AOPA Pilot Magazine - January 2020 - 93

already stretched. So, the bolt doesn't "feel"
the 1,500-pound load at all-not even if that
load is applied cyclically 1,200 times a minute (as it would be if this were a cylinder
hold-down joint). Since the bolt doesn't
"feel" the cyclic loads, it will never be subject to repetitive-stress fatigue. And since
the joint doesn't budge, there's nothing to
cause the nut to loosen and cause a loss of
preload. (If you ever wondered why cylinder hold-down nuts don't need cotter pins
or safety wire, now you know.)
Now imagine the bolt is undertightened so that it has a preload of only 1,200
pounds. This changes everything. Each time
the joint sustains a 1,500-pound load, the
1,200-pound clamping force isn't enough to
hold the joint firmly together. The joint will
separate slightly under peak load. The separation will stretch the bolt, which will now
feel every bit of that 300-pound preload deficit. If the load is applied to the joint cyclically
1,200 times a minute, then the bolt will be
subject to 300-pound repetitive stress that
could cause the bolt to develop fatigue cracking (typically at the threads) and ultimately

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fail catastrophically.
Worse still, the
repetitive separation of the joint
would likely result THE REDUCED-DIAMETER shanks of Lycoming connecting rod and
crankcase through bolts make them less stiff and therefore their
in self-loosening of joints harder.
the nut, causing the
bolt's preload to be
progressively lost and hastening catastrophic load versus 2,000-pound preload), the
load causes the joint to decompress ever
failure of the joint.
That's why having adequate preload on so slightly, which causes the bolt to stretch
the bolt (and therefore enough clamping ever so slightly. Thus, the bolt actually
"feels" a small part of the load.
force on the joint) is so crucial.
How much stress the bolt feels is a function of the relative stiffness of the bolt and the
NOT QUITE THAT SIMPLE
I just lied to you. My explanations made joint. If the joint is much stiffer than the bolt
the implicit assumption that the bolt is the (which is usually the case), then the bolt feels
only part of the joint that acts like a spring. very little load-this is referred to as a hard
In the real world, that's not quite true. As joint. But if the bolt is stiffer than the joint-a
the bolt is tightened, not only does the bolt soft joint-then the bolt feels the lion's share
stretch a bit (like a stiff tension spring), but of the load. That's why it's important for the
also the joint compresses ever so slightly joint to be as stiff as possible and for the bolt
not to be excessively stiff.
(like an ultra-stiff compression spring).
You'll often find that critical bolts are
What this means is that when load is
applied to the joint, even if the load is less manufactured with reduced-diameter
than the bolt's preload (e.g., 1,500-pound shanks to make them less stiff and easier

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AOPA Pilot Magazine - January 2020

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