[TowerTalk] Slipp-Nott

[Roger (K8RI) on TT]

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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