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Old July 11th 03, 04:23 AM
Andrew Lee
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Default Kevlar Spokes


"S. Delaire "Rotatorrecumbent"" wrote in message
...

Kevlar has poor fatigue characteristics.

Were are the facts on this statement?
Having worked with Kevlar I would never had thought this to be true.


I guess to be accurate, it would be better to say how it is used. I got
this impression from a climbing perspective, where it is fairly common
knowledge that kevlar has poor flex fatigue and is that is a consideration
for cord where there is constant flexing. Thus you wouldn't want to tie
and untie knots in the cord regularly, but you can tie it once into a loop
for your Hexes (a form of rock protection or anchor). You can google this
for more information... I also found references to it loosing strength in
sail cloth applications (where they also found Kevlar had problems with long
term UV exposure, a problem that can be overcome though I think). I'll
leave it up to you to google for perhaps a more scientific paper with
testing...

I also found these old Jobst posts:

Ed Chait writes:

I would think that at least on a single speed, or fixed gear bike, a
narrow, toothed Kevlar belt and aluminum sprockets would be lighter,
cleaner, quieter and never require lubrication. I'm sure a belt
drivetrain has been tried before. Why has it never caught on?


It's good you say that in the subjunctive, there being no proof that
it is possible or practical. Kevlar is definitely not the medium, it
fails in fatigue as soon as it absorbs a little moisture, and it IS
hygroscopic. Kevlar has limited uses and timing belts is not one of
them.

Jobst Brandt avid G? writes:

Yes, that's the kind I also had in mind when replying, and it is such
belts where the Kevlar fibers break in fatigue with the absorption of
humidity. I doubt that anyone makes timing belts with Kevlar these
days. Their frailty must be common knowledge in the business.

A quick Excite net search revealed quite a few companies with sales
info for Kevlar reinforced belts. Maybe it's not a major problem for
certain products or conditions. I'd be curious to learn more.


We don't know where the Kevlar is and how it is used. It's somwhat like
the sticker "Intel Inside" on a product.

Jobst - do you have quantitative data, references or experience that
would be informative for the rest of us ?


I was involved in the development of paper moving plotters at HP
before their ink jet plotters. These plotters used timing belts to
move the carriage and paper (X&Y). We tested all sorts of belts and
discovered that, in the humidity chamber, all Kevlar belts failed. We
used conventional Rayon backed belts after that with no trouble.

No papers were published on these findings, they being insignificant
in that sort of work.

Jobst Brandt
ow, it may be that Kevlar is not "bad"
in fatigue characteristics when used in pure tension,it is doubful that it
can compare to steel operating at stress below the fatigue limit. I amnot
familar with how the fibers would be secured at the ends, but if there are
lots of bendsthat might cause problems with the constant stress cycling of
loading rolling wheel...I don't know for sure though. In what applications
have you worked with Kevlar?


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