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Old April 21st 17, 04:21 AM posted to rec.bicycles.tech
John B Slocomb
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Posts: 356
Default Selecting An Appropriate Bolt

On Thu, 20 Apr 2017 07:03:57 -0700 (PDT), wrote:

On Wednesday, April 19, 2017 at 8:56:36 PM UTC-7, wrote:
On Thu, 20 Apr 2017 09:52:15 +0700, John B Slocomb
wrote:

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

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


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
mechanics.

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.

See:
http://www.croberts.com/bolt.htm
In particular Picture #10.

As for broken head bolts - see:
https://www.bimmerforums.com/forum/s...ken-Head-Bolts

Also see: http://www.boltscience.com/pages/Failure%20Modes.swf
and:
https://www.hiretorque.co.uk/failure...bolted-joints/
-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.

Also:
http://www.bluetoad.com/article/Bolt...0/article.html
and:
http://www.onallcylinders.com/2014/0...ener-failures/

Also:
https://www.excelcalcs.com/engineeri...-joints-fail?/
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.

I have not only worked on cars and agricultural equipment and
industrial equipment (loaders and dozers etc) and been rather
extensively involved with amateur built/homebuilt/experimental
aviation, I have also taught automotive mechanics at the secondary
school AND post secondary (trade) level.


That Figure 10 was not a "failure" per se. The bolt did not break from under tightening - it wore the threads off. Eventually indeed it would have broken.

Head bolts on cars can be said to ALWAYS break from over-torquing. What occurs is that you over-torque the head bolts and then when the engine heats up and expands it blows the top of the bolt off from exceeding the mechanical strength of the bolt. Sometimes you can hear it go.

I rarely use a torque wrench because you can FEEL the torque that should be applied and using a torque wrench a number of times shows you that it almost always feels under-torqued. So you develope a feel for it. A correctly designed piece is supposed to use the mechanical strength of a large number of bolts and not the near maximum strength of a few. This is the mistake that is almost always made on that great "German engineering". They use calculations instead of common sense. Ten headbolts torqued within 10% of the proper torque are better than 6 headbolts designed to carry the load if properly torqued to exactly the correct value.

Which reminds me of what is becoming with the carbon fiber engineering these days. It isn't "great engineering" to make a 12 lb bike that can kill it's rider with a single manufacturing flaw.


A friend, who was an EWO on B-52's, once commented that it wasn't
exactly confidence building to go to war armed with equipment built by
the lowest bidder :-)
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