Roger
It is true that the tower acts as a torsion bar in the high rotor
configuration. The extent that the tower deflects and acts as a damper
(what is the source of the damping?) will depend on the nature of the
structure and the location and configuration of the guys (anti-torsion
brackets?). The result is a dynamic loading of the tower that will also
have resonances.
I seriously doubt that the shear placed on a Rohn 45G, for example, will be
particularly significant for a lower position of the rotator, though,
depending again on the nature of the structure and the antenna. All of the
loads except the vertical load are linear and can be analyzed using a Mohr's
Circle method. Even the vertical load an probably be treated as solely
compressive for that purpose since the buckling loads will probably not be
affected by a shear at a very low position.
In the latter configuration it is the mast that becomes the focus of dynamic
torsion. Again, as you say, it acts like a spring with very limited
damping. But so does the tower. Placement of radial bearings (not thrust
bearings) along the mast is essential to prevent buckling of the mast if it
is too long or too thin. Ignoring the rotator backlash, the dynamic result
can easily be determined by use of a simple second order linear differential
equation equivalent to the equations used in linear electronic systems. If
the length of the mast is not particularly long and the mast has a
sufficient polar moment of inertia then either the spring constant in the
linear equation becomes very high using Hooke's Law or the length over
which it is applied is short and the deflection becomes very low. The
result is a mast that has its critical load applied in shear, an easy
analysis using the torsional stress equation, particularly if there is no
vertical load on the mast (thrust bearing at the top). Choice of the mast
material is thus critical as is the decision to limit the perceived dynamic
damping.
I have done it both ways without problems.
73
Bill
W7VP
----- Original Message -----
From: "K8RI on Tower talk" <k8ri-tower@charter.net>
To: <towertalk@contesting.com>
Sent: Saturday, April 15, 2006 10:35 AM
Subject: Re: [TowerTalk] Rotor Location
>A tower is like a very large torsion bar. It has lots of give for rotation
> with the guy lines serving as dampers that also limit the travel. When
> the
> rotator is moved closer to the bottom of the tower the moment changes from
> a
> twisting to shear. If this is not taken into consideration the system can
> end up with undue stress on the tower legs. In most systems which are
> lightly loaded this isn't a problem. However the mast will serve as an
> undamped torsion bar with only the rotator being a limit at one end. Like
> any spring system you do have to be careful of resonance and the amount of
> travel allowed. A long mast should have thrust bearings, or sleeves to
> hold
> it *centered*. *One* thrust bearing may be necessary for support if the
> weight is near the limit of the rotator's ability to support. With the
> more massive antennas this becomes a trade off between the amount you are
> willing to let the antenna swing in the wind and how much weight (and
> length) you are willing to put into the mast.
>
> Roger Halstead (K8RI and ARRL 40 year Life Member)
> N833R - World's oldest Debonair CD-2
> www.rogerhalstead.com
>
>> I'm seriously considering moving my rotor from the top of my tower to a
>> spot in the bottom section so that it will be easier to reach for
>> maintenance.
>>
>> I'm not worried about the cost of the additional mast, but what other
>> considerations are there? Is there any reason not to move it?
>>
>>
>> 73 de N8AU, Jim in Raymore, MO
>> _______________________________________________
>>
>>
>>
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