[TowerTalk] more on torquing bolts with anti-seize

[Eric Scace K3NA]

 ... input from my brother Greg, who knows a shitload more about this stuff that
I do... -- Eric K3NA

[in response message sent by K0FF:
Subject: [TowerTalk] Final words on the Bolt issue

Well, this thread has gone around and around, so I called on some experts, and
the best answers I got were from the Customer Service Engineers at Permatex.
They took a lot of time and filled me in on more than I ever wanted to know
about lubricating bolts etc.  First off, the "old wives tale" is true. If you
lube a bolt with a good grease, a lot more of the applied force on the nut winds
up on the bolt, and not wasted as friction, and can more easily break the bolt.
Because of this, they told me that industry-wide  manufacturers ( Like GM etc)
have issued bulletins to their mechanics to derate the torque figures by 20%
when
using an anti-seize. Everyone agrees that an over tight bolt is a bad thing.
Even though Anti-Seize products are not lubricants per se, the do add
"lubricity" to the process.  Of course that's not the reason we want to put goo
on the bolt in the first place, rather we want to minimize galling and
corrosion,  and have a safe joint to boot, and maybe even have a chance to
remove it in one piece some day a bonus.  They have 3 products in their line,
and the base is either petroleum or mineral oil. It's the additives that do most
of the work evidently and include Moly, nickel, and graphite. A new mix is made
specifically for Stainless Steel, and contains copper. Looks very interesting,
and has a part # of 31163. If Champion Radio doesn't carry it, I'm sure they
soon will.  Good news is that it is inert, and has an unlimited life in the
threads.  For sure I'll have a tube in my kit from now on.

73 Geo>K0FF
...end message]

Yes, it is correct, but the amount of reduction depends on a lot of factors.
the whole story about threaded fasteners is that you should consider the
fastener and the fastened items as a spring mass damper system for dynamic
loading and a simple spring system for static loading.  You are stretching the
bolt when tightening it.  the stretching force is resisted by the compressive
force of the items being joined and the additional loading from use of the boat.
the idea is that the available stretching force must be enough to overcome
compressive force and service forces plus some factor of safety.  You solve the
system by modeling the assembled parts as truncated cones with the truncation
point at the diameter of the washer under the head.  I forget the exact shape of
the cone, but that's not that important.  You then solve for stress at the bolt
head and at the last thread before the bolt shank (the two most common failure
points), using your favorite method of modeling failure (energy method, etc.).
If you do this exercise for our boat [N.B. Greg and I race Tornado catamarans,
but this applies to towers as well -- K3NA], you will most likely find out that
other criteria end up being more important than failure analysis of the bolt
head / threads and that the failure that you are likely to see is not going to
be those items.  In our case, corrosion failure, or failure from repeatedly lead
cycling aluminum threads (thus work hardening them) are the most common
occurrences.  Therefore the beam bolts are oversized so that failures of this
sort happen relatively rarely.  Additionally, torque specifications are
generally listed for new fasteners under ideal conditions (clean, burr free,
etc).  There is a distribution of stretch from "torquing" a bolt that results
from non-uniformity of the internal external thread contact, alignment of the
screws, smoothness under the bolt
head and so on.  If you are required to design so stringently that you must
account for this, then torque specs are given for lubricated threads and new
fasteners, such as is common in auto connecting rods.  Even more stringent
requirements (race engine connecting rods, for instance) are tightened by
measuring the stretch of the bolts.

We are on the other end of the spectrum.  There is no benefit in saving 5 gms of
weight from bolts, so we use oversized, relatively unsophisticated threaded
fasteners in a not so well controlled environment.  I torque by hand, but a
torque wrench is a smart idea. Torque them to 14 - 17 ft. lbs. and forget about
'em.  I have never seen a bolt failure on a catamaran.  However, I have seen
plenty of corrosion failure of internal threads in hulls.

Unless you are building super critical devices, don't waste money on your torque
wrench.  The Most accurate torque wrenches are dial type actuated by an internal
beam.  They are EXPENSIVE - several hundred dollars.  Also expensive and not
worth a shit are click type wrenches.  these wrenches depress a ball into a
detent using spring pressure.  When enough torque is
reached, the ball pops out of the detent and the wrench clicks.  They are very
inaccurate and not very stable.  Sears Craftsman sells a cheap torque wrench
that is nothing more than a flexible wrench with an unloaded pointer connected
at the socket end of the wrench and free at the other.  A plate is mounted at
the other end of the wrench that relates deflection of the wrench to applied
torque.  These are surprisingly good and cheap.

-Greg




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