okay we'll take a look at this furnace
for checking the carbon content and you
see this is just an ordinary induction
coil put the stuff in there heat it up
to about 2600 degrees with oxygen and it
will burn and then you can proceed now
on the resistance high temperature
furnace which is another one that's used
you have carbide or moly die silicide
elements then the accelerators use you
have a catalysts in it and then you can
detect the separated gases by one of the
two detection systems that we had
previously mentioned that provide a
specific and consistent signal and
process it electronically it's a lot of
a lot of electronic stuff but you can
come up with an answer on the infrared
detection you can apply it on the basis
that various gases can absorb energy
within a specific wavelength of the
infrared spectrum now this is similar to
a scanning electron microscope analysis
when you analyze something and you
actually have a wavelength for a given
element so that you can determine how
much of each one you have in it there on
the thermal conductive detection system
it's based on the principle that each
gas has a distinct capability of
carrying heat from the body so you can
take the carbon dioxide and determine
the thermal conductive chains generated
by it and come up with the amount of
carbon that was expected and in any
sense you can determine then the total
amount of carbon that was in the sample
and get it a an accurate reading on it
now let's go to thread inspection and
this one is something that people talk
about a lot but kind of like politics
they talk about it a lot but
but it's not a lot done so I found out
when I went to checking into it the
common methods of inspection that are
given once again in this Mel handbook
age 28 and are the system's 21 22 and 23
they're also covered in antsy B 1.3 em
now in general the system 21 requires
the least amount of inspection system 22
is an intermediate amount and 23 is the
most stringent and so our each one of
these is practically a separate document
so I'll just try to summarize them a
little bit but one of the things we
found out is that most people just use
system 21 and on system 21 all you're
doing is checking the OD your ID and
using a go/no-go gage anyway some of the
things that you are looking for on
threads now this was up earlier in the
course but nevertheless just to go
through it again to familiarize
ourselves with it here is the the pitch
of the threads the distance between
threads here is that angle which is
usually this alpha 1 alpha 2 the sum of
those normally is 60 degrees on threads
here is your banish cone where you run
out your threads and here is the major
diameter which is the outside rest of
the threads the minor diameter which is
from the diameter at the root and then
the pitch diameter which is a very
important thing and that's of course
where if you mated the threads up
perfectly you would have the same
thickness through the cross-section of
the thread as you do in here as you do
in the mating thread here and here is
here's another one that I wanted to show
you for this reason this is kind of a
close-up that shows the difference
between having an unrated through
and having a radius thread the other
thing is this circle here represents the
pins that are used for measuring the
pitch diameter there's a method to call
the three pin method that they can put
the pins a two on one side one on the
other side but flat plates across
measure it then you go to a table and
you can find out from this table based
on the size thread you're using the
diameter of these pins you can come up
with an accurate reading of the pitch
diameter of the thread now for external
threads system 21 just includes go/no-go
diameter and the major diameter that's
all that they do and you can either
measure this you usually measure it with
what the color ring gauge which is
nothing more than a calibrated thread in
a ring that to replace replace what you
would normally use as a nut and you try
to thread the fastener into it if it
goes it's fine you turn the the use the
other one the no go and if it doesn't go
in that one that means you are within
the acceptable limits it doesn't tell
you exactly what your dimensions are but
it just tells you that the thing will
work so that's called the the functional
diameter if you will system 22 includes
the system 21 measurements plus pitch
diameter you can either measure it with
a pitch micrometer which is a micrometer
that has a grooved head on one end of it
that fits over a thread then you have a
pin type on the other one that fits in a
thread and you can span this across the
OD of the threaded area and get a
reading which will give you the actual
diameter pitch diameter that you want
and you can look on the table in and see
whether it's within the tolerances that
you want the thread groove diameter
which is the measurement between threads
at the pitch diameter point the
functional diameter which you get from
up here with a go/no-go gauge leading
flank angles and that's just go no go
minor diameter and then you can measure
the route profile but I found that that
is not done that much unless somebody
insists that it's done system 23
includes all the others plus now you get
into the roundness of the pitch cylinder
itself the taper of it in other words if
you take the whole thing as a cylinder
do you have a taper on it is it round or
is it lopsided the cumulative thread
form variation going through the thing
and checking to see whether it varies
any from one end to the other the
leading helix angle variation the flank
angle variation on the threads run out
an even surface texture now surface
texture on the threads usually is not a
problem but you could measure can see it
would only be if the thing had been
coated with some sort of a coating that
was not electronically directly
deposited in other words if you had
galvanized threads then you could have
have a problem because you'd have extra
plating material in the threads and
after you've gone through all of these
things this is just four threads
nothing's been done on on the rest of
the fastener so you could have a huge
crack in and it wouldn't make any
difference because if you passed
everything else the guys say well I
inspected the threads and they're good
so you still have to look to see if
there's anything else wrong
now for internal threads you have the go
no-go and the minor diameter and that's
about all it's usually check the the go
no-go gauge one end fits the other one
doesn't and then with the minor diameter
you use a regular plug to slip in to
check to see if it is okay
then you move to the system 22 it
includes the 21 and then you go for
minimum material pitch diameter a thread
groove diameter and the angles on the
threads but since this is internal
thread this is hard to do so usually
people don't do it now here's a go/no-go
gauge one side will thread in if it's in
normal tolerance the other side won't
and this is used to check internal
threaded or tapped holes and that's the
only usually that's about the only
acceptance that people use I found here
is the go no-go pen for just checking
the minimum diameter in a threaded hole
to see if it is within tolerances and so
this one is fine if it and this one's
not supposed to go it is bigger than the
tolerance bandwidth will a lot would
allow it the hole to be then for the
system 23 it includes the the others
plus the roundness of the pitch cylinder
and they taper the pitch cylinder and
but still nothing on the internal thread
radius regardless of what you call out
it's not measured unless you would go in
and tell somebody you have to have it
and then as as I mentioned earlier using
this dental plaster type stuff you can
you can actually cast it and then take
it put it on an optical comparator to
see whether you have the radius that you
want
so now we move on and and to the the
cold hard facts of life that even though
you've inspected the heck out of the
threads if with these three systems if
you run it through all of them
you still haven't looked for
manufacturing defects for defects in the
the threads FF s eighty-six federal spec
gives examples of acceptable and
unacceptable defects and we'll look at
those and subsequent figures and you
will note that the acceptance of the
thread defects becomes more critical as
the fastener strength increases and the
ductility decreases so there has to be
some engineering judgment exercised on
it now here's one of the things threads
should have no laps or seams at the root
or the flanks the here's the root here's
the flank of the thread and so in
general what you're saying on this is
any defects below the pitch diameter
because you're loading this part of the
thread a lot more than you are this part
so anything below the pitch diameter in
the way of a defect a noticeable defect
you're not going to accept when you get
things above the pitch diameter or
outboard of it here now you can accept
more defects there because it is more
likely loaded but even so there are
limits on how much you can accept on it
so so it once again you look at it and
if you find too many cracks in a
fastener you really should reject it now
here's something that is a lesser
problem it just looks bad having little
nicks or something like that as long as
it's not a crack that's just a nick from
handling and it doesn't affect the
functioning of the threads you could
probably accept a nick on the outside
surface of the threads now here is
another method of thread inspection a
laser inspection method most
manufacturing facility
would not have this at all because it is
a set up with a full computer printout
availability and I think cost about
$100,000 so you wouldn't find them in
your normal inspection shop but it is a
very accurate method of checking threads
it uses laser triangulation sensors and
a motion sensor to digitize the thread
and form and it's a non-contact method
you're using a laser beam and the the
measurements are made by comparing the
data obtained by laser scanning the
thread to a perfect mating part that has
been mathematically created in software
and the thread axis is the method you
use for spinning it around so you can
check it at different points now these
machines though are used for inspecting
inspection equipment because they're
accurate enough for instance you can use
them to inspect the thread plug gauges
and go/no-go gauges dyes and taps and
they can handle parts up to six inches
in diameter and four to 64 threads per
inch now it's a time consuming thing so
the the places that you would use it as
say you don't have very many bolts
holding an engine on a plane so on a 747
if you wanted to inspect the super high
strength alloy steel bolts that are
holding it on you would run them through
an inspection procedure like this check
every one up because they're your it's a
super critical application on the figure
seventy eight is a picture of this one
you set the fastener down on the the
head you can turn it you scan the thread
in and because this is on a rotary
spindle here and once you you scan it
then the the table will index to another
location then you get a thread profile
that you can compare it to a perfect
thread so if you're really doing
something critical this will work in
fact I believe marshal I think got one
of these machines because they wanted to
use it for checking some of the super
critical flight Hardware for shuttle and
installations that they were putting
together there so I went looked at him
there's a company here in West Lake I
believe handles them and they do work
well except that you would not inspect
something that was just an ordinary
production part because it's too eh too
time-consuming too expensive now there
have been various discussions through
the years on how variation in pitch
diameter on a fastener can do them in
and I guess this arguments been going on
for 30 or 40 years or something like
that so the industrial fasteners
Institute here in Cleveland initiated a
research effort in 1993 to manufacture
measure and test a bunch of fasteners
that were deliberately made out of
tolerance on pitch diameter just to see
how bad it was and they put out an
article on that in mechanical
engineering in the December 1996 edition
and the conclusion was the variations in
pitch diameter don't have a very big
effect on the joint strength fatigue
life and clamping performance in other
words it can be out of Tolerance quite a
bit and still pass the standard tensile
and proof load requirements which kind
of surprised a lot of people I I thought
it would have more effect than that
because varying the pitch diameter of
course you are loading your threads
unevenly
but evidently what happens is that
though you are loading them unevenly
you're spreading the load around of
where you get more yielding and it'll
still carry the load
so they some of the people who did the
testing were surprised if it was that it
was that good now moving to the other
parts of the fastener the head and shank
inspection there's one of the places
where you can really get in front into
trouble with a fastener is having any
kind of a defect in the Phillip radius
under the head because since that is one
of your highest loaded areas any kind of
a crack there usually will propagate to
cause failure so a list of defects and
their definitions are given an ASTM F
788 that is for the fasteners and nut
inspection is covered in ASTM F 812
there are very similar methods of
inspections so I'll cover primarily the
ones here just for fasteners and leave
the the other part out now quench cracks
quench cracks are caused by excessively
high thermal and transformation stresses
during heat treatment and so getting one
of those means that you've got problems
with the material so you could have a
problem with it so in general quench
cracks of any detectable size by visual
inspection make the fastener
unacceptable and here's another one this
is a pet peeve of mine socket head depth
even though if you go to any of the Aunt
suspects and
or mil spects any of these on socket
head fasteners
they give dimensions for the depth of
socket but I have yet to find anybody
that's ever checked one we had a problem
here a couple years ago with some na s
fasteners that the heads popped off of
them in a wind tunnel installation and
when we looked at him the socket depth
was too deep well see in a socket head
if that depth gets too deep you wind up
with a small annulus of area there is
all you have left if you get below the
the bottom of the head with the socket
depth you're in trouble and that's what
was happening and although everybody
talks about him there like UFOs no
evidence there
here are some examples of the things in
head and shank inspection and cracks in
general these are quenched cracks which
you can see can happen in the heads in
the shank around the top of the head but
here's the one that really gets you if
you have any cracks here in this radius
fill it radius under the head you're in
real trouble
so that is from that FF s 86 or ASTM 788
I don't remember now which forging
cracks now remember I mentioned on
fasteners that the higher strength ones
usually have forged heads because you
don't want to have the discontinuity in
green flow at the particularly tough
illustrate Phillip radius so you can get
forging cracks during the cut off or
forging operation or even cold forging
you can get some on the material if the
material is a little bit too hard when
you're when you're cold forging it and
these are located on the top of the head
or on the raised periphery around the
indented head bolts and screws and you
can you can except some of them if they
are very very slight so that they're
more or less a street grid than a crack
just just a indentation mark as long as
they have a very shallow depth but once
again depends on the criticality of the
installation as to how much you accept
in the cracks here's one that shows a
forging track on the top of the head and
if you look at those a limits on depth
you'll see that if you take 0.04 times
the diameter or something like that for
a bolt that is a 1/4 inch in diameter
that's a pretty shallow crack it's
enough
more than a streak that you can see so
so so that type of crack wouldn't so
called crack would be acceptable now
here is a fear burst and that's an open
break in the in the metal from forming
and you can you can accept these only if
there is it's in the flats and extends
in the crown chamfer circle at the top
of the head or in the under head bearing
circle and none of you located at the
intersection of the wrenching flats that
reduces the width across corners below
its specified minimum in other words you
can you can accept some of these once
again if they are so shallow that they
don't look like a crack itself but just
beware of them because the this is one
here and you see this is really this one
amounts to just a little dent on the
corner of the flat so so that would
probably be acceptable as long as it did
not look like a crack itself
Foles that's the kind of a doubling over
material which occurs during the forging
operation and usually occurs near the
intersection of diameter changes
particularly with non circular heads and
the only problem that you look at with
that is you can allow them in some cases
at the corners but you don't want any
near the Phillip radius of the fastener
in in this area here now here you're
getting it because you're trying to form
a round cross-section in the square so
it's kind of hard to form that without
getting some deformation and burgers
around it so so once again you look to
see where it's at and evaluate it before
you accept this on a fastener
now seams are usually in the raw
material before forming and they're
pretty straight and see seams are
acceptable because usually they're
they're not a crack per se and they're
they're shallow and don't have and have
a pretty good radius now see here if you
look at this it's point O three times
the diameter so if you go with a 1/2
inch fastener you see you still have
something there that you would have
trouble even seeing it's so shallow so
so that would be okay surface voids you
can get this in a material due to the
way it's formed but you got to watch if
it's it indicates there's probably
something wrong with the chemistry of
the material if you're getting a lot of
voids in the surface and once again the
void depth look at the amount here that
you're allowed point O two times the
shank diameter that's still or ten
thousandths all right at ten thousandths
void is a pretty shallow one and then
the this one I would look at if you had
void areas that are that high a
percentage of the under head bearing
area I would look at the material to see
whether I had the right material
chemistry or not and whether I'd want to
reject it on that basis now tool marks
Nicks and gouges they're permitted on
the under head surface but you notice
the restriction on that astonish your
micro inch surface roughness does not go
under the 125 well you see a 125 is
really not not too rough it's a rough
machining surface and
so the the other place that you would
look at if the head is banged up a
little bit and it's on the corners out
of the way you could probably accept it
as long as the plating surface is not
gouged now plating inspections this is
another this is another one of those
that we talked about and people don't do
other than look at it same yep that's
that's a gold color so it means it's got
chromate in it and don't see that it's
got up too much so guess it's alright
most of the platings we've discussed
earlier and but we didn't discuss
discuss anything on the inspection of
them so we'll kind of limit our coverage
here to zinc and cadmium platings
except for just visually looking at the
things and the substitution of zinc for
cadmium and using a dye and amassed the
color is a common way to cheat it's done
off a lot and because the the chromate
dye that you use usually you look at it
and the fasteners are gold type color
and it you can't tell by looking at it
whether it's a zinc or cadmium so the
only way to find out is actually to run
a test now you can do two different
things on it you can destroy the plating
on a fastener and take a chunk of the
plating and go put it in a scanning
electron microscope and see whether it's
mostly cadmium are mostly zinc but then
there are other things that you can do
here too then in inspecting now zinc is
usually covered by ASTM
B 633 and cadmium is covered by a
federal spec uqp 416 you can do process
control inspection
and the plating outfits are supposed to
do that and most of them do so that they
control the amount of additives they put
in if their bath gets tired they can add
chemicals to it and so on and take new
readings determine how it is plating and
you can do a lot sampling inspection
visual inspection and plating thickness
tests there is in fact I believe a guy
from here at Lewis just recently
developed a method of inspecting the
thickness of plating Dan Roth works over
in M and s or the what used to be M and
s I believe developed one but there are
methods of looking at and I think
ultrasonically measuring plating
thickness on materials you can do an
adhesion test you can do a corrosion
test and you do hydrogen embrittlement
test although the hydrogen embrittlement
test you can get that with both zinc and
cadmium so that in itself would not be
conclusive the lot sampling technique
you can take a lot of plated fasteners
of the same metal composition and so on
and take a bunch of samples out visually
inspect them look to see if the plating
is smooth and to see whether that here
is properly whether it has blisters in
it fits and that sort of thing and then
you could alright you can measure them
non-destructively by these various
testers an electronic test eddy current
magnetic beta radiation back scatter and
all these things that's covered in one
of the sections of the mill handbook h28
you can take plated specimens for the
required adhesion corrosion and hydrogen
embrittlement tests from a production
lot at scheduled times you can determine
the adhesion and this is a real
scientific method by scraping the
surface with a knife and then looking at
it to see whether it is adhering
properly with a magnifying glass that's
a method of inspection that you can do
yourself
now corrosion resistance is determined
of course by doing your salt spray test
which runs 96 hours and after the
exposure the presence of corrosion
products visible to the unaided eye at
normal reading distance is caused by
rejection because you should not get any
rusting on it or deposition of corrosion
products and for the 96 hours now
hydrogen embrittlement testing this one
is there are different schools of
thought on where you should start on the
hydrogen embrittlement testing some of
the faster manufacturers of the lower
strength fasteners say gee you don't
have to do it on lower strength
fasteners because you can't get hydrogen
embrittlement well a guy by the name of
Lou Raymond who is kind of the Guru in
the US on hydrogen embrittlement ran
some tests and decided that you get
hydrogen embrittlement all the way down
to about a great 5 fastener the only
thing is it takes it longer to show up
so in this ASTM spec it only they go
anything above a hundred and forty four
which means your grade eight would be
the first one that you would test and
put it at crank it up to 85% of tensile
element for a minimum of a 70 to 72
hours and you shouldn't have heads
popping off if you have heads popping
off then you have hydrogen embrittlement
now the sample size and rejection
criteria
normally you pick a bunch of random
samples out of a bin and test them and
the ASTM f 788 has a table which will
show later that gives you the number of
samples that you should take for a given
production lot another one is given in
this aunt cas qcz 1.4
which superseded mil standard one 105
and then we have aunt suspect be
eighteen eighteen point one that gives
some sampling techniques the basis of
all of these is to randomly pick a small
sample and any failure of the samples
rejects the whole life here is one from
ASTM f 788 which shows you the sample
size that you should take for a given
lot size and you check it for all of
these different tests that you want to
run and once again the amount of testing
that you do depends on the criticality
of the design so if you find that there
okay you can proceed and accept the
quantity of fasteners that you have if
you find problems and you could go ahead
and insist on more testing to verify
that it's not as serious as it appears
on the surface now for macroscopic
examination of predix with seam
indications here is a sample table from
ASTM f 788 and you can take them a look
at them according to this sampling
technique and if they are not judged
acceptable then you can either conduct
more tests or reject the entire lab
there's been a lot of talk on the lot
traceability and commingling and
certifications and so on
concerning the fastener quality act
which is also known as public law 101
amended its 101 592 as amended by 104 -
113 and of course one of the things that
is covered in that law is lot
traceability of fasteners the customer
can ask for the steel manufacturers name
the lot number chemical analysis of the
wire from the which the fasteners were
made and of course from domestic
suppliers this information is readily
available because most companies when
they make fasteners they get a bill of
lading with the coil material that gives
all this information on it and but on
imported fasteners then it's a bit of a
problem to get it because you have to
get the certification from the person
who made it in the foreign country now
on co-mingling this is something that's
kind of a new word but what it is in the
past the fastener distributors would get
fasteners from all different suppliers
put them together in a barrel and when
someone ordered some they get a bunch
out of the barrel so you it would be
theoretically possible for you to get a
hundred fasteners made by 25 or 30
different manufacturers under this this
part of the law the commingling would be
cut way down to where you can only have
the fasteners from two different
manufacturers in the same lot because
each each manufacturer must register his
trademark with asme then he has a stamp
his trademark on all the fasteners
covered by the law now the the minimum
size is covered are quarter inch in the
inch system and five millimeter
diameters in the metric of course that
is not altogether true - because if the
fastener is three
hardened in other words if it's a
heat-treated fastener then smaller sizes
are covered but nearly all the small
sizes are excluded from the law because
they're not heat treated that much
now if the fasteners haven't been
exempted they're now restricted by this
commingling rule and of course you can't
have more than two locks in the same
container at least this way you have a
better idea of who the manufacturer was
on your lot of fasteners that you're
getting for your usage a customer can
demand certifications such as the
material lot numbers chemical analysis
reports in tensile test data and this
documentation is notarized and legally
binding on the suppliers part now in the
past we could get certifications with
fasteners but they normally were a sheet
that the clerk who filled the order
initialed and said these are certified
to be good and that was it and so nobody
was legally responsible so if it is done
this way with the certification then the
supplier is legally responsible for the
fastener so one of the gimmicks that
some of the distributors distributors
are using is they're saying okay you
want certified fasteners it'll cost you
three times as much for certified
fasteners
that's uncertified fasteners now do you
really want certified fasteners so so
that's one of the loopholes if the
distributor is not required to provide
certifications he's not responsible for
fasteners anymore the other thing that
is in that law which is a real big
loophole is agreement between customer
and manufacturer now if the customer is
a clerk who knows nothing about
fasteners they can make all sorts of
agreement with the manufacturer without
even knowing it so that so in other
words it's don't-ask don't-tell type
thing with these fastener certifications
if you don't ask for them and insist
that you get them and pay the surcharge
forgetting them you're not going to get
them so that's where the fastener
quality act stands and I am although a
lot of fastener manufacturers are scared
and a lot of companies are scared on it
I don't honestly think myself that it's
going to amount to that much in the long
run it is being handled by NIST and of
course the the government will enforce
the laws how much they enforce them
nobody knows yet I went to the school
that they taught on how it would be
implemented it's something like a
hundred page document even the lawyers
can't agree on how it should be
interpreted so they'll probably ask for
another delay in 98 now as far as
inspection and test standards there are
all kinds of specifications for testing
inspection methods and what we have done
is listed as many of those as we can in
the appendices we also have general
references in the appendices for where
some of this material came from and
additional references in case you'd want
to check further since so many fastener
tests are done per mil standard 13 12
will give a summary of its contents here
just kind of go over it it establishes
standard methods for testing fasteners
in both the metric and the inch pound
system and the standard test methods
yield data and design allowables
that are safe to use in fact mil
handbook five uses mil standard 1312 for
running their tests on both materials
and fasteners that they publish in the
book and also fastener allowables in the
joint section of mil handbook by the
each test has a standard method spelled
out and each and if their book forms for
each one of them so you have a - -
number for it that gives you a
standalone document if you will so if
you turn over to the next page here are
the different categories the salt spray
test that we've covered the interaction
tests humidity lap shear test stress
durability hardness testing tensile
strength stress corrosion stress rupture
fatigue the thickness of metallic
coatings and doubles your testing and on
the then we go to torque tension
clamping forces for installed or
installation for fasteners stress
relaxation elevated temperature tensile
tests sealing single shear sure joint
fatigue receptacle push out panel
fasteners for electrical tensile
strength - panel fasteners and and
receptacle torque out fasteners then you
have grabbing recessed torque for a
quality conformance test structural
panel lap shear sheet pull up now this
is something that is important and some
of the cherry rivet type manufacturers
the pulls damn tape rivet manufacturers
have had trouble meeting the this sheet
pull up because if you have several
sheets together maybe they're not
exactly flat and you try to pull them up
and clinched the rivet sometimes you
have trouble passing this test with
these pull stem rivets so a lot of the
manufacturers have had to go back and
revise things to get a little more pull
in the system so that they can lock the
rivets so that's what that one is
is used for that one there would be for
the blind fasteners then you have
locking torque tests and this last one
here has been I'm not sure whether the
final copy of that one is even in the
book yet but I don't the last committee
meeting we had it was discussed that
that one was being published barrel nut
tension test which which we didn't have
Dave and we did the CM 1b bans now for
the the metric side you have these that
are covered and they for some reason or
rather they changed it to a DoD standard
thirteen twelve for the metric in order
to differentiate in the mil standard and
these are the ones that they have for
testing of metric fasteners now we go
into the the do's and don'ts of fastener
designs and I I kind of come up with a
set of guidelines you're just common
sense type guidelines that for you to
use and it's not a complete list because
you can always come up with an addition
for a list but these at least could be
used as a designer's checklist and one
of the things that's a pet peeve of mine
is a field that enough information
should be given on a drawing to fully
define the fasteners you want and I know
in the past I've been disappointed in
some of the drawings in which they say
all fasteners to be per ffs 86 or 85 now
if you go look at that spec you can
anything from alloy steel stainless
steel down to even nylon fasteners on it
so you're giving the guy a lot of leeway
if you don't define it any further than
that so this is why that I said fully
defined fasteners you want in other
words when you call them out for a
specification give the paragraph of that
specification that covers the fasteners
that you want to use or if you are not
satisfied with how its defined in the
spec give the strength level required
for example on drawings on materials I
know I have seen on drawings where it is
critical enough that you even specify
the grain direction on the drawing
because you know on materials you have a
longitudinal transverse and short
transverse directions on them the short
transverse is usually the weak one so
you specify on the drawing and the the
area of major stress that you want that
to be the longitudinal direction in
order to get better properties so
specify what you want on the face of the
drawing now here I mentioned earlier in
the course using up it's softer than the
bolt that'll keep you out of trouble and
that distributes the loads on the thread
because usually if you torque a fastener
to failure with a nut it will fail in
the first two threads in the thread
run-out area due to stress concentration
so the nut will not fail usually it's
the the bolt that fails and don't use
feather edges on sheets in a joint match
drill for counter sunk holes use
floating nut plates for critical designs
particularly for counter sunk fasteners
so that the countersink can Center the
fastener and the nut plate will not be
try
bend it determine the environmental
conditions before selecting materials or
coatings for fasteners because you want
to make sure that you're covered with
your temperature range and design sure
fasteners to be critical and varying
that means that the fastener is stronger
in shear than the material so therefore
you can elongate the hole and the
material to allow your fasteners to pick
up the load without failing the fastener
don't use Jam nuts for locking check
alignment of fasteners before final
assembly and of course as a corollary of
that avoid head bending because the
fastener bending I think the SAE
handbook says don't go more than plus or
minus two degrees on misalignment on a
fastener head to avoid trouble with
bending and followed the edge distance
and spacing guidelines on fasteners now
you can temper this but one of the
things that you don't do is put a
fastener so close to the edge that if
the tolerance goes against you when the
hole is drilled you'll have it pushing
out of the hitch and I've seen some that
were almost that bad now don't use
fasteners that look alike but are made
of different materials they'll use 300
stainless and 2a 286 stainless the same
size same head everything that so that
you can't tell the difference between
them and don't use fine and coarse
threads in the same assembly unless
there's a big difference in the fastener
diameter so it's not possible to get
them in the wrong holes and here's
something that you can get in trouble
with although we did it on fittings on
cm-1 don't mix metric and inch fasteners
in a design that that'll get you in real
trouble verify that you have the
fasteners you specified and demand
traceability if it is a critical design
make sure that you get the proper
traceability of the fasteners use
inserts and soften
cereals to avoid fastener pull out if
you can't use through holes if the
dominant fastener load is sheer don't
use a high torque on the fastener
because you have to combine the fastener
and shear loads to the total strength of
the material so you don't want to use up
all of it in tension if your primary
load is shear avoid tap poles as much as
possible because you can't inspect them
you're not sure how good they are so if
you can avoid them don't use them use
harden washers under both the head and
the nut on a bolded installation if
possible don't Terk a fastener above its
yield point stay below the yield point
and don't get close to it unless you run
sufficient tests to determine
pretty much where it is then in a
fatigue joint if you have to go up
because of fatigue then you can go up to
a near yield
thou'rt the use of lubricants lowers the
coefficient of friction so the Clerc
values have to be adjusted accordingly
one of the cases we had at the Cape of
this using silver-plated nuts stainless
steel nuts of course the silver
tarnishes so if you have them in a
barrel for a long time they look bad so
some manufacturer decided he would stop
that so he coated these silver plated
nuts with wax from tarnishing they
didn't tarnish but nobody told the guy
using the torque wrench so they were
yielding these things all over the place
and couldn't figure out why they were
yielding well somebody found out that
because the wax actually reduced the
coefficient of friction to about half of
what it would normally be and Clerc
tables are only guidelines the design
engineer should determine the turqu
values for his design because that's why
you don't blindly use a turk table and
will get you in trouble
fasteners loaded in fatigues should be
trucked the near yield values I
mentioned that earlier and before we go
into the frequently asked questions on
for design we'll take a short break and
come back then and finish up the
question and answer section