Using FEA to design a press frame

[mcfridge]

Hello everyone,

I had a couple posts on this site about building a press to make skis/snowboards. I have mine built, if you look here (

http://www.grafsnowboards.com/pressidx.htm

)and go to either the 6th or 7th picture down, you can see how it basically looks.

My buddy wants me to help him build one now, but on mine it is built out of some pretty heavy I-beam (if you look at the picture, it is 2 long pieces on top, 2 smaller pieces holding them up, and 2 long pieces on the bottom). The pressure comes from an inflated bladder (firehose for now). We had a suggestion from an engineer to use FEA to come up with a design that would cut down on the weight and the material (to save money). I think my buddy said he had access to Algor FEMpro. Could someone give me some pointers on how to use the software to come up with a good design, and what I should be looking for when I am doing it? Thanks a lot for any help provided.

 

[GBor]

The Algor forum is forum810 ...there are several people that can help with specifics on this.



Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.

http://www.borowskiengineering.com

 

[mastroit2002]

I will be glad to help You in this endeavour.

From your website I got that the press You are design should enforce a laminate composite giving an appropriate curvature.

My simple advice to save some money (I think that the whole structure weights more than one hundred kilos) colud be to design the frame with a welded sheetmetal structure.

The cost of a specific welded structure (I had some experience on it) can be less than standardized geometry identical, I beams.

I think even that since the frame is under a equal distribuited pressure, the upper beam could tapered at the ends achieving robusteness and a good stiffness.

if in need I can give a hand for a simple design or simulation with a solid modeller/FEM package.

Regards,
mastro

 

[jen4950]

Personally, I don't think you need to go to great expense or effort structurally to get what you want. This includes FEA especially.

What you have is essentally a distributed load over a simply supported beam. How much more straightforward can you get? Not to mention that you are using a bladder that will evenly distribute the pressure as the frame deflects- so deflection shouldn't even be a concern as long as you keep overal system deflection around L/480 (very conservative- L/600 would be swatting a mosquito with a howitzer in this situation)

What sort of pressures on the snowboard are you trying to achieve for curing? (my hypothetical assumes 1 Atmosphere = 14.7psi (if I remember correctly; )

For instance- say a hypothetical board is 12" wide and 6 feet long (I have no clue- just guessing based upon the pics; I'm from Texas- what is snow ? ) And you wanted to apply 1 ATM of pressure to the board for curing, which results in roughly 2.2 kips per foot on the said beam.

If you're going to buy some new standard rolled shapes, you'll be buying 50 ksi steel (A572 or equivalent), and E = 29,000 ksi.

To maintain L/480, your max deflection should be limited to 0.15 inches (with a 6 foot long frame).

So if you set the following equation to your limiting delfection, you get the minimum moment of inertia required for your total section. A simple assumption is a simply supported beam without any moment being absorbed by the connections.

Delta = 5 wl^4 / 384 EI

With 50 psi:

I get that you need a W10x15 for L/600. (I >=60.3 in^4)

And that you need a W10x12 for L/480 (I >=48.2in^4) - which happens to be the smallest W10 that they make. But- I'd bump it up to at least a W12x30 or W12x35 for strength considerations- and you're making snowboards; I'm sure you can figure out a way to overstress your apparatus, and we're only talking like 200 pounds of steel here per beam- no big deal. If you have more precise loads I'll work up a beam and explain the details if you would like.

 

[rb1957]

i'd agree with jen, above. i think FEA is going to be alot more complicated than you think (sexier tho' it sounds). so simple hand calcs, as jen suggests, will show you most of what you want to know. personally, i'd have thought that a few feet of I-beam would be cheaper than welded sheet, but you know your business better than i do. i love the idea of using a fire hose as a bladder. i think you don't really want to reduce the weight of the press, but rather want to make it cheaper ... are you going to mass produce these things ? could you let your buddy use your press ?

why not make a press from a lighter (cheaper) I-beam and verify the pressure obtained.

good luck