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---- Lynn Hanover <lehanover@gmail.com> wrote:
> 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.
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