[TowerTalk] To pin or not to pin

[Roger (K8RI) on TT]

The clamp comes off easily as it bolts to a flat plate. The base of the 
clamp is slotted to allow centering it on different size masts.  The 
mast plate you describe could be slotted to mount the same way in the 
same place. Just weld it to the bottom of the mast

As for the 8mm hole.  There will be two, one on each side. Breaking the 
continuity of a circle always weakens it  On a 2" diameter mast that's 10%.
Depending on the mast material, thickness, and how brittle the material 
the holes weaken the mast.  Being as they break the circumference in two 
spots, they weaken it more than the 10%.  How much more is beyond my 
knowledge.  Remember, we have not only weakened the mast, but are 
applying force in the opposite direction with the shear pin. A reason 
the shear pin must fit tightly and the hole is a smooth surface with no 
rough spots to serve as the starting points for cracks.

Take an antenna that uses rivets to hold the elements together. Although 
small, with the first rivet only a half inch from the end of the larger 
tube, if the end brakes off, it almost always at the "single rivet" hole.

These are extremes but the small aluminum tube in the element shows that 
the hole does weaken the tube.  If you use two rows offset by 90 
degrees, the holes in the two rows should not line up.  As an example 
only: If one row of 3 starts a half inch from the end and has one inch 
spacing the second row should start 1.5 inches from the end.  Holes need 
to be clean and smooth.

Whether the weakening caused by the holes needs consideration, depends 
on how close the force we are applying is to the ultimate strength of 
the mast.  This is where size does matter. Larger is better. Much 
better.  With a lot of torque, a 3" mast is much better than a 2" with a 
thicker wall. Plus the 3" presents a much larger gripping surface 
assuming the rotator will handle the larger size.

In most cases it's of little concern, but with large antennas weighing 
several hundred pounds and long booms, the starting and stopping torque 
can exert thousands of ft lbs. Then the holes need to be taken into 
consideration.

73

Roger (K8RI)


On 8/28/2016 Sunday 7:59 PM, mike repinski via TowerTalk wrote:
>   The clamp looks like a poor design. What would be a trick solution would machine a 1/2 thick plate that would replace the clamp and have a hole bored to let the mast pass thru and weld the mast to the plate that would then bolt in place of the clamp that dos not work. The only issue is you would have to take it apart to do this.
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> -----Original Message-----
> From: john <john at kk9a.com>
> To: towertalk <towertalk at contesting.com>
> Sent: Sun, Aug 28, 2016 7:44 pm
> Subject: Re: [TowerTalk] To pin or not to pin
>
> I am not a mechanical engineer but I do not see how an 8mm hole will weaken
> your mast. I had a station that used two Yaesu G-2800 rotators and I pinned
> the mast per the manufactures instructions and had no issues.  Having an
> antenna slip in the wind is unacceptable in my opinion. I would either
> change the rotor, clamp or try pinning it. Zinc hardware corrodes very
> quickly outdoors, don't use it.  If you are drilling a hole why do you need
> M8 hardware instead of 5/16 or even go larger which is readily available in
> stainless steel.  I do not believe that stainless steel bolts come in the
> same grades standards as steel although some stainless materials may be
> equivalent. If you drill your 4130 mast, do not run the drill too fast and
> use a lubricant to avoid work hardening it. I have had no issues machining
> 4130/4140 using standard HSS bits.
>
> John KK9A
>     
>
>
> To:	Towertalk Reflector <towertalk at contesting.com>
> Subject:	[TowerTalk] To pin or not to pin
> From:	Rudy Bakalov via TowerTalk <towertalk at contesting.com>
> Reply-to:	Rudy Bakalov <r_bakalov at yahoo.com>
> Date:	Sun, 28 Aug 2016 16:47:27 +0000 (UTC)
>
> I am getting really tired of my mast slipping during major wind gusts. The
> mast
> is supporting a 4 el 40m M2 yagi and a 6 el M2 KT36XA. The clamp is the
> standard clamp that comes with the ProSisTel PS61D rotator.
> So the question is if I should pin the mast to the rotator clamp using a M8
> bolt. The clamp has 8 bolts, but clearly the friction between the clamp and
> the
> mast is not sufficient under extreme conditions. My one and only concern
> about
> pinning is that the hole in the mast may weaken the mast. The PST61D is
> rated
> for 36 sq.ft so, right or wrong, I am not concerned about the wind damaging
> the
> rotator.
> If you suggest I pin the mast, there are two follow up questions:
> 1) What grade bolt should I use for the pin? The mast is 120K psi. Should
> the
> bolt be rated for less (e.g., Grade 5) or more (e.g., Grade 8 or 10.9) psi?
> That is, do I want the bolt to break before the mast does under extreme
> conditions?
> 2) Further, is it OK if the bolt is zinc plated? I could not find stainless
> steel Grade 8/10.9 M8 bolts. Is the contact between zinc and the mast going
> to
> lead to corrosion?
> Rudy N2WQ
>
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-- 

73

Roger (K8RI)


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