[TowerTalk] Pinning mast?
TexasRF at aol.com
TexasRF at aol.com
Tue Nov 8 05:58:50 PST 2011
Don't take this as a recommendation but rather as a data point: I have had
great success with pinning masts using 4 X 3/8" grade 8 bolts. Two pinning
holes are drilled completely through the mast to size for a 3/8" coarse
threaded tap. Then the four holes are tapped as far as the tap will go. Some
of them tap completely through a 3" mast. The four bolt lengths are about
half the mast diameter and are screwed into the threaded holes and torqed as
tight as possible with a standard end wrench.
The mast adapter fits inside the mast and is made of solid material. The
fit is pretty close, maybe a couple .001" clearance. The wind loads fall
between 30 and 200 sq ft for the eme antennas. So far, there have been no
failures, thank goodness! The 200 sq ft antenna has been in service since 1992
and the others from 1999 and 2011.
This is a non reversible procedure. The 2011 antenna (32ft dish for 70cm)
uses a recycled mast and rotator from a previous antenna. I had to cut the
mast at the rotator and have the remaining material machined off to reuse
it. Stuck like glue as they say. The rotators are all homebrew worm drive
designs and quite large/heavy, but that is another story.
In a message dated 11/7/2011 10:25:35 P.M. Central Standard Time,
grants2 at pacbell.net writes:
A good question - even a pair of "slim" shims at 90 degrees apart should
work with the compression clamp. These can be ~2x the thickness of a
360 deg shim and easier to drive in. Can the clamp be spread with a
drift or N x chisels to help get the shims in? Or maybe several
threaded studs with slim nuts can be backed against the inside of the
clamp flanges although these will need to be much smaller than the clamp
bolts in order to get the nuts to fit.
The "problem" with pins is getting the pin to hole clearance essentially
zero - as in rivets in a Boeing airplane. Otherwise the mast and clamp
work against each other enlarging the hole. The idea of a taper pin is
reasonable since they have zero clearance, but I wouldn't want to try
that up the tower with a #9 or 10 reamer ($$ and hard to do, step drill
Since the K7NV mast clamp can be deformed in one plane, if shims can't
be placed the best bet is to add a couple of through bolts 90 deg to
this plane. Drill the holes a bit (1/16") undersized and use a core
drill exactly the size of the bolts. A core drill will essentially ream
the hole to round and very close to diameter, even hand held. (but I
think any pin solution is not as good as making the clamp work +/- as
On 11/7/2011 4:46 PM, john at kk9a.com wrote:
> You didn't say what kind of mast clamp you are using. While pinning a
> is good thing in many cases, I think you need to correct the fit of the
> clamp before going any further. My prop pitch has a 3" K7NV mast clamp
> my mast is 2 7/8 diameter so I purchased as 3" diameter stainless steel
> with a 1/16 wall and made a shim out of it. The clamp then tightened to
> mast securely. Why won't yours shim properly?
> John KK9A
> To: towertalk at contesting.com
> Subject: [TowerTalk] Pinning mast?
> From: K7LXC at aol.com
> Date: Mon, 7 Nov 2011 14:50:44 -0500 (EST)
> List-post:<towertalk at contesting.com">mailto:towertalk at contesting.com>
> Howdy --
> I've got a sloppy fit between a mast clamp and a mast - both
> won't shim properly so I think my only option is to pin the mast. I've
> done it a bunch of times and mostly they hold butr sometimes they don't
> proposing to use 2 in one direction and another two 90 degrees from the
> first two with good-sized high strength bolts. Other than the challenge
> doing it at altitude, is there anything else I should be considering?
> there another way to do it?
> This is a prop pitch with big antennas on it.
> Steve K7LXC
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