To add another thought to the long litany:
Here in Texas, a number of folks have improved the gripping power of various
rotator systems by adding a small section of tractor implement belt between
the rotator clamps and the mast. This dramatically increases the coefficient
of friction at the critical point. Over time it may need to be replaced, but
certainly less often than the generic slippage causes.
Just 2 cents from a windy state.
From: Mark . [mailto:email@example.com]
Sent: Friday, March 03, 2006 10:21 AM
Subject: Re: [TowerTalk] More on Orion mast clamps
Valid questions all. The engineers at Orion would have to provide the
estimated failure torques for the various parts of their rotator. Where do
you want the weak link to be? I think most people would prefer it to be in
controlled slipping of the mast clamps or shearing of a sacrificial external
pin or key rather than gears inside the rotator.
In the absence of real tests, observations from owners like you and others
on this list help out.
To inject a little more science, you and installers like Steve may find
yourself determining the torque required for mast clamp bolts for specific
installations that is just the right amount to hold prevent 'nuisance
slippage' in minor storms, yet allow slippage to protect the rotator in
As an aside, how about adding a 2 pin connector (without screw connections)
similar to Molex or trailer types in the coaxial cable rotator loop? In a
windstorm, if the mast slips far enough, the coax jumper will just separate
at the connector without being destroyed. I'd be willing to bet that at HF
freqs, the impedance bump would be negligible.
My own experience has been with much more modest installations. I want to
try and provide more specific help for you rather than just helping with the
science in general.
Having no experience with the Orion 2800, I brought up M2's web page to see
it (http://www.m2inc.com/products/rotors/or28002.html). Wow, those are
massive cast iron mast clamps! Far, far stronger then the cast aluminum ones
on the Yaesu rotators that can crack if you're not careful.
I see that the 2800's clamps are already toothed to help bite into masts. I
also see what very clearly appears to be a pre-drilled hole in the clamp for
a relatively small diameter mast pin. The web page doesn't say whether
pinning is authorized. My own intuition tells me that a pin of size small
enough to fit through this hole will shear long before the splined joint
between the rotator shaft and clamp assembly- good pre-engineering, in my
opinion, if this is indeed Orion's intent.
At this point, I can see how the mast material itself will cause a variation
in the gripping power of the clamps.
In general, metals with higher hardness (and usually higher yield strength),
and smoother surface finishes will have a lower coefficient of friction.
It's easier to grip an aluminum mast than a steel one, especially if the
mast clamps have teeth. An aluminum mast clamp has an especially hard time
biting into a mast because the teeth aren't hard enough. In the case of a
chrome-moly steel mast that has a higher hardness and yield strength, the
aluminum mast clamps, even with teeth, will just slip without biting. If you
could knurl the end of the chrome-moly mast, it being the harder material
would bite into the mast clamps.
The Orion's clamps are cast iron. I'd have to look up the hardness and see
if the teeth would bite into a chrome-moly type steel.
Steve, how does this line up with your extensive field experiences? Do
aluminum masts tend to slip more readily than smooth chrome-moly masts for
similar antenna loads? If the steel mast has a layer of rust or is
galvanized, I'd guess it would have more friction.
<< ...snip...How did M2 come up with Braking Torque specification and what
kinds of force
(wind, inertia) is the rotator subjected to by the large beams? How is it
It seems to me that the "band aid" solutions may increase the holding
strength of their mast clamp but ultimately would cause the rotator to
break--whether pinned or the clamping just strengthened as the load is
The real question is how capable is the is the rotator to withstand the
forces applied by large beams. k7puc >>
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