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[TowerTalk] mast weight = a non issue!

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Subject: [TowerTalk] mast weight = a non issue!
From: (John D. Peters)
Date: Sat, 12 Apr 1997 03:35:01 -0700 wrote:
> I am a believer of the school that the thrust bearings are for sideways
> thrust, not an element for the support of the weight of the mast and
> antenna/e....
> This belief is based in part on the reality that your mast is probably not
> perfectly true or vertical, and your rotor shelf is probably not level flat.
>  If the above are one or both correct (true????????) the rotation of your
> mast creates a situation where your mast, in relation to your rotor shelf, is
> actually moving up and down to some extent as it rotates.  If you tie your
> rotor to the mast at the bottom of that up down cycle, and you begin
> rotation, when the mast is at its high point you are actually trying to pull
> the housing of the rotor apart!
> Did you know that the very popular ham series of rotors is set up for a 2" OD
> mast? Think about it - you are moving one side of that clamp only....and this
> means that if you are committing the towertalkian sin of using 1 1/2" water
> pipe which has an OD less than 2" you are turning that mast in an oblong
> orbit! After 25 years of doing this that hit me recently.  Oooooooooops -
> QFU!
> Solution, don't worry about the weight of the mast being borne by the
> rotor....huh???? Check it out, read the spec sheets - the ability of almost
> all commercial rotors to handle WEIGHT is very large, so - why sweat it?  If
> the mast wants to move in relationship to the rotate plate - so not
> pinning its vertical movement you don't stress anything.
> <SNIP>
> Mind you I am NOT an engineer, and dropped out of ME studies in the second
> term of college over 20 years ago (gawd I am an old fart with a two letter
> call living in FL) ..this is just plain and simple common sense stuff, like
> knowing to take an extra coupla 1/4-20 nuts and bolts up in your pocket when
> you climb the tower - oooooooooooops!
> <SNIP>!  I
> am told that the hardness of a chrome molly steel mast is SO great that the
> act of painting it is more for ones own psychological/cosmetic sake than it
> is for corrosion prevension....but. of course we all paint it cuz it is just
> one thing we CAN do!
> The posts on the reflector about masts recently have been great, ya' know I
> still am into this Heavy Metal stuff.............whoa the seventies, Rockin'/
> Tower Workin'/ Drivin' to Dayton/ Traffic Handlin'/ Contestin'' and
> BABES!!!!!!!!!! I don't handle traffic anymore, and the babes are grandkids
> but ya know I still love contests and towers and when noone is lookin play
> that air guitar to Stariway to Heaven on my car seat belt!
> 73,
> k4oj
> --
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I think every post should be taken with a grain of salt.  I probably 
should not even respond to a post that starts off "I'm not an 
engineer...I dropped out of ME...  but "common sense".....etc."  But I 
wouldn't want anyone to take "no comment" as agreement!

I am an engineer.  I did not drop out of anything.  I have watched my 
antennas stand up just fine to winds over 140 mph, hurricanes, ice. And I 
also believe in "common sense" mostly in the form of overkill.  Using 
water pipe (schedule 40 at that) or EMT or PVC or whatever as a mast is 
just a terrible thing to do.

To drill a hole in a good chrome moly mast you will want to pay the price 
for a tungsten carbide drill bit.  You galvanize or paint the mast if you 
don't want it to corrode.  Corrosion is not because of strength nor 
hardness.  Experience (common sense) tells you to be sure it will fit 
into the tower with the zinc coating, engineering tell you to use the 
chrome moly material. 

If you are using a TA-33 and turning it with a TV rotor, including the 
larger ones sold to hams, perhaps the aluminum case and internal 98 balls 
will support your system.  Put the TA-33 close to the top of the tower.

If your are using a system with 500 lbs of antennas on a 250 lb mast with 
30 or more sq ft of wind load, you might want to ignore a post that 
thinks mast weight is a "non issue."  

I think the weight of the mast and antennas should be transferred to the 
tower through a very good axial and thrust bearing system.  The torque in 
the vertical axis from wind load should also be transferred to the tower 
through another axial and thrust bearing system far enough inside the 
tower to handle the torque.  If your mast is not vertical, or your tower 
is not vertical, you should correct the problem. (Not intended to be read 
by EME and satellite guys.)

I prefer to have a bearing at least 6 ft inside the tower and a bearing 
at the top of the tower.  Either bearing large enough to support the 
total weight with a huge margin.  The mast does NOT move vertically as it 
rotates.  Tha mast should not move vertically as it rotates.  

If you use an in line rotator, you should align the mast in the chuck so 
the center of the mast is at the center of rotation.  If indeed someone 
sells a rotor with fixed chuck NOT designed for 1 5/8 inch water pipe, 
that is not a mast I would use.  If you expect such a rotor, with an off 
center load, to rotate the mast (and antennas) and tilt/bend the mast as 
it rotates, and not damage the hole in the top of the tower, and support 
your system, good luck. (At least you could shim the thing to 2" diameter 
in the chuck.)

I think the rotor should only be required to rotate and prevent rotation 
in wind.  Any wind, including hurricane, that could occur in the area. 
The tower should support the weight of the system, including any snow and 
ice which may be encountered.  The tower and mast should be able to 
handle all torque, bending moment, imposed by any wind, from any 

Otherwise you do not have a safe installation.  I think mast (and 
antenna) weight is a big issue.  Strength of material, diameter and wall 
thickness of the mast is also a big issue.  The combination of vertical 
thrust, and axial load...side thrust, and how you transfer them to the 
tower is a big issue.  And how you rotate and hold a big antenna system 
is a big issue.   Ignoring any part of the system is a mistake.

73 John K1ER (2 letter call watching BABES in Hawaii...less bugs)

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