Hi Joe,
I was not aware that US Tower was using 1/4" cable over 3 1/2" dia. pulleys!
Maybe that's why I've heard of a few relatively new US Tower towers crashing
because of cable failure. I've never seen one up close and just thought they
(crank-up mfgs.) all used 3/16" cables. Maybe thats why I saw somewhere on
the reflector that someone said that US Tower's recommendation for changing
the cables is 3 years! That may be the reason for this recommendation!
They're "system" maybe was not "engineered" correctly if they are, in fact,
using 1/4" cable over 3 1/2" dia. pulleys and it can and will cause early
fatigue failure of the cable, if this is fact.
My failure occurred in less than 2 years, but with all due respect, I had a
lot of stuff up top. 24' mast, HDR300, TH6DXX, Discoverer 7-3 (3 ele. 40m),
DW-3 (warc), 3 VHF beams, VHF vert., and 7 runs of coax. All in all, close
to 30 sq. ft. wind load. I'm also sure that all this "above and beyond what
the average ham does" with one of these towers and the excessive weight was
a big contributing factor to its early demise. Thats exactly why I theorized
that 1/4" cable should provide me with the safety advantage I was looking
for.
Maybe US Tower is experiencing complaints of cable failures in the above 3
years old average range, thus they're recommendation. These early failures
can most likely be eliminated by either changing the pulleys to something 5"
dia. or greater, which would be a difficult redesign chore, or simply going
to 3/16" cable. As you can see from the breaking strength data I've
provided, 3/16" cable is very adequate for the job and will then be
operating with the proper, more than minimum diameter pulley, to provide for
longer service life. I hesitate to recommend or say that US Tower has a
design flaw in they're choice of cable, or that anyone who has 1/4" cable
running over less than 5" dia. pulleys should consider changing to a smaller
cable, but I'm just stating the facts as I know them and from my
investigations and experiences. 73, Roger - K9RB
-----Original Message-----
From: Leikhim, Joe <jleikhim@nettally.com>
To: UpTheTower <UpTheTower@aol.com>
Cc: towertalk@contesting.com <towertalk@contesting.com>;
rogereka@email.msn.com <rogereka@email.msn.com>
To: <towertalk@contesting.com>
Date: Saturday, January 10, 1998 1:27 AM
Subject: Re: [TowerTalk] Interesting crank-up failure story
>Interesting;
>If this is true, US Tower should be using larger pulleys on all towers.
>3.5 inch pulleys are used at most points with 1/4 inch cable.
>
>UpTheTower wrote:
>>
>> Subject: BOUNCE towertalk@contesting.com: Non-member submission from
>> ["Roger Borowski - K9RB" <rogereka@email.msn.com>]
To: <towertalk@contesting.com>
>> Date: Thu, 8 Jan 1998 22:40:16 -0500 (EST)
>> From: owner-towertalk@contesting.com
>> To: owner-towertalk@contesting.com
>>
>> >From k7lxc Thu Jan 8 22:40:11 1998
>> Received: from UPIMSSMTPSYS04 (upimssmtpsys04.email.msn.com
[207.68.152.41])
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>> for <towertalk@contesting.com>; Thu, 8 Jan 1998 22:40:10 -0500
(EST)
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>> Thu, 8 Jan 1998 19:40:05 -0800
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>> Thu, 8 Jan 1998 19:40:00 -0800
>> Message-ID: <004c01bd1cb0$40e4cc00$ef20460c@win95>
>> From: "Roger Borowski - K9RB" <rogereka@email.msn.com>
>> To: <dick.green@valley.net>
>> Cc: <towertalk@contesting.com>
>> Subject: Wire Rope....with a story
To: <towertalk@contesting.com>
>> Date: Thu, 8 Jan 1998 22:37:48 -0500
>> X-Priority: 3
>> X-MSMail-Priority: Normal
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>>
>> Hi Dick,
>> In reference to your queries about crank-up tower cable and bearing
>> specifications, they are originally designed with an ample margin of
safety.
>> The breaking strength of various sizes and types of wire rope cable is
>> available from manufacturers and even some large suppliers such as
>> McMaster-Carr. I agree that those specifications should be included in
the
>> product information paperwork, along with a recommended and outlined
>> maintenance schedule and a warning not to deviate in the selection of
>> replacement materials and why!
>> About ten years ago I had an LM-470D motorized Tri-Ex tower which I
>> bought used. In examining the hardware, I found the cable to be somewhat
>> rusty and a couple of the pulley bearings were rough feeling. I replaced
all
>> the pulley bearings and also decided to replace the original 3/16" cable
>> with 1/4" cable. I thought that would provide me with a even better
margin
>> of safety, since the grooves in the pulleys accommodated the 1/4" cable
>> fine. The breaking strength specifications for the galvanized steel
aircraft
>> cable commonly used is 3700# for the 3/16" size vs. 6100# for the 1/4"
size.
>> (Note that these specs. are for new cable. Degradation starts almost
>> immediately after flexing over a pulley under load and exposure to the
>> elements.)
>> Those figures far exceed any crank-up tower's complete weight, with
a
>> good margin of safety. The 1/4" (upgrade) should last indefinitely. So I
>> thought! You can imagine my suprise less than 2 years later when my main
>> lower cable broke and the tower and antennaes came crashing down with
only
>> 2-3 ft. to go for full 70' extension. The top three sections sheared the
>> coax arms and stops. The bottom of the top section crashed through to the
>> concrete base and was accordianized for about 2 ft. The downward impact
of
>> the top mast (2 x 1/4" wall x 24') and antennaes caused the mast to slip
>> through the trust bearing, all the way to the mounting hardware of the
>> TH6DXX, which was only about 4-6" above the top of the tower, not before
>> caving the top plate in and making the rotor plate resemble a soup bowl.
The
>> antennaes were all ruined. The Discoverer 7-3 boom folded over on both
ends.
>> All antennaes had all elements looking like inverted U's. A real
disaster!
>> The only thing that survived was the mast (and me!)
>> I was standing at the base, operating the electric motor drive when
it
>> happened. I will never forget the sound. In retrospect, I was very
fortunate
>> not to have been injured or worse by the "raining hardware", whipping
>> cables, and antenna element ends that wound up like spears sticking in
the
>> grass. It all happened so fast, there was no time to react!
>> During my post-mortum on the cable, I did some testing in a
>> metallurgical lab we had at my place of employment at the time. The
failure
>> was definitely caused by overstress fatigue. This was apparent even in
>> sections of cable ten feet from the point of failure in comparison with a
>> piece of new cable I had left over. There was no rust on any of the cable
>> and I frequently used a wire rope spray graphite lubricant which was
evident
>> on all the tested cable, including at the point of failure. So what
caused
>> the failure? I examined all the pulley bearings and there was nothing
wrong
>> there, except for some damaged pulley sheaves due to the impact with
>> adjacent bent tower metal pulley housings.
>> In further investigate of wire rope applications, I found in the
>> Society of Automotive Engineers handbook, a section on wire rope stating
>> that there is a minimum radius of pulley for each different size of wire
>> rope. If you imagine the cable running over the pulley, you can envision
the
>> strands riding against the pulley surface being in compression and the
outer
>> strands being in tension. There was a formula, if I remember correctly,
it
>> was cable diameter times 20, i.e.; for 3/16" cable the minimum pulley
>> diameter should be 2.8125 inches, for 1/4" cable the minimum pulley
>> diameter is 5" !!! Guess what? My pulleys on the top of the bottom
section
>> were 3 1/2" diameter. In this case bigger was not better!
>> I don't own a crank-up tower any longer, although my understanding
of
>> the workings of them does not preclude me from owning one, and they do
>> deserve a lot of respect. I recently relocated to NE Florida from the
>> Chicago area and purchased a Rohn foldover tower to allow me to work on
the
>> antennaes at near ground level as I'm not as agile as I was 10 yrs. ago!
My
>> last tower up north was a stationary Rohn 25 with tilt-over base.
>>
>> 73 to all and I hope this posting prevents some other "do gooder" /
>> "make it bigger and better" person from recreating my disaster.
Roger Borowski - K9RB
>>
>> and....replace those cables on your crank-up, if its more than 5
years
>> old! Its not an easy job and they're not inexpensive, but neither is all
the
>> stuff you have up on top! Consider it routine maintenance and Insurance
>> against tragedy. Just use the same size and type cable as the
manufacturer
>> put on it.
>
>--
>Joe Leikhim KE4TZH
>Jleikhim@nettally.com
>
>"tv dinner by the pool,
>i'm so glad i finished school" -F.Zappa 1967
>
>"The Revolution will NOT be televised" -Gil Scott Heron
>
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