Well, to put it as it affected me as I read it, that's enough to give me
the willies. While some might have read that as a victory of sorts, a
successful test of a non-factory method, the very idea of 5 torquings, or
that one can only use 1/3 of the rated strength of the line, is way more
than enough to stifle me from ever considering using it.
Particularly, the pretension stresses are not really the deal breaker, it's
what happens when you get an hour or two of near hurricane winds, or actual
hurricane winds, or a really nasty derecho wind, with its shearing pulse
winds. We'll throw in the towel with tornados.
Back in the day W4BVV lost his 150' 40 meter tower (and the huge 70' boom,
full size element yagi on top of it) when STEEL cable pulled through triple
cable clamps. As a result, I've never felt good about clamps on anything
big. He later did something complex with a folded back end that was woven
and clamped, forcing a failure force to unbend the fold AND unbend it with
the fold clamped down, before pulling it through the main clamps. So far as
I know those were still working fine when they came down on the way to
re-erection at W3LPL. These days we have the wraps, and the right size,
new, never come off.
Yup, stick to the factory method. Guy systems are one place where cheep
makes no sense at all. Chopping up mangled tower plus mangled beams is very
discouraging.
And whatever political viewpoints one might have on climate change, winds
are gradually creeping up both in velocity and frequency. It's no fun at
all beating the loyal contest opposition because some of his antennas are
in a heap on the ground.
73, Guy.
On Thu, May 29, 2014 at 9:48 AM, Tree <tree@kkn.net> wrote:
> Forwarding from KZ1W:
>
> ---------- Forwarded message ----------
> From: Grant Saviers <grants2@pacbell.net>
> Date: Wed, May 28, 2014 at 9:56 PM
> Subject: cable clamps on old Phillystran
> To: topband@contesting.com
> Cc: tree@kkn.net
>
>
>
> Per prior topband posts and discussions about this topic, I've concluded a
> round of testing of cable clamps on parallel strand (old style)
> Phillystran. Here is the Conclusions and Summary I wrote:
>
> A hydraulic jack H frame press was modified to provide tension in excess of
> 20,000 lbs. Tension was applied to a 4 foot long 5/8” od parallel strand
> (old) Phillystran cable terminated with four 5/8” cable clamps and ¾”
> thimbles at both ends. Clamp nuts were torqued to specific values and the
> holding capacity of the cable assembly was measured over periods of weeks.
>
> There is significant creep of the plastic sheath from the cable clamp
> forces between the clamp and the Kevlar core. In the first test sequence,
> the residual torque of the clamp nuts reduced by 65% in 21 days.
> Subsequent
> tightening of the clamp nuts showed smaller sequential reductions of
> residual torques. Five cycles of tightening were demonstrated as necessary
> over a period of weeks to achieve sufficient residual torque of the clamp
> nuts.
>
> A conclusion at 66 days since initial assembly was that four 5/8” wire rope
> clamps, torqued in 5 cycles over weeks to a 50 ft-lb value, will support a
> long term tension without significant slippage at the desired holding
> strength of 6600 lbs, about 25% of the cable rated strength. After removal
> of the cable sheath, there was no visible damage to the Kevlar core at the
> clamps or at the thimble. It is speculated that a slightly higher torque
> value than 50 ft-lbs would improve the slip strength. Adding a 5th clamp
> would further improve the slip strength.
>
> The core around the thimble showed evidence of small differential slippage
> of fibers. The test sequence was such that the fibers could slip against
> each other as tension and clamp nut torques were increased sequentially.
> Thus,
> the test process was not the same as tightening the clamps and then
> installing the guy. However, the Phillystran tested is to be used at 25%
> of its rated strength, so the risk seems minimal in this case. Note that
> wire rope is expected to hold at least 80% of rated strength when properly
> terminated with cable clamps, and is not sequentially pre-tensioned when
> put into service. Whatever unequal forces exist in the individual wire
> strands around the thimble are equalized in some manner.
>
> From this testing, it seems unlikely that parallel strand Phillystran can
> be reliably terminated with cable clamps at more than 1/3 of rated breaking
> strength. The simplified conclusion is that the cable will slip unless the
> clamp has extruded out most of the plastic sheath in the clamping area.
>
> The planned tower has maximum pretension in the guys of 600 lbs. Thus, the
> average long term tension is substantially below the measured slip value
> produced in these tests, so it seems unlikely that the preload tension will
> cause slip over a period of years.
>
> Since the plastic sheath was breached by at least one clamp, water will
> intrude into the core. Moisture does slightly reduce the strength of the
> Kevlar fiber. The clamp fully covers the split area so degradation from
> sunlight seems unlikely, although UV degradation is a major concern with
> Kevlar.
>
> Although the tests were successful in achieving a stable termination at
> 6600 lb tension, doubts remain in my mind about the long term reliability
> of using cable clamps. Hence, I plan to assemble and test a Phillystran
> cable terminated with the standard Crosby Spelter sockets for 5/8” wire
> rope using epoxy potting.
>
>
> A pdf of the full report is available, contact me offline for a copy.
>
> Grant KZ1W
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|