stiffening sheet metal tray

[mmolt]

Friends,

I'm in the process of developing a quite optimized (in terms of cost and strength) sheet metal tray. The size of the tray and its design in general can be seen in the attachment. The tray will actually undergo some vibration, and from my own testing I've found the vibration to actually cause the shelf to resonate to the point of creating quite a bit of noise. In addition, the vibration seems to be fatiguing the spotwelds to some degree. Anyhow, I'm fairly sure if I roll some ribs or cross brake the large 20 Ga sheet metal I'll gain some much needed stiffness. The problem is I have no experience in stiffening by rolling ribs or braking, or potentially even something else to accomplish the same thing.

Additionally, what orientation do you think ribs or brakes should have in relation to the tray?

Your thoughts and suggestions are appreciated.

-Mike

 

[TheTick]

Hard to tell exactly what you need for ribs, etc. They would definitely help stiffness, though.

My first impulse would be to add ribs diagonally so that they are deliberately not in line with wave patterns.

Remember that ribs only add stiffness, not strength.

 

[MadMango]

Not knowing the scope, can the bottom tray be divided into 3 or 4 sections? You could then create some lap joints with hemmed edges, and may even get rid of the spot welding and 14ga cross straps. The extra forming upfront would save the secondary welding operation and allow for easier handling (smaller panels), and access if maintenance is required.

If the above can't work, use diagonal ribs as TheTick suggests. For guidelines, see

http://www.npd-solutions.com/sheetmetal.html

about 2/3 down.

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of Eng-Tips Forums?

 

[CorBlimeyLimey]

The Cross-Brake method would make it more difficult to spot weld the strapping.

Rolling ribs in either direction would make it more difficult to seal at the frame.

I'd probably try something like this;

 

[MouseTrap]

I've worked with speakers in which thee was a (smaller) metal (.125 thick aluminum?) panel in which shallow radiused ribs were pressed into the one panel itself to add stiffness - some toling sharges were involved, but these ribs didn't involve any extra pieces that needed to be welded into place. I don't recall specifically what shop these ribbed panels came out of, so I can't help you there. The ribs were about 1.5 inches wide by about 3/8 inch deep, and had a radiused bottom.
Maybe somebody else knows what I am talking about and can add some additional light on this possibility.

 

[drawoh]

mmolt,

Could you apply the gussets by indenting the sheet metal, rather than by spotwelding? This probably would be cheaper, it would eliminate the failing spotwelds, and you could do a fairly elaborate pattern.

I have analyzed and solved a similar design problem using vibration tables out of a handbook. I used the Mechanical Engineering Essential Reference Guide, and its article on Vibrations. The article is also in Mechanical Design Handbook by Harold Rothbart, but without the tables on vibrations of flat plates. Some current publication must have this information. FEA would work too.

JHG

 

[ajack1]

Could you not just use tread plate for the base?

 

[berkshire]

mmolt (Mechanical)
Is material cost and weight a factor in this application ?
If it is not, I would suggest doing away with the frame and simply making the entire tray out of 16 Ga material.
You can the cross brake the tray before forming the side flanges.
B.E.

 

[mmolt]

The design is absolutely an attempt at optimizing the tray design. Weight is a major factor as I've been given a limit, cost is huge as the overall cost of the product has already been set. In my experience, material is always about 70 to 80 percent of a products actual cost, so it does not bother me to throw labor or processing (forming) at it to achieve my goal.

Thanks for all your input, any other suggests are appreciated.

-Mike

 

[jistre]

Instead of putting the spotwelded braces in, could you just cut the metal in the bottom to form a flange that could be bent down? Stiffening all day long without having the hassle of fitting up and spotwelding all those braces. Sell it as a value-added drain feature.

 

[MadMango]

You haven't really defined the purpose of the tray, or listed all the constraints, making it hard to suggest further ideas. I fully understand if specifics cannot be given. If possible, you could place several fasteners through the center area of the panel into the supporting structure (using j-nuts). That would fix your drum skinning. Also, polystyrene or similar blocks and foam tape would serve the same purpose.

DARK

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[Artisi]

Have you tried going up one gauge of material for the panel.

 

[dvd]

I have a hard time believing that the material is 70-80% of the cost of the product; maybe in some industries, but since you are asking rather simple questions that a sheetmetal fabricator would have a good handle on, I don't believe that you are optimizing much of anything with regards to costs for sheetmetal fabrication.

With that said, I would make the pan out of one material with welded corners, not spot welding any ribs; I would look at gluing (hot asphalt comes to mind) a heavy asphalt shingle type of material to the bottom of the pan to stiffen it. I have seen this type of stiffening done on dishwashers and thought that it seemed like a good idea for stiffening and sound-proofing.

 

[GregLocock]

I suggest you look at the floorpan of a car.

How much time and effort and money are you prepared to put into solving the problem?

Cheers

Greg Locock

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[MintJulep]

Firstly you need to truly establish what your design goals are. You state that you want to optimize for cost and weight, but then go on to express concerns about noise and "resonance".

It would seem quite possible to design an adequately strong, light tray that is noisy. What is the tray loaded with, and does the load damp the noise and vibrations? Is quietness of an EMPTY tray a design requirement or not?

What is the vibration mode that causes the noise? Is the entire plate fluttering, or is it oil-canning (buckling)?

It is possible that simply tensioning (stretching the sheet tight) before welding it to the frame could solve either mode.

Another possible solution would be to used ridigized (embossed-textured) sheet rather than flat sheet.

A cross-break may or may not work.

Stamped or rolled stiffing features in the right spots would work, and almost certainly would be cheaper than spot welded stiffeners. To know where to put them you need to know the vibration modes.

An entirely different approach would be the application of a damping material.

 

[mmolt]

Group,

I simply cannot tell you the application as it's related to contractual work... Most of my question on this revolves around rolling ribs, cross brakes, or other production techniques to stiffen a sheet of metal as my original message implies....with weight being an issue.

dvd, care to elaborate on your statement "don't believe that you are optimizing much of anything with regards to costs for sheetmetal fabrication."

MintJulep, both cost and weight are probably similarly weighted in terms of importance. With the tray loaded, the resonance is not an issue, but there are times with the tray will be depleted of its load, so resonance is a concern. Obviously any weight added in terms of the load will dampen the structure of the tray. Best I can tell, the tray is oil-canning. When excited about about 1.5 Hz my best guess is the tray is actually resonating at about 3 times the input.

-Mike

 

[dvd]

mmolt:

I don't think that one can obtain an optimum solution without knowledge about how to solve the problem. When you arbitrarily state that "material is always about 70 to 80 percent of a products actual cost" it says to me that you don't understand enough about the processes involved to come up with an optimum solution. You can't optimize based on assumptions, you can only approximate.

To me, an optimum solution would be one where there was only one piece of material required to make the assembly. It would be formed in one operation on an open set-up with no special tooling. There would be no fit-up of additional parts.

If you were to make the tray out of 14 ga. and perforate it to obtain a large percentage of open area to lessen the weight, your fabrication costs could be very high but the part weight would be low. (The seaworthiness of the vessel may also be questionable.) This might be the most optimum strength-to-weight ratio but the least optimum cost-to-weight ratio.

I guess that my ramble points out to me that an optimum solution is in the eye of the beholder, you may interpret this optimized message entirely differently.

 

[Tmoose]

I'm thinking panel vibration at 1.5 Hz would likely be inaudible, unless there were nuts and bolt bouncing on it, exciting nice annoying higher frequencies with impacts.

 

[Helicopterjunky]

Let me give you my two cents from a slightly different perspective. I would cut a sheet metal blank from a single gauge material, one piece. This piece will look like an unfolded box lid when laid flat. Use a brake to bend the four edges up and use a single pass MIG weld down each corner. If you want more damping, try already impressed patterned plate with different patterns such as treadplate. These ideas were proposed seperately by DVD and AJack1. I combined the best from both. Good Luck!!