karl mentioned on my last post that he thought aluminium would outperform carbon for gantries. I produced the above graph comparing the strength and cost of tubes of carbon, stainless, aluminium and titanium. The catch is only readily available sections were used. The metal tubes were sourced from http://www.onlinemetals.com and carbon came from http://www.carbonfibreexpress.com/.
Each type of tube has two columns. The first is the buckling strength of a length 550mm long (I mentioned in an earlier post that the minimum value for this parameter is 350kg). The second column is the cost of 2475mm of tubes (approx what you need for a gantry). The bottom axis is the weight of the 550mm long tube.
Filament wound carbon tube is the clear winner from the list. The tube pultrusion is next. I am sure I was told pultrusion has off axis material however I think for the marginal cost saving I would still go with the filament wound carbon - a little more robust.
First aluminium comes in at 50% more weight, but that only has a 0.9mm wall. You would probably only be able to glue such tubes together into sockets as the wall is so thin. However it is CHEAP!!! $25 for all the tubes you require. And just as strong as the carbon.
I always struggle to find an application for titanium (outside of high temperature/corrosive environments). This shows it has very similar strength and cost as the carbon, however its high density (4.51g/cc) makes it twice the weight of carbon, even with 50% less wall thickness. Stainless doesn't even get a look in before 3x weight. Rather cheap tho.
I am a little disappointed to see how far along the list my gantry is. But I am told bladerider uses 20x1.6 aluminium so I am slightly lighter than them. If anyone else wants to divulge what they use I will mark them on the graph above too.
Hey cool analysis. Would submit that joining the carbon tubes adds some weight if not done carefully, but the reasons for shifting to carbon in MTB and road frames seem clear!
ReplyDeletewhat about plain shteeel?
ReplyDeleteOf all the people to do a material selection, you're the one I would trust (what was it, 110%?), but which method was used to come to your conclusions?
i scored 135 out of 100 for my material selection assignment...
ReplyDeleteThe first column effectively looks for the best EI/weight ratio as the structure is limited by elastic buckling. Second column looks for the best $/weight ratio, but this is more for interest than anything else, the material costs are relatively cheap.
mild steel has the same elastic modulus as stainless steel. For the same tube sizes would be the same.
karl, certainly joining the tubes will add weight however I would not expect it to be any more than 10% of the gantry weight.
Total length of gantry tubes: Approx 3025mm
Num of Connections: 12
Length of overlaps: 30mm
Length of tubes + overlaps = 3025+12*30 = 3385
3385 / 3025 = 12% (i was close...)
My aluminium gantry uses rather heavy connections too(see earlier post).
hey
ReplyDeletegreat stuf!
i reckon i'd go for the 15mm pultrusion for the cost saving over filament wound stuf. did phil stevo recently build a gantry from it?
i think i used about 22 x 2mm wound tube, it was overkill and heavy.
also the Mach 2 gantry has some nice joiners for carbon tubes that could be little bladder moulded parts, pretty easy, pretty self-aligning and very light, minimum sticky mess.
i checked thru some old communication from Carbon Fibre Express and apparently the pultrusions have 20% off axis (somehow) in which case it is probably up to the job.
ReplyDeleteNice little study. Although you’re about 10% lighter than the BR example, it looks as though the BR one will be about 50% stiffer than yours, plus a little cheaper, which could well be a better design. However, I don’t think I could bring myself to build an ally one anyway so I should only really look at the carbon ones, as long as your careful and think about what your doing then you wont get too sticky or add much weight to the connections. I think the buckling length could be significantly reduced by being a little bit ingenious, but this may not be that much use as you may only reduce the overall stiffness of the gantry.
ReplyDeleteThen there's the welding trashing all the properties, work hardening and corrosion factors to add on to alloy.
ReplyDeleteOr a moulded carbon one that takes 4hrs to make finished with less drag ;) hopefully last forever. Guess we’ll see at the worlds!
Have you done a load study on the wing bars fwd strut and side struts with a 24:1 vang? The are some serious loads going around the place.
The welding is only at the ends, where buckling is of little concern. Plus the significant properties that affect buckling aren't really affect by welding, E and I.
ReplyDeleteDetermining which is better is tricky. Ultimately it is based on the influence of the design on the race course...I wouldn't want to model or experiment with that for a pissy little gantry. Lighter is better works for some ppl, stiffer may work for others.
The same method of construction for the aluminium one could be used for carbon with a nice bit of vacuum bagging work.
Work hardening is a phenomenon in metals undergoing plastic deformation, which gantries should not be experiencing in normal operation. Ignoring corrosion correction and replacing with an inspection regime is common for light weight performance aluminium applications. Or just replace when it breaks like most things in sailing.
Dude you forgot fiberglass tube. Bill and Gui both used it and it works fine, and is like 0.1x the cost of carbon.
ReplyDeleteI don't know where to get it from. If you can advise a supplier I will include it in the graph and email to you.
ReplyDelete