[TowerTalk] Fwd: torque arms or not?

Sun Jan 8 21:42:18 EST 2017

A good friend of mine had a 100 ft tall tower topped with a wind 
generator.  It was constructed of 20 ft lengths of 4 inch tubing with 
1/4 wall thickness each having welded on plates to join the pieces with 
bolts through the flanges. The legs at the bottom are on 14 ft centers.  
X bracing is 2x2 angle.  The tower lasted a few years but the design did 
not allow for the wind changing direction without slowing down first. 
Gyroscopic effect translated torque about the vertical axis to forces 
trying to either lift the prop end of the generator or lower it.

These forces were not properly accounted for in the design and one 
evening it wiggled about some and collapsed. There was enough reusable 
materials to reconstitute the bottom 40 ft.

Repetitive motions/forces unaccounted for in the design caused a 
catastrophic failure of the system. There is good news.  The tower was 
over 100 ft from his house and I have the 40 ft tower refurbed and on a 
good foundation permitting it to tilt over on its two hinged legs..  Be 
wary of repetitive motions, especially if they in any way come close to 
the elastic limit of any of the materials.

On my DX70 tower project I will be programing my controller to ramp up 
and down relatively slowly so as to not introduce a large twisting 
effect of the roughly 350 lbs of antenna, rotator, mast, and TiltPlate.

Patrick        NJ5G

On 1/8/2017 8:21 PM, Hans Hammarquist via TowerTalk wrote:
>   Isn't the fatigue problem with the tower itself or the joining points between each section?
>
> I assume that a torque arm is an arm with two parallel, or almost parallel, guy wires attached. Whe the tower tries to twist the load moves from one guy wire to the other. Correct?
>
> Even if my tower is "correctly" guyed I would like to attach an extra set of guy wires half between the guy point and ground. The tower vibrate uncomfortable when I am at the half way point. Yes, it's an aluminum tower. I was told aluminum is more "springy" than steel.
>
> Hans - N2JFS
>
>   
>
>   
>
> -----Original Message-----
> From: jimlux <jimlux at earthlink.net>
> To: towertalk <towertalk at contesting.com>
> Sent: Sun, Jan 8, 2017 6:39 pm
> Subject: Re: [TowerTalk] torque arms or not?
>
> On 1/8/17 12:53 PM, Keith Dutson wrote:
>> I like to climb.  When up the tower and testing rotation (ground crew
>> operates), I do not like the feeling when the antenna starts and stops.
> so that's a "feels uncertain under my feet" kind of thing, which is
> totally rational.
>
>> Small Yagi antennas, such as a tri-bander are not a worry, but BIG antennas,
>> for 40 and 80, really do a number on twisting on my 45 and 55G Rohn towers.
>> I think this is not good for the guying system.
> There, I think that's not really the case.. the loads on the guys are
> probably not significantly varying.  The guys are steel or composite,
> and fairly low load, so fatigue failure isn't an issue.  Maybe there's a
> "wear on the attachment point" issue, but I that that wind gusts (which
> happen all the time, much more frequently than rotor moves) would be a
> bigger factor.
>
>
> I think the take home here is that "more guys make it more rigid, which
> reduces the pucker factor when climbing" is the dominant effect.
>
>> Even with the bracket, there is some twisting, so the rotator is not getting
>> much more torque.
>>
>> 73, Keith NM5G
>>
>> -----Original Message-----
>> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of
>> jimlux
>> Sent: Sunday, January 8, 2017 2:00 PM
>> To: towertalk at contesting.com
>> Subject: Re: [TowerTalk] torque arms or not?
>>
>> 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
>>> snip>
>> _______________________________________________
>>
>>
>>
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