Roger makes a good point about many rotator bearings needing some axial
load and the high axial loads they are rated for.
I would add a concern about the dead weight load imposed on the tower if
a thrust bearing at the top supports the heavier mast sizes/materials
discussed. All tower specs I've seen have a dead load spec in addition
to wind load. Self supporting towers either fail in buckling,
compression or tension of some structural element and excessive dead
load may be a problem. IMO, put the mast weight on the rotator and use
a true radial bearing (wood, plastic, etc) at the top. Then it is also
easy to hoist the mast up a bit to remove the rotator. (aren't the
goals to make it easy to work on the rotator and take loads off the tower??)
Grant KZ1W
On 9/8/2013 12:04 PM, Roger (K8RI) on TT wrote:
On 9/8/2013 1:08 PM, WA3GIN wrote:
I used this method on the HDBX48 and its been up for over 30
years.... funny thing is I've never had to work on the rotor
(Tailtwister). I use a thrust bearing to keep the pipe weight off
the rotor bearing although I'm sure they would handle the weight. I
used allen screws to set the couplings. Abt 12sqft of antenna on the
mast. I ran a ground strap from the mast coupling, just above the
rotor, to ground.
I really like the innovative suggestions.
There is one thing I caution on and that is somr rotators are designed
to work with a load on them and a negative load can be detrimental.
With the cable suspension, I's add a spline say from a car drive
train. A good one with no slop, that would allow vertical movement of
the mast and no negative (pull) on the rotator. The whole works would
leave about 10# on the rotator so there would be no tendency to pull
against metal with no bearings. Just keep it well greased.
Mounting the rotator independent of the tower removes all the torque
from the tower so all it has to do is hold things upright. Of course
this does put the entire torque load on the rotator, but it was
designed for that.
There is, or can be, a problem with mounting the rotator to the tower
near the base. The tower can no longer spread the torque along its
length, but rather it now is between the rotator and the concrete
almost all in shear mode. If mounted right at the bottom, it is all
shear mode and unhealthy for the tower.
A local ham poured the pad a bit wider than needed. He then built a
platform to fit inside the tower, but was entirely supported by two
heavy steel angles (facing each other) bolted to the concrete. He
used 2" X 3/16ths or 2" X 1/4 angle IIRC.
The platform was just an accessory shelf with the ears cut off to keep
it clear of the tower. This was securely bolted to the steel angle.
Just think of two steel angles about 6 to 8 inches apart (what ever is
convenient and fits) These bolt to the concrete roughly 6 to 8 inches
outside the tower. They then go up a foot or so, then horizontally
through the tower and back down on the other side. If tower bracing or
construction prevents them from being at the same level, a block or
riser can be fashioned to go on the lower one so they are at the same
height where the rotator shelf mounts. Dimensions are not critical.
For masting he used 2" Chrome molly with 1/2" wall. Heavy and over
kill would be an understatement. I could not lift one end of a 24'
length and I was doing full squats with 205# of free weight. We had
to come up with a different rotator and he came up with a lighter mast.
One note on the mast being supported by cables that cause the mast to
move vertically. This adds a varying percentage of the entire weight
of mast and antennas to the rotational mass so the rotational torque
varies through the rotational cycle. How much? I don't know as it
depends not only the rotational angle (180 deg at max) plus the
vertical angle to the anchor point. Higher is less.
I believe the tail twister is designed to support 500# dead load
although it might be 800#. I'm sure some one on here will have the
figure.
73
Roger (K8RI)
73,
dave
wa3gin
----- Original Message ----- From: "Wilson" <infomet@embarqmail.com>
To: <towertalk@contesting.com>
Sent: Sunday, September 08, 2013 12:41 PM
Subject: [TowerTalk] Ground Level Rotators
I love the pipe mast and cable suspension.
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