my ski press has been started
Moderators: Head Monkey, kelvin, bigKam, skidesmond, chrismp
my ski press has been started
I finally got most of it cut and some bolted together. I just have some pics of it clamped and put together before its tuned and painted.
Please don't take this as critic on your design, because I'm only trying to help you!! 
1/2", so about M12 in the metric system.
I assume you have class (or grade, don't know the correct word) 8.8 for the threaded rod then you can use the next calculation:
You want to have the tensile stress of max. about 80 percent of the yield point.
A M12 bolt has a working surface of 84,26 mm^2, lets say 84 mm^2.
So the maximum load at one threaded rod is: 0,8 (from 80%) * 800 (from 8.8 ) * 0,8 (from 8.8) * 84 = 43 kN.
You use 12 rods, so the total force is 12 * 43 = 516 kN.
Your maximum pressing surface is 200 mm (width) * 2000 mm (length) * 2 (top and bottom) = 800000 mm^2
Maximum allowed pressure will be 516/800 = 0,645 N/mm^2
This is about 94 PSI and means you should take good care while operating your press, if you won't like to demand everything of your materials! (also the maximum load will be lower depending on the friction coefficient of your rod, in a worse case you can max. load 28 kN per rod)
Please correct me if I'm wrong!!
Also 6" inside height seems pretty small to me. My bottom mold is already 7" high. Hope you can make everything working!
Good luck!
Buuk
1/2", so about M12 in the metric system.
I assume you have class (or grade, don't know the correct word) 8.8 for the threaded rod then you can use the next calculation:
You want to have the tensile stress of max. about 80 percent of the yield point.
A M12 bolt has a working surface of 84,26 mm^2, lets say 84 mm^2.
So the maximum load at one threaded rod is: 0,8 (from 80%) * 800 (from 8.8 ) * 0,8 (from 8.8) * 84 = 43 kN.
You use 12 rods, so the total force is 12 * 43 = 516 kN.
Your maximum pressing surface is 200 mm (width) * 2000 mm (length) * 2 (top and bottom) = 800000 mm^2
Maximum allowed pressure will be 516/800 = 0,645 N/mm^2
This is about 94 PSI and means you should take good care while operating your press, if you won't like to demand everything of your materials! (also the maximum load will be lower depending on the friction coefficient of your rod, in a worse case you can max. load 28 kN per rod)
Please correct me if I'm wrong!!
Also 6" inside height seems pretty small to me. My bottom mold is already 7" high. Hope you can make everything working!
Good luck!
Buuk
mmm
Unless I misunderstand you Buuk, the surface area should not be multiplied by 2 as you put it 'for top and bottom'. The pressure will exert an equal and opposite force up and down over the area of the bladder which is just the width X the length.
Given that adjustment your numbers Zacks press would only reach 40% of yield.
Having said that it is good to second guess each other on this site - I am pretty nervous about the safety limits on these presses and the potential for serious injury if something goes wrong is pretty considerable. I'll certainly put my design up for critque before I blow myself up!
B (who is still trying to make his press)
P.S. Zack - pretty fast work on the press, but I would agree with the others that 6" may be a bit tight. I thought that I could do it in that size, but whan I planned it out in detail I found that my mould ended up needing about 10" (although I could take a bit out of that).
Given that adjustment your numbers Zacks press would only reach 40% of yield.
Having said that it is good to second guess each other on this site - I am pretty nervous about the safety limits on these presses and the potential for serious injury if something goes wrong is pretty considerable. I'll certainly put my design up for critque before I blow myself up!
B (who is still trying to make his press)
P.S. Zack - pretty fast work on the press, but I would agree with the others that 6" may be a bit tight. I thought that I could do it in that size, but whan I planned it out in detail I found that my mould ended up needing about 10" (although I could take a bit out of that).
Re: mmm
These ski presses can be very dangerous if not built and operated with caution so I have to really emphasize that everybody be careful. It's part of the reason why I've started to recommend that people use a vacuum press. I'm actually going to switch over to vacuum once I get the time and money.Bambi wrote: I am pretty nervous about the safety limits on these presses and the potential for serious injury if something goes wrong is pretty considerable. I'll certainly put my design up for critque before I blow myself up!
As for the press, 6" is about what my press measures but I really wish there was more room. You can make it work just make sure your molds aren't too tall. Your press seems similar to mine and so far it works well without any problems (aside from the inside height issue). But I only inflate my bladder up to 30-40psi. I'd be a little afraid to go much higher than that.
- Kam S Leang (aka Little Kam)
Re: mmm
do you have any of the design for any of your molds. I am wondering about what sizes they are and what you used to make them. If I really needed to I guess i could go buy more steel rod for 10ft@$5.00 and then recut it so its longerlittleKam wrote:These ski presses can be very dangerous if not built and operated with caution so I have to really emphasize that everybody be careful. It's part of the reason why I've started to recommend that people use a vacuum press. I'm actually going to switch over to vacuum once I get the time and money.Bambi wrote: I am pretty nervous about the safety limits on these presses and the potential for serious injury if something goes wrong is pretty considerable. I'll certainly put my design up for critque before I blow myself up!
As for the press, 6" is about what my press measures but I really wish there was more room. You can make it work just make sure your molds aren't too tall. Your press seems similar to mine and so far it works well without any problems (aside from the inside height issue). But I only inflate my bladder up to 30-40psi. I'd be a little afraid to go much higher than that.
My molds are pretty much the same as those described on the site. But to fit your press you should make them a lot shorter. Make your molds first and do a practice layup (with the ski materials and bladder) in your press. Then if it doesn't fit go ahead and make your press taller.
- Kam S Leang (aka Little Kam)
@Bambi: The force created by the bladder will not be devided over the top and bottom piece. For example when the bladder is at 5 bar, the force created by this pressure will work at the top AND at the bottom (just like a pressure vessel, in which there is the same pressure working at all sides). So the total force trying to break apart the press (the rods) is 2 x length x width x pressure. So the force per rod will be found by deviding this force through the number of rods. Just like I did. Right?
Buuk
Buuk
Buuk,
As you said to Zack - all this is in the interest of helping and I hope that people take it as such
My view is that the pressure is not 'divided', but it has an equal and opposite reaction on each half of the press. Your pressure vessel analogy is a good one:
For the hoop stress in a long tubular pressure vessel the area over which you would calcullate the force would be the diameter of the pressure vessel (equivalent to the area of the press). so the force would be:
Pressure * Diameter * Length (not multiplied by 2 for the top and bottom)
The stress in the wall is therefore going to be:
Force / Wall crossectional area Area = Pressure * Diameter/ Wall thickness (the lengths cancel)
Or in the case of the example in the link below Pressure * 2 * Radius / Wall thickness
http://www.efunda.com/formulae/solid_me ... vessel.cfm
Likewise for the stess along the length the area would be given by the formula:
Area = Pi * Radus^2
Giving the final Stress of: Pressure * Radius / 2 * Wall thickness
With your calcullations I think that you would get double the stress for these two examples.
Having said that, I hate to be suggesting that people have more room for safety than they think. There are lots of other factors that can trip you up - for example these calcullations all assume a nice even distribution of the load over each tie rod. In practice the load will be higher in some rods because they are slightly shorter, and there will be local stress concentrations as well due to sharp geometries. All these factors mean that I would target a much greater safety factor than 80% of yield based on a calcullation like this.
The other area that could make everything a bit safer would be to inflate the bladder with water, thus elimating the large resevior of compressed air. I toyed with the idea of a bladder within a bladder - un pressurized water to fill the main bladder and then a small compressed air baloon to do the final pressureising. Unfortuantely I think that you would need to empty the water out of the hose each time you used it which is less than practical. If anyone has any ideas of ways to make this practical I would love to hear them.
Anyway good luck one and all - big saftey factors are a good idea.
B.
As you said to Zack - all this is in the interest of helping and I hope that people take it as such
My view is that the pressure is not 'divided', but it has an equal and opposite reaction on each half of the press. Your pressure vessel analogy is a good one:
For the hoop stress in a long tubular pressure vessel the area over which you would calcullate the force would be the diameter of the pressure vessel (equivalent to the area of the press). so the force would be:
Pressure * Diameter * Length (not multiplied by 2 for the top and bottom)
The stress in the wall is therefore going to be:
Force / Wall crossectional area Area = Pressure * Diameter/ Wall thickness (the lengths cancel)
Or in the case of the example in the link below Pressure * 2 * Radius / Wall thickness
http://www.efunda.com/formulae/solid_me ... vessel.cfm
Likewise for the stess along the length the area would be given by the formula:
Area = Pi * Radus^2
Giving the final Stress of: Pressure * Radius / 2 * Wall thickness
With your calcullations I think that you would get double the stress for these two examples.
Having said that, I hate to be suggesting that people have more room for safety than they think. There are lots of other factors that can trip you up - for example these calcullations all assume a nice even distribution of the load over each tie rod. In practice the load will be higher in some rods because they are slightly shorter, and there will be local stress concentrations as well due to sharp geometries. All these factors mean that I would target a much greater safety factor than 80% of yield based on a calcullation like this.
The other area that could make everything a bit safer would be to inflate the bladder with water, thus elimating the large resevior of compressed air. I toyed with the idea of a bladder within a bladder - un pressurized water to fill the main bladder and then a small compressed air baloon to do the final pressureising. Unfortuantely I think that you would need to empty the water out of the hose each time you used it which is less than practical. If anyone has any ideas of ways to make this practical I would love to hear them.
Anyway good luck one and all - big saftey factors are a good idea.
B.
Just did a FEM analysis of Zachjowi's press at 90 PSI.
Please check the link below for more info!
http://www.groene-engel.com/Buuk/Skibui ... alysis.pdf
Buuk
Please check the link below for more info!
http://www.groene-engel.com/Buuk/Skibui ... alysis.pdf
Buuk
Bambi and Buuk:
it would be nice if you could write up your work, especially the calculations and the FEA results, in an article format and submit it to me. i'd like to post it on the main "articles" section of SB.com. it's very interesting and useful information.
again, let me re-emphasize Little Kam's comment about safety. BE EXTRA CAREFUL!! a pneumatic press can seriously hurt, and even KILL, you! this point is even stressed in our About page:
http://www.skibuilders.com/about.shtml
anyway, consider a large factor of safety as mentioned...
it would be nice if you could write up your work, especially the calculations and the FEA results, in an article format and submit it to me. i'd like to post it on the main "articles" section of SB.com. it's very interesting and useful information.
again, let me re-emphasize Little Kam's comment about safety. BE EXTRA CAREFUL!! a pneumatic press can seriously hurt, and even KILL, you! this point is even stressed in our About page:
http://www.skibuilders.com/about.shtml
anyway, consider a large factor of safety as mentioned...
so according to that what is happening? what can I max it out at before breaking it? I will most likely max it out at 45PSI I thinkBuuk wrote:Just did a FEM analysis of Zachjowi's press at 90 PSI.
Please check the link below for more info!
http://www.groene-engel.com/Buuk/Skibui ... alysis.pdf
Buuk