Aluminum recommendation needed

[josephmartins]

Good evening all,

I'd like to decrease the weight of our automobile rotisserie for our shop. Here's an example of a typical rotisserie:

http://www.harwoodperformance.bizland.com/1941buick/Editorial_20k.htm

We used off-the-shelf 3"x3" 0.188 wall square tubular steel for the vertical upright attached to horizontal legs of the same material. At the top of the upright we horizontally mounted a foot long, 3"OD 0.188 wall tubular steel sleeve containing a 2.5" steel pin. The pin was then welded to another tubular structure that is, in turn, fastened to the frame of an automobile. 1/2" steel plate gussets were used as reinforcements.

As you can see in the photos of the example URL, one such unit is attached at each end of a vehicle to enable someone to rotate the frame and body 360 degrees.

My questions are...

What type of aluminum would make a suitable replacement for the 3x3 0.188 wall steel? How about the 3" OD 0.188 wall sleeve and 2.5" OD pin?

Together, the two stands must support at least 5000 lbs.

Currently the stands simply weighs too much. It really makes life difficult when we need to transport the rotisserie from one location to another. I'm hoping we can find an aluminum suitable for the design....but I have no idea what type of Aluminum we should use, or what size and wall thicknesses would be required to approximate what we have now.

Any recommendations would be appreciated.

Kind regards,

Joe Martins
Langdon, NH

 

[CoryPad]

If it is simple off-the-shelf steel, then it isn't that strong. The standard 6061-T6 aluminum alloy for extrusions/bars/shapes would work as a substitute. You may need larger thickness or larger section size to accomodate the stiffness decrease. 5000 pounds actually isn't that much for a well-designed structure. You can use Google to find an aluminum shapes supplier.

Here is one in New Jersey:

http://www.coltwell.com

You can also try the larger catalog/warehouses:

http://www.mcmaster.com

http://www.misumi.com

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[josephmartins]

Thanks Cory - I appreciate the reply. That's what I had hoped.

 

[pdybeck]

Joe,

To re-state what Cory said - you may need to accomodate for stiffness. Aluminum as a material has about 1/3 the stiffness of steel - with equal geometry. Its modulus of elasticity is 1/3 that of steel. Stiffness is more than just material properties, its a function of material properties AND geometry. You may not even need the level of stiffness you have now with your steel frame. Either using larger walled tubing, larger diameter tubing, or placing supports in strategic places would allow you to cut the fat out of the design. 6061-T6 could be an excellent choice and its readily available. FEA could provide to be an extremely useful tool for optimizing the design and evaluating several possibilities before commiting to one.

 

[josephmartins]

Thank you pdybeck. How might I go about finding someone with FEA experience on the forum to input the design and optimize it for aluminum?

One other thing I should mention. I originally planned to weld the parts together. I had been told that the structure would be much stronger if I designed it to be bolted together, rather than welded, if I use aluminum. The person I was speaking with told me that welding puts the aluminum back into its annealed state, thereby weakening it. But in places where I had hoped to weld triangles of plate aluminum to increase rigidity, I simply don't see how I could easily bolt parts together instead. Does welding weaken 6061-T6 aluminum? If so, I might have to rethink the whole design.

 

[pdybeck]

I am not the person to ask about welding aluminum. You would probably want to really evaluate your current design to see if just switching to aluminum would really be better. You would want a good analysis of your current design to compare to any analysis of a proposed design. You may be able to just use thinner walled steel members, or less of them, or different geometry, or ... In looking quickly at your rotisserie, you may not want it to be too light, as it could cause stability with the stand wanting to rock and roll when rotating the car bodies.

As far as finding someone with FEA experience... This forum is not supposed to be used for that (wink) ... But there are people that could help you. First you would need your design in 3D. Do you have that? Once in 3D, it becomes easier to evaluate. I use SolidWorks and CosmosWorks

http://www.solidworks.com/

- which is very capable. To say the structure would be much stronger if it was bolted together is too general a statement. It may be true, it may not. You could change things to make that statement true, or false. There are ways you can bolt gussets to structural members to provide supports - I am actually evaluating this for my company as we speak. A completely bolted together frame would have its merits. Easier to assemble and potentially easier to mass produce, easier to ship in pieces and assemble...

 

[pdybeck]

You could also have combination of materials in your design. Some steel, some aluminum... Re-thinking could definitely be a good thing. Is this something you are looking to manufacture?

 

[EdStainless]

Don't overlook the option of using lighter steel. Slightly smaller size and thinner walls and you could cut a lot of weight out. Moving from 3x3x0.188 to 2.5x2.5x0.125 cuts 40% of the weight.
Some of the parts that aren't in the load path (that tie the two ends together) could be made from light gage Al without concern.
I would also suggest looking at gusset plates (with the corner cut out) in place of square tubes for braces.

= = = = = = = = = = = = = = = = = = = =
Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

 

[josephmartins]

Good evening pdybeck,

I understand. This is basically one of those problems where the answer really is "it all depends". I have to keep that in mind when making generalizations. I did also consider a hybrid steel-aluminum design...it's an interesting option.

By the way, I spoke to a company in New Hampshire that specializes in welding aluminum and they said it can definitely be welded, with retreatment of the 6061 after welding. The person who told me it would create a weaker structure was probably thinking of a different aluminum alloy.

So it seems there is some legwork to be done to nail down the specs, but I don't want this to take forever. I'm anxious to get started on the Mustang. Regarding manufacturing...at this point I have no plans to build any beyond my own. Several companies already make rotisseries. Mine would essentially be the same, except for the materials. But, I'm sure other other automobile hobbyists would be interested in the results of my project, especially if they had been considering doing the same thing.

Ed, I think that's something I should consider. But that's going to be beyond my own skillset. I'll have to find someone who can alter the design and run some analyses to determine the best combination of configuration and materials.

Thank you all for your help.

 

[josephmartins]

oh, pdybeck, I do not have a 3D model. Just a dimensioned drawing in PDF format.