[TowerTalk] Thrust bearing question

[Jon Pearl - W4ABC]

Hi Roger and Mike.


On 2/7/2013 10:44 AM, K8RI wrote:
>
>
> 2 points with the first beingm John is correct, a 16' mast putting the 
> antenna 16' above the top of the tower substantially derates the tower 
> below the 18 sq ft original rating.

I've yet to finalize the spacing between the HF, six and two meter 
antennas but I may well find some economy of space, allowing me to drop 
the rotator further down into the tower.  I can't make it lighter but I 
can make it stiffer by shortening it's effective length above the top of 
the tower.

>
> The other is, you already have a thrust bearing in the form of that 
> sleeve.

I guess that's the point I was trying to rationalize between my own two 
ears.  I received another email privately that essentially said the same 
thing.

> It wont support vertical load, but that sleeve will substantially 
> reduce any lateral/side load on the rotator produced by the leverage 
> of the mast. 

The M2 OR2800 is rated at 1800# vertical load and it incorporates a 
Center Guide (cone) for the mast to rest on.  It's been a while since 
I've looked at the slop between the present mast that's on the tower and 
the sleeve.  If it's substantial then a thrust bearing might be of 
service, if only to cut down on the lateral movement.

> the sleeve will serve as a pivot point with 16' above it and 5' below 
> it for a 16:5  ratio for lateral force on the rotator, or slightly 
> less than 4:1 which is a big number.  Fortunately the sleeve limits 
> the pivot ability with most of the load showing up as lateral load on 
> the tower.  Thats good for the rotator, but for the tower? Not so much.

Yes, I've looked at the same ratio and it may change.

>
> Assuming you install an antenna of 18 sq ft which is the tower rating, 
> with 18' of antenna 16' above the top of the tower that is 18' 
> multiplied by a 16' arm.  So the tower is going to see much more than 
> 18 sq ft of load..

The actual numbers from bottom to top are 9.25 sq. ft., 2.5 sq. ft., & 
2.7 sq.ft.

>
> The rotator was already designed to support a substantial vertical 
> load so the bearing capable of supporting a vertical load is not 
> necessary, but sure is handy if you need to work on the rotator 
> without taking all the antennas down.  OTOH you can build a simple 
> fixture to hold the mast in that case

When it's all horizontal at waist level, it's all much more manageable.

>
> IE "to me" 16' sounds like a bit much.
>
> 73 and good luck
>
> Roger (K8RI)
>
>

Thanks again and 73,


Jon Pearl - W4ABC