[TowerTalk] mast intrusion into tower

[Michael Tope]

See my comments below:


Mike, W4EF...............

----- Original Message ----- 
From: <K7LXC@aol.com>
To: <k1ttt@berkshire.net>; <alwilliams@olywa.net>; <towertalk@contesting.com>
Sent: Saturday, January 13, 2001 6:37 AM
Subject: Re: [TowerTalk] mast intrusion into tower


> In a message dated 1/13/01 4:58:50 AM Pacific Standard Time, 
> k1ttt@berkshire.net writes:
> 
> > several reasons:
> 
> Hi, Dan --
> 
>     Good points. 
> >  
> >  1. jacking the antenna further up the mast than you can work standing on 
> top 
> > of  the tower is a real pain
> 
>     Mostly. Temporary steps make the job doable but there is a fairly 
> significant 'pucker factor' involved in climbing the mast.
> 
> >  2. lowering an antenna jacked up on the mast to work on it and then 
> putting 
> > it  back is even more of a pain.
> 
>     It's tedious but sometimes the only way to do it. 
> 
> >  3. the lever force of the antenna working against the top section of tower 
> > gets  larger as you extend more mast out.
> 
>     True.
> 
> >  4. The side force on the rotor gets larger as more mast is above the tower 
> > than in it.  and remember most rotors are only bolted to the mounting plate 
> with 
> > 4  relatively short bolts into a cast housing.
> 
>     Actually the bolts are fine. A destruct test we did at Hy-Gain with a 
> TailTwister showed that the rotator took 3,000 ft/lbs of force before it 
> broke at the neck - the bolts were fine. (I don't know if they used 4 or 6 
> bolts in the test fixture but the results are probably the same.)

As long as a fairly tight sleeve or thrust bearing are used, the forces on the rotator even 
when its mounted close to the top should be almost purely lateral. One problem with
having the rotator too close to the top is that the overturning moment and side load of the 
mast and antenna are transferred to the tower over a small area. This concentrates huge
forces near the top of the tower. If the mast/rotator are placed lower into the tower, then
these forces are distributed across a larger area. 
> 
> >  5. if you have to take the rotor out for repair the further into the tower 
> > the  mast is the easier it will be to control it when the rotor is removed.
> 
>     True. Especially if the mast is top-heavy due to antennas stacked up the 
> mast. 
> 
Agreed, that is probably the scariest thing about this sort of installation. 

> >  6. if the mast should bend it becomes an expensive and dangerous job to 
> > remove
> 
>     If you've got a bent mast (and shame on you for using the wrong one), the 
> ONLY way to get it out safely is with a crane. DO NOT send your nephew up the 
> mast or any other non-professional technique. 
> >  
> >  some of these can be helped by putting an extra bearing below the tower 
> top 
> > to control the bottom of the mast and take some of the side force.
> >  
>     I don't know that it takes much side force but I don't recommend a second 
> (intermediate) bearing. The reason is that it's very easy to introduce 
> excessive bind into the rotator/mast path with a second bearing. The rotor 
> and top plates are not precision tower parts and you need relatively large 
> tolerances to get everything working together.
>
Steve, you are correct. I have seen several installation that used a tight
mast sleeve, and a thrust bearing for the mast. Both of these installation
ended up binding so badly that the rotator's (T2Xs) would not turn the
antennas. The solution was to float the thrust bearing flange on the bearing 
shelf (e.g. double nut the flange bolts before they were tight). In this case
the sleeve took care of the side load, while the bearing took the vertical load. 
The a two bearing installation is subject to the same binding problem. A
good solution is to install a second bearing and bearing plate but leave it
loose (oversize flange holes and double nutted fasteners). When the rotator
needs to be removed, the second bearing can be put into service within a 
matter of minutes. 


 
>     With Rohn 45G and large-type towers, I just install an intermediate rotor 
> plate (accessory shelf) with NO bearing. The hole in the plate is enough to 
> capture the mast and keep it from wandering around so you can do a rotor swap 
> or other mast manuver.
> 
> 7. The farther the mast is down the tower, the farther down you move 
> wind-induced torque. (The most desireable place for the rotor plate is at the 
> guy attachment point so that forces that come down the mast into the rotor 
> and plate are transferred directly to the guys. This is usually not very 
> practical.)
> 
> Cheers,    Steve     K7LXC
> Tower Tech 
> 
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> 



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