Cloth press design: rough draft- please critique!

[webboy]

Hi,

I got some Cordura from Montucky Madman last Christmas (thanks again MM,) and I'm finally getting around to working on my press design. This is a rough sketch, but hopefully those of you familiar with the cloth press concept can chime in here and let me know if this design looks decent. Of course I could just build the thing and test it, hoping that it doesn't blow up, but it would be great to get some help from the engineers in here.

Here is what I've done in Sketchup. This is only the top half; the bottom half of the press body will be basically the same as this, but with a stand- probably along the lines of what chrismp has done (Big Ear Snowboards.) I threw in a few extra support structures so they can be seen more easily, and also hid a few blue boards to give a cutaway view.

Oh yeah, it's a double wide. EDIT: Hope to press around 50 psi (Thanks Richuk!)



The blue beams (pine wood) measure 88mm (3.46 in) along the top edge, 45mm (1.8 in) thick, and are about 2.2 m (7.2 ft) long. The green supports and the pink bottom are 21 mm (.83 in) thick plywood. The orange "feet" are 120x45 mm (4.7x1.8 in) pine, and have 10 mm channels cut out to support the green plywood. The "feet" are thus 35mm (1.4 in) thick under the vertical plywood supports. The width is 450mm or about 17.7 inches wide.

Any and all opinions would be appreciated. I have no idea how to calculate loads on this thing, so I'm open to whatever suggestions or criticisms you guys have.

Oh, and I have a ridiculously huge hose that I will use for this thing. 12" diameter Ultraman from Mandals in Norway- http://www.mandals.com/mandals/product_ ... g/ultraman. Yes, it's the same hose as Endre's, just a bit shorter. The hose was a stupid impulse buy to be honest, but it should kick ass anyway.

Hopefully I didn't leave anything major out of this post.

Thanks!

[Richuk]

Wondering what psi you are intending?

[webboy]

Guess that's important info! 50 psi I'm thinking.

[webboy]

Couldn't stop the Sketchup this evening. Here's another concept drawing. Note once again that there are some blue boards missing to allow for the cutaway view.

[MontuckyMadman]

looks good to me.

[falls]

Have you got a way to support it on the table?
All those round curves it might roll away!

[webboy]

Now it won't!

[webboy]

For anyone who comes across this post and doesn't get the concept; here's what we're looking at:

I believe this is the original- Plywood's press:


thread: http://www.skibuilders.com/phpBB2/viewtopic.php?t=1225

Shopvac's:


thread: http://www.skibuilders.com/phpBB2/viewtopic.php?t=1542

chrismp's (Big Ear Snowboards):


thread: http://www.skibuilders.com/phpBB2/viewt ... 6223#16223

[Richuk]

Is plywood (green) in compression a good choice? Only 50% of the fibres with be pointing in the right direction - will it fatigue over time?

Sorry, if this is a stupid observation ... great drawings!

[doughboyshredder]

Great drawing, but I am not sure if the ribs are the way to go.

I am not an engineer by any means, but here are my concerns.

If the ends of the top mold terminate between two of the ribs then the pink layer of plywood may end up taking quite a bit of force at that point. Probably would be o.k. but, it sticks out to me. On the bottom there isn't much mold material above that layer of plywood and it may deform around the ribs.

If the green ribs are not oriented perfectly vertically then I could see them collapsing by getting pushed to the side, although based on the design of interlocking in to the orange pieces that probably wouldn't happen. Each rib though is still going to see about 4,000lbs of pressure. Not sure if that is within the materials capability or not.

Why not just build the molds with rounded tops and bottoms. I think it would be easier and then you wouldn't have any open spaces to worry about compressing.

As I look at your drawing (which again, is awesome) I think I see that you are trying to make it so that you can change out the molds and keep the weight down. Pretty cool design really, I am just not too sure about the ribs. Looking at the other fabric presses none of them have any open space.

Good work and good luck!

[webboy]

Is plywood (green) in compression a good choice? Only 50% of the fibres with be pointing in the right direction - will it fatigue over time?

Sorry, if this is a stupid observation ... great drawings!

Not stupid to me, I'm not sure at all! Perhaps OSB is better. Anyone know?

Thanks for the compliment on the drawings- I put a few hours in.

[webboy]

Great drawing, but I am not sure if the ribs are the way to go.

I am not an engineer by any means, but here are my concerns.

If the ends of the top mold terminate between two of the ribs then the pink layer of plywood may end up taking quite a bit of force at that point. Probably would be o.k. but, it sticks out to me. On the bottom there isn't much mold material above that layer of plywood and it may deform around the ribs.

If the green ribs are not oriented perfectly vertically then I could see them collapsing by getting pushed to the side, although based on the design of interlocking in to the orange pieces that probably wouldn't happen. Each rib though is still going to see about 4,000lbs of pressure. Not sure if that is within the materials capability or not.

Why not just build the molds with rounded tops and bottoms. I think it would be easier and then you wouldn't have any open spaces to worry about compressing.

As I look at your drawing (which again, is awesome) I think I see that you are trying to make it so that you can change out the molds and keep the weight down. Pretty cool design really, I am just not too sure about the ribs. Looking at the other fabric presses none of them have any open space.

Good work and good luck!

Hi DBS. Thanks very much for the feedback. First, as this is still conceptual, I didn't put too much thought into the pink plywood layer yet. I figured I could easily change that to another material, or more likely just add another layer between it and the mold. Maybe MDF is better here? Beefing that up if needed should help with the deformation you mentioned. Also, I suppose I could modify the top mold a bit so that the ends terminate under the vertical supports. Secondly, if I agreed that it was easier to build the molds with a rounded top / bottom, (which I don't think I do,) it would be a great idea in terms of eliminating the gaps, but no so great if I decided to change my press design or move to an I-beam press (which is what I would ultimately like to have, but I want to start "small" for now.) Regarding the open space- Actually chrismp's press has gaps. Quote: "they do have supports in the like every 20cm, just not on the farthest points outside...we figured we could save some weight without losing strength." Link: viewtopic.php?p=13546#13546 The following post has pics with end-on views which allows you to see the space- viewtopic.php?p=13517#13517 Hopefully chrismp will chime in here.

Thanks to you as well for the compliments on the drawing.

[Alex13]

The drawing looks good, and the design is not bad either.

My concern is that you could crush those ribs fairly easily. It's hard to do an analysis without FEA (and I'm too lazy to draw something), but here's what my instinct tells me. The cordura won't direct the force perfectly down the ribs, it will stretch in the middle and look something like this (cheap side on view). I've ignored the dark blue beams in this pic.



The forces it will generate are shown in red, and the moments in green. This will roughly balance itself out, as it will be similar in between each rib (so each rib will have a force pulling itself left and right).

If I were you, I'd look to strengthening the ribs in those directions. The hard angle between the rib supports creates a weak spot. Look at putting a piece of angle to support along as well, something like this...



Alternatively, use thicker pieces spaced closer together. It depends on what type of wood you use for the horizontal outer beams (the dark blue ones), if it's a fairly hard wood that doesn't flex much you might be ok.

Also look at using thicker pieces or otherwise supporting the pink sheets you have, they look to be possible weak points to me, especially at the edge of the top mould here looks prone to breaking.

[webboy]

Thanks for your feedback Alex.

I've tried to address yours and other's concerns with a few design changes/additions. I really liked your idea with the wedges, but I thought perhaps putting rectangular pieces in between might add even more strength and be a bit easier to implement. They are floating between the orange "feet," so perhaps I need to think about that as well, but already I've got tons of angled cuts to make, so...

Also, I've beefed up the flat sheets to 2x25mm plywood. Perhaps I should use a combination of plywood and MDF. Again, I haven't put much thought into the sheets, since that is the simplest part of the design to address.

Regarding materials- maybe it would be stronger, (not to mention cheaper) to use OSB in place of the vertical (green) plywood. If anyone could confirm that one way or the other, I would appreciate it.

[chrismp]

i totally agree with alex13.
i thought about doing a design similar to yours, but finally decided against it for the above mentioned reasons.
we figured that using boards in a direction they have more strength in would be better even if we couldn't get that big of an ark out of it without making it too heavy to lift.
the supports beneath our pyramid are about 15 cm wide and spaced at about the same amount.