Newflee

I have made a Kevlar/ Carbon gear and after trimming and sanding to shape it is rough where the Kevlar has been sanded (edges). The carbon has not presented a problem.

The gear is layed up with west system. Do I coat the gear with west until smooth, or is there a better way? I have heard mention of wet sanding but don't see how that would be very effective.

Anyone have to do something like this?


Lee

Mike James

Kevlar will always fuzz up when sanded.

Best choice is to add a finish coat, such as more epoxy, and sand that.

Ed Smith

There is no advantage to adding kevlar to a composite landing Gear.

Kevlar is an impact resistant material and does not add stiffnes or the type of strength one would require in a Landing Gear.

Ed S

Newflee

It's done already.

davidfee

You can add epoxy to the trimmed/sanded edges... or you can wick thin CA glue into the kevlar fibers and then wet sand them with 180/320 etc. paper. You'll likely need to add more CA as you go, but it sets up a lot quicker than epoxy.

You can also try just wicking thin CA into the loose fibers and then slicing them off flush with the carbon with a brand new (very sharp) single edged razor blade.

Been there, done that...

-David

davidfee

Actually, I can think of one good reason to use Kevlar in your carbon gear. If you do happen to snap your carbon gear, the Kevlar will keep the broken piece from falling loose... so you won't have to go looking for it somewhere else on the runway.

It is generally true though that, since carbon is much stiffer than Kevlar, the carbon will normally "see" most of the load... and will normally fail before the Kevlar carries much load at all. Mixing these fibers to get the appropriate stress distribution becomes something of an art-form.

-David

Newflee

Thanks for the replies guys. I decided to wrap the gear in a single layer of CF cloth. Since CF sands smooth, I now have a nicely finished unit. The extra CF can't hurt either. I don't really like adding west to fully cured west but I would have had to do that even if I was just surfacing with resin so it is what it is.
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Just for the record the kevlar that was used is a hybrid kevlar/Cf used on bias. I agree, kevlar on it's own would not be ideal. I just think the hybrid will temper the unit against shattering. I could be wrong.


lee

davidfee

That and it just looks so darn cool!

-David

Newflee

Your so right. I have to paint the gear white anyway but even chicks would love to look at this stuff. Some day aliens will dig up some parts and they will become the new precious gems. Kinda like our amber.

hayman

I either use a pair of cuticle scissors freshly sharpened over a disk sander and/or wet sand from 220 up to sometimes 600 grit. I use Kevlar for the leading edge of hand launch and unlimited sailplane wings so I want to do as little of sanding of Kevlar that is necessary. There is a neat way of making composite cloth, Kevlar/Glass that makes doing L/E's so much easier and the only thing you sand is glass.

Bill
http://www.paonline.com/hayman/video.htm

rjmech

My Feedback: (3)

Everybody,
David has it right, Kevlar (actually aramid fiber) is pretty much useless accept for hi impact areas ex:bullet proof vest, aircraft ice shields, a few layers on leading edges,etc. Stick w/ mostly using cfrp in the models. Also Kevlar absorbs water so over time it gains weight, not a big deal on a few oz. landing gear but on larger things it can make a difference. Also when you are wet sanding and you start to get that fuzz on you part you have just destroyed all of the strength in the sanded layer of your part. What makes glass, cf, and aramid fabrics so strong is that each fiber is nearly perfect and smooth, essentially it has no weak areas or stress risers, when you sand it you have just cut thousands of fibers. Not a big deal on on a thin edge, but on say the whole outside of a cowl or fuselage it would be.
Thaddeus

gjs

Anybody tried to burn fuzz off, torch it? Seriously

Mike James

I sincerely hope not.

Even if you could do it without starting a fire, you'd still be left with exposed raw Kevlar.

This job should be properly done, by applying an exterior finish to the surface.

If you want to see how well composite materials burn, take a glass fuselage, or a glassed, sheeted foam wing, and put it in a (safe, please) trash-burning barrel. If you haven't already seen this, you might be amazed at how fast these things burn. (I've done it... That's how I know.)

gjs

Sorry , I havent seed the kevlar fuzzies actually, i just know synthectic fibers generally melt with the touch of a flame without having to accumulate enough heat to start full blown combustion. Torch the wrong tool, carefully applied lighter. My neighbor had his corogated fiberglass fence catch fire, quite a specticle, 30ft. flames lapping the trees. Point taken. I stand corrected.

Icemanxxxv

Don"t sand Kevlar....It dosen't sand well at all. Try putting a layer of fiberglass over your assembly and use that as a sanding coat. just don't sand all the way thru it.

MJFinegan

My Feedback: (2)

Anyone who has sanded kevlar intentionally or otherwise has had to deal with the tennis ball look. If it's any consolation, an r/c landing gear is a whole lot smaller and easier to deal with than a 1:1 MD-80 engine cowl. At that time, our solution was heavy prime coats, sand, reprime, sand etc until the offending fuzz was taken care of.

davidfee

Yup, the best way to get around the situation is to not get into the situation in the first place. Have the kevlar stop short of the edge (or surface) etc. Carbon and glass sand much better than kevlar. Repairs don't have to result in a huge ball of fuzz either, but each situation will be slightly different. I've repaired lots of kevlar fuselages, but I've never repaired an MD-80 engine cowl.

Here's another example of gratuitous carbon/kevlar hybrid use. Why did I make a brushless motor case out of it? Because I think it looks cool.

-David

GaryO

The key to sanding Kevlar (if you really need to) is to use very fine sandpaper, 400 grit and up.