[TowerTalk] Trust Bearings

[Jim White, K4OJ]

This is an olde topic on this reflector but - here we go again...

Over the past 30 some odd years of hamming, and my father and his 
fathers ham radio experiences, I offer up my take on this - three 
generations and a lot of time on towertalk have lead me to this:

The rotator is capable of withstanding a LOT of downward force - there 
is a listing of that spec in their manual I guarantee... you probably 
can put a compact car on it for cripe sake (note seasonal integrity)

The kind of thrust you want to control with the thrust bearing is NOT 
downward thrust it is lateral (sideways) thrust...

If thrust bearings can keep you antenna mast vertical then all your 
rotator needs to deal with is providing rotation of it and its antennas...

On the two big towers at the family antenna farm we have dual Rohn 
thrust bearings, one on the top of the flat top plate, and another down 
roughly five feet and the rotator is actually at the base of the top 
section down perhaps 10 feet from the top plate.

The masts are prox 20 feet long Chrome Molly... with "half" of their 
length sticking out of the tower.

Because of the two thrust bearings are there should you have to remove 
the rotator for servicing the antennas will not be able to "force" the 
mast out of its vertical plane.

To those who use those screws on the side of the Rohn thrust bearing to 
support the weight of the antenna system while they are changing out a 
mast - I think you are very much so playing with fire.  the tiny surface 
area at the end of those three bolts should not be asked to handle 
support of this heavy mast and its antennas and you could risk losing 
some fingers or worse.

If it is necessary to support a vertical load like the mast, use 
something with some meat to it like the pair of cast Hygain jaws from 
one of the TH6 or XXXBA type antennas... place them around the mast at 
the top of the HORIZONTAL thrust bearing so that when called upon to 
take the weight they will they transfer that weight to the bearing they 
are sitting on and in turn to the shelf they are mounted to and 
ultimately to the legs of the tower.

Again this is my take on thrust bearings as regards their usage in a ham 
antenna system... some may find it overkill but I have found it perhaps 
the best investment on the "farm" - rotator swap outs are made a 
relatively simple business.

For your situation if you have a flat top plate I would put a thrust 
bearing there and another just above the rotator... NOTE - by using a 
comealong or a pulley arrangement you can "boost" the entire antenna and 
mast combination up... this means you can secure the quad boom to mast 
plate at the top of the mast by just having the top of the mast barely 
sticking out of the tower and then raising the quad and mast up the 
"six" feet... and all the time the mast will be constrained form 
sideways movement by the thrust bearings...

I have forgotten about ice since I have been in FL for about 25 years 
now - dunno what is best for that other than perhaps trying to shield 
the bearing with something like an inverted funnel so freezing rain 
would not run into the bearing area - just watch out for the wasps!

GL - HNY

Jim, K4OJ



Noel wrote:

> I'm trying to decide what type of thrust bearing to use at the top of a
> 60' tower to support a small quad.  The antenna only weighs 35 Lbs but I
> want the boom as high above the tower as possible to keep the elements
> above the guy wires and get about half the antenna above the tower
> rather than around it.  So I'm thinking of using a 12ft mast with about
> 6 ft of mast inside the tower and a thrust bearing at the top for
> stability and keep some pressure off the rotator.  
> 
> I'm Looking for suggestions on what type/make trust bearing works well
> and will hold up in icy winter conditions as well as the heat of summer.
> 
> 
> Appreciate any recommendations.
> 
> Noel,
> 
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