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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