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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