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.)
> 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.
> 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.
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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