[TowerTalk] To pin or not to pin Correction

Mon Aug 29 03:23:01 EDT 2016

Paragraph 4 should have the second row of holes start 1:, not 1.5 inches.

73

Roger (K8RI)

On 8/29/2016 Monday 3:04 AM, Roger (K8RI) on TT wrote:
> 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.
>>
>>
>>
>>
>>
>>
>>
>>
>> -----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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