[TowerTalk] Analysis of mast slippage in rotor

Wed Oct 2 02:14:39 EDT 2013

Is there a reason you can't enlarge the clamp holes to take a larger 
diameter u-bolt?  It will take a milling machine to properly move the 
hole center of the existing hole, but that is very easy to do with an 
end cutting end mill.  OTOH, you might break the clamp castings with a 
lot more torque/tension.

McMaster stocks 2.25" id u-bolt "muffler clamps" in 304 SS in 3/8 x 16 
material.  That diameter will have several times the strength of 1/4-20, 
if you can fit the nuts and modify the holes in the rotator clamps.  
http://www.mcmaster.com/#3042T56

The increase in mast slippage with time is probably due to wear of the 
casting contact points or if the mast is galvanized, wearing away of the 
Zn.  Also, since bolted mechanical joints tend to relax/settle in after 
a few temperature cycles, it is always best to re-torque to specified 
values after this period.  For some slip critical joints right near the 
maximum gr8 allowable fastener tension, we would temperature cycle 50 to 
130 F several times over night and then re-torque.

Generally, the HyGain and Yaesu clamps are a bit on the wimpy side, so 
consider the aftermarket fixes.

As to the maximum stress in a u-bolt, it seems you have verified that 8 
ft-lbs or so is the max.  For a curved bolt the stress will be higher, 
even if it is full supported over the full 180 degrees. McMaster specs 
the plain and stainless steel 1/4-20 u-bolts as 425# max load.  You can 
calculate the approximate straight bolt stress level from the area of 
the minimum diameter of the threaded section, assuming equal 212# 
tension in each leg.

Double nuts, or Nord-Lock lock washers are a good idea.  From what I've 
read and experienced, split ring lock washers are a waste of time.  You 
won't find them in most any structural connection.  Also, extra thick 
and perhaps hardened washers are needed if the clamp tension bolts pass 
through slots or oversize holes.   Any hardware store washer will 
deform.  McMaster and machine shop tooling suppliers stock them as setup 
or fixture washers.

Grant KZ1W

On 10/1/2013 5:00 PM, John Becker 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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