[TowerTalk] limits of Rohn 25G with tall masts

[Michael Tope]

Hi Steve,

I was thinking about this problem a little more this morning. 
The force that gets transmitted to the rotator depends a lot on
how much slop there is in the top tube, how much "give" there
is in between the two halves of the rotator mechanism, and how 
"springy" the mast material is. If the top tube is somewhat 
bigger than the mast, then a side load on the mast will get 
trasmitted to the rotator. If there is enough slop in the sleeve,
the rotator will see the orverturning moment of the mast before 
the top tube does such that the rotator will see the same forces 
as a mast mounted rotator installation. Conversely, if the rotator
mechanism has enough slop in it, the rotator housing will just lean
over a little and let the mast hit the side of the sleeve. Ultimately,
what the rotator feels in terms of overturning moment or a side load, 
will depend on the geometry of the particular installation.

I didn't realize that the T2X could take 3000 ft*lbs of overturning
force, but this makes sense given that these things are designed
to be used in a mast mounted configuration. If thats the case,
Pete installation would probably be okay. I just like overkill I guess.
Also, seems to me that a second bearing is nicer when you want to take 
the rotator out.

Anyway, I have more pressing mechanical problems to deal with here, my left 
mouse is starting to stick - grin grin.

73 de Mike, W4EF........ 
----------
From: 	K7LXC@aol.com[SMTP:K7LXC@aol.com]
Sent: 	Monday, November 23, 1998 2:46 PM
To: 	W4EF@pacbell.net; n4zr@contesting.com
Cc: 	towertalk@contesting.com
Subject: 	Re: [TowerTalk] limits of Rohn 25G with tall masts

In a message dated 98-11-23 07:46:52 EST, W4EF@pacbell.net writes:

>  My concern would be with the side load on the rotator.
>  If you are using a point top with no thrust bearing, and the
>  rotator only a couple of feet below the gib sleeve, my guess
>  is that you will be subjecting the rotator to a pretty nasty
>  side load. Imagine a 12' lever arm working against 2' lever with
>  the gib sleeve as the fulcrum - this is a 6:1 force multiplier. 
>  I am planning on using a flat top with a TB3 and a second
>  bearing 6' or 8' below the top. The rotator will probably be 10'
>  below the top bearing. This should ensure that the all the side
>  load gets taken up by the bearings instead of the rotator.
>  
       The rotator will be okay. For one thing, the tapered top tube will
constrain the side forces almost entirely. In other words, the tube will only
allow the mast to move a little distance. Secondly, the T2X for instance will
take 3,000 ft.lbs. of lateral force on the top of the rotator before breaking
so there's not much danger of a problem with it either. 

>  I'll let you know when I get my spreadsheet done. It will be 
>  interesting to plug in your numbers once I get this thing checked
>  for errors (I will probably need a legal disclaimer too - I am not
>  a registered P.E. - strictly amateur hour here)
>  
       Let me know too - I'd like a copy of it.

Cheers,  Steve   K7LXC


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