(Part - 1) I am having trouble with my rotor not holding my beam in
position during wind storms. The beam wants to turn such that the boom
is pointing into the wind and the elements are broadside to the wind. I
have been fighting with it for a while and will make two posts. One post
for each mode of failure. In this post I would like to talk about the
mast slipping in the rotor bracket.
I have an investment in HAM IV rotors (four of them). I do not want to
change to another brand/model if at all possible. My SteppIR DB-18E is
installed 18 inches above the tower top plate thrust bearing. The mast
goes down 10 feet into the tower to a HAM IV rotor. There is no vertical
weight placed on the rotor and minimal horizontal force being placed on
the rotor given the 10 ft. to 1.5 ft mechanical leverage advantage. The
rotor has no problem turning the antenna and I always allow the antenna
to coast to a stop before engaging the rotor "brake". I designed the
system this way so that the only "work" required of the rotor would be
to turn the antenna (over coming the antenna's resting momentum) and
holding the antenna in position when the wind try's to turn it (the
brake). The HAM IV is rated for 15 ft-sq antennas. The DB-18E is rated
at 12.1 ft-sq. I used this exact same set up with a Mosley PRO-67B for
decades with no issues. The Mosley is rated at 12 ft-sq wind loading.
The HAM IV turns the SteppIR beam with no problem (even in the coldest
weather). This set up is not holding the DB-18E in place during wind
storms (30 - 50 mph gusts). The holding capability of the rotor can not
stop the DB-18E from turning into the wind.
Here is what I have tried so far: I tried a stainless steel bolt (1/2
inch) sharpened to a point that is mounted in a "nut". The nut is welded
to the steel bracket that is part of the rotor to mast clamp. I would
first tighten down the U-bolts and then tighten the 1/2 inch bolt such
that the point would dig into the mast. This failed because the
stainless steel bolt was softer than the mast and the turning mast just
rounded off the point on the 1/2 inch bolt. Next I ground the end of the
1/2 inch bolt flat and drilled a 1/4 inch hole into it. Into this hole I
placed the shank of a 1/4 inch drill bit that I had ground to a point.
Again I tightened this new improved bolt so it would dig into the mast.
This failed because the pointed 1/4 drill bit shank sheered off. My next
attempt was to use strips of dry wall sanding mesh between the mast and
the rotor to mast bracket (the mesh is like mosquito screen with sanding
grit attached to it - there is no paper involved and it is abrasive on
both sides). This arrangement held, but the "weak link" just moved to
the connection between the bottom of the rotor and the rotor plate (I'll
talk about this in another post). I am not happy with the sanding mesh
method because I discovered while taking the rotor down that the mesh
had disintegrated along the four pressure points where the mast touches
the rotor bracket.
In general, I think the rotor bracket is a poor design. The cast
aluminum portion that is part of the rotor upper bell is a V-trough with
a raised cross hatch pattern. The steel outer bracket is also a
V-trough. They effectively only touch the mast along four vertical
pressure lines and provide very little surface area to increase friction
between the bracket and the mast. They do allow for differing diameter
of masts to be used which I am guessing was the primary design goal.
I am hesitant to pin the mast to the rotor bracket (using a bolt that
travels completely through the rotor bracket and mast) as I have heard
that the cast aluminum portion of the bracket that is part of the upper
rotor bell housing will break under the load. Looking at the aluminum
bracket, I can find no location that is very "beefy" where I would feed
comfortable drilling a hole for a pinning bolt to pass through.
Well this is where I stand. I would appreciate some insight from those
of you who have mastered this problem.
P.S. I don't mean to be rude, but, I am not interested in hearing how
well your brand X, Y, or Z rotor works for you as I want to make my pile
of HAM IV rotors work for me. It also would not be helpful to tell me
that you have never had this problem with your HAM IV rotor (lucky for
you). Sorry for this per-emptive gripe.
73 - Jeff KA9S
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