There is a simple analogy to consider when thinking about turnbuckles
loosening or not. Their threads are same 60 degree form and helix
angles as common nuts and bolts. So if you think nylocks, lockwashers,
Nordlocks, aviation nuts, or etc. are never needed then there is no need
to secure turnbuckles against unscrewing. We all know that is not the
case. While the helix angle on these threads is nominally self locking,
whether it locks for a turnbuckle (or any bolt) depends on surface
finishes, lubrication, vibration, contamination, tension cycling, torque
from the guy twist, temperature cycling, etc, same as other fasteners.
Hot dip galvanizing is a known thread lubricant, the bolt torque tables
show fewer ft-lbs to tighten bolts to specified stress/tension. My
tower installer applied additional lubricant. At the specified torque
values for structural bolts, the fastener is far less likely to loosen.
That isn't the static load in a turnbuckle which is exposed to many of
the conditions that cause common fasteners to loosen.
Grant KZ1W
On 9/22/2017 12:55 PM, Roger (K8RI) on TT wrote:
There is often a big difference between what is a proper turnbuckle
installation and what the typical ham chooses and does.
Likewise, I've never seen a turnbuckle unscrewing by itself. Vandals?
Kids? All a safety wire would do is slow determined vandals down.
There are other, quicker methods, but they leave no doubt that vandals
did the deed.
I've seen a number of ham installations where a safety wire was needed
in case the turnbuckle failed. There is a hexagonal Aluminum rod,
threaded on both ends (LH & RH threads). They are handy, easy to use,
light weight, and cheap. I've even used some on small, single and
double braided rope where the lines only serve as a backup to keep a
vertical straight. IOW, strength wasn't as important as light weight.
However, I have seen these used on ROHN 25G. Where their failure could
lead to expensive repairs and safety issues. The use of under size
turnbuckles is common. In those cases the safety wire does serve as a
"back up" in case the turnbuckle fails. THOSE safety wires need a
minimum of 2 and preferably 3 cable clamps at each end
In the case of the Al rods, they don't unscrew. The threads fail. The
ends are steel rods with the typical, "ends bent in a circle" which is
not welded or forged. The internal Al threads contacting steel corrode
relatively fast.
As I've said before, the typical ham has nowhere near the knowledge
present on TowerTalk and even that is going down rather quickly with
today's question pool and the practical knowledge required to earn a
license.
Towers seem to be a subject where the input of an "Elmer" is seldom
sought. The old TV tower approach with a soft, galvanized guy simply
wrapped around a single tower leg, rather than a guy bracket (as ROHN
uses). Those old Radioshack guys are surprisingly weak. I've seen
three men pull a 3/16ths in two while walking up a 40' tower.
73, Roger (K8RI)
On 9/12/2017 Tuesday 8:48 PM, Steve Maki wrote:
The typical turnbuckle safety wire is designed to prevent inadvertent
unscrewing of the turnbuckle, either from vibration or random
vandalism. It's not to support the tower in the event of a turnbuckle
failure. That would at least require more than one in-line cable clamp.
Note: I've NEVER seen a turnbuckle prone to unscrewing itself, but
don't consider that a reason to not use them.
The turnbuckle shouldn't be the weak link in the system anyway. It
should be oversized and of good quality to insure that the threads
remain in good operating condition.
-Steve K8LX
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