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[TowerTalk] Interesting crank-up failure story

To: <towertalk@contesting.com>
Subject: [TowerTalk] Interesting crank-up failure story
From: Jleikhim@nettally.com (Leikhim, Joe)
Date: Sat, 10 Jan 1998 01:25:22 -0500
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
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> 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
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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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