You probably don't want to hear this, but you're really pushing it with a
Ham IV turning 12.1 sq ft of antenna. I agree with Roger that the ratings on
these rotors are generous, and they really need to be derated for wind. I
wouldn't use anything less than a Tailtwister, G2800 or Orion with one of my
4-el SteppIRs, which are 10 sq feet, much less a DB-18.
Now you may be thinking that this has nothing to do with the slippage
problem, but if I'm not mistaken the Ham IV jaws are shorter than the
Tailtwister jaws. The extra height translates into more holding power. I
would bet that if you used a Tailtwister you would have fewer slippage
problems and you wouldn't be asking the rotor to do a job it can't handle.
You haven't posted on the rotor-to-shelf issue yet, but I have a feeling I
know what it is because I've had the same problem with my Ham IVs. It's
fixable with Loc-tite, but I've not had that problem with the Tailtwister,
maybe because it's got a larger base.
All that said, you'll still have problems with any of the rotors I've
mentioned. While I've never had slippage problems with a Tailtwister, I've
had the infamous brake problem. Don't know if that's changed since MFJ
bought the company, but it varied from one rotor to another. I finally found
a couple that didn't have the problem. The other rotors, G2800 and Orion,
have mast slippage problems with big beams, too. The G2800 jaws are too
short. It works to pin the mast, but eventually the bolt will shear off.
That seems to be true with any pinning method. The Orion jaws are poorly
designed, with not enough contact points (very similar to the Ham IV in that
respect.) The only way I was able to solve that problem was with K7LXC's
replacement rotor jaws, which clamped the heck out of the mast with six or
seven U-bolts. K7NV has a different replacement that works just as well, but
is more expensive and harder to install.
That makes me wonder if you can get more clamping power with a Ham IV by
omitting the steel plate and just using u-bolts. I suspect you may not be
able to find a u-bolt that has the right width and an arc that fits snugly
on the mast, but it's worth trying. Maybe additional holes can be drilled in
the stationary portion of the clamp for more u-bolts. Take a look at K7LXC's
design on his website to see if it can be adapted:
I guess if you're dead-set on the Ham IV and the above isn't feasible, a
Slipp-Knott makes the most sense. However, my research on the product
revealed that it's designed to slip if the torque exceeds a certain level.
The reason is to protect the rotor.
73, Dick WC1M
> -----Original Message-----
> From: KA9S - Jeff [mailto:email@example.com]
> Sent: Wednesday, April 04, 2012 10:18 PM
> To: firstname.lastname@example.org
> Subject: [TowerTalk] HAM IV rotor / mast slipping
> (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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