Hi Bill,
Bill Aycock wrote:
> John Langdon ( N5CQ ) pointed out a usage (very common, in fact) of both
> Phillystran and EHS, together, that is proper, and OK. This is the use of
> the two in series to break up conductivity, using Phillystran, with EHS at
> the ends to give mechanical and fire protection.
This is a common, recommended practice.
> He is certainly right- I was meaning only parallel usage.
"Don't do this!" Cables with different elogations will not load share
properly.
> He said, also, that he thought Phillystran stretched LESS than EHS. -
> any data to help in this regard?
Well, here goes!
First, lets see what Philadelphia Resins Corp. says about the Phyllistran
cable stretch.
In Tech Bulletin 323-5/80, Comparison of Various Ropes, at 50% of breaking
strength the aramid cable elongates exactly 2 times more than galvanized IPS
wire rope. I would assume that the comparison is for cables of similar load
ratings, the chart doesn't say.
It also does not indicate which wire rope construction was used for the IPS
rope.
Here's where it gets fun.
The best info I have found on wire rope elongation comes from the Macwhyte
Wire Rope Co. They have stretch coefficients for various wire constructions. I
used this info to calculate solid diameter equivalents to duplicate the
elongation for use in my FEA models.
Here's an example of what happens to the different constructions of 1/4" wire
rope:
Construction Equivalent Solid Dia
1x7 .204"
1x19 .198"
7x7 .160"
7x19 .140
It is readily apparent that the construction can have a dramatic effect on
elongation. Basically, the more wires in the construction the more it
stretches.
Not knowing what wire construction is being compared to the aramid cable makes
it hard to evaluate.
I went through a bunch of data processing to arrive at what I think is the
situation. I've got an Excel spreadsheet that is available to those who want
to spend time on the subject.
For 1/4" EHS 1x7 wire rope and HPTG6700 Phillystran, I get about 3 - 3.5 times
more elongation in the aramid cable.
This makes some sense. Kevlar is stronger than steel. The cable is sized for
breaking strength. The 6700 Lb aramid has a nominal diameter of the fiber
bundle of .22", just a little less than the nominal EHS wire size, but is
stronger.
The kevlar 49 used in the lower cost cables has an elastic modulus of around
18 Msi. Most steel has a modulus of around 30 Msi. So the aramid is about 60%
as stiff as steel.
To get close to the same elongation as the 1/4" EHS, one needs to use the
HPTG15400 cable. It is just a little over 1/2 the weight of the 1/4" EHS.
On a real tower we get some of the elongation difference back in reduced cable
sag and weight carried by the tower.
There are lots of towers out there still working just fine with the aramid.
But to answer the question, I'm still pretty shure that the aramid stretches
more for equivalent breaking strength.
73, Kurt
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