[TowerTalk] Analysis of mast slippage in rotor

Wed Oct 2 03:13:27 EDT 2013

On 10/2/2013 2:14 AM, Grant Saviers wrote:
> 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.
>

The Ham series mast clamps are cast as part of the bell. The U-bols fits 
into a grove to match the size of the u-bolts and the radius of its 
curve so enlarging the holes creates a problem matching the grove. The 
grove is formed on top of a ridge that also serves as reinforcement of 
the top section used for the mast clamp, so there is little room for 
changing the bolt size and virtually none for placement.

> 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, 

The casting contact points for the u-bolts are virtually 180 degrees of 
solid contact,  Wear on the inside of the clamp is problematic. It's 
just two nearly flat surfaces of cast Aluminum so there is little 
contact area except a line on each side of the mast. Adding a flat piece 
of soft lead on each side would give far more gripping power, but then 
it'd be off center.
This mast mount is one of the reasons I believe in limiting any of the 
Ham series to relatively small antennas

There is one mod for the mast that sounds simple, but isn't.
It's the addition of a flat piece to match the angel of the clamp on 
each side, but this is tricky.  You can add a 1/4 to 1/2" wide flat 
piece on each side of the mast, but if they are 1/8th " thick you must 
mill 1/8th inch off the mast on each side to match the angle of the mast 
clamp. With a 1/2" wide flat it's not going to slip, but exceed the 
rotater ratings and it'll bust the mast mount.

73

Roger (K8RI)

> 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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