To Build A Ski Press...

[doughboyshredder]

hydraulics. Smooth as butter.

[i killed judas.]

hydraulics. Smooth as butter.

Im no engineer, but if those are hydraulics, then what is the need for the bladder? Shouldnt the hydraulics be able to apply more than enough psi?

I am not trying to contradict, I am just curious as to what the answer is.

[doughboyshredder]

good question. The hydraulics can only generate a total of about 4,000lbs of force over the entire surface area. When the airbags are inflated to 70 psi, I am generating over 120,000 lbs of force! Air pressure is a trip. It doesn't take much to get an incredible amount of potential force.

[iggyskier]

good question. The hydraulics can only generate a total of about 4,000lbs of force over the entire surface area. When the airbags are inflated to 70 psi, I am generating over 120,000 lbs of force! Air pressure is a trip. It doesn't take much to get an incredible amount of potential force.

Also - a hydraulic press requires you to have perfect top AND bottom molds. In a pneumatic, you have some margin for error with the top mold.

[iggyskier]

Holy shit man- you need a TIG welder! I know, I know- I read the part about the $$ and the experience, but DAMN THATS ALOTTA BOLTS!

The middle sections are definitely one of the things looking back that I totally could have had welded and wouldn't have altered my ability to move it around. But it was well after everything was together. Plus, I really sort of wanted to build it entirely by myself, and if I was going to do that, bolting it was.

Let me know if you want somebody who does this shit professionally to look at your drawings. The key here is DRAWINGS. If you can put it on paper, I can make it. I've been at this for ten years and work at a family shop where I have the keys and would do it off the books. Your press is sicktah, but I'm sure you've found things you'd like to have done differently.


Werd. Again, nice work Iggy. Let me know if you want it welded next time. You'll probably spend the same money you spent on nuts & bolts on shipping, but it'll be done to the nines with a TIG welder...

Awesome. Thanks for the offer. Definitely expect a PM here sometime soon.

As far as $$ to ship those bolts....yeah. I think the package weight over 50 lbs. Not cheap.

[j o n o]

Here is a simple single ski press that cost about 600$ (it was 2 years ago and I am estimating the cost).
It has plywood, and doug fir 4x4s and a single 2x4 running lengthwise on the top and bottom.
It resists internal pressure with 10, 3/4" threaded rods connected to 3" wide 1/4" steel bars AND 16, 3/8" threaded rods that connect to 1/8" angle iron.
I have also used 1/8" angle iron to stiffen the press lengthwise.
So far there has been no noticed deformation in the overhanging angle iron or in the protruding 3/4" threaded rod or in the press in general when under pressure.
The 3/4" threaded rods have a shear strength of 45,200 lbs (2260 lbs each) and the 3/16" threaded rods have a tensile strength of 78,400 lbs (4,900 lbs each). The total pressure my press can handle is 123,600 lbs.
My biggest mold is about 9" by 84". So, 9X84X2=1,512 sq inches. This is the maximum amount of pressure exerting area my press is subject to. 123,600/1512=81.74 lbs/sq inch is the theoretical max pressure I could run the press at.
The reality is that my molds are mostly smaller (especially the top mold) and put less pressure on the press. I have run my press up to 70 psi with a mold of about 1200 sq inches (84,000 lbs) with no deflection or weirdness.

The bottom line is that I never go anywhere close to 75% of the max pressure of my press. My compressor could exceed and destroy my press so diligence is needed to run this press. The wood parts and the overhanging angle iron are the weak links in my design and need to be monitored for stress (none seen so far in 20 cycles).



Remember: Pneumatic ski presses are very dangerous and could easily kill you if you're not careful. If you have not calculated your press's tolerances then you should not be using the press.

[Rubicon]

Im no engineer, but if those are hydraulics, then what is the need for the bladder? Shouldnt the hydraulics be able to apply more than enough psi?

I am not trying to contradict, I am just curious as to what the answer is.

I think I understand what you are asking. The hydraulics only raise and lower the top mold to make it easier to insert the ski material. The pneumatic bladder is used to actually press the ski.

[Trainwreck]

I think I understand what you are asking. The hydraulics only raise and lower the top mold to make it easier to insert the ski material. The pneumatic bladder is used to actually press the ski.

Thats what I was assuming.

[doughboyshredder]

yep, purely to make it easier to load the cassette with the materials. That way the air bladder doesn't have to fully inflate to fill the void giving more surface contact.

I did do some basic calculations on the stresses applied at the weakest points in my press. After that I really did go in to a different room with a 50' air hose and inflated the bladders to almost double the working pressure and it barely even creaked.

It is important that while you are designing to think about what would happen if any bolts you are using do shear. You don't want a bunch of projectiles aiming at where you are working.

And air bladders do fail. I know someone that was standing by one when it exploded at Mervin. Can you say heart attack?