[TowerTalk] Analysis of mast slippage in rotor

[John Becker]

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