I'm not a fan of a weight on the hoist line to place tension on a wire
antenna. It seems to me that it adds one more spring/mass resonant
system element which can't be a plus. I think a long bungee is better
as the differential force (k*dx) deviation over length change is less
and there is negligible mass. The downside is bungees don't last very
long, maybe a year or two. A safety loop is always a good idea for
retaining the hoist line if the bungee fails.
I've found that ball bearing blocks are another help as the turning
friction is way less than plain bearings. Thus the line turns the
sheave rather than sliding in the block. Blocks need to be sized right
for the line so it can't over-ride the pulley side or rub on the block
cheeks. Polymer balls never need any lubrication. As K9YC notes taller
trees move more, so this may be helpful. Also larger diameter sheaves
subject to the line fitting properly will reduce line wear. Considering
what it costs for a tree climber, $20 to $45 for a good Harken, Lewmar,
or Ronstan block is a smart investment, IMO.
Also, it is amazing what a difference a ball bearing block makes in
lower friction loss. Some measurements I've seen show a 40% lower
tension on the output line of standard plain bearing blocks vs 10% on
ball bearings blocks. There are claims that special plain bearing
materials make loss much lower.
Friends I have loaned my ball bearing 4:1 block set with low stretch
line to for tower work are amazed how much less pull force is needed.
They are essential on larger sailboats with thousands of pounds load in
sheets or vangs.
Generally, wire rope is out in sailing as high tech polymers equal the
performance and are much more user friendly. It's increasing rare to
see wire rope mooring for big ships. Plus aramid and polyesters float
(instead of nylon) or have near neutral buoyancy making easier mooring
line passing or don't add stress to the line from their weight for deep
anchoring rodes. Plus is it hard to find blocks with sheaves grooved for
wire rope - it is a different groove profile than for fiber lines.
Grant KZ1W
On 7/8/2019 9:44 AM, Jim Thomson wrote:
Date: Sun, 7 Jul 2019 14:21:34 -0700
From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Break-aways for Tree Installed Wire Antennas
<I hire climbers to put pulleys high in the trees, then use Synthetic
<Textile rope with a weight on one end, the other end tied off. I find
<that the rope does NOT last forever in my tall redwoods -- tree sway
<causes the UV-resistant outer jacket to fray around the pulleys,
<exposing the UV sensitive inner braid. I've had this happen in as few as
<five years. The trees are 200 ft redwoods, and the pulleys are around
<140-150 ft. My guess is that this isn't a problem for shorter trees. :)
<73, Jim K9YC
### Why not use .125 or .1525 galvanized winch cable ?
Use correct pulleys....... and end of problem.
## Or SS winch cable....and SS pulleys.
Jim VE7RF
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