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Re: [TowerTalk] MonstIR/Orion problem

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Subject: Re: [TowerTalk] MonstIR/Orion problem
From: "Michael Tope" <>
Date: Sun, 26 Feb 2006 20:44:05 -0800
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----- Original Message ----- 
From: "Rick Karlquist" <>

> The physics of the slip-nott don't necessarily make sense.  If
> you have a certain coefficient of friction, and a certain amount
> of clamping force, in theory, it shouldn't matter if you distribute
> the force over a large area with a slip nott or a smaller area.
> The slip-nott is not engineered to have more total clamping force.
> Maybe it works for reasons I don't understand.
> Rick N6RK

I think if the materials involved are infinitely stiff, you would be
correct. Real materials deform in response to the applied forces. 
I think that is why the M^2 OR-2800 clamp is so prone to slip. 
I have torqued the six grade 8 bolts on the Orion clamps as tight 
as I could get them with a 3/8" drive ratchet to the point where I 
thought there was just no way the mast would budge only to find 
the coax service loop on a KLM 4 element 40 meter yagi wrapped 
two and half times around the mast after a good windstorm. Those 
Orion clamps don't distribute the force very uniformly on the mast. 
To see this, imagine clamping a circular cross-section in a square 
diamond shaped clamp. As the clamp tightens, the mast cross-section 
will tend to oblate into an ellipse in response to the applied force. 
The square diamond cross-section of the two clamps halves will also 
tend to deform into a non-square parallelogram. These deformations 
will cause a portion of the clamping force to act in a direction that 
is not radial with respect to the mast cross section. Contrast this 
with a set of fitted clamp shells that precisely match the cross-section 
of the mast. When this set of clamps it tightened, the there will be less 
of a tendency for the mast to oblate, and when it does it will build up 
radial pressure in the direction perpendicular to the bolts as the mast 
tries to buldge out and runs into the closed sides of the fitted clamp. 
This is precisely why a hose clamp won't slip when minimally 
tightened around a piece of flimsy aluminum tubing, that would 
otherwise deform severely when placed in a non-uniform clamp 
(like a vise). An oil filter wrench compared to using a set of 
channel locks to remove a flimsy oil filter is another good 
example of this principle. 

I still, however, have my reservation about the Slipp-Nott's claim 
that it will save rotators where pinning the mast will tear them up. 
This is only true if the failure point for the rotator is well above the 
point where the winds are making the mast slip. In this case adding 
the Slipp-Nott will prevent slippage in moderate winds but still 
allow slippage at high levels of wind force that would break the 
rotator. Adding the Slipp-Nott is analogous to increasing the size 
of a fuse in an electrical circuit that is prone to blowing the stock 
fuses under normal operation, whereas pinning the rotator would 
be analagous to bypassing the fuse altogether. You can't, however, 
make the fuse any bigger than the circuit (in this case the rotator 
gears) can handle without failing, so even the Slipp-Nott isn't a 
cure-all. It is just a way of making the holding force of a rotator 
mast clamp more closely match the ratings of the rotators internal 
components. If the rotator will break in 120 MPH winds, but the 
mast is slipping in 50 MPH winds, then that is a good application for 
the Slipp-Nott. If, on the other hand, the mast in that same rotator 
doesn't slip until the winds reach 110 MPH, then you probably don't
want to add the Slipp-Nott. Of course, if the system is designed
so the wind never gets strong enough to break the rotator, then
you can just pin it (assuming pin is big enough so it doesn't break) 
and be done with it. 

73, Mike W4EF........................


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