In a message dated 1/5/04 6:26:16 AM Pacific Standard Time, w4pa@yahoo.com
writes:
> Question about guy wire vibration. K4JNY and I were doing tower work
> at his place all day Sunday and have noticed that on our 120â Rohn 45
> that we are experiencing quite a bit of vibration to the tower and guys
> that we believe is being transmitted down the tower by the 4 el 40m
> beam at the top. This vibration occurs at wind speeds of 10 mph or so
> or higher.
>
> Jeffâs first thought was that guy wire tension was too high, but
> checking with the tension gauge resulted in only 2 of the 9 being too
> tight â loosening them didnât seem to mute the vibration on them much.
>
>
> The guys are EHS at the 40â level, and EHS for the bottom third and
> Phillystran top two thirds for the guys at 80â and 120â.
>
> Our concern is that the Phillystran is not going to respond well,
> long-term, to vibration. Can excessive vibration be an early failure
> problem for Philly guys?
Interesting question. I would say that the wind induced harmonic
vibrations wouldn't do much if anything to the Phillystran but would play havoc
with
the attendant hardware - i.e. shackles, tower guy attachment, rotator hardware,
etc.
This is a common problem with utility lines - especially if they're long
span. What you need to do is to change the resonant frequency of the guy.
Typical fixes add some sort of weight to the cable or have a device in the
middle
of the span that does the same thing. I'm sure you've been driving along some
power lines and have seen those little trianglular devices hanging on the
wire.
Preformed Line Products makes a Air Flow Spoiler that wraps around the
cable. Here's what they say about them:
>> Air Flow Spoilers are designed to counteract the damaging effects of
galloping.
Galloping or dancing is a wind-related phenomenon when low frequency, high
amplitude wind-induced motion can cause trip-outs, service interruptions, cable
damage, damage to supporting structures, and damage to support hardware at
their point of connection. High winds combined with cold weather and ice
accumulation are the main contributors to galloping on regular cables. The
accompanying ice build-up and wind can cause cable failure and structural
damage.
Air Flow Spoilers maintain aerodynamic stability by continually changing the
profile of the cable to the wind. The one-piece helically formed rod, made of
rigid, non-corrosive, non-metallic plastic, controls motion problems that can
damage lines. The three-section structure of the Air Flow Spoiler enables it
to grip the cable or conductor firmly.
Several Air Flow Spoilers are required in each span to offset the aerodynamic
lift forces which cause galloping.
The number and placement of Air Flow Spoilers in each span are determined by
a computer program which considers the results of on-going field and
laboratory research. An Air Flow Spoiler Placement Request should be completed
and
returned to Preformed Line Products Company prior to installation of Air Flow
Spoilers. A placement scheme will be returned to determine placement of Air
Flow
Spoilers in each span. Please advise whether the cables contain optical fibers.
This may influence Air Flow Spoiler Selection.
Our Air Flow Spoilers are made of a rigid, non-metallic, non-corrosive
thermal plastic. The unique design of the PREFORMED Air Flow Spoiler maintains
aerodynamic stability be providing a continually changing profile to the wind.
This
helps to dramatically reduce the incidence of galloping on lines. >>
>
> Any other concerns with this situation we should be aware of? Aside
> from the loosening of connections, antenna hardware, etc. â our
> concern is for the tower and the guys itself.
Since longterm vibration can cause fatal tower problems (and we all know
what it can do to antenna elements!), you need to figure out some way to
dampen the vibrations. Doing with the guys already installed makes it somewhat
problematic. I'd live with it for the short-term and come up with some plan to
fix
it this summer.
I couldn't find anything specifically in the Phillystran info I have but
you might want to talk to Ken Knight, the Phillystran sales guy that's at
Dayton every year. He's at kknight@phillystran.com - tell him I sent you.
Cheers & GL,
Steve K7LXC
TOWER TECH -
Professional tower services for commercial and amateur
888-833-3104
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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