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Author Topic: Rohn Tower Top Section Vs. Thrust Bearing  (Read 5029 times)
KI4AX
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Posts: 113




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« on: March 21, 2012, 11:33:20 AM »

I am sure many of you have answered this question in the past either for your self or someone else but I have to ask again because I don't know.

I am getting ready to go up with 40' of Rohn Tower, a Wilson System 33 Tri-Bander (similar to Mosley's TA-33), and a 22 Element 2 Meter Beam (don't know the exact specs). I plan to use a 10' piece of 1-3/4 Galvanized Electrical Conduit for the mast. The rotor is a CD 45-II. According to the specs on the rotor I can have a maximum of 8.5 sq. ft. of antenna with a Thrust Bearing. The Wilson Tri-Bander is 5.7 sq. ft. and I am sure the 2m beam is not more than 2.8 sq. ft. But without a Thrust Bearing the rotor is only good for 5 sq. ft. I plan to put about 6' of mast above the tower and about 4' down into the tower with the rotor at the bottom.

The Rohn 25g tower that I have includes a tapered top section with a 2" O.D. tube at the top. My question is really about the top section. Does the tapered top and the tube take the place of a thrust bearing and bearing plate? Can I depend on the tube at the top to take the lateral forces? Or should I get rid of the tapered top section and start looking for a Thrust Bearing Plate and Bearing?

Thanks

Dan KI4AX

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K3GM
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Posts: 1804




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« Reply #1 on: March 21, 2012, 12:00:04 PM »

.....The Rohn 25g tower that I have includes a tapered top section with a 2" O.D. tube at the top. My question is really about the top section. Does the tapered top and the tube take the place of a thrust bearing and bearing plate? Can I depend on the tube at the top to take the lateral forces? Or should I get rid of the tapered top section and start looking for a Thrust Bearing Plate and Bearing?
Thrust bearing are design to support axial loads.  The "rocket top" section comes with a rolled steel bushing that takes lateral forces off of the rotor. However, it does not take the axial load off of it.  Certainly, a flat top section with a thrust bearing would be a far superior, but there are many rocket tops in use without problems.  Your vertical load is not a problem, and I think your rotator should be avle to support it.  Although it has been a while since I've seen a new one, the bushing is still probably only peened into place and can be easily banged out of the top tube.  I did this some time ago replacing the steel bushing with a PTFE collar that I machined and then press fit (hammered) into place.  This presented little wear from lateral forces pressing the mast against the sides of the bushing.  I once took a rocket top tower down which had been in place for many years.  The bushing inside the top tube had reduced the mast wall by nearly half from rotation and more likely rocking back and forth in the wind.
« Last Edit: March 21, 2012, 12:04:51 PM by K3GM » Logged
AD4U
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Posts: 2167




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« Reply #2 on: March 21, 2012, 12:02:08 PM »

Good questions Dan and I am sure you will get many (conflicting) answers.

I have been hamming since the 1960's.  I have used all sorts of towers, masts, antennas and rotors.  Many of the towers were Rohn.  Right now I have two Rohn 45 towers and two Universal self supporting aluminum towers.  

I have never used a thrust bearing at the top.  I have always depended on the tube in the tapered top section to support the lateral forces.  I have never had a failure.

The masts I use are schedule 40 galvanized water pipe sized to fit loosely (but not too loose) inside the tube on the tapered top section of what ever tower I am using.  Some of these masts have been in constant use for more than 30 years. The only thing that may cause a problem with this situation is the rotor must support the entire weight of the antenna(s) and mast.  

If you use a bearing at the top, the bearing MAY be designed to support the weight of the antenna(s) and mast.  Every bearing I have seen only has a couple of set screws that screw through the bearing and "grip" the mast.  IMO this will not support a heavy antennas and a heavy duty mast - at least not for long.

Others may have different opinions.

I have always used a heavy duty rotor and I have never had a problem.

Dick  AD4U
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K1ZJH
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« Reply #3 on: March 21, 2012, 02:22:54 PM »

You're tapered top section will take the lateral load off of the rotor base. Rotors can stand a lot of
downward force.

My Rohn 25G top sectionhas a flat top, and I am using the Rohn thrust bearing.  I bought my tower used
and worked with what I had.

Pete
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K4RVN
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Posts: 778




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« Reply #4 on: March 21, 2012, 03:08:10 PM »

The bearing will help take the load off your ball bearings in the rotar and reduce the required torque to turn the antenna.In effect you are adding another set of ball bearings to support the load and reduce the forces on your rotator. If you ever want one, now is the time to install it. I bought one several years ago and it is still in the box because it is just too much trouble to remove the existing antenna up on my tower even with a hazer to let it down.

Frank
« Last Edit: March 21, 2012, 03:23:40 PM by K4RVN » Logged
KI4AX
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Posts: 113




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« Reply #5 on: March 22, 2012, 05:29:47 AM »

Thanks to everyone for your comments. I realize I have made a mistake which may have some bearing on the situation. I purchased a piece of 1-1/2" Electrical Conduit to use as a mast where as I should have purchased 1-3/4" Galvanized Water Pipe. The reason I chose the Conduit is because it was lighter (thinner wall) and I did not want to load up the rotor with 40 or 50 LBS of dead weight. But I guess if the Water Pipe fits better the downward amount of weight is not as critical. I am not sure what the MAX down load is on the CD 45-II rotor... I could not seem to find that info in the Manual. But the main thing is to reduce the lateral load on the rotor so that I can get up to the 8.5 Sq. Ft. rating on the rotor. Not being able to achieve the higher rating on the rotor will cause me to have to use a bigger rotor. So I think I will take the conduit back and purchase a piece of water pipe and see how that fits. Perhaps it will fit sufficiently and I can use that as my mast.

Thanks again for all the comments.....

Dan KI4AX
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W3HKK
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Posts: 596




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« Reply #6 on: March 22, 2012, 06:30:48 AM »

Here's one more option, for larger antennas, and for stacking them vertically. 

I used quarter inch wall x 2 ID x 20 ft length of
6061T6 aluminum alloy , set into  the Ham 4 rotator installed at the base of my top 8 ft aluminum tower section.  A thrust bearing was mounted  into the top flat section of the tower.

So 8 ft of  mast  was inside the tower and 12 ft. extended above it, where I mounted a KT-34XA 32 ft boom tribander about 2 ft up from the tower, and an 11 el. 2m Boomer  7 ft above that.

Originally, I started out with a  HyGain 204BA yagi at the bottom and a DB10-15 duoband yagi 8 ft above and  the 2m Boomer  3 ft above that.
I designed wind loads for 100 mph winds with 20% safety factor.

This was on top of a Heights  56 ft aluminum tower.  Again, designed to handle  100 mph winds  with the original antennas, and no guys.  Then I guyed the tower at the top only, for another safety factor.

Never had a problem with the tower, though I did have one element come loose and rotate,  one time.  And one of he two antenna to mast u bolts snapped in extreme winter cold ( presumably from overtightening).

I also drilled thru the mast and installed a set screw to keep the mast from changing direction in winds. 

The tower was up for 18 years.

When I took it down the last time ( to move to a deed restricted area) I noticed the three 3/8" dia. bolts through the base of the hinged tower upon which the mast sat had  worn through by about  a third, and the holes in the tower that they ran through had enlarged by about a third. I couldnt tell any difference in the towers behavior in winds although Im sure  it may have moved a bit.  So old  especially aluminum towers should be examined at the base bolts, but since the steel bolts  wore, Id extend that to say ALL towers should have their bolts examined and replaced as necessary, with stainless steel bolts ( as these were.)

The antennas  on that 20 ft mast ( 12' exposed) would dance definitely around and sway but the strength of the aluminum alloy was enough to handle it through several instances of 70-75 mph winds and many days of winds in the 50+ range.

Overdesigning the mast ( the most common failure point) is always a good move.  And using set screws  to lock the mast to the rotator will save you tower climbing for sure.

Good luck.
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WB2WIK
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« Reply #7 on: March 22, 2012, 09:31:45 AM »

A couple of notes:

The CD45-II will handle 500 lbs of vertical "dead weight" easily.  Weight is not a problem.

The pointy top (welded tube) R25G section does provide great lateral support if the mast is just slightly smaller in diameter than the I.D. of the tube.  However, unless you shim the mast at the rotator, the mast will not rotate concentrically and will absolutely, positively, bang against the inside of the tube as it rotates unless you do something to remedy this.  The CD45-III, like the HAM-IV and others of that family, are designed to concentrically rotate a 2-1/16" diameter mast. 

You can certainly use a smaller diameter mast, but then it won't rotate concentrically; all the manuals show you how to "shim" the mast at the brackets to make the mast rotate concentrically, and if you do this, everything will last a lot longer.

The SY-33 will eventually wear out the CD45-III because that rotator doesn't have a wedge brake like the HAM-IV has.  So, every time you stop the beam from rotating, and every time the wind blows hard, the gears are taking a beating.  Eventually teeth break, and the rotator needs service or replacement.  How long this takes depends how often you rotate the beam, and how strong and often the wind blows. Wink
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K2FOX
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Posts: 110




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« Reply #8 on: March 22, 2012, 05:33:49 PM »

The Rohn 25g tower that I have includes a tapered top section with a 2" O.D. tube at the top. My question is really about the top section. Does the tapered top and the tube take the place of a thrust bearing and bearing plate? Can I depend on the tube at the top to take the lateral forces? Or should I get rid of the tapered top section and start looking for a Thrust Bearing Plate and Bearing?

Thanks

Dan KI4AX



Use both the tapered section AND a thrust bearing. Taper will help with the lateral load and the bearing will help with the downward forces put on the rotor. It will extend rotor life considerably.
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KB1NXE
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Posts: 311




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« Reply #9 on: March 22, 2012, 07:26:10 PM »

There is also some merit to the thought your rotor must have some down force load to seat it's own bearing properly to prevent wear, chatter and failure.  While I have a 'rocket top' section AND a thrust bearing, I did load the rotor before I tightened the bolts on the thrust bearing.  My rotor is a T2X.

Get a hold of the book "Up the Tower" by Steve Morris - KL7XC and read up.  It will be the best money you spend in your entire tower budget.
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K4RVN
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« Reply #10 on: March 22, 2012, 10:03:59 PM »

Steve,
I am just curious, is the disc brake on the CD45-ll not good enough to hold the beam? I almost bought a used one recently
and know nothing about them except I do have the spec sheet.


Frank
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K2DC
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Posts: 1365


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« Reply #11 on: March 23, 2012, 03:56:03 AM »

Dan,

   Don't be so sure about that 22 element 2M beam.  The 17 element Cushcraft is 3.9 sq. ft., and their 26 element is 4.8 sq.ft.  I would guess that your 22 element would be somewhere in the middle, perhaps around 4.4 sq. ft., but much more than the 2.8 that you guessed.  That would put you at a total of 10.1 sq. ft. and well above the maximum recommended for your rotor.

73,

Don, K2DC
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K3GM
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Posts: 1804




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« Reply #12 on: March 23, 2012, 07:25:13 AM »

Another possibility is replacing the current rocket top section with a section of 25G, then obtain a flat top Rohn clone stub from a place like Norm's Fabrication.  I have one of his top stubs, and can vouch for its quality; very nice.  The 25G section and stub is a much less expensive way to go.  However, it's not without its own drawbacks.  In this case, the rotor shelf is positioned only 12" below the top plate.   The alternative is you have to place it at the top of the last section 10' below, which isn't bad at all if you have a long enough mast.
« Last Edit: March 23, 2012, 07:31:39 AM by K3GM » Logged
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