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Re: [TowerTalk] One thrust bearing vs. two

Subject: Re: [TowerTalk] One thrust bearing vs. two
From: "Roger (K8RI)" <K8RI-on-TowerTalk@tm.net>
Date: Wed, 12 Aug 2009 14:12:09 -0400
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>

Tod -ID wrote:
> Good Idea>
>  
>  
> I am planning to install an XM-240 ten feet above a Force 12 C-31XR on a 20 
> foot
> 0.25" wall 1026 DOM mast. Sixteen feet of the mast will be above the top of 
> the
> tower.
>   
There are pros and cons where two thrust bearings are involved even when 
one of them is the rotator.
This is a case where there are a lot of "it all depends".

I've done it with the rotator near the top with the sleeve portion 
serving as the single thrust bearing which it is. I had a tri bander at 
the top of the tower and a TV antenna about 6' above that.

Currently  I have a heavy 2" mast from the rotator about 18' down in the 
tower. I need to measure that both for reference and as I'm going to be 
replacing the mast and bearings. This arrangement has now destroyed one 
TB3 and the second isn't in that great a shape. The second is used to 
take side load off the rotator and does not support load *except* when 
the rotator is removed.

The mast is going to be replaced with 2", 1/4" wall, DOM. The bearings 
are going to be replaced with DOM sleeves. The position of the sleeves 
and rotator is adjustable "within limits".  Even though large the array 
including mast is well within the ratings of the rotator (PST-61) as 
both wind load and vertical load.

I don't believe in using two thrust bearings, unless at least the lower 
one(s) are position adjustable and none support the load. Where the 
rotator is mounted near the base of the tower, they do prevent the mast 
from kicking sideways with torque in larger systems. It is almost 
impossible to distribute the load between bearings and rotator. Normally 
only one is going to be doing the supporting.
I would bet (although I have no statistics to prove it), many rotators 
are under size  for the wind loads, but few are under size for the 
vertical (dead weight) of the system. I think many times what we use 
works in spite of the installation, not because of the installation as 
most of the systems I've seen fit that description.  The problem is 
those that can do both with the larger systems are a bit pricey.

Even with a single top thrust bearing it shouldn't take the load off the 
typical rotator as most are designed to work under some load.  It is 
possible under some conditions for the mast to put considerable *pull* 
on the top of the rotator, which is another reason I prefer the sleeve 
that doesn't support weight.

Some rotators have bearings that will handle both the weight and pull 
such as the PST61 and relatives. I'm sure that is to protect that big 
and expensive worm gear wheel by keeping it aligned.
>  
> Tod, K0TO
>  
>
>
>   _____  
>
> From: N7KA@comcast.net [mailto:N7KA@comcast.net] 
> Sent: Tuesday, August 11, 2009 10:52 PM
> To: Tod -ID
> Subject: Re: [TowerTalk] One thrust bearing vs. two
>
>
>
> A lot has to do with mast length above tower and antenna load.  The lower 
> thrust
> bearing is mostly used to take the lateral thrust load presented by a long 
> mast.
> I use 2 since my 21 ft mast is 15ft out the top of tower and 6 feet inside top
> section.  Not a single problem in 22 years.
>
>  
>
> However if a short mast 2 thrust bearigs is not really needed.  In fact I used
> Rohen 25G tube top for years with NO thrust bearings, and that with several
> yagis 20/15M, or 20/40M.
>
>  
>
> For best advice detail what you plan to do.  General questions will get a 
> mixed
> bag of answers.
>
>  
>
> Arne N7KA
>
>
>  
>
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>   
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