Help me to Identify a Composite

[MadMango]

I cross-posted this to the Composite Engineering forum as well.

I have been tasked with designing a ramp, and I have strict criteria that I have to meet. Due to these criteria, I don't believe that I can achieve this design with conventional sheetmetal or metal construction and think only a composite structure will work.

The ramp is 33.75W x 23.63L, with a max thickness of .50 inches. The ramp will see a max load of 770lbs in the form of a wheelbarrel/wheelbarrow. I am not allowed to have any supporting structure, except at the top of the ramp where it will be hinged with a piano hinge. This allows the ramp to be folded away. The environment will be outdoors, where it can be subjected to Arizona heat, Florida humidity, Wisconsin winters and everything inbetween. Oh, and I forgot to mention that it can't weigh more than 25lbs...

Since I know the material Unobtainium doesn't exsist, I am hoping someone from this forum may be able to point me in the proper direction in what types of composite materials I should be looking into. This would include a realistic thickness (I'm thinking 1.00" min) and weight.
"The attempt and not the deed confounds us."

 

[MASSEY]

Have considered skywalk/catwalk grating?

The piano hinge would be out, but it may meet your other criteria. It could be hinged with round stock running through the slats and that would allow it to pivot out of the way.

Another option that come to mind is the grating made of fiberglass, I believe, and it is perforated with square hole throughout. This is available in many thicknesses and square hole dimensions.

 

[MadMango]

Massey, thanks for the suggestion, but I forgot to include some vital information. I cannot have openings larger than .63" in the surface of this ramp, so that may eliminate off the self items that are used in decking and catwalks. But I'll still look into the options you suggested. "The attempt and not the deed confounds us."

 

[kenvlach]

Noticed you didn't specify the amount of flexing allowed.

Fiberglass grating is too thick for your criterion.
I think that either high-strength aluminum or steel will work. In either case, use a diamond tread pattern or mix in some grit in your coating to make it non-slip.

See

http://www.gatorramps.com/

and

http://www.mcmaster.com/

for ideas and materials. For the latter, search 'dock plates' and 'anti-slip floor plates.'

 

[GregLocock]

What thickness? you mention 0.5 and 1 inch in your first post. If it is 0.5 inches maximum then you will need a fairly exotic composite, as I get a very low estimate of 44 MPa for the outer fibre stress, which assumes the thing behaves as a beam, unlikely to say the least.

I'd use steel plates over a wooden core if that meets your climatic requirements.

Cheers

Greg Locock

 

[MadMango]

I'm not sure of exact flex in the ramp surface, but the customer shouldn't preceive anything dangerous, so I am thinking on the order of .500" deflection max at the center of the loaded ramp.

The .50" thickness is the max thickness dictated to me by my wonderful Marketing Department (theu would rather see .375&quot. My brain is telling me it has to be closer to 1.00" thick.

Steel over wood core: I'm an absolute novice when it comes to composites. Is there going to be an issue in applying a finish to the steel, like powder coating the composite plate? "The attempt and not the deed confounds us."

 

[GregLocock]

You'll need some sort of frame for the edges, then powder coat the whole thing in one go. Most powder coating systems seem to be quite low temperature, I imagine the wood would survive.

I should add I haven't designed anything like this - our solar car was composite obviously but all point loads were taken via aluminium plates which then fed them into the composite structure. The point loads from the wheels would worry me on this thing.

Your 25 lb limit is pretty tight by the way, but if the point load issue is OK then I get two 1.2 mm plates of steel with a 10mm core should be a good place to start. Cheers

Greg Locock

 

[sancat]

Adding to Greg's idea, what about to paint the frames previously, and then glue them over a polyuretane foam slab, or alternatively, make the whole steel box, weld it with a point welder, paint it, and then fill it by injection with polyurethane foam.


Polyuretane foam is used for insulation for refrigerators, and can be injected through a small hole. You can change its density by changing the components mix.

You could form nerves on the steel sheets that form the box, or weld some omegas between both layers prior to close the box, to add rigidity.

sancat

 

[MadMango]

I tried making a "corrogated" ramp, using 14ga top skin out of 304 SS, with numerous hat sections of 18ga SS underneath (spot welded together), but the whole thing was too heavy (35lbs via SW Mass Props).

We don't have any experience with injected anything at work, so I am not sure the injected polyurethane foam is something I want to get into.

I think the quickest thing to assembly will be some turnkey material that I can get purchased at the size required. If it has to fit into a frame, that will be easy enough. "The attempt and not the deed confounds us."

 

[GregLocock]

The trouble with that approach is that you aren't using the material efficiently. Maximum performance is from an I beam. To get the two skins to work as a section you'll need something stiffer than foam.




Cheers

Greg Locock

 

[sancat]

Greg, Mad

The kind of foam I speak of, is so tough like cork. The one we use industrially here has two outer layers of aluminum and is 6 inches thick. It is used as walls for big cold-storage plants. However a small portable machine is used to inject this foam when making refrigerating appliances, the machine mixes on the nozzle the components, and they expand and glue to the walls.

But if foam is not the way to go, and my numbers are not wrong, a 0.9mm thin (gage 20) sheet sandwich, with a corrugated gage 20 layer below with waves 1 1/2 inches wide, and 3/8 high could work (as <<ham>>).
I would use glue on one of the sides, and spot weld the other (upper side/contraction side welded). Locally we glue with SikaFlex, but there should be a number of alternatives there.
If my numbers are not wrong you will have a 3.6 kg/mm2 which is very low. The corrugated sheet waves should be close enough so that the wheel cant fall between two of them.

sancat

 

[harrisj]

MadMango:

You haven't mentioned cost. If it's not important, can I join your company please?

Assuming you can spend a bit, the aerospace industry has good aluminium skinned honeycomb composites. Because the honeycomb cells are orthogonal to the surface, they have high compressive stiffness which is what you need to prevent buckling. The loadings are difficult to model but manufacturers will give the data. There are also carbon fibre skinned Al honeycombs - pricey.

Failure mode is something to be considersd. A wooden plank creaks and groans before it breaks - the more sophisticated a composite structure becomes, the less warning it gives.

You'll have to think about point loadings if you're using thin-skinned composites. Also tolerance to damage and abuse. Dynamic loads may be present.

All in all, I agree that 1/2 inch sounds a bit marginal unless it's used in a very tightly controlled environment.

Cheers - John

 

[GregLocock]

I was thinking about 747 (or earlier) flooring on the way in this morning - that's an aluminium skin, aluminium hex core, aluminium skin I believe. Strong stiff and light - but it won't cope with point loads (how did they cope with stiletto heels?) or enormous weights on unsupported spans, like your wheelbarrow.

The modern stuff is kevlar/glass/epoxy of the same general construction - the aluminium hex being replaced by a Nomex core which is a kevlar paper hex.


Cheers

Greg Locock

 

[MadMango]

Hassisj, it's hard to beat the cost of 8 sq ft of 304ss, so we know this is going to be relatively more expensive. The real question is by what factor. Of course if this single peice ends up being >20% of the total manufacturing cost, we'll have to abandon the project.

Well, I've been on the phone all morning and have contacted about 8 companys in California. I have gotten some quick responses from these and am in email contact with 3. You have all led me onto the right track for now.

Thanks all. &quot;The attempt and not the deed confounds us.&quot;

 

[Speedy]

What about a supporting truss / web on the underside that would fold away, into a recess, when the ramp is up. It would give the desired resistance to bending...Though, it would have to open automatically. Speedy

&quot;Tell a man there are 300 billion stars in the universe and he'll believe you. Tell him a bench has wet paint on it and he'll have to touch to be sure.&quot;

 

[FourJawChuck]

Wheelchair vans use similar ramps and have even longer spans, 1/8&quot; diamond plate aluminum sheet with tack welded 3/8&quot; square aluminum tube stringers is strong, light and likely to be more durable than your other choices. Even u-channel will work well.

 

[dkelly]

Check out this ramp company, They will do customs as well

http://www.handi-ramp.com/Portables.htm

 

[RobWard]

Don't forget that people pushing wheelbarrows up slopes tend to take a run up. Make sure the ramp will resist this load and that the impact won't lead to failure. Many composites don't like impacts.... &quot;I love deadlines. I love the whooshing noise they make as they go past.&quot; Douglas Adams