> Personally I see no reason for a second bearing and I see a huge headache
> trying to line up a top bearing, middle bearing and a rotator on something
> built as unprecise as the Rohn mounting plates. Your rotator already has
It depends on the size of the array and how far above the top of the tower
it extends.
Idealy I'd use two bearings made from sleeves as the top antennas are 30
feet above the top of the tower., but things are seldom ideal.
> thrust bearings and it should easily accommodate the side forces at the
> bottom of the mast if it's properly sized.
I agree for normal size arrays, but that side force can get pretty large
with the leverage from a mast extending well above the top of the tower.
The alignment is not all that difficult. It's easier for me to do than
explain. Center the mast between the tower legs at the top (every thing else
is in relation to this point), center the shaft between the tower legs at
the bottom, but leave the bottom bearing loose, fit the rotator, and center
the shaft in the rotator. Using a dial indicator, or just a good straight
ruller (indicator is easier) adjust the mast mount on the rotator to make
sure the mast rotates without moving from side to side. Center the the
bottom bearing on the mast.
I would add a caution about using a thrust bearing to hold the mast while
removing the rotator. I'll admit to having done it, but getting my hands
between a mast with several hundred pounds (or more)that is just held with a
few set screws and the rotator makes me nervous. There is a much less
expensive way to do this that is safer. Instead of installing a thrust
bearing on the bottom rotator plate, just use a block of wood, or metal
plate. A quarter inch thick Aluminum or steel plate, large enough that it
won't slide out of position, or drilled to match the holes in the rotator
plate so bolts can pin it in place should be suficient. Raise the mast
above that rotator plate, insert the metal plate or block of wood and then
set the mast back down on it. The metal plate is easier and more sturdy than
the wood but if the mast and antennas are not super heavy a 2 X 6 or 2 X 10
in good shape should do the job.
HOWEVER... Like most things, thrust bearings (even one) come with trade
offs. I prefer to mount the rotator well down in the tower, but not far
enough that I have to deal with exagerated forces due to torsion and
resonance in the mast. Most rotators need some weight on them and if the
total weight of the array and mast is well within those ratings the load
should rest on the rotator. The design of all rotators I've seen will handle
side thrust within their ratings much better with weight on them than
without. Also, there is the problem of uneven expansion between the tower
and mast when using a thrust bearing, or bearings, that support/hold the
mast in the vertical plane. I can easily visualize a situation where the
rotator would be supporting no load or even a negative load and this *might*
remove the load from the gears in some designs.
Another point is rotators that use tapered wheel bearings which BTW make
excelent thrust bearings. Side forces on these *may* translate into vertical
motion. Even with two opposing bearings they have to be carefully positioned
to prevent this up and down movement. They really need to be preloaded with
compression between them.
So, like anything else when used in a manner other than for which they were
designed, we are essentially moving into the experimental test area.
Generally the forces in most installations are not large enough to create a
problem, but nature is good at creating exceptions.
Working "up there" with heavy weights and forces of nature is hazardous,
(far more so than most of us give it credit) so it's a place where every
thing should be well thought out ahead-of-time. There is a tremendous
difference between working with a 3 or 4 element tribander on top of a tower
with a Taitwister type of rotator down in the tower than there is working
with a 600# to 800# array and mast just as there is a tremendous difference
between putting up a 40 foot tower for the tribander and a 100 foot heavy
duty tower. The forces are multiplied many fold going from that 40 foot to
the 100 footer while the tolerance for mistakes causing injury has gotten
much smaller. We can safely apply the procedures used on the "big stuff" to
the smaller ones, but not necessiarily the other way around.
73
Roger (K8RI)
>
> John KK9A
>
>
> To: "Dick, W1KSZ" <w1ksz@earthlink.net>,"TowerTalk"
> <towertalk@contesting.com>
> Subject: Re: [TowerTalk] Rohn 45 Question
> From: "jeremy-ca" <km1h@jeremy.mv.com>
> Date: Sat, 25 Aug 2007 10:42:08 -0400
> List-post: <mailto:towertalk@contesting.com>
>
> Put the thrust bearing on a 45AG4 top section; the holes are already there
> for that purpose.
>
> Use a second bearing on a redrilled rotator shelf where the rotator would
> normally go and put the rotator at the bottom of the section if you have a
> large array. I do this even on 25G for a large stack of VHF yagis. It
> saves
> thrust bearings and rotators as well as makes removing them or dropping
> the
> mast a lot easier.
>
> Carl
> KM1H
>
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