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Hybrid Metal bolts

Use the advantages of two metals
  [vote for,

So as it was a pleasant and sunny weekend, I chose to spend some time unbolting things from my motorcycle only to reverse the whole process later on, with a variety of expensive fluids replaced. In general I recommend it, it satisfies the work ethic like science does, only there is a good chance that the whole endeavor will actually work.

Anyhow, when you take old bolts out, it's good practice to replace them with new ones if they're subjected to a lot of stress. So I ordered a load of new bolts, and because I was being fancy I added 3-4 titanium ones. Now, taking bolts out I found many dull steel ones. I also noticed they weighed quite a lot and were tedious, uninspiring objects. The new ones were better, I had a few stainless steel examples for spots prone to rust, and the titanium ones were a lovely color, and noticeably lighter.

Fast forward to the end, fluids have been replaced, different colors of grease and threadlock have been smeared in hopefully the correct places. Last bolt in is a titanium one, and, oh. Unaware of my obviously tremendous strength I destroyed the bolt. The point of titanium bolts is to extract money from the gullible, but as a by-product they save mass & weight. But, steel is way tougher. Hmm, bolts only need toughness/strength and things in a few places, the threads, the part that interacts with the tool, the mating surface of the head and a little bit behind the threads. The middle of the bolt does very little.

Now, hollow bolts exist and that's fine, but we can take it further. Suppose we remove really quite a lot of the center of the bolt. It's now a thin walled threaded tube. If you make it really thin, then it will be subject to the "coke can" effect, the bolt may partially collapse on itself. So we support the tube walls, but not with more steel. This is largely a compression load, so use a lighter material. We have established that titanium is essentially a con, lets go with aluminium. So, take a steel bolt, remove much of the center of the shaft leaving the threads and thin walled tube, driving surface and head section etc, then pop in an aluminium tube. I reckon a nicely reamed inside of bolt, polished aluminium then cryofit one inside the other.

All the advantages of steel, lower weight and none of that stupid stripping or galvanic welding of titanium- aluminium interfaces*. Downsides: bolts are now multi component examples of precision engineering and as such will cost 4x what titanium does.

*I believe that even caught out the F22 maintenance people.

bs0u0155, Apr 25 2016

US 20100047034 http://www.google.c...tents/US20100047034
Hybrid composite-metal male fastener [xaviergisz, Apr 25 2016]

Comparison of Grade 5 & 8 Steel vs 7075 Aluminum vs Grade 5 Titanium http://www.tikore.com/titanium-facts/
Sheer and other strength [CraigD, Apr 26 2016]

Expensive wheel lug bolts http://www.tikore.c...2x1-5-conical-seat/
...with an option for laser engraving [CraigD, Apr 26 2016]

There are MANY steels..http://engineering.uprm.edu/inme/vgoyal/inme4011/Online_inme4011/Topic2_MaterialSelection/TablesMaterialSelection.pdf http://engineering....terialSelection.pdf
[bs0u0155, Apr 27 2016]


       //This is largely a compression load, so use a lighter material// That would tensile compression, then?
MaxwellBuchanan, Apr 25 2016

       //That would tensile compression, then?//   

       That's all a matter of how the bolt is loaded. So it varies. The point is that the vast majority of the load at rest, is induced by the clamping force, and it's carried from the underside of the head and in the surface under the threads.. very little in the middle. There can be many other stresses. I suppose i should clarify that the cryofitted aluminium is there to resist any compressive forces on the tube structure as a whole which may hasten its catastrophic failure. You can demonstrate this by getting someone to stand on a Coke can, then dent the side with a ruler or something. The can collapses and the vict...volunteer is sent crashing to the ground. If you filled the can with expanding foam, or even Coke, that won't happen.
bs0u0155, Apr 25 2016

       //highly compressed lighter-than-air gas.//   

       Well, air is 1.29 g/l Helium, and you want helium to prevent any hydrogen embrittlement issues, is 0.179 g/l. So you'd be able to cram 7 atmospheres of pressure in there for the weight of air. But it would still be further compressible, and would be exerting a constant load. You could avoid that by using a liquid, but then you have no resistance to deformations with a constant volume. Both liquid and gas can have pretty big changes with temperature, especially if the liquid boils. Hence why the solid is a better choice here. Especially if the solid is actually not bad at being a structural component.   

       Because the volumes are tiny, I thought helium wouldn't be much of an advantage. But there are other larger volumes, I went and checked, noone seems to have tried a pressurized helium crankcase. I'm very surprised. Obviously helium would vent over time, but for racing, I'd have thought that having a low density gas would be helpful for all those fast moving components, an inert gas might help prevent any oxidation and a little pressure might stop some blow-by... can't find any references though. Wish I knew an engine designer.
bs0u0155, Apr 25 2016

       //This is largely a compression load, so use a lighter material//   

       If you're looking for something totally lightweight yet resilient and still somehow extremely difficult to crush, how about Hillary Clinton ?
8th of 7, Apr 25 2016

       She's too expensive...
bs0u0155, Apr 25 2016

       No, you can't afford her. That's not the same thing.
8th of 7, Apr 25 2016

       But why do you want lighter bolts on your motorbike?
hippo, Apr 26 2016

       He doesn't, he wants them for his F-22.
8th of 7, Apr 26 2016

       From brief surfing of ye olde internet (see links), replacing steel bolts with titanium looks to me to be mostly an expensive vanity conceit – for example, a way to reduce the mass of a sports car’s wheels by about 2.5 lbs by replacing its original lug nuts with a $750 set (16 or 20) of titanium ones (and for $50 more, get your initials laser engraved on the heads of 1 bolt per wheel).   

       I saw the sheer strength (which is what’s exceeded when you break a bolt by twisting its head) listed as 79,800 psi for titanium, vs 48,000 for aluminum, 72,000 for grade 5 steel (the cheapest/weakest machine bold steel), and 90,000 for grade 8 (the best), so don’t blame breaking a titanium bolt on the bolt – just learn to control your manly might, or use a torque wrench.   

       The idea still gets my bun, ‘cause drilling and filling steel bolt with aluminum sounds like time-consuming but low-budget fun, and as long as you don’t get too carried away, won’t hurt anything. Bolts connections are almost always way over-engineered, so as long as you don’t over or under tighten them (so again, the torque wrench), their strength isn’t a big deal.   

       I’d keep some original and some drilled and filled bolts around to hand to friends and brag about how much lighter they are. A more enterprising person would make a ton of ‘em and sell them to people who’ll pay $750 for a set of titanium ones.
CraigD, Apr 26 2016

       There's a certain kind of cyclist who has a weakness for this weight-saving logic. They replace crucial tiny components on their bike, like bolts, spoke nipples, allen nuts, etc., with titanium equivalents to save a few grams, while ignoring the fact that they would benefit from losing a couple of kilos of fat.
hippo, Apr 26 2016

       /they would benefit from losing a couple of kilos of fat/. At least these folks own a bike. Replacing the castor wheels on my desk chair with a beryllium - boron composite has not reduced the load on those wheels.   

       Back to the main premise: that titanium is weaker than steel. CraigD's numbers say no and my bedtime perusals of the Materials Handbook left me with that impression too. Titanium is strong. I thought the downside of titanium was the expense of refining it - much like aluminum 60 years ago. If it were cheaper to refine it would give steel some real competition.
bungston, Apr 26 2016

       I thought it was more expensive because it was more difficult to work with (i.e to weld, cast, bend, etc.)
hippo, Apr 26 2016

       Iron has the rare property that the oxide has a lower melting point than the parent metal; that's what makes welding easy and steel so ubiquitous.   

       Titanium has to be welded and then cooled in an inert atmosphere. it's even more temperamental than Aluminium. Any time it's a liquid, oxygen has to be excluded.   

       So it's not so much the cost of refination (which is high) but the difficulty and expense of working it, plus the fact that fabrication and repair outside a specially equipped facility is nearly impossible - that, and the fact that outside military and aerospace, existing steels and other alloys are usually adequate to the task.   

       As [hippo] points out, in terrestrial and marine applications the weight saving might be better looked for elsewhere.
8th of 7, Apr 26 2016

       8th: Why does low melting point of oxide make welding easier? Cooled welds just look like the parent metal to me. How does oxide get involved in welding.
bungston, Apr 26 2016

       [CraigD], shear strength, or sheer strength?
normzone, Apr 26 2016

       //Why does low melting point of oxide make welding easier? // There's always an oxide layer on any metal (except noble metals, I guess). It has to be broken down for welding to work. So, a low-melting oxide surface makes for easier welding.
MaxwellBuchanan, Apr 26 2016

       Damn, it's sexy when you talk like that...
normzone, Apr 26 2016

       It is, isn't it ?   

       The oxide liquefies and floats on the surface of the metal; if it didn't, it would get included in the final weld and greatly reduce its strength.   

       With Aluminium, the surfaces have to be abrasively cleaned, then fluxed, then either TIG welded (inert gas) or oxy-acetlylene welded with a strongly reducing gas mixture. The fluxes are highly corrosive so meticulous cleaning is needed when welding is complete.   

       With steel, minimal cleaning is needed, and flux isn't required at all.   

       Noble metals do have surface oxidation, but the layer is very thin and mechanically weak, to the point it can just be wiped away. Stainless steel forms an oxide layer, but again it's very thin and very passive, so it doesn't penetrate below the surface.
8th of 7, Apr 26 2016

       //Damn, it's sexy when you talk like that...// Steady on, [norm], you're a marred man.
MaxwellBuchanan, Apr 26 2016

       //that titanium is weaker than steel. CraigD's numbers say no and my bedtime perusals of the Materials Handbook left me with that impression too. Titanium is strong.//   

       There are, it's fair to say, a LOT of steels. Titaniums... titania? are more limited because of the expense of the base material, why would you make the nasty stuff if the alloying is the cheap part? I think that the link is being a little misleading. They are, after all in the business of selling expensive titanium. They list the yield/tensile strength of "grade 8" steel as 130,000/150,000 psi which looks almost the same as "Tikore's preferred material" 6AL4V. But this isn't fair. Compare to 4340 Chro Mo steel <link> and it's up against 287000/270,000 psi. This is a much fairer comparison. 4340 is an expensive steel, but that's like expensive pilsner to titanium's high end wine list. You can put together a whole bike frame for a $200, titanium is 10 fold more. Scroll down in the link and steel still kicks ass in the strength to weight contest. But this is 4340. 4340 rusts, Ti doesn't. Now you're comparing to stainless steels, here Ti starts to look pretty good. Corrosion, strength, strength to weight...   

       So yeah, the basic premise, steel is stronger/tougher/harder than titanium... true, just pick the right one. Although for bolts, you may not actually want that. Drilling out stainless is difficult, I imagine 4340 would yield at roughly 1:1 with most drill bits.
bs0u0155, Apr 27 2016

       ... very, very thinly...
8th of 7, Apr 27 2016

       I wonder if you could start with a titanium bolt and then do something like 3d print or sputter deposit steel onto the outside: the threads etc.   

       I have pondered this same thing for a sword (not that anyone needs a functional sword): could one make a sword out of some light, flexible alloy then deposit a thin strip of hard brittle steel along the edge? Maybe easier here to just lay the thin hard strip alongside the springy one while hot.   

       Maybe same for the bolt: wrap the light bolt with a steel strip while both are hot.
bungston, Apr 27 2016

       If we could 3D print the graphene on, then we're in pseudoengineering buzzword paradise.... lets get a kickstarter going!! Once I've got the money, I'll just mail out a bunch of bolts with a bit of aluminium paint on the bottom, we'll be rich-ish.   

       //start with a titanium bolt and then do something like//   

       Start with a 2 metal tube, make the threads the conventional way. I'm not entirely sure how the heads get made.
bs0u0155, Apr 27 2016

       You had the venture capital on the hook, right up until the ukulele started playing.
bungston, Apr 27 2016

       At least it's not a banjo.
8th of 7, Apr 27 2016

       I wish it were an accordion.
normzone, Apr 27 2016

       Here [norm], just step up onto this large stack of gasoline-soaked bagpipes ...
8th of 7, Apr 28 2016


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