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[Towertalk] GS-065: how to center the mast?

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Subject: [Towertalk] GS-065: how to center the mast?
From: (Roger Borowski)
Date: Fri, 3 Jan 2003 09:55:29 -0500
Have you shimmed this mast in the rotator? The HAM series rotators only run
on true center the mast when the mast used is 2-1/16" O.D., anything smaller
in O.D. will rotate eccentrically, causing precisely the problem you're
seeing, trust bearing or not. A 1 7/8" OD mast needs to have a couple of
3/32" thick shims installed between the mast and the cast "V" section in the
rotator to make it run concentric with the centerline of the rotator. Once
you do this, I believe that you'll find the adjustment of the trust bearing
not to be an issue if the mast is straight. It will only snug up to the mast
at its true centerline, as the mast will only run concentric then, and not
be a factor of the rotator positioning, unless of course, the rotator
mounting plate is not mounted perfectly perpendicular to the rotator turning
centerline. If the rotator mounting plate is bolted to the tower legs with
u-bolts, they should be tightened last, after the mast and trustbearing.
73, -=Rog-K9RB=-

PS; A 1 1/2" pipe is not a mast, but just a pipe. Its not really all that
strong as an antenna mast. Depending on your antenna load, you need to know
this if you don't already know. An arrangement with a trust bearing usually
indicated more antenna load than a pipe can safely handle. Masts are
generally sized 2" OD (still requiring 1/32" shims for the HAM series
rotator) and are made of a far different manufacturing process of much
higher strength steel. Pipes are only rated for carrying gases and liquids
and not for strength. They bend quite easily in comparison. If you have any
significant antenna load more than a foot or so above your trust bearing,
consider obtaining a true antenna mast, made of tubing, typically referred
to at DOM or Drawn over Mandrel manufacturing process. They come in various
sizes and lengths, with varying wall thicknesses. You may wish to use a
simple computer program called "mast.exe" to calculate your needs and see if
your present pipe will be likely to survive your antenna load. 73, -=RB=-

----- Original Message -----
From: "Roger" <>
To: "towerTalk" <>
Sent: Thursday, January 02, 2003 11:37 PM
Subject: [Towertalk] GS-065: how to center the mast?

> TTians,
> Is there any practical way to exactly center a 1 1/2 mast inside an YAESU
> GS-065 thrust bearing while we are unconfortable installing the mast on
> of the tower (65 feet high...)?
> The mast is 1 1/2" (OD is 1.89"), 13 feet long and is pretty straigh. The
> rotor is a HAM-M inside the tower, 6.0 feet below the top plate, which is
> flat 3/16" steel plate.
> I've done some tests on the ground and found that any minor error
> the retaining bolts of the GS-065 causes the mast to turn eccentrically,
> severely forcing the rotor and the thrust bearing.
> Precisely adjusting the retaining bolts has been difficult because:
> - there are two sets of them;
> - the GS-065 has an internal diameter of 2.75" and the mast here is OD
> 1.89" - a big difference in diameters;
> - the adjustment must provide good alignment both on the vertical and
> horizontal planes.
> I think that I will not be able to make a good centering while I am at the
> top of the tower without some kind of tool like a bushing for filling the
> empty space between the mast and the bearing.
> Such a bushing should have to fill all the space (from the top to the
> bottom of the bearing), in order to avoid misalignements both on the
> horizontal plane as well as on the vertical plane. Besides, the bushing
> should have to be sliced in four pieces, to avoid the retaining bolts...
> I took the measures with a caliper rule, made a sketch and ordered the
> bushing from a lathe shop... it came pretty well done... but, to my
> surprise, when I forced down the slices into the clear slots (between mast
> and bearing, between the bolts), they only penetrated half an inch! Then I
> discovered that the GS-065 inside cilindrical surface is irregular. Its
> internal diameter VARIES and has some other irregularities.
> Now what? I've never read anything on this subject, so your advice will be
> very wellcome. To see pictures of the bearing, top plate and bushing,
> please go to
> TIA,
> Roger PY1OL
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