attaching a structure to carbon tube

[YateHaugan1]

Hi everyone,

I'm working on a seat design that uses 10cm (O.D) aluminium tubes as supports, to which my main seat structure is clamped.

There are an increasing number of carbon fibre alternatives available now and I was wonder how one would approach mounting to these instead. I presume some kind of secondary load-spreader would be required? - like a metal sheath to the carbon tube where the main structure clamps to it.

I've not been able to find any examples of similar structure and the engineer's response to the issue.

cheers

 

[GregLocock]

10 cm? Wow.

Yes, the design of interfaces between components and carbon fibre tubes is complex and poorly documented, so far as I know.

One way of looking at it is that you have to take the known loads in the bolt into the layup in such a way that the fibres are loaded in tension, whilst acknowledging that the stiffnesses need to match, and that stress raisers are a bad idea.

In my experience aerospace were routinely getting that wrong 20 years ago, and automotive 30 years ago. Perhaps things have improved since then.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[rb1957]

do you mean you want to replace Al tubes with Carbon ? I'd be surprised if the cost was worth it.

like Greg ... 10cm ?? 10mm maybe.

you want to attach a seat pan to a tubular frame ? Most seats use machined frames.

The problem with Carbon and Aluminium is corrosion, so you have to isolate the two. Cover the Carbon with a layer of glass cloth, and/or uses Ti for interface fittings (even more expense).

Not QED (Quite Easily Done).

another day in paradise, or is paradise one day closer ?

 

[YateHaugan1]

thanks both for replies,

the application is more weight critical than cost.

the tubes are actually 60mm, not 100.

 

[rb1957]

wow that's a large tube for a seat !

if you're chasing small weight improvements, then a machined frame will be lighter, and adapt the the seat pan better

another day in paradise, or is paradise one day closer ?

 

[RPstress]

Crushing and other energy absorption of seat tubes and other compression loadpaths under crash loads is usually fundamental to seat design. Carbon structures need very careful design to absorb similar energy. Simple mechanical fastening of carbon tubes is possible but may not be optimal. Usually locally thickening the carbon tubes is sensible and not too difficult but without this load spreaders and similar will help, albeit with some weight penalties. How are the current structures "clamped"together (you imply you'd like to maintain this design)?