This is a bit preliminary but:
I was doing some calculations to work out how big the I beams should be for my press and surprised myself at how much movement you get with a beam. I could not find actual data on I beam stiffness off the web (rather strange) so I did some rough calcs from 1st principles.
What I found was that even a 10 inch deep beam will moveby a few millimeters assuming that you are using a pressure of order 50PSI.
So this leads to 3 questions:
1) does anyone have any sorces on actual data for beam stiffness?
2) what size beams are people typically using?
3) I have noticed a lot of talk about loss of camber. Could it be that if people are not using really stiff beams that the mould is bending with the load and effectively reducing the camber in the mould? (this is only really relavent for th ebottom beam as the top beam is compensated for by the bladder) By my calcullations if you were running a press at 50PSI with an 8 inch I beam you would probably get about 5mm of deflection so you coul expect a mould with 10mm camber to yield skis with only half that. Just a thought.
B.
I beam stiffness
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Re: I beam stiffness
You really need to do an analysis on a whole structure rather than just an I-beam.Bambi wrote:This is a bit preliminary but:
I was doing some calculations to work out how big the I beams should be for my press and surprised myself at how much movement you get with a beam. I could not find actual data on I beam stiffness off the web (rather strange) so I did some rough calcs from 1st principles.
What I found was that even a 10 inch deep beam will moveby a few millimeters assuming that you are using a pressure of order 50PSI.
So this leads to 3 questions:
1) does anyone have any sorces on actual data for beam stiffness?
2) what size beams are people typically using?
3) I have noticed a lot of talk about loss of camber. Could it be that if people are not using really stiff beams that the mould is bending with the load and effectively reducing the camber in the mould? (this is only really relavent for th ebottom beam as the top beam is compensated for by the bladder) By my calcullations if you were running a press at 50PSI with an 8 inch I beam you would probably get about 5mm of deflection so you coul expect a mould with 10mm camber to yield skis with only half that. Just a thought.
B.
I get no deflection with 12" U-channel. Sounds like you are an engineer... if so and you really want an optimized design, there are some quick simple FEA programs out there.
You probably need to find a copy of the "Manual of Steel Construction" by AISC. I dont think they publish the beam tables on the web because they want people to buy their book but you might be able to find a hard copy at a library.
If you have calculated the moment of inertia (I) that you want I can look up some beam sizes for you.
Keep in mind that any beam, no matter how big will deflect under load. It is just a question of how much compared to how much is acceptable.
If you have calculated the moment of inertia (I) that you want I can look up some beam sizes for you.
Keep in mind that any beam, no matter how big will deflect under load. It is just a question of how much compared to how much is acceptable.
With most frame designs you not going to be simply loading a beam. The stresses will be spread out through the whole structure.
Rich is right...there is always deflection. In my previous post I stated that i get no deflection. I guess this isn't really true however, the deflection i get is so small that it can be neglected when is comes to our purposes.
My advice...find someone with Solid works...draw up a frame and do a quick FEA or...just build it a little stronger than you think you should. For our purposes a little deflection doesn't matter. An MDF mold probably compresses more than a well-built frame would deflect.
Rich is right...there is always deflection. In my previous post I stated that i get no deflection. I guess this isn't really true however, the deflection i get is so small that it can be neglected when is comes to our purposes.
My advice...find someone with Solid works...draw up a frame and do a quick FEA or...just build it a little stronger than you think you should. For our purposes a little deflection doesn't matter. An MDF mold probably compresses more than a well-built frame would deflect.
First off, as mentioned earlier, how are you loading the beam in your calcs?
Secondly, are you calculating the beam movement assuming it is fixed at each end with no support?
My suggestion is go grab any mechanics of materials book and just do a simple hand calc. using the analysis of beams under constant load per length. It should take 15 minutes (if that) to do all the necessary analysis as most books have all of the size/inertia/strength/area properties in tables. In my experience using aluminum I beams at work our defelction has always been negligible. Steel I beams are pretty ridiculously strong when being used in the ski press application. I would pay more atention to quality welds or fastners when determining the safety of the press. --Geoff
Secondly, are you calculating the beam movement assuming it is fixed at each end with no support?
My suggestion is go grab any mechanics of materials book and just do a simple hand calc. using the analysis of beams under constant load per length. It should take 15 minutes (if that) to do all the necessary analysis as most books have all of the size/inertia/strength/area properties in tables. In my experience using aluminum I beams at work our defelction has always been negligible. Steel I beams are pretty ridiculously strong when being used in the ski press application. I would pay more atention to quality welds or fastners when determining the safety of the press. --Geoff
Thanks for the pointer on the steel book.
As for the analysis - I am aware that I need to look at the whole structure for an acccurate view - I was really just doing a calculation based on the concept of having the beams supported at ether end so that I could 'side load' the mould and skis. In this situation the deflection is predominantly a function of the beam bending and the clamping arrangement at the ends. I was really just a little surprised at the deflection that I was seeing and having seen other presses based on this kind of approach I was wondering what people were seeing in practice.
I have access to Solidworks so I will do a more thorough analysis once I have settled on a design.
Thanks for all the advice - once I get somewhere I will share some photos!
B.
As for the analysis - I am aware that I need to look at the whole structure for an acccurate view - I was really just doing a calculation based on the concept of having the beams supported at ether end so that I could 'side load' the mould and skis. In this situation the deflection is predominantly a function of the beam bending and the clamping arrangement at the ends. I was really just a little surprised at the deflection that I was seeing and having seen other presses based on this kind of approach I was wondering what people were seeing in practice.
I have access to Solidworks so I will do a more thorough analysis once I have settled on a design.
Thanks for all the advice - once I get somewhere I will share some photos!
B.