[TowerTalk] Thrust Bearing adjustment

Roger (K8RI) on TT K8RI-on-TowerTalk at tm.net
Thu Apr 10 21:46:45 EDT 2014

On 4/10/2014 7:29 PM, john at kk9a.com wrote:
> If your concern is containing the mast for rotator swaps K7LXC has a great
> suggesting of just installing a lower rotator plate with no bearing.  Towers
> are so imprecise, I cannot imagine trying to line up three bearings and not
> have it bind.

IF the mast fits the rotator so it rotates in a circle and the bottom 
bearing is more than a few feet above the rotator, the top bearing is 
centered on the mast. Then the bolts holding the bearing to the mounting 
plate are tightened. At this point the mast should be centered in the 
rotator and the top bearing. Just center the lower bearing on the mast, 
then tighten the bolts holding the bearing to the accessory shelf.  
Centering is easy.
My bearings are over 10 feet apart and the bottom one is close to 10 
feet above the rotator.


Note the rotator is near the bottom of the photo, the bottom bearing is 
about 5 feet below my feet, and 10 feet above the rotator.  The only 
time that one is tight is when the rotator is disconnected, but lining 
them up was easy.

However, this is about twice the safe distance above the top of the 
tower.  I wanted to try it. It worked, but I'd not do it again. Even 
with the vibration/flexing dampened, it was beating the snot out of 
those antennas.

My rotator mast mount can be centered for any practical size mast. I put 
a stub of the mast material in the mount. Then center the mast 
rotation.  It's easy to do within a few thousandths and a lot easier to 
do before its in the tower.  You can use a dial indicator, or something 
as simple as a piece of steel and eyeball it in.

Ham series rotators are designed to support the load, so the only thing 
a bearing or bearings should so is remove the lateral load. If using a 
ham series, do not tighten the bearings on the mast enough for them to 
hold the vertical load, let the rotator do it.

If you are temped to remove the load anyway, call Hy-gain who make those 
rotators first.  It is possible for the mast expansion/contraction to 
actually put a negative load on the rotator and those rotators can not 
withstand much force in that direction. It's done and often gotten away 
with doing so, but often we have things work in spite of what we do, 
rather than because of what we do.

I do not like the rotator shelf approach. If I have to remove the 
rotator, it might be a week or more before it gets put back,  A quarter 
inch clearance can really beat the snot out of things on a windy day 
with a load like I have on that tower.

There are lots of simple and cheap ways of holding the bottom of the 
mast, but having the bearing right there means that all I have to do is 
tighten the bolts before removing the rotator and loosen them about a 
half turn (or so) after the rotator is put back in.

Ideally the ends of the bolts would just touch the mast, but with no 
force applied so the mast could move freely in the vertical.    Good 
enough to dampen lateral motion and that 2" mast will move far more than 
that. Without restraint in a tad less than 20 feet, the center would 
easily spring what appeared to be a quarter inch or more. With the 
extension holding antennas, that translated to a lot of antenna movement 
as under the right wind conditions it could easily resonate.  
Restraining the lateral movement between the rotator and top of the 
tower increased the resonant frequency and reduced the lateral motion of 
the antennas considerably.  The top antennas (144 and 440) were moving a 
good 3 to 4 feet!

I wish I had a video of that array going into resonance in a strong wind.


Roger (K8RI)

> John KK9A
> To:	 <towertalk at contesting.com>
> Subject:	 Re: [TowerTalk] Thrust Bearing adjustment
> From:	 "Ed Muns" <ed at w0yk.com>
> Reply-to:	ed at w0yk.com
> Date:	 Thu, 10 Apr 2014 14:35:21 -0700
> The second, lower, thrust bearing is VERY handy when you need to remove the
> rotator without first removing all the antennas on the mast above the top
> thrust bearing.
> Ed W0YK
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