Topband: Fwd: cable clamps on old Phillystran

[Carl]

What about ice forming inside the sheath from those breaks?

Carl
KM1H


----- Original Message ----- 
From: "Tree" <tree at kkn.net>
To: "160" <topband at contesting.com>
Sent: Thursday, May 29, 2014 9:48 AM
Subject: Topband: Fwd: cable clamps on old Phillystran


> Forwarding from KZ1W:
>
> ---------- Forwarded message ----------
> From: Grant Saviers <grants2 at pacbell.net>
> Date: Wed, May 28, 2014 at 9:56 PM
> Subject: cable clamps on old Phillystran
> To: topband at contesting.com
> Cc: tree at 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
> _________________
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