[TowerTalk] Analysis of mast slippage in rotor

Wed Oct 2 13:48:03 EDT 2013

I haven't tried this but would it work to have a flat steel plate about 1/2 inch thick with four holes on the back side of the rotator (where the U bolt normally goes) and then use straight bolts getting rid of the U bolts?   

Seems if you drilled all the holes to accept 3/8 inch bolts you could tighten it to hold anything a Tailtwister brake was rated to handle.

Stan, K5GO

Sent from Stan's IPhone

On Oct 2, 2013, at 12:09 PM, Steve Dyer <w1srd at yahoo.com> wrote:

> I don't see that it address stainless hardware.
> Steve W1SRD
>> Lubricating threads makes a big difference, and here is a chart that helps
>> 
>> http://www.portlandbolt.com/technicalinformation/bolt-torque-chart.html
>> 
>> Grant KZ1W
>> 
>> On 10/2/2013 2:51 AM, Jim Smith wrote:
>>> Hmm.. I suspect that you know more about this stuff than I do.  However, you state, "I did some testing with a spare rotor, a short piece of mast and a torque wrench. I lubricated the threads to prevent thread galling."
>>> 
>>> Lubricating the threads drastically changes the torque required to establish a given amount of stress in the bolt.
>>> 
>>> Fastenal used to have a very good explanation of this topic on their web site but I don't see it there now.
>>> 
>>> Perhaps others with more understanding of this topic than I would care to comment.
>>> 
>>> 73, Jim    VE7FO
>>> 
>>> On 2013-10-01 17:00, John Becker wrote:
>>>> My mast has slipped about 30 degrees in the rotor after over four years
>>>> with no slippage. Prior to noticing this I wasn't aware of any recent
>>>> windy days. Before taking corrective action, I'm thinking about why this
>>>> has happened now and how to most likely prevent a future occurrence.
>>>> 
>>>> The rotor is a HAM-III in a Rohn 25 top section with a Rohn TB-3 thrust
>>>> bearing. The antenna is a KT34-XA mounted two feet above the thrust
>>>> bearing. This antenna has been up since 1981 and mast slippage has
>>>> occurred previously a few times but only when there has been unusually
>>>> high wind.
>>>> 
>>>> One of the first things I found is that these rotors apparently use
>>>> non-standard size U-bolts. They are 1/4-20 stainless steel with a 2.25"
>>>> inside dimension. The only source I found for replacements is Hy-Gain,
>>>> now a division of MFJ. I wonder if they are making their own U-bolts? I
>>>> was unable to find anyone else selling 2.25" ID U-bolts smaller than
>>>> 5/16-18.
>>>> 
>>>> I looked for the correct torque spec for bolt tightening. For 1/4-20
>>>> stainless, the Standard Dry Torque spec is 75 inch-pounds or 6.25
>>>> foot-pounds. This is for a bolted joint and I wonder if it also applies
>>>> to a U-bolt? I did some testing with a spare rotor, a short piece of
>>>> mast and a torque wrench. I lubricated the threads to prevent thread
>>>> galling.
>>>> 
>>>> 75 inch-pounds is not very tight, definitely less than I would have
>>>> tightened them if just going by what feels reasonable to me. I gradually
>>>> increased the torque to 200 inch-pounds, which is the upper limit of my
>>>> smaller torque wrench. I was expecting the U-bolt to fail at less than
>>>> 200 inch-pounds but it did not. I left it at 200 inch-pounds for several
>>>> days to see if there would be a delayed failure but it held. However,
>>>> 200 inch-pounds feels too tight to me for a 1/4" bolt.
>>>> 
>>>> Due to the design of the HAM series rotors, the rotor casting contacts
>>>> only the center 1.5" of the 2.25" ID U-bolt. This permits progressive
>>>> tightening of the U-bolt to cause the shape of the U-bolt to distort,
>>>> going from a "U" shape to a rounded "V" shape. It was necessary to
>>>> tighten the nuts on both sides of the U-bolt by roughly 1/8" to increase
>>>> the torque from 75 inch-pounds to 200 inch-pounds.
>>>> 
>>>> I'm wondering if this distortion of the U-bolt also occurs slowly over
>>>> time, resulting in a gradual loosening of the U-bolt? This could explain
>>>> why mast slippage becomes a problem as time progresses.
>>>> 
>>>> Another possibility that comes to mind is that the normal stresses that
>>>> occur each time the rotor starts and stops might gradually cause the
>>>> nuts to loosen in the absence of rust to hold them in place. I plan to
>>>> add stainless steel nylon insert lock nuts on top of the standard nuts
>>>> on the U-bolts to prevent this.
>>>> 
>>>> The U-bolt that had been tightened to 200 inch-pounds was distorted to
>>>> the point that it was very difficult to get it out of the rotor casting.
>>>> There were obvious bends in the threaded portion just below the nuts.
>>>> This is another indication to me that 200 inch-pounds is too tight, and
>>>> I would not have used this U-bolt on my rotor.
>>>> 
>>>> I decided to continue the experiment by straightening the test U-bolt
>>>> and tightening it with a larger torque wrench until it failed. However,
>>>> I didn't get to the point of using the larger wrench because as I was
>>>> re-tightening it, this time it failed at between 150 and 175
>>>> inch-pounds. Undoubtedly the operation of straightening it weakened it
>>>> further than it already was, and I don't have another spare U-bolt to
>>>> sacrifice.
>>>> 
>>>> There have been discussions of mast slippage on this list in the past,
>>>> but I don't recall anyone discussing the optimum U-bolt tightening
>>>> torque. Possibly I just missed seeing it.
>>>> 
>>>> Suggestions and discussion about how to alleviate this problem would be
>>>> appreciated. Thanks!
>>>> 
>>>> 73,
>>>> 
>>>> John, K9MM
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
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