I have an old article about this from the yccc newsletter for oct 1994.
unfortunately this one hasn't been converted and put on our web site
yet. I will try to get this scanned and put on my web site as I can't
find the original for it here either.
The formulas for the static case are not too bad. Imagine the angle 'a'
is the angle from the tower to the tram wire, where 0 is straight down
and 90 is horizontal from the tower. And angle 'b' is the angle from
the ground to the wire, where 0 would be laying on the ground and 90
would be straight up. W = weight of antenna hanging from the wire.
]\
]a\
] \
] \
] \
] \
] \
] W-_
] -_
] -_
] -_
] b -_
=========================================
(don't you just hate ascii graphics!)
P=tension on ground end=W*sin(a)/cos(a+b)
L=tension on tower end=(W+P*sin(b))/cos(a)
Note that when the antenna is stationary the L tension is shared between
the lifting rope and the tram wire, assuming the lifting rope is
attached near the same point the tram wire is attached.
The important things are to note that as a+b approaches 90 degrees the
tension becomes infinite... so the more sag in the wire the better.
trying to design it generically is tough since so much depends on the
tower arrangement and how much room is available on the ground. I found
it easiest to make a scale drawing of the side view, figure out how much
clearance is needed between the antenna being lifted and the tower at
various points(usually worst case is near the top of course), this will
give you a minimum value for the 'a' angle and the closest point you can
anchor the tram to the ground. then determine how far out I had to put
the ground anchor to give a reasonable 'b' angle.
other notes:
The 'tiller' method of tilting up the antenna elements as it nears the
tower is a great way to reduce the needed clearance over the top guy
wires.
A back stay on the tram wire is almost always a good idea, the side
forces on the top of the tower for most reasonable angles get huge
quickly. An extra clamp above the rotor to make sure the extra down
force doesn't end up crushing the rotor doesn't hurt either.
A quick example of the forces from the calculations I did for lifting a
170# telrex up a 150' tower with the ground anchor about 150' from the
tower base at about mid point of the lift:
Angles a=40, b=40
P= 629#
L= 750# (this is shared between wire and lifting rope)
Horizontal force on mast = 482#
Vertical force on mast = 1409# (tram+backstay+down force on lifting line
pulley)
More from old discussions on this topic are at:
http://www.k1ttt.net/technote/tram.html
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Pete Smith
> Sent: Sunday, April 04, 2004 13:47
> To: towertalk@contesting.com
> Subject: [TowerTalk] Tension on tramline
>
> I was talking the other day with a local antenna guy about tramming.
I
> mentioned reading various cautionary tales on towertalk about masts
> getting
> bent as a result of the tension on tramlines when relatively heavy
> antennas
> are being moved up and down. He said he had never had this problem,
but
> opined that this was because he always left sufficient slack in the
> tramline so that the tension didn't become too severe.
>
> I can visualize that in cases of extreme slack (i.e., where the tram
line
> goes almost straight down from the tower to the load) the tension on
the
> tram line would be almost zero, because virtually the entire antenna
> weight
> would be borne by the rope that controls the antenna sliding down the
tram
> line. I have also experienced what happens when you try to pull a
line
> bearing weight in the middle (like a center-fed sloper suspended from
the
> top of a tower, at a 45-degree angle) really taut, in order to get the
> droop out. The tension required seems to increase almost
exponentially as
> you get closer to dead straight, and you can't ever get all the slack
> out. The wire will break first.
>
> Is there a mathematical formula that will tell you roughly how much
> tension
> is required at a given load weight and degree of slack?
Alternatively, is
> there a safe rule of thumb to tell you how much slack to leave in a
tram
> line for a given load?
>
> 73, Pete N4ZR
> The World HF Contest Station Database
> is back, at www.pvrc.org
>
> _______________________________________________
>
> 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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> http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
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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