----- Original Message -----
From: "Rick Karlquist" <firstname.lastname@example.org>
> 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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