[TowerTalk] Analysis of mast slippage in rotor

[Grant Saviers]

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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>>
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