On 10/2/2013 5:51 AM, Jim Smith wrote:
Hmm.. I suspect that you know more about this stuff than I do.
However, you state, "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."
Lubricating the threads drastically changes the torque required to
establish a given amount of stress in the bolt.
Likewise I don't remember the figures, but tightening a 1/4" u-bolt to
the same torque lubricated would put far more stress on the bolt than
dry. Possibly enough torque to weaken the bolt. OTOH SS should be
lubricated if you don't want the nut to become part of the bolt so the
difference in torque needs to be taken into consideration.
A search on "torque requirements for dry vs lubricated bolts" produces
pages of documents. Far too many for me to read.
I found the following segment in :
http://www.stlaroc.com/features/torqtips.html /Reprinted from i Saluti,
October 1996,
from Alfacionada, AROC Southern California / "
I checked Carroll Smith's /Nuts, Bolts, Fasteners and Plumbing Handbook/
to confirm Wysocki's point. The answer was found in chapter three,
"Thread Physics." The text states that many variables affect the amount
of torque required to produce a given tension, including the type of
plating, the cleanliness of the threads, the fit of the threads, and the
type of lubricant used (if any), as well as the thread pitch and the
bolt size. Further, it states that, in dry, unplated threads, about 50%
of the torque is used to overcome friction between the bearing surfaces
and the work faces (where the nut meets the washer or part), and about
40% is used to overcome thread frictions A quick calculation shows that
only 10% of the torque is used to create tension in the bolt. It follows
that reducing this friction (by applying a lubricant) will cause a much
greater portion of the same torque to be applied to creating tension.
Alternatively, the tension created by applying identical torque to a
lubricated bolt thread (and bearing surface, also) will be much greater
than that produced in a dry thread. How much greater? A graph from the
Smith text (pp. 48-52) shows that about 200% more tension *- TWICE THE
TENSION! -* is created when using antiseize, and close to 150% more
tension is created when using engine oil as a lubricant. It should be
noted that torque tables for particular applications are quoted as "dry
and clean," unless they specifically state otherwise. So, if you choose
to lubricate the threads and bearing surfaces of a fastener application,
the "dry" table no longer applies; instead, refer to a table that shows
lubricated torque values or reduce the torque used to produce the
recommended tension.
Because of the number of variables involved, I'm not sure that Daniel
Wysocki's rule of thumb (65% torque when lubed vs. dry) will apply in
every case, particularly with smaller fasteners, where the tolerance of
error is smaller, but his guideline is more appropriate than using the
"dry" table on a lubricated bolt."
The following chart may be of use as it lists torque values for dry,
plated, and waxed/lubed threads.
http://www.portlandbolt.com/technicalinformation/bolt-torque-chart.html
or just
https://duckduckgo.com/?q=Difference+in+bolt+tension+for+lubricated+threads+vs+dry+threads
Results will vary with different search engines. I use
https://duckduckgo.com as they do not record any of your information.
73
Roger (K8RI)
Fastenal used to have a very good explanation of this topic on their
web site but I don't see it there now.
Perhaps others with more understanding of this topic than I would care
to comment.
73, Jim VE7FO
On 2013-10-01 17:00, 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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|