I don't know if your rotator has the hole for
a "pin" or not. Can you pin the mast to the rotator?
On my rotator, a Tailtwister, I use, IIRC, a 5/16 SS
bolt through the mast and through the rotator "V".
I also have a ~5 second brake delay built into the
control box to avoid any sudden stops.
On the SS U-bolts, I use spring-type lockwashers with
the SS nuts. I tighten them 'till it "feels right"....not too
loose and not too tight. Works for me.
73,
Charlie, N0TT
On Tue, 01 Oct 2013 19:00:36 -0500 John Becker <johnb3030@comcast.net>
writes:
> 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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