[TowerTalk] mast bearings

[K7LXC@aol.com]

In a message dated 06/22/2000 7:37:30 PM Pacific Daylight Time, 
kn9t@webtv.net writes:

> In a large hf stacked array with 15 feet out of tower and 10 feet inside
>  is it necessary for more than one thrust bearing . Would binding become
>  a problem if one was placed between top bearing and rotor.

      Having a thrust bearing at the top of the tower is pretty much accepted 
as a way for the TB to take the weight of the mast and antennas off of the 
rotator. I've talked to some pretty knowledgeable mechanical people who make 
good cases for having 'some' weight on the rotator so having all the weight 
on the TB and none on the rotator *might* not be the optimum situation. Does 
either way have any dramatic affect on the service life of a rotator? I'm not 
sure.

     But to answer your question, with an aggressive array like you're 
describing having a top TB is a good idea - if for nothing else than to give 
it sufficient lateral support. With a top-heavy situation like this, it's a 
really good idea to have something  (typically another rotator shelf) between 
the TB and the rotator. This will allow you to do a rotator swap while the 
mast is held in a vertical position. (This is a place where having no weight 
on the rotator comes in handy.) I wouldn't use a second TB though - your 
chances of introducing additional bind in the system is high and the 2nd TB 
doesn't really contribute anything useful. Put in an intermediate shelf but 
save your money on the 2nd TB. Sometimes you'll get a lot of bind just 
between the rotator and TB anyway so having another TB inline is probably 
just asking for trouble. You might not know if there's any binding until the 
system is done and then it's too late to do anything about it.

     BTW, you could have a perfectly aligned rotator, TB and mast but it'll 
start binding as soon as you put an antenna on it. The problem is that the 
antenna weight is not in the center of the mast but is offset from the mast 
by a couple of inches. This is quite common. What you can do about this is to 
have someone on the ground run the rotator back and forth while you adjust 
the mast with the bolts in the TB to minimize the binding. 

     Something else you can do is to tighten everything up in a certain 
order. Center and tighten the TB bolts, then the bottom rotator bolts and 
then the rotator shelf bolts last. This will allow some self-aligning that is 
helpful. 

Cheers & GL,    Steve   K7LXC
Tower Tech

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