twybrow
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| posted on 27/3/07 at 06:56 AM |
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Steering column extension
Can anyone see a problem with using a carbon shaft for my steering column extension? I think I could bond (and pin) the joints with the U/Js... Not
really any great need to use carbon, just a have a suitable shaft at work! Would Mr SVA be happy?
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David Jenkins
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| posted on 27/3/07 at 08:06 AM |
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Never mind Mr SVA - he probably wouldn't spot it anyway - would you be happy?
Unless you are 100% happy that the grade of carbon fibre tube is fit for purpose (e.g. is it suitable for considerable twisting forces) and that your
fixings are strong enough, you'd be far better off with good old-fashioned thick-wall steel tube, properly welded.
Entirely IMHO, of course...
David
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Mr Whippy
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| posted on 27/3/07 at 08:15 AM |
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I definitely would not use that even if it did look cool. It will be imposable to spot defects or splits in the laminate plus how are you going to
make the joint? Glue bond or hole, both methods are not acceptable for the loads it will receive and I doubt the SVA guy would be happy at all.
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nick205
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| posted on 27/3/07 at 08:17 AM |
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ditto ^^^
I don't think my SVA examiner would have spotted it to be honest.
For peace of mind I would prefer the tried and tested steel tube construction for this part of the car.
If you have access to CF materials, maybe you could use them elsewhere on the car? Some nice CF interior panelling perhaps 
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BenB
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| posted on 27/3/07 at 08:46 AM |
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I would so not do that!!!!
Steering is one thing you really don't want to go wrong (especially when the method of failure for a CF extension is likely to be sudden and
total loss of steering)...
I'm about to head out on a four hour blast (A1 here we come) and am still rather wary just because of the swivel nut coming off last time I
drove the car. Can't imagine I'd be going near the car if the steering had gone wrong at speed...
Whereas if one brake caliper fails, or even a wheel falls off you can try and control it, if you're steering column sheers you're 100% a
passenger...... Strangely, its only when you make the sudden transition from driver to passenger that you realise how much kinetic energy is built up
in a car....
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twybrow
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| posted on 27/3/07 at 09:52 AM |
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Thansk for the responses guys. Well the tubes i have in mind are composite torque tubes (driveshafts) for an aerospace application. They will be more
than a match for a steel equivalent under torsion. As was mentioned, the fastening method would be the part that concerns me. Mr Whippy, what makes
you think that an adhesive bond would be insufficient to take the torsional load (which would be relatively low compared to a driveshaft I think), and
with a pin through at both ends, even if the adhesive fails, the pin will give you a backup. I agree with the point about failure being difficult to
spot... As for the ultimate failure, well that would be more progressive than with a steel equivalent. It would not just fracture in two...! I will
probably end up going for a steel solution, but I am still tempted!
I will be splashing carbon liberally around my car (I have a huge box of reject material in my attic!) but probably not until after it has been built
and on the road.
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Mr Whippy
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| posted on 27/3/07 at 10:36 AM |
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Don't get me wrong I do like carbon but it’s not for all applications. You never mentioned that the tube was originally drive shaft but even
then is it intended for shock loading and what is the design torque??? As for bonding it, carbon is not absorbent at all and epoxy glues need a large
keyed surface, it's not that easy to do a 100% trustworthy joint. You can pin it but since its hidden away how would you ever know the bond has
failed?
Uncharted territory and on a critical safety system, I wouldn’t touch it.
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twybrow
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| posted on 27/3/07 at 11:34 AM |
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The original design failure load was 1000NM (740ft.lb). I have just spoken to one of the chaps who worked on the project - he said the failure was
catastrophic (very large bang, major fracture running around the tube) but the tube remained one piece. My point is, the composite is not the problem.
Do you think a person could even begin to stop 1000Nm through the steering wheel!?
As for the bonding, I agree you do need a reasonable keyed/prepared surface. A film adhesive should provide the level of bond required.... More
in-depth research needed!
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MikeRJ
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| posted on 27/3/07 at 11:36 AM |
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Can you effectively pin a carbon shaft like a steel one? I know very little about CF, but I though point loads on laminates were not a good idea,
hence everything being bonded.
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twybrow
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| posted on 27/3/07 at 12:01 PM |
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You are correct. Composites do not like point loads (or bearing loads). If i pinned it, it would be as a backup. There would be a clearance hole
around the pin (tight into the steel, clearance on the composite). As such there would be no load on the pinned composite area. It would just be to
make myself feel better that i had a back up!
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Mr Whippy
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| posted on 27/3/07 at 12:13 PM |
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Interesting.
Most of my carbon stuff is for my r/c heli's and its strength is quite amazing. Funnily enough one also uses a carbon drive shaft for the tail
and I only ever use carbon for the blades. The main failures though are the laminations splitting apart rather than the fibers themselves. Many of the
shafts at machined to increase the glue bond area and once together they will never come apart. So give it a go but inspect it from time to time just
to be on the safe side.
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andyace
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| posted on 27/3/07 at 12:52 PM |
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I would guess that the issue would come at SVA time if the examiner had the same suspicions as most peope on here that it would not be up to the
job.
I would guess that you would need some documented proof on the suitability of the material.
For example you could put on fuel lines that were used on formula 1 cars but if they aint got the correct number written on them then mr SVA man aint
going to pass it.
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DarrenW
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| posted on 27/3/07 at 01:11 PM |
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I dont even like the idea of DIY modified steel steering shafts, the idea of CF gives very bad vibes.
Ive worked for an OEM automotive steering system manufacurer before and seen prototype shafts fail that were made by very experience steering system
model makers. My colleague at the time got into serious trouble over this. Also being aware what tests OE systems have to go through before being
approved makes kit car modded shafts a worrying area for me.
Im sure Coozer will be along soon to back me up on this (he works for same company).
Of course im saying all this with no experience of CF and composites. Purely my own oppinion.
[Edited on 27/3/07 by DarrenW]
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twybrow
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| posted on 27/3/07 at 01:36 PM |
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I take it the blades break when you hit them into something?! You should see what happens when you fire chickens at them at 350mph!
I work in the composite development department of a composite propeller manufacturer.... If you pay me lots of money, I am sure we can stop your
blades delaminating!
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Mr Whippy
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| posted on 27/3/07 at 03:04 PM |
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Yeah the blades do break sometimes (though I don't crash very often now... )
The tips by the way can break the sound barrier! sounds like a machine gun 
The rule is though that if they have had a scud and even if they appear undamaged it's bin time. In one of the mags a test was done on blades
like that and it was found using ultrasound or something that cracks had formed under the surface that were invisible otherwise. So hence the
rule...
Didn't Airbus have a similar problem with their airliners loosing their carbon tails? At least one went down cause of that.
[Edited on 27/3/07 by Mr Whippy]
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twybrow
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| posted on 27/3/07 at 03:09 PM |
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I think Airbus have more problems than that at the moment!
NDT is the only way to test in this instance. As you say, the surface can look wonderful, but be hiding a variety of defects....
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