twizzstyle wrote:Have you ran the numbers? Pressure is a dangerous thing. Yes the bolts are going through 4 holes, but there is ONE shear point - in between the two bars. That's where they would fail.
Hose pressure times surface area, divided by the number of bolts (16? or is the back part welded)
He does a grade 5 vs 8 1/4" shear calculation, and there is a table for a bunch of fasteners. Long story short, use grade 8 over 5 if using bolts.
Grade 8 might "snap" during a shear failure, but if it gives you an extra 1/3 ton capacity and brings your factor of safety (F.S.) into the correct range then who cares? For a press you want at least a F.S. of 2, or the press should be capable of holding 2x the amount of pressure that you intend on running at.
regardless of the increased force a grade 8 bolt can withstand they shouldn't be used for our application. If the pins in the above pics had been grade 8 somebody could have died.
The press should be designed so that grade 5 bolts are already overkill.
If using grade 8 and another part of the press fails you have now created the possibility of chunks of hardened steel penetrating your skull. Yeah, that would be rad.
I just prefer things that fail to bend, rather than snap, and I was taught that grade 8 bolts had limited uses because of the hardness of the steel.
This however suggest otherwise, and is a good read on bolt strengths.
Well we wrapped the press in 3 20,000lb straps and pressed a pair of skis at 40lbs after dry testing it the day before using just the bolts, uncomfortable amount of deflection then. Well the press failed not only in how much it bends but the lack of support it gave the mold caused the mold to concave and twist the skis. I'm back to square one, mold is done for and The press is good for scraps at this point, its all bent and was too weak to start with.
I really want to thank all of you tough for your advice, I'm definitely gonna take some more time to get the next press good on paper before I actually build it instead of just assuming. I wanna go all the way with the next one, by that I mean cattrack and maybe even I-beams.
Watch craigslist in your area for steel under the "materials" section. Used steel in my area goes for 1/2 of new and a lot of the guys who list on there have the ability to cut to your lengths. Many smaller metal fab companies have scrap beams you can pick through.
Underdog wrote:
I really want to thank all of you tough for your advice, I'm definitely gonna take some more time to get the next press good on paper before I actually build it instead of just assuming. I wanna go all the way with the next one, by that I mean cattrack and maybe even I-beams.
Lets not forget the forces you are dealing with. Guesswork will kill you. I trust my design and construction of my press 100% and its way over built, but it still scares the crap out of me every time I press a ski.
I always say the best lessons are learned by mistakes though, so building this thing wasn't a complete waste for you. It was just really really dangerous
