[TowerTalk] [Bulk] Re: [Bulk] Re: Top Bearing Plate for 55G vs Rotor Plate

Thu Aug 14 17:49:24 EDT 2014

The moment at the tower top is what it is from mast and antenna wind 
load above the top bearing.  You can move around a bit how the mast 
shares the loads with the top section of the tower structure but you 
aren't going to reduce it with a center bearing.  Also, bending 
(deflection) is not bending moment.

There are several types of top bearings.  With the tube version on US 
Towers or a Rohn tapered top section essentially no moment can be 
transferred to below the tube.  Some thrust bearings with a ball bearing 
single race can tilt a bit, but will be damaged if the bending moment 
causes more than a few degrees deflection.  For thrust bearings with two 
races the bearing will prevent most tilt. If a spherical outer race 
flanged pillow block is used, then more tilt deflection can happen and 
thus more bending below the bearing. If a plastic top radial bearing is 
used then how much bending is below the bearing depends on mostly the 
thickness of the plastic. If a combined thrust and radial plastic 
bearing is used, then there will be little bending below the bearing. 
(i.e. 3" mast in 4" of UHMW and a 5.6" diameter 6061 thrust plate on the 
mast).

Since the original question was about R45 top plate thickness, for any 
sensible thickness the concern is more the bending of the plate vs the 
weight it can carry.  My "backyard engineering" guess is 3/16" is 
probably ok, 1/4" likely ok, and 3/8" is overkill.

So for some top bearing types, you can slightly reduce the mast bending 
inside the tower and a small amount outside with a center bearing.  The 
moment of inertia ("stiffness") of Rohn 25 is 30 times that of a typical 
2" x 0.25" wall steel mast.   Bending deflection for a point loaded beam 
and a fixed base is proportional to the length cubed divided by the 
moment of inertia.  So there is 30x less deflection in a stick of R25 vs 
a 2" x 0.25" wall mast for the same length and load, which is why the 
adage "masts are no substitute for a tower" is true.  Thus, there is 
also very little contribution to the strength of a tower section from an 
internal mast.  Long internal masts may be nice for where the rotator 
goes, but not much more.

A center bearing might slightly change the moment applied to the 
rotator, but again that is usually not a concern.  And yes, 6061-T6 
aluminum masts, with about 1/3 the modulus of steel will bend more for 
the same physical dimensions and forces.

There are handy calculator apps for

moments of inertia of pipe at 
http://www.engineeringtoolbox.com/pipe-formulas-d_1335.html  (helps 
understand why bigger diameter is better than thicker walls)

And for simple beam deflections and stress (not for lattice towers) at 
http://www.engineersedge.com/beam_calc_menu.shtml

The Rohn 25 mechanical data is at 
http://static.dxengineering.com/global/images/chartsguides/r/rohn-25gdrawingc630625.pdf

caveat emptor etc.  My comments are not to be used for designing structures.

Grant KZ1W

On 8/12/2014 10:59 AM, Doug Renwick wrote:
> Here is my take.  A blanket statement  does not fit all situations.  A
> middle bearing does have merit.  I cases where a long mast fits inside the
> tower or there are stacked antennas on the same mast, a middle bearing will
> stop the bending moment in a strong wind.  And a bending moment will be
> larger with an aluminum mast.
> I put a middle bearing in my towers.  Alignment has not been a problem.  The
> middle bearing is loosely fitted so there is some room for motion.  It is
> easier to have it there than try and install it later.
> Doug
>
>
> -----Original Message-----
>
> I agree with Steve that a middle bearing is not a good idea as aligning
> 3 bearings is very difficult and a middle bearing serves no purpose at
> rotator rpms.
>
> However, a middle support plate with a removable u-bolt or clamp, that
> can constrain the mast when you want to raise it to remove the rotator
> for service is a big help.  Then the mast stays in column and doesn't
> overload/twist the top bearing.
>
> I also dislike the commercial "thrust" bearings.  A block of black UHMW
> 3" thick bored for the mast od will never need servicing.  Or stack up
> some HDPE kitchen cutting boards bored to fit, but they will need a coat
> of paint to protect them from UV.  A rotator sized for a 4L full size
> 40m beam bearings can easily handle a 500lb thrust load.  btw, my 40m 3L
> full size OWA weighs 350# plus 150# for the 3" x 0.25" wall alloy mast
> (a must for that size antenna).
>
> Grant KZ1W
>
>
> On 8/12/2014 8:04 AM, K7LXC--- via TowerTalk wrote:
>>>    I'm working on getting the parts together for a Rohn 55g  tower.
>> ?  The Rohn 55g Bearing plate sure is expensive.  I see  two other
>> possibilities.
>>
>>>    1 - Use the less expensive beacon  plate and drill a hole for the
>> mast/bearing.
>>>    2 - Use a rotor  plate for the same purpose.
>>    
>>>    I plan on having a top bearing, an intermediate bearing  then the
> rotor.
>>    
>> I have no idea what the thickness of the Rohn bearing  plate is as
>> compared to a beacon or rotor plate.  I'm hoping to put up  a full size 4
>> element 40m beam some day, so the system will need to handle a  good
>> 300+lbs.
>>
>>
>> Hiya, Rich --
>>    
>>       The beacon plate is FB if you don't mind  drilling it.
>>    
>>       As for an intermediate bearing, I'd  discourage it. The plates are
> not
>> precision items and you'll just introduce bind  into the system. Since all
>> you're trying to do is capture the mast, put the  rotator plate in the
> middle
>> and skip the bearing - cheaper too. And the upper TB  carries all the
>> weight anyway so a middle bearing doesn't provide any structural  help.
>>    
>> Cheers & GL,
>> Steve     K7LXC
>> TOWER TECH
>
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