On 12/20/2013 1:54 PM, Richard Karlquist wrote:
On 2013-12-20 08:55, Patrick Greenlee wrote:
Interesting dialog about slipping mast, antenna, and rotator clamps.
What is wrong with a sheer bolt? (through bolt in sheer)
Patrick NJ5G
My big mower comes with a sheer bolt.
As does my snow blower and rototiller.
The concept is
that the sheer bolt MUST be a relatively soft materical
and the shaft assembly must be a relatively hard material.
And likely is machined to have a specific break point.
Most hams "pin" the mount to the mast. Some call them shear bolts, but
it's a rare case when the bolt is truly a shear bolt.
The sheer bolt is designed to break off if you hit
a stump, etc. It also gets gradually chewed up due
to normal wear and tear and eventually breaks apart
for no particular reason. The warranty is void if
you replace the sheer bolt with a grade 5 or something
hard.
Usually anything other than the specified bolt.
My neighbor performed that experiment and broke
his gear box.
If you want to replicate that on your tower, then you
immediately have the problem that your mast is not
hardened (and if you have an aluminum mast forget it)
I believe Chrome Moly is hard enough, but where would you get the proper
size (and resistance) shear bolt.
It you do get it to work, then whenever there is a
sufficiently big storm, the sheer bolt saves your rotor,
but the cables get trashed. You get to climb the
tower and fix all this stuff, but at least the rotor
doesn't break.
Get the proper size rotator to begin. The big boys use heavy rotators
and no shear pins that I know of. They use a large enough rotator to
handle the load.
Look at first two photos:
http://www.rogerhalstead.com/ham_files/skyhook.htm
If you replace the sheer bolt with a grade 5 or 8, then
it gradually enlarges the hole in the soft mast, with
produces more play, which exascerbates the problem.
And you no longer have a mechanical fuse, so you better
have a really rugged rotor.
What you really need to do is to do some precision
drilling and tapping so that the bolt is threaded
Drilling and reaming so there are no threads in contact with the mast or
mount. It should be a smooth fit, or even press fit. Drill rod is great
for this. Press fit does not mean installed with a hammer. Threads in
both the mount and mast creates a problem as tightening the bolt will
change the spacing and thus the thread alignment. Even a slight change
adds stress to the threads which are a weak point unless they are rolled
threads (aircraft bolts). There are tollerances on taps and bolts so
getting the threads to match the bolt precisely is unlikely. Using a
sacrificial bolt to lap the tapped threads and carefully selected bolts
for fit might work.
None of these little things usually mean much until time and/or a storm
adds stress to open the holes just a little, but once started, they grow
ever more rapidly.
I've seen masts pinned with nothing more than a large nail (that fit the
holes well) that lasted for years. One was mine when I was 60 years
younger. (Didn't know what was proper and couldn't afford it anyway.)
But usually, proper pinning of masts takes drill rod and a reamed hole
for the best results and life. Most of us drill a hole, stick a bolt
through, put a washer and nut on it, and tighten. A couple years later
we notice the bolt is wearing and the hole is about half a diameter
larger. We may drill a new hole a few inches, higher or lower with a
snug fit. If it is tight enough to require a few hammer taps on the
head to seat it, all the better. this should last a lot longer.
Hopefully the rotator is up to the task and the antenna(s) are not too
much for it.
into both the mast and the mast clamp so there is
zero play. Unfortunately, this makes it very difficult
to line up the holes to get the bolt threaded in,
with the antenna flopping around making the mast
turn.
So in conclusion, pinning the mast is not a panacea,
but has its own problems.
With that I agree. <:-))
73
Roger (K8RI)
Rick N6RK
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