First Press

[TristanJ]

Hey everyone, this is my first post on the forums! I'm from calgary, AB and I've been shadowing the boards for a couple of weeks now, trying to gather enough information to build my first press. I have arrived at an estimate of about 65,000lbs of total force, and I was wondering if this sounded about right at 60psi? Furthermore, there is probably a point where additional pressure does not improve the bonding of the ski any more, and I was wondering what this point was in your estimation! Thanks so much for your time and I look forward to joining this awesome community!

[rockaukum]

I had clacs done for my press the other day and at 50 psi we came to about 72,000psi. Forgot the length of the hose I have and there are two hoses. I usually press at about 40 psi.
rockaukum

[TristanJ]

Awesome! Sounds like you're pressing a slightly bigger area than I am. At the moment I'm looking at about 12" by 210 cm, although might kick that up to 18" for snowboards, do those dimensions sound reasonable? I'm kind of trying to do all this off the top of my head so I may be out to lunch!

Thanks!

P.S. Any builders in Calgary?

[j-raw]

210cm/2.54 cm/inch = 82.7 inches

82.7 inches * 12 inches = 992 square inches

60psi (pounds per square inch) * 992 square inches = 59,500 pounds

Keep in mind this is the total force felt by the entire press frame. If your press is like the one in the skibuilders how to you could divide this number by the number of vertical supports to find out how much force each smaller section of the press feels. Your 65,000 pound number seems to be pretty reasonable though. Good luck.

[SHIF]

The force applied to the ski press frame is simply the air pressure times the actual area of the “contact patch” of the bladder, not the air pressure times the total area of the ski press (unless you have a grossly over sized bladder hanging out all around).

If you use cat tracks or a similar force distribution device you can achieve a contact patch area greater than the actual surface area of the ski base and edges. Multiply your air pressure by the ratio of these two areas to understand the real psi value used in pressing your skis.

-S

[j-raw]

I don't understand what you mean when you say, "use the ratio of these two areas to understand the real psi value". The psi value in the bladder is the same no matter the area which it is exerting that force. Do you mean the real force value exerted on the press frame by the bladder. Just curios...

[doughboyshredder]

I think what shif is referring to is the pressure that is applied to the laminate. In other words just because your bladders are pumped up to 60 psi, does not mean your laminate is feeling 60 psi. Depending on whether the surface are of bladders is larger or smaller than the surface area of the laminate, you will either have higher or lower pressure at the laminate.

Rough numbers: 100 square inches at 100 psi equals 10,000 pounds of pressure. If that pressure is applied to the top of a mold that is 50 square inches than the mold is feeling 200 psi. Right? I have been wondering about this for a while. I would think you would always want to adjust your air bag pressure to compensate for the area of your laminate.

Regardless it's a lot of damn pressure. I plan on being in the other room when I first bring my set up to full pressure.

[j-raw]

There are defiantly a lot of variables that make for interesting calculation. SHIF does have a good point about the contact area, however I'm pretty sure that if you check the pressure in your bladder and it is for instance, 35 psi than it is safe to say that anything that is touching it also feels 35 psi regardless of the "contact area". Either way, like you said, close the doors and watch from outside when it comes time to pump that baby up, shit might hit the fan and when it does you don't want it all over your face!

[SHIF]

Yes, my ratio estimate is in fact for figuring the psi used in the actual ski laminations. A bit off topic I suppose.

Yes, every tiny bit of "contact area" is subjected to the same psi as the air inside the bladder.

Cheers,

-S

[doughboyshredder]

Please forgive my ignorance. I have a hard time with basic math, so this is a bit of a stretch for me as it is. In the following picture I drew the airbladder, the cat track, the laminate, and the press frame. I used easy numbers for the purpose of discussion.



In the drawing the air bladder is producing 10,000lbs of force being distributed across the cat track. This puts the pressure of the cat track at 83.33 psi. Right or wrong?

The laminate has a surface area exactly half that of the cattrack. Is the laminate feeling 10,000lbs of force (i.e. 166.66 psi of pressure), or is the laminate feeling 83.33 psi of pressure and therefore only 5,000lbs of force? If so where does the other 5,000lbs of force go? Am I confusing this because of the difference of pressure and force. Does pressure and force correlate in the way I am assuming they do?

Hoping one of you smart guys can fill me in on where my thought process is wack.

[SHIF]

Good job on the drawing.
The force on the cat track exactly equals the force on the upper frame beam. The area of the cat track is irrelavant as long as it is at least the size of the bladder "foot print".

If the bladder is putting 10,000 lbs force into the press frame, the laminate is also putting 10,000 lbs force into the frame. But this 10,000 lbs is spread out over less area resulting in higher force per unit area (psi). In your model, the laminate sees 10,000 lbs / 60 in^2 = 166.67 psi.

Cheers,

-S