My 70' tubular U.S. Tower crank-up does not use 1/4" cables for extending and
retracting the tower. I think they're 3/16" or thereabouts. However, the
tilt-over raising fixture does use 1/4" cable and I would estimate that the
pulleys are not much more than 3 1/2" in diameter. Of course, this is a very
low-duty cycle, occasional-use cable that is never under load for very long
(and can be removed and stored indoors when not in use.)
73, Dick, WC1M
-----Original Message-----
From: Roger Borowski - K9RB . <wa9eka@worldnet.att.net>
To: Jleikhim@nettally.com <Jleikhim@nettally.com>
Cc: towertalk@contesting.com <towertalk@contesting.com>
To: <towertalk@contesting.com>
Date: Saturday, January 10, 1998 9:21 AM
Subject: Re: [TowerTalk] Interesting crank-up failure story
>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])
>>> by dayton.akorn.net (8.8.5/8.8.5) with ESMTP id WAA13412
>>> for <towertalk@contesting.com>; Thu, 8 Jan 1998 22:40:10 -0500
>(EST)
>>> Received: from UPIMSSMTPUSR04 - 207.68.143.160 by email.msn.com with
>Microsoft
>>> SMTPSVC;
>>> Thu, 8 Jan 1998 19:40:05 -0800
>>> Received: from win95 - 12.70.32.55 by email.msn.com with Microsoft
>SMTPSVC;
>>> 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
>>> X-Mailer: Microsoft Outlook Express 4.72.2106.4
>>> X-MimeOLE: Produced By Microsoft MimeOLE V4.72.2106.4
>>>
>>> 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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--
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