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[TowerTalk] Re: Long mast, intermediate bearing

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Subject: [TowerTalk] Re: Long mast, intermediate bearing
From: (Stan Griffiths)
Date: Sun, 12 Apr 1998 03:35:30 -0700 (PDT)
>At 8:01 PM -0800 4/6/98, Matt Kolb wrote:
>>Even your smaller rotators (I
>>am looking at the Yaesu G-800 manual as I speak) will easily handle 400
>>lbs of vertical load while they will handle very little in lateral
>>binding on the mast before they crap out.  The centering effect as well
>>as the ball bearings make sure that the only pressures seen by the
>>rotator are those of a purely vertical load....something that they
>>appear to be designed to handle, hence the vertical load rating.
>I have been chatting with Steve about this, but perhaps I should insert
>this point of confusion (on my part) into this conversation.  I am
>concerned about this "lateral binding on the mast."
>For example, I have a 20' mast; 16 feet above the tower, 4 feet down to the
>rotor.  That amounts to a 4:1 lever arm; for each pound of force pushing
>the top of the mast to the left, there is a four pound force pushing the
>bottom of the mast to the right.  It is the job of the rotor to contain
>this lateral force at the bottom of the mast.  Correct?
>To continue the example, with 20 sq ft of wind loading on the mast, one
>could reasonably expect to see a 200 pound lateral force applied to the top
>of the mast.  This would equate to an 800 pound lateral force being applied
>to the rotor.  Correct?
>This lateral force would seem to be just as critical to a rotor as any of
>its other specifications, yet none of the rotor manufacturers appear to
>state it.
>Now, since rotors aren't being sheared apart in rampant numbers, this
>doesn't appear to be a real-world problem, but I am curious about the
>physical forces in effect here.  Any thoughts on this?
>73, Dick
>PS:  Be gentle.  I'm a software engineer and this is definitely hardware!
>Dick Flanagan W6OLD CFII Minden, Nevada DM09db (South of Reno)

Hi Dick,

I'm a little late in reading this and I have not had a chance to read all
the other comments you may have gotten on it so everything I have to say may
be old info now, but here goes anyway.

I think your analysis of the lateral forces on the rotator are correct for
the mechanical configuration you have discribed.  There are two things I can
think of  to help reduce the lateral forces on the rotator:  1) move the
rotator further down in the tower by either using a longer mast or (Heaven
forbid) reducing the height of the antenna a couple of feet.  2) use a pipe
top to help keep the mast from flexing in the wind to keep some of the
lateral forces off the rotator.

There seems to be a lot of reluctance among this group to use a pipe top.  I
like them myself for a couple of reasons and maybe some discussion of pipe
vs flat tops would be worth while.  Let me start it off:

Pipe top advantages:

1.  They keep the mast vertical and in place with the rotator removed
without the need for any other special braces or hardware.

2.  They help keep the lateral forces off the rotator due to wind on the
array above the tower.  They do this because the pipe top sleeves over the
mast and helps it to resist flexing, at least for the area of the overlap.

Pipe top disadvantages:

1.  They are harder to climb since the spaces to put your feet are much
smaller at the top.  This sometimes makes it difficult to stand high on the
tower when servicing a beam mounted several feet above the top of the pointy
section.  This is not really a problem for me since my feet are not huge (9
1/2) and if it were a problem, I would solve it by mounting a step on the
pipe top like K7LXC has told us how to do on the mast many times.

2.  It is not real easy to install a thrust bearing in a pointy top.  (How
necessary is a thrust bearing, anyway?  This is another interesting discussion.)

Flat top advantages:

1.  Easy to stand on when working on an antenna mounted several feet above
the top.

2.  Easy to install a thrust bearing.

Flat top disadvantages:

1.  Requires an extra shelf and some kind of lateral support to keep the
mast vertical when the rotator is removed for service.  This shelf is
usually installed where the rotator would go requiring the rotator be
installed lower in the tower.  This problem is worse on 25G than 45G because
standard Rohn shelves can only be installed where there are no diagonals in
25G, unlike 45G, where you can install a standard Rohn shelf almost anywhere.

Do we really need thrust bearings?

Probably not, in most cases, in my experience.  The main purpose of a thrust
bearing is to keep the weight of the mast and antennas off the rotator.
Most rotators I have seen can take many hundreds of pounds in weight such as
the Ham IV and Tailtwister.  Most two beam HF stacks with a stout mast would
not weigh over 400 pounds or so, so this does not seem to me to be a good
reason to spend another $75 on a thrust bearing, not to mention the $35 for
the shelf.  If the weight of the mast and antennas is over 500 pounds, then
I think a thrust bearing should be considered.

A secondary reason often stated to use a thrust bearing is to prevent or
reduce binding as the beams and mast are rotated.  My experience says that
if properly aligned, there will be very little binding with a pipe top with
no thrust bearing installed.  The key phrase here is "properly aligned".
This means use shims to center the rotation of the mast exactly to the cent
of rotation of the rotator and make sure the axis of rotation of the mast
and rotator are as close to exactly parallel as you can get them.  It helps
a LOT to tighten the hardware in the order I have discussed here before:
First, rotator mast clamps.  Second, rotator to shelf bolts.  Third, shelf
to tower u-bolts.

Hey, I sell Rohn thrust bearings to everyone who insists on buying one.  I
make money every time someone decides to use one.  My only motivation here
is that I genuinely believe that most of the time it is money wasted.

If you were to do a financial analysis of a pipe top without thrust bearing
vs a flat top with a thrust bearing (including the necessary hardware to
keep the mast vertical with the rotator removed), I think you would find the
flat top/thrust bearing method to be a LOT more spendy.

Here, let me do it for you.  I will not include any pieces such as the
rotator mounting plate that are common to both situations:

Situation #1:  Pipe top with 18 foot mast and rotator at junction of the
bottom of the top section and the top of the section below it.  No thrust

25AG3 Pipe Top = $106.86

Situation #2:  Flat top with 18 foot mast, thrust bearing, and additional
shelf to keep the mast vertical with the rotator removed.  Rotator mounted
same as in situation #1, above.

25AG4 Flat Top = $119.34
TB3 Thrust Bearing = $75.66
AS25G Shelf = $32.45

Total = $227.45

So the flat top and thrust bearing are an additional $120 or so for no
improvement in performance or safety that I can see.  You may say it is a
small amount considering the overall cost of the tower and antennas.  I say,
you save that much on 3 or 4 towers and you have enough to put up another
tower!!  If you are trying to build a competitvie contest station, I say you
can't get enough towers.

So there ought to be enough controversial statements above to get some
iteresting discussion going.  I will sit back and listen now.


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