[TowerTalk] Orion 2800 Rotator Cable Replacement

[jimlux]

On 12/10/19 2:41 PM, Bob Shohet, KQ2M wrote:
> Hi Jim,
> 
> ”A major advantage of using stretched wire is that un-stretched copper
> will stretch under tension over years. Before changing to stretched
> wire,”
> 
> I have seen you mention this a number of times in the past and I have been meaning to ask you some questions about it.
> 
> My non-engineering understanding of wires and stretch is that a given wire after being subject to variable strength forces over a given period of time, will stretch only so far before it breaks.  Assuming that that is conceptually correct, let’s say that we have two wires – Wire A and Wire B.  Wire A has been pre-stretched before being put up and is now compared against Wire B which has just been put up without being pre-stretched

Wire A will now be smaller in diameter, and probably have a higher 
breaking force.

Don't forget that there's a difference between yield (which leads to 
deformation) and failure.  The load for stretching is above the elastic 
limit (above the yield) but below the failure load.

> 
> Now we subject both wires to the same forces over the same period of time.  I would expect that going forward Wire A will not stretch as far as Wire B, but is more likely to break and break sooner under significant force than Wire B because it has already been pre-stretched.  Wouldn’t this be correct?

Yes, wire A will be "stiffer" (have a higher spring constant in a 
"newtons of force per meter of stretch" sense), so for the same load, A 
will stretch less.

However, the ultimate failure load for both is probably the same. B will 
stretch further before it fails. That is B will pass through the exact 
same state as A on the way to failure.


However, there's also a whole thing about "non uniform loads".  B, being 
softer and stretchier, as it deforms might concentrate the loads at some 
point, which would then fail.


When Jim does his "hook it to the bumper and pull til failure", I'm 
going to guess that it fails at the end, not in the middle. That's 
because the inevitable bends, twists, or wraps to attach it are a stress 
concentration of some sort.  I doubt he's attaching some thing like a 
symmetric mesh cable grip to each end to insure perfectly uniform load.

There's also a work hardening aspect (which is what drawing is doing) - 
the softer (B) wire might flex more in the wind, which would cause work 
hardening at the ends or attachment points.  The work hardened points 
have a higher spring constant than the non hardened points, and, so, 
take more of the total load.  The "load per unit length" is higher on 
the work hardened sections (because they're stiffer).

So now you're in the classic "fail at the reinforcing gusset" problem.






> 
> If this is not correct, can you please explain why?
> 
> Tnx & 73
> 
> Bob  KQ2M
> 
> 
> From: Jim Brown
> Sent: Tuesday, December 10, 2019 4:49 PM
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Orion 2800 Rotator Cable Replacement
> 
> On 12/10/2019 1:13 PM, john at kk9a.com wrote:
>> Of course I meant to say "with no issues" in my post.  The nylon coating on
>> THHN does quickly deteriorate outdoors however I believe that the nylon's
>> purpose is to lessen friction and improve chemical resistance. Neither of
>> these are issues to me and the PVC underneath is pretty tough.
> 
> The 350 ft run to my Yaesu rotator are two 250 ft runs of #14-2 Romex
> (plus ground) to the base of the tower, laying on the ground.
> Transitioned to something smaller to go 120 ft up the tower. Nine years
> so far with no issues.
> 
> I've used THHN extensively for wire antennas and radials. My current
> favorite for high dipoles is bare #8 that is stretched to make it hard
> drawn #9 -- attach one end to a fixed object and the other to the
> trailer hitch on the SUV, pull slowing until it breaks.
> 
> A major advantage of using stretched wire is that un-stretched copper
> will stretch under tension over years. Before changing to stretched
> wire, I found that I had to circumcise my 80M dipoles (#10 THHN) every
> few years to keep them on frequency. For my fan dipoles, I now use
> stretched wire for the longest element that bears the pulling stress and
> THHN for the shorter elements that do not.
> 
> 73, Jim K9YC
> 
> 
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