[TowerTalk] HAM IV rotor / mast slipping

[KA9S - Jeff]

(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