"Ed Huntress" wrote in message
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"chaniarts" wrote in message
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On 8/31/2011 3:39 PM, DougC wrote:
For a project I can use either kevlar cord or thin steel cable.

The breaking strength for a given diameter (at least in the small sizes)
comes out very very close to each other: in one instance, .31" steel
cable was rated at 184 lbs, while .3125" kevlar cord was rated at 175
lbs.

Everywhere that sells the stuff usually gives the breaking strengths of
both, but not the working loads.

From other places online I gather that the typical rated working load
for steel cable is 20% of the breaking strength. Is this assumption
reasonable?

And what is the % for kevlar rope? The same, or higher or lower?

(-I know that there are other factors like kevlar's higher price, lower
abrasion resistance ect, but those aren't significant in this instance-)

doesn't kevlar rope stretch under constant loads? there are many
different kinds of synthetic ropes used in boats in place of rod or cable
rigging.

Kevlar's elastic modulus in tension is extraordinary -- for Kevlar 49,
it's on the order of steel. But rope is more commonly made of Kevlar 29,
which has a little more than half of the modulus of Kevlar 49.

The "constant load" issue is a combination of elastic modulus and creep.
It will creep, but not very much.

Here are some useful comparisons of Kevlar, Spectra (polyethylene) and
steel. They're worth reading if you're going to do anything that's
load-bearing with either of those synthetics:

http://www.xmission.com/~tmoyer/test...ength_Cord.pdf

http://www.unols.org/publications/wi...iber_ropes.pdf

--
Ed Huntress

My kiteboarding lines are made of Dyneema, the Spectra/Nylon combo.