We have not covered a library full of bolt data, in these few words. In
aluminum or other soft material, a bolt that does not stretch is a far
better choice than one that can be stretched to some greater degree. When
you look closely at the threads in a tapped hole that is under tension, you
see in one case only the first 4 or 5 threads are under tension. In another
case, when the bolt can stretch further then, even fewer threads will be
loaded. So, generally the first few threads in a hole will fail first, as
those are the only ones loaded. Few tension bolts are designed to screw into
threaded holes. Some of the few are rotary case bolts and head bolts on some
piston engines. So then the ideal bolt would have one heat treat on the
threads, (no stretch) and another on the shank, a sort of spring like heat
treat. Bolts and studs in soft material need to have very course threads, to
get as deep a throat as is possible. A fine threaded hole in aluminum is
short lived. Better to bore it over sized and threaded for a nutsert or
Helicoil, or drilled through and nut and bolted where ever possible. Also,
there is a need to have a number of fasteners for each junction, where in
iron or steel, perhaps one or two fasteners might have been enough.

Since a multipal heat treat on a single bolt is unlikely, then you see such
ideas as the threaded part is bigger than the shank, like case bolts, and
many head bolts. So most tension bolts are designed to have a high quality
nut on one end. The very best have dimples on both ends so a micrometer can
measure length while torque is applied. So, the amount  of torque is not a
factor. The manufacturer is after a minimum stretch value, perhaps .007".
for example.

So, a hard bolt in soft tapped holes is fine. Since they don't stretch, all
of the threads engaged will be used unifomly. These junctions withstand only
minimal torque, and leave the designer with the pregnant problem, of keeping
the bolt from backing out. So now after this long trip, we are back to how
do you do that.

 There are methods that seem from the 20s, that you still may see used
today. Often a strip of soft metal with two holes to pick up two bolts is
used for a washer. The excess material past the bolt heads is folded over a
flat on the bolt head and bent sharply with a punch. Works great. Time
consuming.

A high tech lock washer (Mazda) light weight easy, reusable, you already
have a bunch of them. The 1902 angle cut hardend tractor lock washer. Works
great, $1.60 a pound at Tractor Supply. Looks too crude to be on an
airplane. Damages the bolt and the piece you are attaching. Some of the
locking compounds, work great, looks fine if you wipe off the excess.
Pricey, And you cannot tell which bolts are glued and which are not. And the
very best..................Safety wire. Light weight, looks good, easy to
use, cheap,
impresses the AI so he passes your machine, even though it has Chinese hose
clamps everywhere.

Lynn E. Hanover



But do you really want to use hardened bolts anchored in aluminum?  Lynn
said it the other day.  It would be better to choose a bolt where you
could get some stretch out of it before it pulls out of the anchor
material.
>>
>
> Ernest,
>
>   Generally, there isn't much choice.  Threaded fastener
> specifications normally include some degree of heat-treating -
> hardening and then drawing the temper to achieve a "grade" of
> hardness.  I have seen bolts made from aluminum, but I don't know as
> I'd want to use them on any critical component of an airplane.  Yes
> internal threads in aluminum aren't as strong as those in steel, but
> we see steel cap screws in aluminum threads in all kinds of automotive
> applications.  The strength of an internal thread in aluminum can be
> improved by swaging the thread with a thread-former instead of cutting
> it with a conventional tap.
>
> Dale R.