Posted: Thu Aug 04, 2005 11:32 am Post subject: Rotor base mounted...great idea?
Hi all,
I have never thought in such idea to prevent mast torque or tower element´s fatigue (stress) due to twisting movements onto high winds (two of the most common tower´s failures) but this seems to be a great idea.
Installing the rotors mounted inside the bottom section of the tower and attached to the concrete base would transmitt all vibrations caused by the winds to the cement and not to the tower structure. Appart of that it would more easy to repair, maintain or change the rotor if you ever need to. It would be important to remark to install the rotor base ATTACHED TO THE CONCRETE AND NOT TO THE BOTTOM TOWER´S SECTION to prevent the trasmission of the vibrations to the tower base elements that could be even more dangerous than installing the rotor on the top section. The only disavantage that I´ve found about this system would be that you would need a good tube section and hard enough from the antenna to the rotor at the bottom and at least 3 trust bearing 1.6 meters (10ft) separated for a normal tower of 10 meters (30 ft). Needless to say it won´t be a good solution for those with higher towers. As a seeing is better than an explanation, here you have a link (hope this help):
(note that rotor on this case it´s not fixed to the concrete pad but to the bottom of the tower -not a good idea though)
And this is what some experienced ham told about the ¨invent¨ (FONT: Tower talk archives)
Friend here had one with KLM KT34 on top. Antenna was rotated by long mast
to rotator mounted at base. No torque from antenna was transmitted to
tower. Worked flawlessly through several bad storms here in Houston area.
Makes a great deal of sense. Probably would make this tower viable
for reasonable ham arrays.
One thing I forgot to add in a previous post about rotors mounted down in a
tower. When mounted "in the tower", but at the base there is no give to
take up the stress and that small area takes the entire brunt of the torque.
If the rotor has a mount on the concrete, or a seperate mount it removes the
stress from the tower.
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
I wish I had mounted my system that way. There is just on thing worse than replace a yagi once up in the tower, TO REPLACE THE ROTOR. 73s!
30KR239 Fernando
30kr239@dxhunters.com
In my opinion, if you put such a long mast and the rotor so low at the base of the tower as you explain Fernando...all that twist movements are transmitted right into the rotor bearing, increasing the chances of getting the rotor damaged more easily.
When manufacturers advise to place the rotors no more that 1 and a half meters below the antenna, that is because they are designed to be installed there, not 10 or 13 meters underneath the antenna.
I dont know if I make any sense, but I preffer to continue using my rotor where it has always been and forget about the so called "torque" or "twisting movements" that in my opinion they have to be very strong to cause any damage to the tower. Fernando your rotor bearing would be completely broken before the tower may suffer any serious damage.
Carlitos,
Sorry to say that I disagree on your appreciations. I don´t think that mounting the rotor down in the base represent a greater effor for the bearing than mounting the rotor inside the top section unless you don´t install the trust bearings mentioned on my post to help with the mast weight and its own torque. The bearing on the rotor is designed for an specific effort (greater when rotor is braking) and this effort is partially transmitted to the tower structure and this is the problem. Just wondering if mounting the rotor down in the base we could develope some system to help the rotor while on its brake´s moment (such a kind of mast clamps to tighten the mast onto heavy winds).On any case, I would rather to change my rotor than seeing my tower collapsing down due to element´s stress.
Appart of that, if tower is guyed part of the stress on the top is transmitted vertically to the lower sections of the tower and partially absorbed by the guy system.
Anyway, I will keep on checking on the net and will keep you informed. Can´t look at hygains manuals on this computer but I will do it later at home. 73s!
30KR239 Fernando
30kr239@dxhunters.com
This is what I´ve found on a site dedicated to rotors. Looks interesting:
___________________________________________________________
¨To improve the longevity of your rotator you must know that the torque, flexing, moment, misalignment and wind load are maximum on top of a tower, at the junction between the mast and the antenna boom. On the contrary a few meter below most of these constraints are either absorbed or much reduced by the tower structure, all the more when is it free-standing where the tower really "lives" and reacts according strains applied on its structure.
It is thus recommended to install the rotator as low as possible inside the tower, preferably 3m below the top, what will also lower its gravity center. You can also install the rotator straight at the base of the tower with the advantage to be easily accessible for installation and service. In this case the central mast must be longer than the tower and kept in position with accuracy, what can be quite hard to achieve if the tower does not have intermediate horizontal plates to circle the mast.
Placing the rotator several meters below the top is also interesting at starting and stopping of the rotator because by high winds the gears will have to bring out the most of the torque to rotate the mast. Then, the lower is the rotator the less significant are the effects of a misalignment on it. Therefore a "depth" of 3m inside the tower is not a superfluous distance to install the rotator. A misalignment is usually caused by loose U-bolts or loosy fixings between the mast and the rotator grip or between the mast and the antenna boom
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