RV folding table - Steel Tube question

[WVVan]

Hi All,
Thanks for your time. I'm not an engineer nor do I play one on TV. Last time I did any load calculations was in a some college M.E. classes and that was so long ago we were all still using slide rules just a couple years before. No really.

I'm going to build a fold-up table for my RV-van project ("Hal The Van"). The table is 24"x20" and attached to a cabinet by two hinges. After the table is swung into position a "U" shaped welded arrangement of three steel tubes slides out from the cabinet and supports the table from underneath. The steel tubes are supported within the cabinet by two sets of nylon bushings that allow the tubes to slide back and forth.

Hopefully this picture makes it a little clearer.

It is neither to scale nor totally accurate. Just a rough sketch.

I'd like the table to be able to support at least 50 pounds. My question is about the type/thickness of steel tube that I could use. I'm sure if I used "black iron" steel gas pipe that it would support the weight but that seems like overkill.

Instead could I use, from the http://www.mcmaster.com/" catalog page 3627,

Material: General-Purpose Low-Carbon Steel
Shape: Tubes
Wall Thickness: .065"
Inside Diameter: .87"
Outside Diameter: 1"
ASTM Specification: ASTM A513

Would this tube support the weight or would something thicker be needed?
If this tube would work what do you think (roughly) is the max load it would support?

Thanks again for your time. If this isn't clear or you have any questions please let me know.

Dave

 

[edgepflow]

Dave, your best bet is to do some basic beam stress calculations - they are pretty quick and easy. Do you have a copy of Machinery's Handbook available? It has formulas worked out.

Take your weight of 50 lb and multiply by an "impact factor" FI to account that when you set the TV (or whatever it is)down, it will suddenly stop and temperarly have a force weight of over 50 lb. Take FI = 1.5 so your weight for design is 50 lb X 1.5 = 75 lb.

One you figure the max stress in your beam, compare to your allowable stress:

allowable stress = 60% X Yield Stress. McMaster should list the Yield Stress. Make sure you are less than your allowable stress.

 

[AlephZero]

I would be more worried about how long the nylon bushings would last, than if the tube would bend or break. Also it's not very clear from your sketch how the table top is connected to the end of the tubes.

If any of those parts start to "wear", you will have a wobbly and/or sloping table top, which is probably not a good idea.

If you have a 50lb load on the edge of the table, you will have another 50lb at the hinge (because it is balanced like a see-saw about the middle) and therefore 100lb on the tube at the center point. By the same argument you will have 100lb on the bushes at the other end of the tube, and 200lb on the bushes under the table hinge. That seems quite a lot for a nylon bush to take without deforming.

My gut feeling (without doing any calcs) is that if you have a 2-foot length of inch diameter mild steel pipe supported at each end, you could stand on it in the middle without any problem. Which is about the same as the 200 pound load in the middle of your design, or course.

OK, there are two pipes not one, but take that as giving you a safety factor of 2, not a reason to halve the loads. Actually a safety factor of 2 isn't very big here. For example what happens when somebody is picking something up from the floor, and puts all their weight on the edge of the table to help them stand up again?

 

[nvn]

WVVan: I found your diagram clear. We can use edgepflow's impact factor, FI = 1.50, and yield factor of safety, FSy = 1.67, for your round steel tubes. You say you want to design the table for at least 222 N. If P = 222 N is the design load, then the worst case is when P is at the front (left-hand) edge of your table, in which case the bending moment on each tube will be approximately M = 313*P = 69 486 N*mm. Your tube stress is therefore, sigma = 101.13 MPa. I am currently assuming your tube tensile yield strength is Sty = 275 MPa (although I did not look up ASTM A513). Your stress level is therefore, Ry = FI*FSy*sigma/Sty = 1.50*1.67*101.13/275 = 92.12 %. Because this does not exceed 100 %, it indicates the steel tubes are currently adequate. It also indicates your current steel tubes could support a maximum table load of Pmax = P/Ry = (222 N)/0.9212 = 241.0 N.

The reaction force at the outboard nylon bushing is currently Rd = 2.054*P = 456.0 N. Divide Rd by the bushing bearing area. If the bushing is 20 mm long, that would be, say, sigma_br = (456.0 N)/[(25.4 mm)(20 mm)] = 0.898 MPa. Ensure sigma_br does not exceed the nylon allowable stress. (I did not look up the nylon allowable stress.)

 

[WVVan]

Thanks for all the help. And so quick to!

To answer AlephZero question, the fold down table isn't attached to the slide out tubes. It just rests on top of them. Since this is for an RV I'm not too worried about anything wearing out. It's only for occasional usage, unless the economy really gets bad.

A question back to Nvn. I found a chart listing "Compressive Yield Strength (MPa)" of 65 for Nylon 6, the type I'll be using for the bushings. Is this the same as allowable stress?

 

[nvn]

WVVan: That is not the allowable stress. I think you can divide that compressive yield strength by 4 (or 5 or 6) to obtain the allowable stress, in this case.

 

[nvn]

WVVan: Did you, by any chance, make a typographic mistake in that compressive yield strength value you listed in post 5? Just curious.