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Originally Posted by TheModMaster8
I would have to disagree with you on this portion as in every building that bares weight or hold things together is either I beam or [, etc. never have I seen a square metal tube used for holding up a floor or a roof or a a building, why? because it's unnecessary weight when an I beam can bare nearly the same load. also if you have ever seen a tower-crane, they do not use a solid tube rather they use scaled up version of sheetmetal that uses webbing to give it it's strength/rigidity and it's light weight ( for heavy machinery at least) this principle also applies to sheet metal as well. "available in a more weight-efficient manner than sheet metal does," from this i'm gathering that you are saying tubing is lighter then sheet metal? (correct me if I'm wrong on this) if you are indeed saying this then i would have to disagree with you as our robot using extruded tubing this year/every year has a much heaver weight to it, then sheet metal frame (according to CAD)
as for ease of fabricating, I would agree with you in saying that it is much more time consuming if you don't have a laser/water cutter or CNC machine, and even if you do it still would take much more time then extruded tubing, that much i do not disagree with you on.
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You might want to ask your father why he uses I-beam and channel and if he (an other Civil Engineers and Architects) uses other profiles. The industrial buildings that I work in (manufacturer of large electrical equipment) have all sorts of square, rectangular and round steel tube, as well as I-beams, being used as pillars to support the roof structures. This complex of buildings has been expanded at least 4-6 times over the last 30 years.
You can only compare the weight of your robot built from tubing to a sheet metal one designed to same specification if both designs have been optimized properly for the stresses that they will experience and to minimize weight. I suspect that this optimization exercise is beyond the capabilities of most FRC teams since it would involve using tools such as finite element analysis and a very thorough modeling of the stresses that will be experienced by the structure. While you know your robot made from tubing was overweight, you do not know if your sheet metal CAD design is strong enough since, I presume, it was never built.
Quote:
Originally Posted by Adrian Clark
False. I beams in this application are used because they are easier to produce than hollow profiles. It has to do with the extrusion process, a hollow tube requires an extra die for the middle section. Even disregarding price, square or rectangular tubing is much stronger. There is more material, and it's on the periphery instead of the middle yielding a much stronger beam.
-Adrian
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The "strength" of a square, rectangular or round tube is also different from an open profile such as an I-beam or channel. My empirical experience has been that the tubes, in general, resist torque much better than the open profiles. This characteristic may be more important in FRC robots than the ability of a particular profile to support a static load. Perhaps someone with the appropriate background can offer their comments (I am just an EE but I have had to deal with mechanical issues a number of times over the last 30+ years).