"chaniarts" wrote in message
...
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.

Here's a graph that shows relative creep for Kevlar 29 and 49 versus steel.
Keep in mind that this is at 50% of breaking strength, so the absolute
values are 'way higher than they would be in a normal application. I'm
guessing that the relative rates are similar at lower loads:

http://gertrude-old.case.edu/276/materials/21.htm (Scroll down until you
see the rear end of the blond girl; the graph is directly across from her
butt)

--
Ed Huntress