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## Re: [TowerTalk] Rotor Location

 To: "K8RI on Tower talk" , Re: [TowerTalk] Rotor Location "Bill" Sat, 15 Apr 2006 11:09:29 -0700
 ```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" To: 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 >> _______________________________________________ >> >> >> >> _______________________________________________ >> TowerTalk mailing list >> TowerTalk@contesting.com >> http://lists.contesting.com/mailman/listinfo/towertalk >> > > _______________________________________________ > > > > _______________________________________________ > TowerTalk mailing list > TowerTalk@contesting.com > http://lists.contesting.com/mailman/listinfo/towertalk _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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