[TowerTalk] More on Orion mast clamps

Mark . n1lo at hotmail.com
Fri Mar 3 11:21:18 EST 2006

Hello Al,

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 
larger storms.

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.



Al writes:

<< ...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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