You cannot get a 1/4 SS u-bolt very tight. Can you use 5/16 SS or a
stronger grade of steel?. You can buy u-bolts at McMaster Carr and you can
slightly adjust the size using a mallet and a vise if needed. While some
people are against it, pinning the mast is another option. Actually Yaesu
recommends doing so.
John KK9A
To:towertalk@contesting.com
Subject:[TowerTalk] Analysis of mast slippage in rotor
From:John Becker <johnb3030@comcast.net>
Date:Tue, 01 Oct 2013 19:00:36 -0500
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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