[TowerTalk] Analysis of mast slippage in rotor
jimsmith at shaw.ca
Wed Oct 2 05:51:45 EDT 2013
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
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
> 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
> 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
> 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!
> John, K9MM
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