On 4/10/2014 7:29 PM, john@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.
http://www.rogerhalstead.com/ham_files/Tower29.htm
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.
73
Roger (K8RI)
John KK9A
To: <towertalk@contesting.com>
Subject: Re: [TowerTalk] Thrust Bearing adjustment
From: "Ed Muns" <ed@w0yk.com>
Reply-to: ed@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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