Reaming

On Aug 25, 8:35 pm, Fred the Red Shirt
wrote:
As to the statement that I clearly don't understand the factors
involved, you clearly do not understand what I said, the nature of
preloaded bolts, or even the S-n curves themselves. Improved fatigue
life due to preloading has nothing to do with friction. Friction may
improve fatigue life in the real world by spreading load over a larger
area, but the benefit of preloading on fatigue life is due primarily
to an effect that exists even if no friction is present at all.

Could you please elaborate on the theory of that effect?
E.g. is this a result of the superposition of stresses?


....and...

If we now begin to subject both bolts to the same cyclic
loading of 1500 lbs, where the applied load is increased from 0 up to
1500 and then reduced to zero again, the bolt with the 2000 lb preload
will see a cyclic load of only about 150 lbs, whereas the un-preloaded
bolt will see a cyclic load of 1500 lbs, and will obviously fail much
sooner.

Here you temporarily lost me because you have not told us
HOW the bolt is loaded. If the load consists of additional
tension, then plainly the bolt will see cyclical stress over the
range of 3500 lb to 2000. That is clearly the type of loading Matt
was discussing. If I make the unremarkable assumption that y
ou are familiar with addition then clearly you are NOT assuming
that the load is applied in the form of additional tension.

When a joint is pre-loaded, two important things happen. The bolt
stretches. AND The plates or whatever are being fastened are
compressed. When you add load that induces additional axial tensile
stress in the bolt, you have to consider that the compression in the
plates is being relaxed at the same time. So the stress increase is
not a 1:1 correlation to the additional applied load. The slope will
actually be something less than 1:1 until the point where all the
compression has been removed, after which it will be 1:1. As you can
imagine, the actual slope to the left of the knee is a function of the
modulus of elasticity of the bolts, the MoE of the plates, and the
effective area being compressed (where thickness comes into play).

However, the clamping force will still cause the shear to be
distributed over the surface area being clamped and not just
through the bolts. The superposition of stresses is not
the total story.

That is a completely separate effect and loading situation than what
Bud is talking about. My understanding has always been that what Bud
is talking about is only effective for additional tensile loading of
the fastener. But I agree with you, the clamping can be very important
for shear of the bolt, even if we ignore that effect in practice.

As for S-n curves, there are more than one type. The one
relating to what I am talking about are the ones that show S vs N for
different stress ratios. The stress ratio is the fraction equivalent
of the maximum to minimum load. For example, something that is loaded
in tension to 25000 psi, followed by being loaded in compression to
25000 psi back and forth, will have a ratio of -1.0 ( +25000 tension/
-25000 compression). Something loaded to 25000 psi tension that is
reduced to 10000 psi tension and back and forth will have a stress
ratio of .4 (10000 tension/ 25000 tension). The S-n curves show that
the amount of cyclic load that structure loaded with a ratio of -1
will fail far sooner than one with a ratio of .4, even though the
maximum stress level is the same. You can look in Mil-Hnbk-5 or
elsewhere for S-n curves to verify that.

The peak-to-peak stress difference in the first case, (ratio -1)
is 5,000psi, for the second case (.4) it is 1500 psi. So it is
no surprise that the first case fails earlier!

Now suppose two cases in which the magnitudes of
the stress cycles are equal:

Yes, that is exactly what I'm talking about.

In the first case the bolt is pre-loaded to 2500 psi then
subjected to an alternating load of an additional +/- 1500 psi,
(e.g. from 4000 to 1000 both in tension) while a second,
otherwise identical but not prestressed bolt is cycled
from 1500 psi in tension to 1500 psi in compression.
Both bolts see the same peak-to-peak stress difference.
The ration in the first case (preloaded bolt) is 4, in the
second case it is -1. Which bolt fails first?

Actually case 1 R=0.25, but otherwise your example illustrates my
point pretty well.

Cheers,
Matt

Fred the Red Shirt wrote in
oups.com:

On Aug 26, 2:09 am, wrote:
On Aug 25, 7:59 pm, Fortunat1 wrote:



Fred the Red Shirt wrote
roups.com:

On Aug 17, 2:50 am, Charles Vincent wrote:
Fortunat1 wrote:
"Rich S." wrote in
:

"Fortunat1" wrote in message
...
Well, obviously I'd protect it, but I'm not going to rely on
epoxy to bear a load. If I can't get the holes 100% I'll
bush them....

...So I guess I'l just be as careful as I can cutting the holes.
Just
looking through Bengelis' book, I see he recommends using a
twist drill to cut the holes, presumably to their final
size,...

I would test that theory first. Reamers may or may not give a
good finish on wood. That was one of the reasons I quoted the
study I did. The twist drill gave the best hole finish.

Bits made for wood, high quality brad-point or forstner bits,
may give you a cleaner hole than a twist drill made for metal.
Cheap bits are crap-they'll burn their way through the wood.

Actauly, having tried each on some scrap pieces of spruce, the
twist drill gave the best finished hole and the roundest hole by a
long shot. The 5/16th bolt was a perfect fit after having used a an
8mm twist drill.

As a rule of thumb, when working wood, use tools made for
woodworking. Duh!

Yes, mostly I do, but in this case, I'll use what works best! I
found the woood bit went a bit eccentric as it went through giving
a slightly tapered bore as it went through.

Wood expands and contracts with changes in humidity and
it does so anisotropically. E.g. a flat-sawn board will
have the highest expansion rate accross it's width, less through
its thickness, and minimal along it's length. Quarter sawn or
vertically grained wood, which is what you usually want for
a spar cap, will have those first two rates reversed.

What this means is that if you drill a perfectly circular hole
in a piece of wood, as soon as the humidity changes it
becomes an oval hole. The same is true of a wooden dowel.

Wood finishes slow the rate at which wood absorbs or
releases moisture to the air so as to prevent moisture
gradients through the interior of the wood, which minimizes
e warpage. But all wood finishes are permeable to some
degree to water vapor.

So don't get too crazy about making the hole perfect. I think
the epoxy approach is a good idea.

'Kay,. but did the bipes of the thirties have the holes filled in
any way? Lots of them are still flying wth their original spars.

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It's interesting to hear of your results on hole quality. I have
also found that a good, sharp HS twist drill works great in metal or
wood. I think the place where the special wood bits like the Forstner
( a fancy hole saw) are used is in drilling large holes. For 1/2" dia
holes or so and smaller, the twist drill is the way to go. If you
need a 2" or 3" hole or so, well a twist drill that size is a huge
chunk of metal, hard to find locally and expensive to boot. Hole saws
do OK in wood ( and even metal if you are carefull) up to 6" dia or
so and are what I use for large holes.


I'm still surprised that a good quality brad-point would not
make a neater hole than an ordinary twist dirill. At the
very least it will make a neater hole at the entrance and
exit.

It didn't and the brad point is a very good quality bit. One of the
books I have somewhere recommends a twist bit for the wood. Might be the
Bengelis book but IIRC it says to use as sharp a bit as possible and to
feed it at a reate that makes smal shavings, which is what I id and it
worked a treat. I did some practice pieces using some scrap steel parts.
I located the first hole as accurately as I could, then drilled the rest
using the steel part as a guide. I started each cut by hand just turning
the chuck until it was in a bit and then turned the power on. The
resulting hole was about as good as it gets with zero tearaway. The bolt
fit perfectly with the fit just enough friction to hold the bolt in by
itself.

Have you taken the Snipping 101 class yet?

Jim

--
"If you think you can, or think you can't, you're right."
--Henry Ford


"Fortunat1" wrote in message
.. .


It didn't and the brad point is a very good quality bit. One of the
books I have somewhere recommends a twist bit for the wood.

On Aug 26, 1:35 am, Fred the Red Shirt
wrote:
...


Using bushings in a hole drilled in wood helps to reduce
that elongation by spreading that bearing stress over
a larger area in the wood, and is a lighter approach than
simply using a larger bolt. But it is still not a substitute
for maintaining the proper tension in the bolts.


I hasten to correct this. It is right if the bushing is merely
pressed into the hole. If the bushing is well-bonded to the
wood then it will distribute the stress better.

--

FF

On Aug 26, 11:22 am, Fortunat1 wrote:

...
I did some practice pieces using some scrap steel parts.
I located the first hole as accurately as I could, then drilled the rest
using the steel part as a guide. I started each cut by hand just turning
the chuck until it was in a bit and then turned the power on. The
resulting hole was about as good as it gets with zero tearaway. The bolt
fit perfectly with the fit just enough friction to hold the bolt in by
itself.

It sounds like your workmanship illustrates the difference between
a builder and a craftsman.

--

FF

"RST Engineering" wrote in news:13d39pmtl6ru7c7
@news.supernews.com:

Have you taken the Snipping 101 class yet?

Jim


No, and I have no intention of doing so so you can stop wasting bandwidth
netkkkopping

Fred the Red Shirt wrote in
ups.com:

On Aug 26, 11:22 am, Fortunat1 wrote:

...
I did some practice pieces using some scrap steel parts.
I located the first hole as accurately as I could, then drilled the rest
using the steel part as a guide. I started each cut by hand just turning
the chuck until it was in a bit and then turned the power on. The
resulting hole was about as good as it gets with zero tearaway. The bolt
fit perfectly with the fit just enough friction to hold the bolt in by
itself.

It sounds like your workmanship illustrates the difference between
a builder and a craftsman.

Thanks, but I'm afraid my results aren't reflecting that!
I suppose my interest in making the airplane as straight and safe as
possible is more related to my uncertainty about what's safe to let slide
than any desire to make a Grand Champion. I simply don't know enough about
the things to say "yeah, that's good enough"
Having said that, I do enjoy the thrill of holding a nicely made piece in
my hands. Hopefully more of them will be like that than not when I'm done!

Fortunat1 wrote:
"RST Engineering" wrote in news:13d39pmtl6ru7c7
@news.supernews.com:

Have you taken the Snipping 101 class yet?

Jim


No, and I have no intention of doing so so you can stop wasting bandwidth
netkkkopping

Ah, failed the class...

Matt

Matt Whiting wrote in
:

Fortunat1 wrote:
"RST Engineering" wrote in
news:13d39pmtl6ru7c7 @news.supernews.com:

Have you taken the Snipping 101 class yet?

Jim


No, and I have no intention of doing so so you can stop wasting
bandwidth netkkkopping

Ah, failed the class...

Matt


Nice try..

On Aug 26, 1:42 pm, Fred the Red Shirt
wrote:
On Aug 26, 1:35 am, Fred the Red Shirt
wrote:

...

Using bushings in a hole drilled in wood helps to reduce
that elongation by spreading that bearing stress over
a larger area in the wood, and is a lighter approach than
simply using a larger bolt. But it is still not a substitute
for maintaining the proper tension in the bolts.

I hasten to correct this. It is right if the bushing is merely
pressed into the hole. If the bushing is well-bonded to the
wood then it will distribute the stress better.

--

FF

Excellent question! The plane I built called for 2024-T4 aluminum
bushings to be epoxied in the cap. As I pointed out, using this
approach not only has a larger bearing area against the wood, which is
the weakest material in the load path, but it actually restores much
if not all of the strength that was lost when the hole was drilled in
the spar cap. If you have gone to the trouble of using bushings,
epoxying them in place is fairly simple and cheap as hell. Adds very
little work.

Regards,
Bud