james h
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| posted on 7/10/15 at 07:56 PM |
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Tophat or bobbin sizes for ali honeycomb panels?
Does anyone have any experience with sizing the tophats/bobbins used to mount suspension/other components on an ali honeycomb chassis?
I'm thinking its best to use a two part bobbin, fitting together through the panel and bonded. I am unsure on the diameter of the
'flange' on the outside face of the panel, and the thickness of the bobbin around the shank of the bolt. I was wondering if anyone had
come across a calculation or method to decide these dimensions?
Cheers
James
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phelpsa
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| posted on 7/10/15 at 09:39 PM |
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I presume you have a method of calculating the max load at each mounting point? A good estimate is to look at the shear strength of the skin material
(one skin) and calculate the shear perimeter required to withstand the load. Bare in mind that honeycombe panels have a few different failure modes
and it takes a bit of experience / common sense to work out which is the limiting one for your application.
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elvadelva
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| posted on 10/10/15 at 08:18 PM |
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Ditto the above,
I'm afraid the number of variables involved makes it impossible to give ballpark advice, any guidance given with out full structural analysis
is actual quite useless and should be avoided. It really is a case of "if you need to ask the question, you should not be attempting the
task"
I really don't mean that in an unhelpful way, but it sounds as though you need more than is available on a forum.
all the best Ian
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coyoteboy
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| posted on 11/11/15 at 02:47 AM |
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I design honeycomb panels fairly regularly for structural (but not safety-critical, with plenty of testing) use and I'd still be hesitant about
designing a structurally significant part with it because it takes a LOT of experience to cover all aspects.
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Theshed
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| posted on 11/11/15 at 08:26 AM |
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Many individuals have made honeycomb cars in the past. So I would not be too put off by the posts above - you are not building an aircraft! I suggest
that if in doubt you over design. There are lots of photos of 1980's cars about. Lets of ideas in "Building Motorcycle Engined Racing
Cars" and in Alan Staniforth's book - that's where I swiped my ideas to start with.
My car is folded honeycomb. Many large loads are fed in. I am not happy about everything but I am reasonably confident that if will not fail. Inserts
on my car vary. I think I have some pictures at various places in my photo archive.
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Sam_68
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| posted on 11/11/15 at 08:42 AM |
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Ditto Phelpsa and Elvadelva.
But also to add:
Apart from the shear strength of the skin material mentioned by Phelpsa, also check the strength of the adhesive (and the manufacturer's
recommendations for prep, which should be adhered to religiously).
Multiple small bobbins (with an external plate or bracket attached to spread the load between them) are usually better than a single large one.
It is VERY important to design mountings in such a way that the loads are fed into the skins in near-as-damn-it pure shear; so, for example, if
you've got a load that is at an angle to the face of the panel, consider using a bracket that wraps around a corner, with bobbins on both (or
all) faces, so that the forces can be fed into the panels as 'horizontal' and 'vertical' components. Otherwise, you'll
need substantial brackets/spreader plates to keep the loads within the quite limited compressive strength of the honeycomb.
Tony Pashley's book on M/C engined race cars gives typical dimensions and numbers of bobbins required for various components, but obviously
applies to a very light single-seater. Use with caution and common sense.
... but edited for crossed post; also echo Theshed's sentiments. It's not impossible if you are careful and conservative with your design,
and give it sufficient thought. The guy who designed the early Lotus F1 monocoques and the Westfield FW400 told me that he basically specs. bobbins by
common sense and judgement, but of course he has a lot more experience than most of us of the materials involved; until you gain that level of
confidence, it would seem sensible to at least try to do the calculations based on skin shear strength.
[Edited on 11/11/15 by Sam_68]
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