[TowerTalk] 550 -parachute cord

Fri Oct 7 10:54:30 EDT 2005

Re  550 parachute cord:

I used to load that stuff with my bod, back in the good old days...28 foot " 
doube L" cheapo back in the early 70s.
Also packed and serviced parachute systems as a rigger...long time ago.
Modern parachute cord is rarely the stuff they called 550 cord.  The 
rectangular wing parachutes of the last 25-30 years demand precision length, 
non stretchable suspension lines, that are usually woven dacron, or 
sometimes kevlar, or kevlar-like fibers.
The 550 cord, if the real stuff, is pure nylon, very stretchy, has an outer 
woven nylon shell covering about 6-8 thin nylon strings, one of which is 
distinctively colored, identifying the manufacturer, and date of 
manufacture.
The nylon shell carries something like 60 percent of the load, and the inner 
lines the rest... The nylon shell takes the UV hit over time and, as all 
nylon, weakens, sometimes severely, more if the shell or nylon is thin, and 
the UV reaches through it.  It to a great degree shields the UV from the 
inside lines.  If your stuff has no inside lines, but frizzy nylon fill it 
is not real 550 cord.  Parachute cloth in the old days of round parachutes, 
was often 1.1 ounce per sq yard, and then the skydivers, who got tired of 
pounding the ground with their body on landing made thicker (1.6 oz) LO-Po  
low porosity ripstop.  The 28 footers with lo po landed considerably softer 
than the mil surplus "cheap-os".  Anyway, we had to be careful to get the 
nylon out of the sun, or it would weaken over time, and in the application, 
that was not good.  1.1 cheapo fabric would turn to dust in weeks if left 
out in the California sun.  Telltale signs of weakness are loss of the nylon 
shiny look, and it becomes dull, and pasty over time.
It is non-trivial to maintain the rated 550 tensile strength of the line.  
most knots in it severely (~40 percent) decrease the load capacity of the 
line.  Parachutes use carefully terminated and sewn, ends, and polished 
metal link hardware to attach to the nylon webbing of the harness.
I have use it for decades for suspending dipoles, etc, and found it lasts 
quite a long time but eventually the UV gets to it and first the outer 
hollow shell separates, and then the UV works on the strands, eventually 
breaking the line, under load.   Chafing or movement through pulleys will 
also weaken it, (quickly) but it actually lasts a good long time, at least 
for me in California.
Good stuff for antennas, esp temporary supports.

73, DX, de Pat Barthelow AA6EG aa6eg at hotmail.com

>From: Greenacres113 at aol.com
>To: towertalk at contesting.com
>Subject: [TowerTalk] 550 -parachute cord
>Date: Fri, 7 Oct 2005 08:17:15 EDT
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>
>any users of this  cord? i recently got a few hundred  feet of the '550'.
>before i change out the rope supports on  my wire  antennas i'd like to 
>know
>more. is it UV resistant. better or worse that other  ropes? seems to be 
>very
>strong so wx/sun survival is my main  concern.
>
>k9il
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