sort of... If the weight of the beam is significant, which it usually is in a long beam, the beam weight becomes the limiting factor).

So, what do you need to do? For a start, determine the load you want the thing to support, how it will be distributed, how far you want the device to pull out from the truck, and where you can put attachments to the truck. Then you can work out material sizes and support designs.

[Gaze]

enlpck, I mentioned a lot of your same comments back on Day 1 within a few hours of the OP. Thought maybe he would weigh his desired pull-out equipment and post a sketch of the loading/dimensions, then, short of suggesting he take a class, the sizing could be determined with help here. Oh well...didn't happen :) Would have been a fun exercise.

Estimating size he risks time and cost by trial and error. i.e., it would suck to go through the building of the device and have it bend and sag down to his feet :eek: then need to scrap his undersized mat'l and start over heavier.

Best to know what will work from a minimal materials standpoint, according to the numbers, then add safety factor as feasible from there. Sometimes it calcs out where the mat'l size is barely adequate and he might want to just step up to the next nominal size and intentionally overkill it. At least he will know how much future loading he can add.

On deflection, it's never zero as the OP requested, but whatever is an acceptable amount, such as 1/2" or 1/4"... so that should be posted as well.

[pioneer1950]

Hi to all that reads this! Newbe here Old pipe fitter from the old school in south Louisiana around Baton Rouge to New orleans and every where else too. I am replying because as a internet retailer i am able to see where my hits come from and have gotten several hits from this thread. I see the Greg Di wanted to construct a 10' slide out. Greg as a seller of slides for years and have had several customers looking for a 10' slide out, there are no manufactures that will build this length. This is not my site, ( http://trambed.com/photogallery.htm )but may give the ideas you can use for your 10' slide. This design i different than most slides on the net. Good Luck and have fun. I can sell you a 108" x 22" slide out but i am thinking ya got time for engineering your own and this could be the start of a slide out business where ya may could retire from the hooded clan.
;)

Not ment to be spam just to help.......
http://www.truckcargobed.com

[bentwings]

I second the idea of looking at what already exists and building something similar.

Yes the structrual integrity can be analyzed, I do it on SolidWorks. It takes time and lots of material properties, a good Cad model and lots of computer power. It's not a cheap process but it will show up the weak areas. It will not show you how to fix it or build it nor will the Cad system go that. The design responsibility is yours.

As an engineer we are taught to give our opinions with out regard to personal feelings ...... I give my opinions base on hard facts and engineering principles and I don't give bad news to hurt your feelings or make anyone look bad. Facts are facts we all need to face them at this point. I can be challenged too and readily accept them and if proven wrong or can be shown a better way, I don't have a problem accepting the critique.

[Ferntj]

Are there some sort of tables that tell you how much weight a given piece or shape of steel will carry without deflection?

I want to build a 10' long by 22" wide rollout bed for the back of my utility van so that I don't have to walk in to get my larger tools out (table saw, miter saw, compressor, etc). The platform would be on wheels, glides or rollers and would extend out about 8 feet from back. Sort of like this: http://www.extendobed.com/

I'm thinking of building a simple frame with expanded metal as the bottom. I haven't figured out the track system yet, but I'm brainstorming.

Obviously, I want to keep weight down, but I want the tray to be strong enough not to deflect. I will probably install a pivoting leg on the end to make ground contact at full extension.

Here are the formulas you need.
http://www.me.metu.edu.tr/me206/SuhaOral/Beam_Deflection_Formulae.pdf
Use Cantilever beam 1 to simulate all the weight on the end and 3 to simulate the total weight evenly spread out.

In 1 the weith is P, l is 96 inches, E is moduls use 30,000,000 for steel, and I is the moment of inertia. Here is a table for I of different structural tubes.
http://www.steeltubeinstitute.org/pdf/brochures/dimension_brochure.pdf

You will probably use something on page 17 like 2x1 or 3x1. Build the slider with the 1 inch horizontal and the long dimension vertical. Then use the Ix for your moment.

Since you will have 2 rails use half the actual weight or multiply the moment by 2.

Example:
You have a 50 pound saw on the end of the slide made from 2x1x1/8 tube (2.2 lb per foot). The deflection is (50x96x96x96)/(3x30000000x2x.28) = .87 inches deflection

Move up to 3x2x1/8 (4.33 lb/ft) Ix = 1.3 and the deflection is just .19 inches.

Of course your decking will add some weight also and you will get some added deflection form the rollers in your slide but at least you have some idea how to pick the materials.

[DaVinci]

Those docs are keepers...especially the second one.

Thanks! :D

[Ferntj]

I liked that one too. I just found that on a quick google search for moment of inertia on structural steel. I saved it for my future reference too.

You could use C-channel for the part attached to the truck box. Could bolt through one side to the floor, have the wheels bolted to the vertical web, then could have an angle iron come down and into the C so the moving part couldn't flip up in back.

[DaVinci]

I thought of this thread when I came across this site:

http://truck.hownd.com/mfglink-93-joey_bed.html