Another way of looking at "torque" requirements at the basic engineering
level of fasteners is to consider that it is a conversion of
(unmeasurable) clamping force on a fastener to a (measurable) rotational
force that considers the tensile strength and elongation under load of a
material (i.e., how much force is required for a given amount of
"stretch" in a material, in this case steel) which can be converted to a
specific (relatively) amount of clamping force on a joined set of
devices. Tensile strength vs. elongation under load is representative of
the clamp force applied to the joint.
Obviously the details are a little bit more involved than that, but you
get the idea. In this case, any slop in the fitment of the bolt to the
WAF/spar holes is an undesirable condition. Read the assembly
specifications and listen to the advice of the guys who have built and
flown these.
As Mark L noted below, the WAFs are positioning devices intended to keep
everything in alignment where clamping force is irrelevant - the bolts
need to be tight enough relative to the holes in the WAFs and spars to
prevent movement/flex of the wing, which would most likely be a 'force
multiplier' in use. I recall someone on the net several years ago noting
that for the WAF to WAF attachment to make sure the bolt was fed through
them from front to back "in case the nut came off" - not sure that is
sound thinking, but that is my recollection of what was said.
In any event... when discussing fasteners used to clamp things together,
torque is used as a measurable equivalent of clamping force. Sorry about
being so long winded.
Mark Wegmet (N952MW res.)
Sr. Quality Engineer... also RETIRED!
--------------------------------------------------------------------------------------------
On 11/6/2021 5:28 PM, Mark Langford wrote:
There's no need to torque the crap out of any of the wing attach
fittings through the spars, or even WAF to WAF, for that matter. They
are simply "pin joints" in engineering terms, meant to keep things
from moving around. Friction is not even considered when analyzing
the stress on these things, even in the case of the WAF to WAF
connections. NASA doesn't even consider friction in stress analysis
of critical structures, at least that's been my experience. If you
start crushing wood, you're over tightening. Don't bother with torque
charts....that is not the proper "case" for these bolts, given that
they can easily crush the spruce.
Mark Langford
Mechanical Engineer....RETIRED!
m...@n56ml.com
http://www.n56ml.com
Huntsville, AL
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