[TowerTalk] Mast and Thrust Bearing questions

[Christopher J Galbraith]

Hi gang,

We are getting closer on our roof-top 40 ft tower design, but could really
use some input on the following related issues:

1. How much mast to put inside the tower?  

We are going to use either 4130 or 1026 DOM, 0.25" wall, 2" O.D. and need
10-12 ft of mast extending from the top of the tower.  We've done the
calculations for the bending moment, but this only applies to the mast at
the point where it meets the tower.

How does one decide how much mast to place inside the tower?  Is there a
rule-of-thumb for how much mast to place inside the tower, to reduce the
side-to-side forces on the rotator?

2. Should we use multiple thrust bearings, and if so, where, and what type?

Some installations we've seen use a thrust bearing at the top of the tower,
then another thrust bearing at some point in between the top of the tower
and the rotator (within the tower).  Is this a good idea, or overkill?

I've seen many reports of failed Rohn thrust bearings.  So, we are planning
to use Array Solutions' heavy duty thrust bearings.  Are there other options
someone can recommend?  

Finally, for background, here are some details on this tower/antenna system:

1. 40 ft of Rohn 55, guyed using a torsional guy system, on top of 5-story
building
2. Antennas on mast include large 10-20m Yagi (15 ft^2) at 1 ft above tower,
and 6m Yagi (3 ft^2) at top of mast (10-11 ft above tower)

Thanks very much for any input!

73, Chris KA8WFC