[TowerTalk] Analysis of mast slippage in rotor

Wed Oct 2 15:31:57 EDT 2013

On 10/2/2013 5:51 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.

Likewise I don't remember the figures, but tightening a 1/4" u-bolt to 
the same torque lubricated would put far more stress on the bolt than 
dry.  Possibly enough torque to weaken the bolt.  OTOH SS should be 
lubricated if you don't want the nut to become part of the bolt so the 
difference in torque needs to be taken into consideration.

A search on "torque requirements for dry vs lubricated bolts" produces 
pages of documents.  Far too many for me to read.

I found the following segment in : 
http://www.stlaroc.com/features/torqtips.html /Reprinted from i Saluti, 
October 1996,
from Alfacionada, AROC Southern California /  "

I checked Carroll Smith's /Nuts, Bolts, Fasteners and Plumbing Handbook/ 
to confirm Wysocki's point. The answer was found in chapter three, 
"Thread Physics." The text states that many variables affect the amount 
of torque required to produce a given tension, including the type of 
plating, the cleanliness of the threads, the fit of the threads, and the 
type of lubricant used (if any), as well as the thread pitch and the 
bolt size. Further, it states that, in dry, unplated threads, about 50% 
of the torque is used to overcome friction between the bearing surfaces 
and the work faces (where the nut meets the washer or part), and about 
40% is used to overcome thread frictions A quick calculation shows that 
only 10% of the torque is used to create tension in the bolt. It follows 
that reducing this friction (by applying a lubricant) will cause a much 
greater portion of the same torque to be applied to creating tension.

Alternatively, the tension created by applying identical torque to a 
lubricated bolt thread (and bearing surface, also) will be much greater 
than that produced in a dry thread. How much greater? A graph from the 
Smith text (pp. 48-52) shows that about 200% more tension *- TWICE THE 
TENSION! -* is created when using antiseize, and close to 150% more 
tension is created when using engine oil as a lubricant. It should be 
noted that torque tables for particular applications are quoted as "dry 
and clean," unless they specifically state otherwise. So, if you choose 
to lubricate the threads and bearing surfaces of a fastener application, 
the "dry" table no longer applies; instead, refer to a table that shows 
lubricated torque values or reduce the torque used to produce the 
recommended tension.

Because of the number of variables involved, I'm not sure that Daniel 
Wysocki's rule of thumb (65% torque when lubed vs. dry) will apply in 
every case, particularly with smaller fasteners, where the tolerance of 
error is smaller, but his guideline is more appropriate than using the 
"dry" table on a lubricated bolt."

The following chart may be of use as it lists torque values for dry, 
plated, and waxed/lubed threads.
http://www.portlandbolt.com/technicalinformation/bolt-torque-chart.html 
or just

https://duckduckgo.com/?q=Difference+in+bolt+tension+for+lubricated+threads+vs+dry+threads
Results will vary with different search engines. I use 
https://duckduckgo.com as they do not record any of your information.

73

Roger (K8RI)

>
> 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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