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[TowerTalk] Long Masts -- Intermediate Bearing?

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Subject: [TowerTalk] Long Masts -- Intermediate Bearing?
From: (K7LXC)
Date: Fri, 3 Apr 1998 10:44:14 EST
In a message dated 98-04-02 21:16:15 EST, writes:

> When using a long mast (say 20 ft) in a flat top section of 45G where a
>  thrust bearing is installed on the flat top and the rotor is mounted about
>  feet down inside the tower....  what is the preferred method of
>  supporting/guiding the mast between the rotor and the top of the tower:
>  (1) Do nothing?
>  (2) Fab up a tube/sleeve slightly larger I.D. than the mast O.D. and pass
>  the mast through it? If this is your preference how do you design the
>  support... weld it on the bottom of a Rohn accessory shlef? What do you use
>  for sleeve?
>  (3) Install an intermediate thrust bearing? How do you mount... on a Rohn
>  accessory shelf? I see in the Texas Tower's catalog that rotor plates are
>  not designed to substitute for a flat top (rotor plates flex).... but is
>  this a problem on an intermediate bearing?
>  I would think some sort of support is needed inside the tower at least on a
>  temporary basis if one plans to use a tram to raise an antenna and the tram
>  is attached to the mast. Without such a support all the pull is on the top
>  bearing and it would seem the bottom of the mast would go all over the
>  (and bearing would seem to be under undue stress). Other ideas for tramming
>  if (1), (2) or (3) not used? 
     A typical rotator like a T2X Tailtwister has a lot of lateral strength. A
destruct test in the Hy-Gain QA lab found that the neck of the rotator finally
broke at 3,000 foot pounds of force. That's quite a bit more than the mast
will exert on it so I think we can say that the bottom of the mast isn't going

     With a long mast and an aggressive antenna load on it so that the mast is
VERY top-heavy, the problem becomes how to hold the bottom of the mast in case
you have to swap out the rotator or similar situation. You can have some sort
of fixture to hold it but most people use an additional accessory shelf with
an additional thrust bearing. That way the mast is always captured. This'll
work on 45G and 55G but not 25G. 

     A bushing would work as well to hold the mast but probably involves more
fabrication than the AS/TB version.

     As long as the mast is installed in the rotator, you can hook your
tramline up to it with no problem. 

73 and GL,  Steve   K7LXC

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