[TowerTalk] torque arms or not?

Sun Jan 8 14:59:33 EST 2017

On 1/8/17 11:39 AM, Keith Dutson wrote:
> Most hams I know are using star guying to avoid excessive twisting when the
> heavy Yagi stops rotation.  I use the bracket from Norn.
>

Is that because of "nice to have" or is there an actual structural 
reason.  I wouldn't think the number of twist cycles  is such that it 
would cause fatigue failure, and if you make the tower more "rigid", 
then the stopping forces on the brake and rotator housing are higher 
(maybe.. the "jerk" might be higher, but the actual torque might be the 
same)

As Mike says, for a high gain dish, with a beamwidth measured in degrees 
or fractions, you need the tower to be rigid enough to keep the pointing 
loss reasonable.  But for HF Yagis, the beam width is 10s of degrees (if 
not 60-70 degrees), so a few degrees of twist is a non issue.

In these "cable stayed truss" structures you don't want them too rigid, 
because that potentially causes stress concentrations in some parts of 
the structure. (piano wire and rope in parallel problem: the piano wire 
takes more and more of the load as the total load increases)
But, nor do you want it so floppy it isn't controllable.

> 73, Keith NM5G
>
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of
> Michael Tope
> Sent: Sunday, January 8, 2017 12:37 PM
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] torque arms or not?
>
> I could be wrong, but I suspect one major application for star guying are
> towers that support very high-gain microwave antennas that require very
> precise and stable beam pointing. You don't want a microwave link to fade
> because the tower is twisting in high winds.
>
> 73, Mike W4EF.........
>
> On 1/7/2017 12:56 PM, DALE LONG wrote:
>> This brings up a related question.  We lost a lot of towers in southern
> Haiti during Hurricane Matthew. Most of them were newly built 100-120 ft
> Rohn 25 towers with adequate guying and torque brackets but not torque arms.
>> Here is a question for the tower professional.  If you are not using a
> rotator, but only some VHF antennas at the top, how important is it to add
> the torque arms. We use a baseplate with pier pin so we are allowing for
> some movement at the base.
>> Our tower in Dame Marie was hit by 145 mph blunt force winds and I dont
> know if anything would have survived.  But several towers folded over, with
> the guy anchors and guy wires intact. We are thinking that we need to move
> up from Rohn 25 just to survive the hurricanes, although the antenna wind
> load is minimal.
>> To include torque arms or not, that is the question. (for tower without
> ham rotator).  Will the oscillations be reduced by the torque arms?
>> Dale - N3BNA
>>
>>        From: Jim Thomson <jim.thom at telus.net>
>>   To: towertalk at contesting.com
>>   Sent: Saturday, January 7, 2017 2:44 PM
>>   Subject: [TowerTalk] Datasheet for Rohn TA-55
>>
>> Date: Sat, 7 Jan 2017 11:10:24 -0500
>> From: Guy Olinger <k2av at contesting.com>
>> To: john at kk9a.com
>> Cc: Towertalk <towertalk at contesting.com>
>> Subject: Re: [TowerTalk] Datasheet for Rohn TA-55
>>
>> On Fri, Jan 6, 2017 at 5:57 PM, <john at kk9a.com> wrote:
>>
>>> . I remember when K4JA's rotating AB105 tower started twisting back
>>> and forth in heavy wind before breaking apart. I would assume that a
>>> star guyed tower is less susceptible to destructive harmonic oscillation.
>>>
>> Believe this was during hurricane Isabel in 2003. The prevailing yak
>> afterwards was that if he had cut the lines at the bottom and unpinned
>> the rotator, let the tower and yagis "windvane", that he would have
>> weathered Isabel with only minor damages. Maximum sustained winds were
>> only 52 mph with gusts to 66.
>>
>> 73, Guy K2AV
>>
>> ###  How tall was the rotating AB105 tower ?  52 mph wind with gusts
>> to 66 mph isnt what I would call hurricane force.  I call that a  real
> heavy wind.
>>
>> ## On paper it appears that a fixed tower with one or more star guys
>> will result in minimal to no torque at all.  2nd best would be a fixed
>> tower with no star guys, and just the usual 3 guys  per level.  3rd
>> best appears to be a rotating tower, which provides no  torque reduction
> at all.
>>
>> ##  I  just cant fathom a 150-200 ft tall  rotating tower with the
>> usual myriad of yagis on it, in a
>> 100 mph wind.  A  go pro camera at the base of a rotating tower in
>> high winds, pointed straight up would be an eye opener.
>>
>> ##  Torque balancing the yagis would go a long way to minimizing the
> torque on the tower.
>> I designed a torque compensating plate for a local fellow years ago,
>> using yagi stress.  Without the tq comp plate installed, it would eat
>> his T2X  rotor.  He ripped 2 of em apart.  With the plate installed,
>> we lucked out, and torque was reduced to almost zero.  It was tested on a
> temp lower tower, at the 30 ft level, with just a pair of thrust bearings
> installed,  no rotor, and no coax.  It was free to windmill.
>> In a 40 mph wind, it did not budge or windmill.  He climbed up the 30
>> ft tower during the windstorm, and could rotate the boom by hand, and
>> it would stay put, pointed in the new direction.  We thought that was
>> good enough and re-installed the yagi on the taller tower.  I factored in
> the coax and balun etc, plus the offset between the 2 inch boom, and the 2
> inch od mast.
>>
>> ##  dunno why yagi makers dont include a TQ comp plate with their
> offerings, its  essentially a non cost item.
>> Done right, it would take a huge load off the tower.  If yagis mounted on
> a mast,  I also alternate sides of the mast.
>>
>> Jim  VE7RF
>>
>>
>>
>>
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