John,
I have no explanation for the slippage, you didn't say if the U-bolts were
loose. The U-bolts are also available from C.A.T.S. rotor service. I would use
a lock washer and/or some Loc-Tite (the removable kind).
You could also bend your own from all-thread. I've bought it in the past from
my local big box store. I think, without any engineering data at all, the
having threads bearing on the mast would give a slightly better grip. Then
there's the slipp-nott http://www.tennadyne.com/slipp_nott.htm
Good luck & 73.
Mike NF4L
On Oct 1, 2013, at 8:00 PM, John Becker <johnb3030@comcast.net> 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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