[TowerTalk] Re: Long mast, intermediate bearing

[Stan Griffiths]

>In a message dated 98-04-06 14:34:13 EDT, nm9h@ctnet.net writes:
>
><<  Unless you are looking at huge
> arrays of 40m beams or greater, the odds of you overloading your
> rotator's vertical weight capacity is pretty slim. Let the rotator and
> the bearing both do their respective jobs for which they were designed.
>  >>
>
>      Actually the topic of rotator pre-load is one of the most mysterious
>I've run into. While knowledgeable mechanical specialists I've talked to think
>that there should be some load on a rotator, no one really knows how much or
>how to measure it. The manufacturers aren't much help either. 
>
>     If you've got your antenna and mast vertical load captured by the thrust
>bearing, then there is no weight (load) on the rotator. While many people do
>this, it may not be the best thing to do, especially since the rotator is the
>most common failure mode. BTW, the M2 Orion 2800 is the only rotator that says
>anything about this at all and they want to have ALL the weight on the
>rotator. 
>
>Cheers,  Steve  K7LXC

Hi Steve,

Rotator vertical "preload" is a topic I have only heard discussed here and
then only by you, I think.  In practice, I have never been concerned about
it and gnerally what I do is lower the antennas and mast down so that there
is about a quarter of an inch clearance before the mast rests on the
rotator.  Next, I secure the thrust bearing set screws and finally tigten
the rotator.

I have thought a little about the stresses on the various parts of the
rotator, thrust bearing, plates, etc. after installation and it seems to me
that there is no way to guarentee that the stesses will be distributed in
ANY particular way between the rotator and the thrust bearing in the typical
installation.  In fact, I would bet, if there was some way to actually
measure it, the weight supported by the rotator vs the weight supported by
the thrust bearing would vary from day to day depending upon wind and
temperature.

Why temperature, you may ask?  Because the distance between the rotator and
thrust bearing is determined in two conflicting ways:  first, it is fixed by
the distance between the point where the thrust bearing is firmly attached
to the mast and the point where the rotator is firmly attached to the mast.
Both of these points had better not move or the mast will slip in the
rotator or the thrust bearing.  Second, assuming the thrust bearing is on a
flat top and the rotator is on a rotator shelf, both of these items are also
firmly in place and also determine the distance between the rotator and
thrust bearing.

When things get hot and cold, they expand and contract . . . even steel
towers and masts.  So unless the tower and mast expand and contract at
EXACTLY the same rate with temperature, very slight differences in
elongation of the steel in the mast and tower will cause the weight
distribution to change from what the thrust bearing holds vs what the
rotator holds.

So I regard this whole issue as something we can't really control and,
besides, does not seem to matter much in the overall picture.

Of course, there ARE mechanical schemes that you COULD use to actually
control the weight distribution between rotator and thrust bearing but I
have not seen any used in practice.  In fact, every large rotating tower I
have ever seen has the rotator mounted outside the tower and it only rotates
and does not support weight.  They seem to work just fine without any "preload".

My recommendation:  don't sweat it.

Stan  w7ni@teleport.com


--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search