Hi Mark,
Thanks for the input. My thinking on the static loading is as follows.
With no displacement of the guy cable from equilibrium, either vertically or
horizontally, the Hubbell/Chance strain insulator is in pure tension. When
the guy cable is displaced vertically, the torque arm rotates about its
pivot bolt. This is a pin joint that cannot transfer moment to the torque
arm or the insulator. The available range of motion is from theta = 0 to
180 degrees. When the guy cable is displaced horizontally, the insulator
rotates about the clevis pin that attaches it to the torque arm. This is
also a pin joint, so the torque arm cannot transfer moment to the insulator.
Either way, there is no mechanism to transmit a moment to the Hubbell/Chance
strain insulator.
There is one caveat. The range of motion of the insulator clevis to torque
arm is limited by the depth of the clevis bow and the width of the torque
arm. At some angle of displacement, they will interfere. At that angle and
greater, the torque arm can transmit moment to the insulator. This is
undesirable, as you point out. I have measured the range of motion
available before interference and found it to be + / - 15 degrees from the
neutral axis.
Without some data on the static or kinetic coefficient of friction between
the tower and the guy ring, there is no meaningful way to calculate what the
displacement angle from equilibrium would be while the tower rotates. Does
anyone think it might be greater than 15 degrees?
Of course, the entire question can be made moot by adding a Crosby or
Chicago shackle between the torque arm and the insulator. This removes the
interference limitation and allows -90 <= phi <= 90 degrees of motion.
Interesting discussion.
73,
Jim, W8WTS
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Message: 1
Date: Sun, 16 Oct 2022 08:30:09 -1000
From: M Gerber <wh7w.us@gmail.com>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Repurposing Phillystran.
Message-ID:
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I might check with Hubbell/Chance regarding the use of their fiberglass guy
strains on a rotating tower if the devices in the link below are what you
are planning to use.
https://www.hubbell.com/hubbellpowersystems/en/Products/Power-Utilities/Line
-Construction-Hardware/Guy-Strains/c/561514
Those guy strains look very robust, but I would suggest they are designed to
be used for static guy wires that only see lateral stress at the clevis
fitting primarily as a result of aeolian vibration. When a rotating tower
starts to move, I would expect there is a more significant lateral load on
the fiberglass junction at the crimped/glued clevis fitting. The repetitive
lateral force that may occur at that junction every time the tower rotates
is of concern to me. The fiberglass fibers in the rod are super strong in
linear tensile loading, but my suspicion is the fibers do not do as well in
lateral bending.
I do not claim to be an expert and hate to pour cold water on your plans.
If you feel you want to insulate the Philly Big Grip from the K0XG torque
arm, the DXE Bent Pin Clevis is likely a reasonable option. I, too, have a
K0XG rotating tower and have plans to replace the fiberglass polygon rod
guys. My reason is more related to the fact that I can visibly see
UV/weather effects on the polygon rod which has me concerned. It appears
Hubbell has those guy strains available with a silicone coating, which you
may want to consider if you opt to use them. I bit the bullet and purchased
the Bent Pin Shackles and new Philly a while back in anticipation of the
planned work ahead at my station.
GL/73
Mark WH7W/WH7T
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