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Old April 21st 17, 11:16 AM posted to
John B Slocomb
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Posts: 333
Default Selecting An Appropriate Bolt

5On Thu, 20 Apr 2017 23:41:53 -0400, wrote:

On Fri, 21 Apr 2017 10:21:56 +0700, John B Slocomb

On Thu, 20 Apr 2017 12:55:36 -0400,

On Thu, 20 Apr 2017 16:17:14 +0700, John B Slocomb

On Wed, 19 Apr 2017 23:56:34 -0400,

On Thu, 20 Apr 2017 09:52:15 +0700, John B Slocomb

On Wed, 19 Apr 2017 18:23:53 -0400,

On Wed, 19 Apr 2017 18:43:02 +0700, John B Slocomb

Metric thread pitch is described totally different than inch size
bolts. Inch size is threads per inch. Metric thread is thread pitch -
so in inch size bolts, a higher number is a finer thread - in metric a
higher number is a coarser thread. A 6X10 metric bolt is 6mm with a
thread pitch of 1mm crest to crest (or root to root - however you want
to measure it)

Who cares, along as the people involved know what you are talking
about? Ant metering system is just that, a system which works for
those that use it.

The old method of measuring gear ratios on a bicycle was to use "gear
inches" which described the diameter of a wheel that would move the
distance in one revolution. Rather archaic today but made perfect
sense to those that used it.

As far as the "grade" of the bolt - a "grade 8" is NOT always better
than a "grade 5" or even, possibly, in some cases, a "grade 2"

A grade 2 or grade 5 bolt may bend and stretch - and still hold, where
a grade 8 would simply snap. It depends on what kind of load is being
carried by the bolt - and how it is torqued. On the same vein, a bolt
that is undertorqued CAN fail faster than one that is overtorqued. A
properly tensioned bolt is "pre-stretched" just enough that any cyclic
load does not stretch the bolt any farther, so the bolt does not
fatigue in use.

An exciting theory and technically correct. although I would comment
that I've yet to see an under torque bolt break.

It's far from "theory" - I've seen numerous head bolts and manifold
bolts fail that were attributed to being under-torqued on vehicles
that were not properly PDId, and quite a few bolts that failed in
shear because they were not properly tightened, and/or the holes were
not properly de-burred, allowing the bolt to loose tension.
No use arguing with Slocumb though - you'll never get anything
through his thick skull.

You must have a tremendous amount of experience with nuts and bolts.
As I mentioned I've been fooling with them things for about 70 years
now and frankly I've never seen "numerous" head bolts fail. Yes, I've
seen head bolts fail, but I would use the term "rarely" not
"Numerous". I would have to say that if you have seen numerous head
bolts fail then you are associating with some very incompetent

And how does one determine that they were under torqued after they
have failed?
Notb incompetent mechanics - but poor factory assembly.

Don't take my word for the FACT the problem exists.

In particular Picture #10.

As for broken head bolts - see:

Also see:
-Particularly item #3
3. Fatigue Failures

Fatigue failures typically occur within a couple of threads, where the
bolt engages into the internal thread. Failure is then reached due to
the high stress gradient within the region.

Fatigue failures can be particularly hazardous because they often
occur with no visible warning signs and the failure is often sudden.
Fatigue failures are often unknowingly avoided in gasketed joints
simply because the required crush for the gasket often dictates a
torque or bolt tension that minimizes the risk of a fatigue failure.
However, changing to a new gasket type later on which requires less
crush may be the initial cause of bolt fatigue failure.

It is not unusual to assume that a bolt has failed due to overload
when it has in fact failed from fatigue, which can also be a
consequence of self-loosening.


The first cause listed:
Insufficient Clamp force? - Usually by applying a measured torque load
to the nut bolted joints are tightened to achieve a specific clamp
load. Even under the most extreme applied loads, the clamping force
must prevent joint movement between clamped parts. Movement includes
both opening of the joint to form gaps and slipping. Loads applied to
the joint may be axial forces (in the direction of the bolt axis)
and/or shear forces (perpendicular to the bolt axis). If slippage
occurs then the joint may fail by the bolt loosening. If a gap in the
joint opens then a bolt failure by fatigue is more likely to occur.
Typically bolt fatigue failures occur because of insufficient preload
rather than poor fatigue strength of the bolt. Improving the method of
tightening can reduce the scatter in bolt preload and help guarantee
the minimum required clamping force

Pay particular attention to the sectionfollowing the "bolted
joint.xls" link which explains things in pretty plain language.

You may have worked on machines, including aircraft without fully
understanding what you were doing or why.

You are probably right although the A.F. thought I was competent. Or I
guess that they did as they kept promoting me and they had me managing
divisions for them. Shoot, they even had me writing the skill level
tests for my career field one time.Then when I retired from that job I
hired on as a mechanic again and ended up some years later being
promoted to "Operations Manager" for a fair to middling sized company
in Indonesia.

Peter principal at work? It was all "Government work"

You are trying to spell "sour grapes" perchance?

Not a chance!!!! I've worked for enough idiots without having to work
for for politicians and bureaucrats.

But one isn't working for a politician when in the service. At least
not when assigned to units that actually have a mission. For example,
I was assigned to one of the two reconnaissance squadrons in the A.F.
equipped with a super long focal length camera. One in Asia and one in
Europe. When you fly missions designated by headquarters USAF they
aren't politicians directing you. I was assigned to the "Gun Ship
Squadron" (puff the magic dragon) where we bragged that if we got
there before they actually got inside the wire we never lost a post.
Not too many politicians there either. Then, lets see, I was assigned
to one of the wings that was flying the SAC airborne nuclear flights,
very serious folks those nuclear chaps, and, oh yes, we provided
support for the U-2 flights over Cuba that told y'all that the
missiles weren't there.

Not many politicians in the operational military (as opposed to the
training units).

Teaching in the public school system was enough politics for me.

Sure, you get a fat pension which I'll have to do without - and I
could have if I'd stayed teaching.

Nope, it is rather a thin pension compared to my active duty pay. You
see, what with longevity pay, overseas pay, flight pay, quarters and
rations, etc., active service personnel may draw nearly double their
base salary. When retirement arrives you receive (for 20 years) only
1/2 of your base salary.


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