[TowerTalk] torque arms or not?

[jimlux]

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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