Hi all,
Some good information passed around on this one.
Regular inspection is certainly the best way to find problems before
they get out of hand.
Correctly selected thread locking measures are essential to the survival
of these connections.
Thread locking compounds are NOT necessarily forever, and impossible,
and to be avoided at all costs!
Here is my offering on what is happening when we bolt a rotor to the
tower and the cause of the failures.
Most installations use plates in the tower that have holes that are
larger than the size of the fasteners that are used to connect the rotor
to the plate. Then the fasteners used have threads running the full
length of the fastener, with no solid (unthreaded) shank to bear against
the holes in the plate. Fasteners that bear against holes must have
solid shanks in the bearing area.
So, the intended method of securing the rotor must be friction caused by
the fastener clamping pressure between the rotor case and the plate in
the tower.
The situation described above cannot rely on the bearing of the
fasteners against the holes in the plate. If it is, the design is
seriously flawed. I.E. not enough clamping pressure (number of
fasteners) to get the job done.
Once the fasteners loosen enough to allow any movement, the entire
connection starts to rapidly turn to custard.
The threads on the bolts start pressing against the holes in the plate
deforming them creating more clearance, resulting in an even looser
connection.
As the mast and rotor assy rotates back and forth it generates inertial
loads on the connection, caused by the free rotation due to the sloppy
fits. The motion ends when the bolts come hardup against the hole edges
in the plate. When the shafts of the fasteners come to rest against the
sides of the holes the shear loads can be several times more than the
simple (system torque) fastener shear capability due to the Force = Mass
* Velocity^2 energy stored in the mast and antenna system, that is
created during the free rotation portion of the load cycle.
Years ago, I had the fasteners come loose in the bottom of a Ham M rotor
with a KT34XA (known for torque imbalance) on the mast. I climbed up
there in a 30-40 mph breeze to see what was going on (only a 40' tower).
It was amazing to watch the connection tearing itself apart. It is
really amazing to witness the amount of energy that can be stored in the
system. The mast and antenna would rotate one direction and stop as the
fasteners loaded up against the plate. The rotor would stop moving but
the mast and antenna would keep going until the bending of the antenna
boom, elements and torsional flexing of the mast would stop. Sometimes
the wind gusts would hold it there for several seconds. Then the whole
assy would go back the other way until the fasteners seated against the
other side of the holes.
I could hear the sound of the fasteners hitting the plate. I just
tightened everything up and waited for the wind to stop.
The original installation used supplied fasteners and lock washers and
grease on the threads.
Fixing it was a real bitch! had to get the rotor out of the tower and
into the shop to try and get ALL of the grease out of the threads.
Fortunately, the threads in the case were not too badly deformed.
Then it all went back together with some blue Loctite (removable) and it
lasted another 7 years until I took it down to move.
If these connections were designed to rely on fastener bearing against
the holes in the plate, they would have "Spit Fit" clearances between
the plate holes and bolts, and the bolts would have short solid
(un-threaded) shanks to bear against the holes in the plate. This would
only require that the fasteners didn't back off enough to expose the
threaded portions to contact with the edges of the holes.
I think that this is a huge potential problem with commercial rotors and
plates supplied by tower manufacturers. Close tolerance fits and proper
fastener use is not a common practice.
I agree with one of the comments that the rotor suppliers should provide
fastener torque specs.
Also, the practice of designing the connection to place the fasteners
against threads in a cast aluminum housing is about as poor as it can
get. Makes the purchase easier on your wallet though, we all know that
is important!
Every aircraft version of a steel fastener to aluminum connection has
had, at the very least, a heilical steel thread insert in the aluminum.
For low yield strength materials like cast aluminum the best choice
would be a tubular steel insert that has a larger OD thread gripping
more cast material.
So, if we must rely on the friction between the rotor case and plate to
get the job done, we need to make sure the fasteners are properly
torqued and never come loose.
Putting grease on the threads is ok for corrosion prevention for this
application as water cannot run directly into the mating threads (they
are upside down), but it won't do a thing to keep them from loosening.
The really good teflon based lubricant (mentioned in another post) used
by marine riggers is "Tef-Gel". It is the best for preventing seizure
and dissimilar metals corrosion and longevity against moisture washout.
Most of the marine rigging connections have positive locking features
like split pins (cotter pins on this side of the pond), or locking set
screws.
Another favorite is "Lanocote", available from the same marine outlets
as the Tef-Gel. You gan get a cheaper lifetime supply (1 Lb) jar at the
local pharmacy for around $12 if you ask for USP Anhydrous Lanolin. This
is great for seizure prevention and corrosion. I use it on turnbuckles
and SS hose clamps, which are subsequently safety wired with SS safety
wire.
It is not nearly as messy as the common anti-seize compounds and has
better resistance to moisture washout.
Lock washers can be reliable for a time, but eventually fall prey to
corrosion. The locking method employed digs thru the plating and exposes
the bare steel to the atmosphere. The corrosion erodes the locking
teeth, they get loose and the fastener looses it's lock.
For the rotor installation I'll place my money on a good removable
thread locking compound any day!
At work, we have installed thousands of fasteners in hundreds of spars
(otherwise known as yacht masts) with all kinds of threadlocking
compounds.
When done properly, they do what they are supposed to do. I.E. the
forever compounds are really forever (damn near), the permanent
compounds are permanent, until you really want to take them apart, with
the assistance of heat, and the removable ones are still intact, but
easily removed.
Swinging from a bosuns chair on a tall structure is way worse than
standing on a tower and trying to get a connection apart. The removable
stuff is just fine, even when you can't get a real solid purchase
against the structure to loosen it! Also no corrosion problems!
Ian, G3SEK's comments about removable compounds not working when the
thread fits have been allowed to deteriorate is valid. His solution to
the degraded thread fit problem is a good answer. Wish the fasteners
were never allowed to get loose in the first place!
The threadlocking compounds were designed to work within a certain range
of fits.
They will provide resistance to galvanic corrosion in the threads by
electrically isolating them from significant areal direct contact and by
sealing them off from moisture intrusion.
There at least a dozen other branch subjects and discussions that
surround this one. They have occured in the past and will probably
surface again.
Hope this one helps.
--
73, Kurt
K7NV "That's K7 "Nevada" (ex - NI6W)
YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress/
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