Question about cable stretch

[K1JJ]

Hola,

Does anyone know which kind of cable has more stretch under large loads - standard 1/4" EHS steel guy cable or the finer woven aircraft type 1/4" steel cable?

They both appear to be rated at about 6,000 lbs breaking strength.

I ran a 200' steel cable tramline horizontal "BOOM" between towers to hold up the elements of a 40M wire Yagi and found that 1/8" aircraft cable had stretched after a few days. I think after re-tightening the elements' ropes a few times and stretching the tramline repeatedly, it will eventually break.

1/4" is substantially stronger and I have a choice of which type to use.  Flexibility is not an issue, just stretch.

Any ideas?

T

[WQ9E]

I have never used aircraft cable but I have used a lot of 1/4" EHS and I have never had an issue of stretch.  Depending upon how much you tension it and how strong your anchor points are you may need to reduce the tension slightly for winter if you first set it up in summer.  It will shrink slightly in very cold conditions and if your anchor points are near their limit in summer you will wake up one frosty winter morn to find your antenna on the ground.

For typical guying use, the static tension is not set excessively high but you will have a tremendous amount of tension if you are trying to avoid any significant sag over a 200 foot span. 

Rodger WQ9E

[WBear2GCR]

I'd actually contact an application engineer at a name brand wire rope manufacturing company and get the real deal on it.

                  _-_-bear

[W3RSW]

agree with Bear. -
Also depending on the metallurgy, some cables cold work under stretch and become somewhat stronger up to a point then rapidly weaker when retensioned than when new.

As you know, but might be useful to 'jn's,-  lots of variables but in a catenary, (the curve described by a hanging chain or wire hanging only by it's own weight with no center loads such as a feed line), there is always sag.  Sag has to be allowed and planned for.  When you add center weight the sag can be horrific and even more droop has to be allowed.  I guess the key it to at least get the ends up as high as possible knowing that the center will be several feet lower.

In my case if a wire stays up over a couple of years but breaks in really big storms, the tension is just right 

Some guys use the five year test.  But seems we've had several 100 yr. storms lately!

[W3RSW]

Thinking about it some more, there's a good documentary about the construction of the Golden Gate bridge..  might have seen it on PBS where the main cables were made of thousands of on-site woven smaller wire. 

 The big thing with multi-strand carbon steel cable is keeping it from rusting, by oiling, pitch and tars, paints, etc.  In stainless exposed to the weather, even it accumulates slight oxidation and dirt over time.  Well I digressed, but it was a good movie.

The big reason for using really flexible cable is just for that, flexibility in airplane control surface applications over small pulleys, etc.  Stretch is allowed for by spring loading the control lines in old airplanes, just like dial cords.  So if your end loading your horizontal wire by small pulleys I guess aircraft cable is the way to go, but if your simply turn-buckling the ends then larger strands are generally cheaper and the way to go.  Guess it's a matter of economics if your 'paying retail,' saving the more expensive flex for portable antennae or where you really need the flexibility.

Sorry for the wordy answer but sometimes the littlest things draw out the windy replies.
We may be tiliting at windmills in worrying about which is better anyway.  In our applications, except for really tight loads over small pulleys either is probably about the same over time.

[K1JJ]

Thanks for the opinions, guys.

I ended up using the 1/4" aircraft cable and put it up today.  I've yet to pull it tight with the come-along, but I think it will work FB.

The bottom line is the working load on a cable should be maybe 10% of its breaking strength.  If I can let it sag a little and keep the total tension load to under 600 pounds, I'll bet I get little stretch.  We'll have to see.

The cable seems so strong now I added two more elements making it a 7el 40M Yagi on a 160' boom. I just couldn't resist...

I'll update the stretch results after a few days of use.

T

[k4kyv]

I recall reading somewhere that aircraft cable and other "wire rope" is not as good as 7-strand HS and EHS cable.  Wire rope is made specifically for flexibility, so there is bound to be a compromise somewhere.  So I'd say, if the flexibility is not needed, you would be better off with EHS.