Our ski press design

[woodi]

I just drew up our press design in proe. I think it should be pretty solid. Our biggest concern is that the two ibeams on top are a slightly different thicknesses the one is .25" the other .375" so the one might difflect differently and cause uneven pressure. What do guys think?

[Enduro_Skiin]

If the I-Beam webs are proportionally that large and they are steel, it will weigh a ton, but they aren't going anywhere with a mere 50 psi. I bet they would be solid at twice that.

I recommend using grade 5 bolts on any bolted connections.

[Class5]

I have an ibeam press very similar in design to the one you picture above. My beams are w10x49, so they are very heavy, but I was worried about underbuilding the thing. I press at 60psi in the hoses. I noticed very little deflection of the beams upwards, although I haven't measured to see how much there is. I have however noticed that the pairs of beams will separate from each other by as much as 5-7mm in the middle. There is a lot of pressure there, so in the end I am glad that I used such heavy beams.

It sounds like you must be getting a deal on the beams if you are considering this option, which is good because they are f#*king expensive. In your picture it looks as if the beams on the top are the same and the beams on the bottom are the same. If you are using a pair of identical beams for the top and another pair of beams for the bottom I wouldn't be worried. If you are mismatching your pairs (ie using two different beams side by side) then they will deflect differently, although probably not that much because it sounds like you are using heavy beams. I would watch the seperation in the middle because that seems to be the spot with the most visual movement. Perhaps a support holding the two beams together would also help?

[tonyt]

What do guys think?

Are your lower beams really as small in relation to the top beams as shown in your drawing?

Keep in mind that the load on the lower beams is identical to the upper beams so the lower beams will deflect much more than the top. Unless your mold is really strong then the deflection of your lower beams will tend to flatten out your camber. If possible you should try and get 4 similar beams for the frame unless you think the lower ones are strong enough as they are. If you tell me the pressure you will work at and the exact size of the lower beams I can check the deflection and stress for you.

Tony

[G]

My preference would be to design the top and bottom beam sections eseentially the same as they are both going to see identical forces. Your bottom T sections look like they are going to deflect a lot more than the top I beams. Depending on sizes this may be a moot point.

I also like the idea of tying the beams together to avoid the deflection differences you will see with the 2 different web and flange thicknesses.

[woodi]

We would love to have everything be unifrom but were working with free steel we got here at school and what we got is what we find. I like the idea of tying the top two beams together to try and even things out. The T's we we're planning on making from one big ibeam 16x7X.5 making them 8X7X.5 or about that. There is one more beam we could use which is quite substantially larger but I don't think we could move it to get it into the shop to cut it.

[tonyt]

The T's we we're planning on making from one big ibeam 16x7X.5 making them 8X7X.5 or about that.

I checked your lower beams with 50psi and assumed that your frame was 6ft long and each lower beam was loaded up by a fire hose pressure vessel about 6" wide, you will get 2mm or a bit less than 1/8" deflection however the stress in your lower beam is 27000psi and the yield stress of steel is about 36000psi. The reason for the high stress is the long thin vertical members of the lower frames which concentrate the tensile stress on the lower end of the beam so don't exceed 50psi.

[woodi]

Thanks for letting us know. I've been meaning to do some analysis on it but hadn't got around to it. I think we're gonna try and find some more steel for that bottom section. If we're going to make a press that big and heavy we want it to be able to withstand a lot of pressure.

[tonyt]

we want it to be able to withstand a lot of pressure.

I keep wondering why everyone goes for so much pressure, most marine type epoxies caution about too much clamp pressure and I now know that it is not a matter of forming the ski since it is completely pressed to the mold at as low as 0.6bar or about 10psi. I get plenty of squeeze out of the glue and the skis are light so I know I am not overloading with epoxy. Any good or bad experiences with lower pressure would be interesting.

[woodi]

maybe it's overkill to go with more pressure than that...I don't really know what kinda pressure is needed cause this is our first attempt at making skis. We just figured we wanted to make it able withstand 100psi so we could experiment with different pressures.

[melvs]

looks a LOT like my press, which I am getting very close to testing. I will post when I do it and we'll see if any big problem arrise.