cr4 - thread_ bolt length
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AnonymousPoster
Bolt Length01/05/2010 9:04 AM
I had adiscussion with one of my friend regarding the bolt length , and the
discussion was about the extra threaded part of the bolt that will be
exposed after the nut , what is the importance or the need for the extra
threaded length of the bolts after tightning the nut ? is it to facilitate the
installation ?
I assumed that we are connecteing two flanges together using a machine
bolts and nuts from the other side , normally the length of the bolt is
specified to be more than the length of the fittings , two flanges + 1 nut +
1 washer , so what is the need for this external exposed threaded length
after the nut ????
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Comments rated to be Good Answers:
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#4 "Re: Bolt Length" by TVP45 on 01/05/2010 10:35 AM (score 2)
#5 "Re: Bolt Length" by BoltIntegrity on 01/05/2010 11:50 AM (score 2)
#21 "Re: Bolt Length" by BoltIntegrity on 01/06/2010 9:43 AM (score 2)
Comments rated to be "almost" Good Answers:
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#13 "Re: Bolt Length" by Trevor Walden on 01/06/2010 4:47 AM (score 1)
#14 "Re: Bolt Length" by Randall on 01/06/2010 5:55 AM (score 1)
#19 "Re: Bolt Length" by Hooker on 01/06/2010 9:21 AM (score 1)
#26 "Re: Bolt Length" by redfred on 01/06/2010 11:23 AM (score 1)
#29 "Re: Bolt Length" by BoltIntegrity on 01/06/2010 11:39 AM (score 1)
#36 "Re: Bolt Length" by nick name on 01/07/2010 4:20 AM (score 1)
#68 "Re: Bolt Length" by nick name on 03/02/2010 10:02 AM (score 1)
redfredGuru
Join Date: Dec 2008Location: Long Island NYPosts: 4473Good Answers: 299
Re: Bolt Length01/05/2010 9:34 AM
One of you is only thinking of the static assembled configuration. The one
who believes this aids assembly, I think is correct.
During assembly there will be a gap between your two flanges. So during
assembly when initially connecting the flanged assembly the first bolt, nut,
and all washers will be asked to briefly hold all of the components together
with this added gap between the flanges. During this brief period, you
should have as many threads of the nut as possible in contac t with the
bolt to handle the loads that will occur during alignment and inclusion of
the remaining hardware.
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#2
#3In reply to #2
#4
#5
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AnonymousPoster
Re: Bolt Length01/05/2010 9:42 AM
In some cases I think it is just a way of dealing with the issue of "is the
bolt too short"? I have heard 1.5 threads exposed and others have told me
3 threads exposed. This was in equipment where the hardware was holdingthe electronics enclosure together. The extra threads served nothing
beyond making the workmanship inspectable.
nick nameGuru
Join Date: Mar 2007Location: City of LightPosts: 2868Good Answers: 107
Re: Bolt Length01/05/2010 10:12 AM
Not only, nut and bolt have chamfers so that if the bolt would have only
the length to be flush with the nut surface at least one thread would be
not engaged so that the loading capability in shear would be less
required. From an other point of view this over length considers all
possible deviations in the thickness of components so that in the worsecase all nut threads will be engaged.
TVP45Guru
Join Date: Jul 2007Posts: 4372Good Answers: 138
Re: Bolt Length01/05/2010 10:35 AM
There is a standard of 3 exposed threads. This is a "rule of thumb" but
serves two purposes:
1. It makes the assembly easy to inspect;
2. The first thread or so is not full and cannot be counted on.
Now, Shigley and Mischke in their mechanical design handbook make the
argument that a good bolted joint only requires about three full threads. I
believe them, but I still like belt and suspenders.
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BoltIntegrityPower-User
Re: Bolt Length01/05/2010 11:50 AM
Three threads, two or one protruding beyond the nut of a fully tightened
joint make no difference. Anything beyond the nut is superfluous. In
fact, it can be argued that the last threads in the nutreally don't do much
either since it's the first few threads that take most of the load.
2
2
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#10In reply to #5
#11In reply to #10
#12In reply to #11
Join Date: Aug 2008Location: EuropePosts: 231Good Answers: 20
Practically, the presence of extended threads simply provides a visual
guarantee that there is at least minimum thread engagement.
On the other hand, sometimes there is a real requirement for much more
than even three threads. In fact, these cases require the equivalent of
one diameter
protruding
beyond the
nut. As you
can see in theadjoining
picture, these
are large
fastener
applications
which are
tightened by
the use ofbolt
tensioners
rather than by
typical
wrenches. Bolttensioners are
used when
speed of joint assembly and precise control of bolt load are required.
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AnonymousPoster
Re: Bolt Length01/06/2010 2:05 AM
it can be argued that the last threads in the nut really don't do much
either since it's the first few threads that take most of the load.
Explain that in terms of an unbroken inclined plain
RandallGuru
Join Date: Aug 2005Location: HemelHempstead, UKPosts: 2602Good Answers: 106
Re: Bolt Length01/06/2010 3:11 AM
Bolts stretch.
I wonder if there is an argument for a slightly variable (pre-tightened)
pitch for units in very precise assemblies?
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bwireGuru
Re: Bolt Length01/06/2010 3:23 AM
Bolts stretch doesn't explain the first three threads taking more load
than the others.
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#14
In reply to #12
#20In reply to #14
Join Date: Dec 2007Location: Upper Mid-westUSAPosts: 7506Good Answers: 96
I think the variable pre-tightened concept is covered by the various
classes of thread.
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RandallGuru
Join Date: Aug 2005Location: HemelHempstead, UKPosts: 2602Good Answers: 106
Re: Bolt Length01/06/2010 5:55 AM
The tension in the bolt at A is constant; is decreasing through B
towards zero at C.
Therefore the slope of the thread in the bolt is steeper at A
decreasing through B to C.
The effect in the nut is the opposite but not as pronounced and not
shown here.
I hope you can just see that the slope of the thread in both nut andbolt is parallel in the upper section, but curves so that it is steeper
in the bolt in the lower section.
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DedalusAssociate
Join Date: Aug 2009Location: ChinaPosts: 25
Re: Bolt Length01/06/2010 9:28 AM
Dear Randal,
As a retention feature this has merits (such as tri-lobial bolts or
uneven pitch bolts).
My concern with all these retention features however is that they
tend to increase friction significantly, reducing even further the
energy available for clamp-force. Then you need to increase the
bolt material grade, then the cost goes up. Also, it is virtually
impossible to calculate and predict the behaviour of such joints, so
joint development relies exclusively on physical testing which is
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#36In reply to #20
#37
In reply to #20
#32In reply to #14
also expensive and time-consuming.
bwireGuru
Join Date: Dec 2007Location: Upper Mid-westUSAPosts: 7506Good Answers: 96
Re: Bolt Length01/06/2010 5:26 PM
Good points, the number of time I've encounter the philosophy
that tight threads equal a preferred engagement eludes me.
Cheater bars aren't needed to thread a nut onto a bolt
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nick nameGuru
Join Date: Mar 2007Location: City of LightPosts: 2868Good Answers: 107
Re: Bolt Length01/07/2010 4:20 AM
You have a wrong view angle of the needs for such anti loosening
features.
Without them the bolt/nut pair is hold in place only by the
preloading generated friction which has to be bigger than the
active moment generated by preloading and thread angle. If for
any reason the preload decreases then holding capacity
decreases , it is an unstable process.
The different approaches (trilobs or similar) have the advantage
that the friction forces which hold the bolt/nut together are NOT
any more depending on the preload magnitude !
Of course friction is higher and torque has to be increased
according to the higher losses at tightening but this is via tests
possible to define and adjust.
With respect to material quality the trend to miniaturisation leads
anyway to smaller bolts and the stronger material allows a higher
ration L/d which is better for fatigue. Since Young modulus is the
same independent of elastic limit, for same load, a high strength
bolt will be more elastic and reduce the variation of bolt forces
under load thus being more resistant to fatigue.
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RandallRe: Bolt Length
01/07/2010 9:31 AM
bwireGuru
Join Date: Dec 2007Location: Upper Mid-west
Re: Bolt Length01/06/2010 4:49 PM
Thank you for taking time for this and yes I understand the point
you're making but it seems odd especially in this day when even
wood screws are having rolled thread that the threaded portion of
bolt encapsulated within the nut should be deemed of less integrity
than the remainder/shoulder or unthreaded length of the bolt.
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#38In reply to #33
#59In reply to #38
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#57In reply to #5
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bwireGuru
Join Date: Dec 2007Location: Upper Mid-westUSAPosts: 7506Good Answers: 96
Re: Bolt Length01/06/2010 5:12 PM
I ran out of time to add
I would expect stretch between A and B to reflect the stretchbetween B and the bolt head
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Randall Re: Bolt Length01/07/2010 9:35 AM
crimich13Re: Bolt Length02/26/2010 6:30 PM
Randall Re: Bolt Length03/01/2010 4:16 AM
omw7Power-User
Join Date: Apr 2007Posts: 237Good Answers: 14
Re: Bolt Length02/24/2010 6:18 AM
I strongly disagree with this. In the circumferential direction the bolt is in
compression and the nut is in tension. When the bolt is just as long as
the nut, some of that compression deforms the end of the bolt and
makes it get smaller in diameter. The thread engagement is reduced. Iunderstood that this is the reason for having threads protruding from the
end of the nut from a calculation point of view.
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omw7
nick nameGuru
Join Date: Mar 2007Location: City of LightPosts: 2868Good Answers: 107
Re: Bolt Length02/26/2010 4:42 PM
The radial forces are the same for bolt and nut. But the radial stiffness
of bolt is a lot higher than the nut stiffness so that the major
deformation will be the nut one and this leads to an increase of mean
diameter of the nut and a reduction of the shear area.
This is the basis for the ratio between nut hexagon and bolt diameter:
the nut has to be stiff enough in order to limit this radial deformation
which penalizes the carrying capacity.
If for any reason you should use nuts made from a material having a
young modulus smaller than the bolt the hexagon has to be increased
to compensate the higher radial deformability.
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#64In reply to #63
#66In reply to #64
#67In reply to #66
#68In reply to #63
omw7Power-User
Join Date: Apr 2007Posts: 237Good Answers: 14
Re: Bolt Length02/27/2010 3:59 AM
However, when the bolt is cut short (say flush with the nut) then the
end of the bolt will dimple under the compression forces which in turn
reduces its external radius and so the engagement of threads with the
nut (which itself has expanded and so also reduced its engagment).
So the final capacity is not as per the calculated value.
Effectively, the different codes that I have managed to find that
reference the number of additional threads beyond the nut have a
varied view on the number of threads between 1.5 to 3.
Of course this is all to do with bolts in tension and not shear.
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omw7
nick nameGuru
Join Date: Mar 2007Location: City of LightPosts: 2868Good Answers: 107
Re: Bolt Length03/01/2010 3:16 AM
A couple of remarks:
1- the load is transmitted by 3 threads at the OPPOSITE of the
bolt free end!
2- the nut diameter increase and the shear force reduction CAN be
calculated thus it is possible to know from the start how much the
bolt will hold in tension!
3- the standard nut dimensions (as mentioned) were defined so that
their radial stiffness will be big enough to avoid such portance
losses. Never the less the bolt stiffness is SOO big in comparison
with the nut that its own compression can be TOTALLY neglected.
So that from the beginning the probable load carrying capacity
I have several FEA results I can send you, I have not any more the
original jpg files (are embbeded in power point and word files and I
do not know how to record them again as pictures and integrate to
a message on CR4) so that I can send only if you give an email
address but not here only on the private channel.
Randall Re: Bolt Length03/01/2010 4:30 AM
nick name Re: Bolt Length03/01/2010 12:10 PM
nick nameGuru
Re: Bolt Length03/02/2010 10:02 AM
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Join Date: Mar 2007Location: City of LightPosts: 2868Good Answers: 107
Thanks to recommendation I recovered the pictures as JPG file and
can display them.
The results come from different FEA with different software and
threads done under different contact conditions, but all of them are
consistent in following directions:
- Load is transmitted by the first threads
- End threads are NOT for load transmission but for all other
mentioned reasons or at least for some of them.
- Nut radial expansion (which visible on the 2 pictures at top) is very
small and the bolt has almost no radial deformation.
- The loading can be from start correctly estimated.
The picture top-right shows also how the nut thread is deformed
under load (the small red circles indicate the region where the nut is
supported by the parts considered as st iff).
It is also interesting how the bolt is loaded and how the stress
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#69In reply to #68
#6
#7
expands within it.
The bottom picture shows also how every bolt thread root is a
stress concentrator.
If from a qualitative point of view all comments are valid it is
different if the quantitat ive aspect is considered.
Hope it will make some aspects clear.
Thanks again for the software indication it can be a real help.
Nick Name
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omw7Power-User
Join Date: Apr 2007Posts: 237Good Answers: 14
Re: Bolt Length03/03/2010 9:46 PM
Great pictures. Thanks for posting them.
I guess that this is a standard height coarse thread with about
about 5 threads. Even with a reduced height lock-nut we wouldstill have 4 threads.
I'm happy to see that the bolt does dimple slightly at the top. This
had been reported to me by a nut-bolt specialist and it is good to
see the confirmation from your pictures.
So if the bolt head was flush with the nut, the other remaining
engaged threads would have a higher load ? I imagine that for a
standard height bolt with 5 threads, this would be reduced to 4
threads (more or less).
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ozzbGuru
Join Date: Mar 2007Location: Rosedale,Maryland USAPosts: 3043Good Answers: 125
Re: Bolt Length01/05/2010 1:08 PM
There is no need for it though it may help in assembly. Bolts though are
manufactured in standard lengths. Better to chose one that has a few
threads sticking out then one that you can't get a full nut on. Failure to
have all the threads in the nut when torque it down may cause the
threads to fail usually on the bolt but have seen them pull out of the nut.
To obtain the nut flush with the end of the bolt means you would have to
start your tightening before you had a nut full of threads.
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AnonymousPoster
Re: Bolt Length01/05/2010 2:59 PM
Thanks for all your replies and Best Regards ,
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#8
#9
#13
#31In reply to #13
Anthony@ALNOCommentator
Join Date: May 2009Location: Gosford,
Sydney, AustraliaPosts: 58
Re: Bolt Length01/05/2010 10:34 PM
Hello Guest,
What a great question
The extra length is generally an allowance as bolt lenghts come in 5mm or
10mm increments pending on the dia. it may be 25mm increments
therefore a bolt lenght would be rounded up to the nearest length in theideal world. In many cases to exert clamping pressure you may only need
one thread but this would be dependant on the dia. and clamp pressure
required.
hope this helps
Anthony@alno
http://www.alnoproductservices.com.au
Anonymous
Poster
Re: Bolt Length
01/05/2010 11:45 PM
The following reasonings are offered from my end : -
1. Bolt length should always be more than the measured length of parts to
be assembled because many a time the parts being loose or difficult to
access, shall require pre-holding of parts on bolt length so that bole end is
available clearly for engaging nut.
2. Actually bolt length required is = measured length of parts + one nut
width and any extra length does not serve any useful purpose. Infact
extra projection shall be an obstruction while dismantling at a later stage
due to corrosion , dust particles sticking and jamming the clearance
between bolt & nut threads. Use of end caps are advised in this regard.
3. Sometimes exact required bolt length may not be available and instead
of cutting and using, it is better to use bolt of nearest length available.
Trevor WaldenActive C ontributor
Join Date: Nov 2009Posts: 19
Re: Bolt Length01/06/2010 4:47 AM
ANSI piping was designed for outdoor Oil Industry piping. The bolts had to
do the same service. But contractors try to cut costs so bolting standards
were included as part of the Piping Specification. For maintenance
purposes it was required that bolts not be too long, the exposed threadwould rust. And also long enough for Assembly, allowing for gasket
compression. The requirement was settled by half the diameter of bolt
maximum, and the bolt end to show. Single plain washers were standard
under bolt head and nut. Lock Nuts (thinner than full nuts) were required
where vibration from machinery would affect the piping.
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bwire Re: Bolt Length01/06/2010 4:31 PM
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Location: Reading,Berkshire, UK. Goingunder cover.Posts: 8914Good Answers: 257
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DedalusAssociate
Join Date: Aug 2009Location: ChinaPosts: 25
Re: Bolt Length01/06/2010 7:12 AM
The thread at the start of a bolt is usually incomplete, i.e. it does not
meet the geometric requirements for an ISO thread and cannot be used to
take torque.
This is because of the method of manufacturing threads which leaves a
short length incomplete, and is made worse by normal piece-to-piece
variation.
Therefore, it is usually recommended that bolts are chosen to slightly
protrude at the back of the nut so as to ensure that the incomplete
portion of the thread is not engaged in the nut. The recommended
protrusion in my company is at least 1.5x thread pitch. In practice of
course we do not design every bolt from scratch, we select a bolt from a
list of standard sizes. Therefore sometimes the only available size happensto be even longer than 1.5 threads more than the clamped piece, and so
you see some joints where the bolt protrudes a lot at the back.
In addition to this, you may find bolts which have a short, unthreaded,
cylindrical portion attached at the tip and this make the bolt stick out
even more. This is called a "pilot point" or I think "dog point" in the US. It
is there to guide the bolt into the nut and prevent cross-threading during
fast assembly
bolt drawing
Hope this helps
AnonymousPoster
Re: Bolt Length01/06/2010 8:45 AM
The people that answered this have some good opinions and a good set of
handy guidelines. There is no use for extra threads unless you may want
the option to put on a "jam" nut or safety wire. The exposed thraeds are
sometimes sharp enough to pose a safety risk in which case you would opt
for an acorn nut or plastic cap. If extra threads aren't used for drawing
components together. Design the bolt/assembled unit without them.
Loctite makes good products for securing theaded items.
bwireGuru
Join Date: Dec 2007Location: Upper Mid-west
Re: Bolt Length01/06/2010 5:18 PM
Good assembly techniques include removal of sharp edges to avoid safety
hazards...
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#18
#21In reply to #18
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CaptMoosieGuru
Join Date: May 2009Location: Red Hook, NewYork (Mid-Hudson RiverValley)Posts: 1945Good Answers: 65
Re: Bolt Length01/06/2010 9:14 AM
Other than using the bolt projection as a visual inspection method, as a
safety precaution, I'd rather have extra bolt thread projection especially
where there are vibrations or load cycles present in a structure orassembly. No matter how hard we try to torque the little buggers down
they always loosen up!
Making my 2 Cents known! Hmmmmm I wonder how magnificent a
Moose quarter would look like if the US Treasury ever gets around to
making 'em!!! LOL Anyone interested in signing my petition????
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DedalusAssociate
Join Date: Aug 2009Location: ChinaPosts: 25
Re: Bolt Length01/06/2010 9:18 AM
Dear Guest,
What holds the joint together is CLAMP FORCE, not FRICTION. Clamp force
comes from bolt extension. This comes from torque on the bolt head
against the thread gripping the nut side. There is probably some
theoretical merit in the previous comments about only a few threads
actually taking the majority of the load, however when designing a new
joint for mass production you need to design it in such a way as to
achieve the full required thread engagement or else you may have some
threads stripping under the torque or others coming lose in operation.
The correct design thread engagement varies from joint to joint and does
need good calculation and/or testing (e.g., torque-angle or torque-clamp
force testing) before it can be decided. As a rule of thumb start with 1.5
bolt diameters for bolt into steel pieces or nuts and 2 bolt diameters for
aluminium.
PS1.: Loctite may be a good solution for one-off fixes on my classic car
but is not going to hold joints together in production. At best it may
prevent a bolt from coming off after it has already gone lose, i.e. the joint
already failed.
PS2: On a typical joint, friction takes up about 80%-90% of the energyyou put in through torque! So only about 10-20% of your torque is left to
extend the bolt and create clamp load. Do not waste this energy further
by bad joint design. Follow the rules, there is good reason for them.
BoltIntegrityPower-User
Re: Bolt Length01/06/2010 9:43 AM
Just a bit of clarification: Clamp force does NOT come from bolt
2
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#23In reply to #18
#19
Join Date: Aug 2008Location: EuropePosts: 231Good Answers: 20
extension; It's a result of bolt elongation. I'm sure that this is what the
author had meant but, since this post concerns bolt "extension", I
thought it important to point out.
However, there is another more serious clarification that needs to be
made: Clamp load does NOT come from "torque on the bolt head..." (or
nut). For example, if the bolt head (or nut) was welded onto the flange
(literally or f iguratively ie galling) and turning force was applied, the
"proper"(!) torque would eventually be achieved but the bolt will not have
stretched. Here's a real-life example that makes this clear.
"Torque" in terms of bolting is very critical yet still widely misunderstood.
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nick nameGuru
Join Date: Mar 2007Location: City of LightPosts: 2868Good Answers: 107
Re: Bolt Length01/06/2010 10:20 AM
"What holds the joint together is CLAMP FORCE, not FRICTION."
The clamping force without friction will not maintain the bolt/nut in place
!
There are 2 aspects:
- holding together the parts against transverse forces the c lamping
force has to be so big that the friction force it generates on the
potential sliding surface will be more important than the transverse
maximal force. This is the reason this force has to be maintained all time
long and which ever will be the loading of the pair.
- holding together the pair bolt/nut this is more difficult since anyeven minute sliding on the thread surface will get the bolt/nut loose.
Axial forces having a neglectable effec t on it. This is the reason the
machines used to check the performance of an anti moose principle are
based on a transverse sliding system. Here again the combination
between axial force (clamping) and friction on the thread contact
surfaces will keep the pair together or not. For THIS is a bonder as
LOCTITE or similar is good since it maintains the threads after preloading
together and does not allow a loss of preload.
It is a bit more complex than it appears at first look.
HookerGuru
Join Date: Mar 2008Location: Bridgewater,Va.Posts: 1057
Re: Bolt Length01/06/2010 9:21 AM
Lots of good answers here.
The standard in aviation is minimum 1.5 threads through the nut. You'll
hear lots of design reasons for this but I can say from experience that
anybody that has to do pre-flight or post-flight inspections really
appreciates the excess.
From my experience in helicopters it is really nice to be able to glance at
an area and quickly visually verify that everything is in place. If a bolt did
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#22In reply to #19
#24In reply to #22
#26In reply to #24
Good Answers: 46 not significantly protrude through a nut the inspection process would be
much more tedious.
Also, on an assembly where a nut is not visible it is nice to be able to run
a finger behind and verify everything is intact. If I couldn't feel the bolt
protrusion, that would prod me into a closer inspection of the area.
Hooker
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BoltIntegrityPower-User
Join Date: Aug 2008
Location: EuropePosts: 231Good Answers: 20
Re: Bolt Length01/06/2010 10:01 AM
Hooker,
It's good that you point out the importance of inspection. Indeed,
physical inspection is a great way to
ensure that the bolts are still there. The
next most important issue is, "is the bolt
still tight enough".
To address this, special bolts areavailable which have an optical sensor
that changes colour based on the degree
of clamp load. In this example, if a bolt
has loosened, the sensor shows red.
When re-tightened to the proper load
(irrespectiveof torque) the sensor
becomes black again.
Proper elongation measurement is a more definitive way of measuring
clamp load but, as a simple "go, no-go" indication, this technology seems
to be quite interesting.
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HookerGuru
Join Date: Mar 2008Location: Bridgewater,Va.Posts: 1057Good Answers: 46
Re: Bolt Length01/06/2010 11:04 AM
Wow, love those optical bolts. I can think of a couple of helicopter
accidents where having those in critical systems may have prevented
the mishaps.
"When re-tightened to the proper load (irrespectiveof torque) the
sensor becomes black again."
So, if not by torque, how is the bolt installed to the proper clamp load?
I'm sure I'm probably missing something outlandishly simple here.
Hooker
redfredGuru
Re: Bolt Length01/06/2010 11:23 AM
Think about what happens with a bolt that binds from galling or debris
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in the thread. You can bring the wrench up to the specified torque
but not have the bolt's head touching the flange.
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HookerGuru
Join Date: Mar 2008Location: Bridgewater,Va.Posts: 1057Good Answers: 46
Re: Bolt Length01/06/2010 11:36 AM
Agreed, to a point.
If I ever found a licensed mechanic that allowed that to happen on
an aircraft, I'd make sure his license was pulled and that he'd never
work in the field again.
There is no excuse for that to happen, and even if inadvertent,
would "probably" be caught by the inspector at sign-off. Unless, of
course, there was corrupt maintenance management shortcutting
the practices and processes.
I am speaking exclusively of the aviation field.
Hooker
BoltIntegrityPower-User
Join Date: Aug 2008Location: EuropePosts: 231Good Answers: 20
Re: Bolt Length
01/06/2010 11:39 AM
Exactly. Even if the bolt's head does touch the flange, variations in
friction factors arising from lack of lubricant, excess lubricant,
lubrication contamination, dirt in the threads, damaged spotfaces,
washer inconsistencies, thread damage (and on and on and on....)
the "proper" torque will likely not produce the desired (required!)
clamp load.
Interestingly, torque wrenches are usually required to be calibrated
to +- 2%, or so. This is absolutely useless when the use of these
"calibrated" torque wrenches can still result in preload variations of
+- 20, 30, 60, 80 or even over 100%!
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crimich13Power-User
Re: Bolt Length02/26/2010 6:48 PM
So load would be the clamping force under the head?
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BoltIntegrityPower-User
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Re: Bolt Length01/06/2010 11:28 AM
Hello Hooker,
I'm sorry for the confusion. Indeed, the bolt is tightenedby "torquing".
This can be done with a torque wrench, a hand spanner or even by a
hammer and flogging spanner. This is because it doesn't matterwhat
resistance is encountered when trying to turn the bolt/nut: the sensor
only changes colour once the appropriate bolt load has been achieved
.
As noted in a previous post, one may have the "proper torque" yet thebolt may still be loose. Actually, if less friction is encountered, a
"properly torqued" bolt can even be too tight. Load verification such
as elongation measurement or by visual inspection utilizing this
technology eliminates these insipient risks.
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HookerGuru
Join Date: Mar 2008Location: Bridgewater,Va.Posts: 1057Good Answers: 46
Re: Bolt Length01/06/2010 11:47 AM
Thanks for the clarification. I certainly understand the risks.
I really like the load verification by elongation, and will certainly keep
these in mind for future design tasks. There's always some worrisome
place where we can't use normal retention practices (safety wire,
loc-t ite, etc)
Hooker
AviatorCommentator
Join Date: May 2008Posts: 86Good Answers: 2
Re: Bolt Length01/06/2010 11:19 AM
Actually, there are two additionnal concerns that may be particular to the
aviation field :
1) Showing a MINIMUM of 1.5 threads with nylon insert locking nuts
insures that any shear force on the pieces assembled with the bolt is
effectively applied against the shank of the bolt (unthreaded part).
2) Showing a MAXIMUM of 3 threads with nylon insert locking nuts insures
that the thread of the nut have NOT started to make their way into the
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untheaded part of the bolt (shank), hereby damaging both nut and bolt.
Interesting discussion !
AnonymousPoster
Re: Bolt Length01/07/2010 1:12 PM
Are you aware that there are special structural bolts available that are
torqued to a point where the threads deform, creating a permanent lock.
These bolts cannot be reused if undone and must be replaced with newbolts/nuts. They are used in bridge construction and in buildings steel
joints where they are hidden from sight and must be relied upon to not
loosen.
nick nameGuru
Join Date: Mar 2007Location: City of LightPosts: 2868Good Answers: 107
Re: Bolt Length01/07/2010 1:28 PM
Indicate the source, manufacturer, address, catalogue, .... Thanks Nick
Name
bwireGuru
Join Date: Dec 2007Location: Upper Mid-westUSAPosts: 7506Good Answers: 96
Re: Bolt Length01/07/2010 2:28 PM
Are you aware that your description apply to all properly tensioned bolt
joints?
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redfred Re: Bolt Length01/07/2010 2:43 PM
bwireRe: Bolt Length01/07/2010 2:46 PM
nick nameGuru
Join Date: Mar 2007
Re: Bolt Length01/07/2010 5:43 PM
No, bolts can be used under conditions several times.
What the guy said (in an improper way which could be considered as
arrogant or at least impolite) is that some bolts when torqued over a
threshold deform the threads so that a mechanical lock appears. I was
informed of such bolts but I was not able to find sources this is the
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reason for my question. In fact as far as I know it is the nut which is
deformed so that a zone is plasticized and generates in the thread a
friction torque INDEPENDENT of the preload. This is a guaranty for a
reduced risk to have the bolt/nut pair going loose. Anyway such
solutions cannot apply for machines and are used in structures where
the loads have not same dynamics as in mechanical systems.
BoltIntegrity
Power-User
Join Date: Aug 2008Location: EuropePosts: 231Good Answers: 20
Re: Bolt Length
01/07/2010 6:01 PM
I've encountered "torque-to-yield" in automotive applications but not
in civil. Perhaps the writer refers to the colloquially-termed "twist-off"
bolts in which a necked portion of the bolt's shank breaks loose upon
reaching a certain torque. The nominal threaded portion remains and is
locked by the nut. Notwithstanding the need for specialized assembly
tools, the obvious problem with these is that the nebulous
torque/tension relationship still applies. As such, actual bolt load when
the spline breaks off is still an unknown value
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nick nameGuru
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Posts: 2868Good Answers: 107
Re: Bolt Length01/08/2010 9:50 AM
The component is called "TC Bolt" from tension controlled and is
intensively used in structures for civil engineering for many reasons
related to cost and quality. Here are a few infos about component
and applications:
The right part is
used as reaction for the torque and will shear at the torque limit
given by the grove at right end of thread.
The shaft is ONLY under tension the torque loop closes at the right
side between thread nut and right end.
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The tool has a
special design with a holder inthe middle for closing the torque loop.
An example of civil
connection between big profiles.
Use in railway
connections.
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#47In reply to #46
A tpical assembly for TC
bolt: a bridge structure.
An other one in London
near to Big Ben
Other structure same
fastener.
The grove is such that the torque at shear is near to the one
needed to stress the bolt in the right strain range.
The only problem with those bolts is that in most cases they are
short and thus cannot be used where transverse loads have a high
frequency. It is but possible to use them in calibrated holes and then
the side movement which is the most dangerous for loosening is
maintained at a very low level.
BoltIntegrityPower-User
Re: Bolt Length01/08/2010 10:09 AM
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The biggest problem is:
" ... the nebulous torque/tension relationship still applies. As such,
actual bolt load when the spline breaks off is still an unknown
value..."
Granted, there is much meat left in the safety factor of civil bolting
applications. Thus, there may not be as much of an issue here as
there is in critical industrial bolting applications. However, it's
misleading when the description suggests "... used... for...reasons
related to...quality".
Quality is knowing what you've got. This is still, unfortunately,
"guessing".
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redfredGuru
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Re: Bolt Length01/08/2010 10:50 AM
Point well made BoltIntegrity. These bolts show what torque hadbeen applied which need not be the actual bolt load. I would
expect that the bolt load will change on these bolts as normal
load conditions change on these civil engineering projects.
But I wish to compliment you, Nickname, TVP45, Trevor Walden,
and Daedalus for such an informative discussion. This type of
engineering insight is precisely why I linger on this blog.
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FJDominguesCommentator
Join Date: Dec 2009Location: HoustonPosts: 61
Good Answers: 4
Re: Bolt Length01/08/2010 5:22 PM
Ok. All this is good discussion, but does not answer the original
answer.
The reason for the extra thread (exposed past the outer face of
the nut) is to prevent yielding of the threads under the nut at
the end of the bolt. The normal standard is to assure that 3
threads are exposed. With fewer threads exposed, there could
be a risk of the bolt threads yielding in shear under the nut.
Exposing any more threads is not necessary and could bewastful. Note here that the bolt yields before the nut (in
correctly specified connections).
Also note that, for many applications, this characteristic would
preclude reusing the same bolt for the same application.
However, since yielding usually does not occur, this normally is
not a problem for applications for which this limitation is not
specified.
Some comments on bolt tension: See my next post, which I
consider to be off topic somewhat.
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FJDomingues Re: Bolt Length01/08/2010 7:20 PM
Anthony@ALNOCommentator
Join Date: May 2009Location: Gosford,Sydney, AustraliaPosts: 58
Re: Bolt Length01/10/2010 4:44 PM
Where can i get a copy of the normal standard?
Anthony@ALNO
www.alnoproductservices.com.au
nick nameGuru
Join Date: Mar 2007Location: City of LightPosts: 2868Good Answers: 107
Re: Bolt Length01/09/2010 6:50 AM
You are right but in this particular case the dispersion is a lot less
important than in other cases for following reasons:
Geometry: the grove is manufactured with a tolerance of about
0.1mm which represents 1% of the diameter even for the smallest
type. surface. So that the torsional modulus will have about 3%
dispersion.
Material properties: elastic limit has a dispersion of about 4%
since the bolt are made from a high quality steel and heat
treated.
Friction: nut and washer come from same supplier and have samesurface conditions, in general the washer is large and thick (to
assure a quasi plane contact since the structural steel is weaker
than the bolt or nut). This guaranties a narrower dispersion of
friction coefficients and contact conditions between nut and
sliding surface (washer only!). Same conditions for the friction
between threads. U estimate the dispersion at about 4..6% based
on measurements.
All together the dispersion of the applied tension will be
(3^2+4^2+6^2)^0.5=7.8%.
Due to the very very high redundancy of structural joints (yousaw the number of bolts in one example) the result will be very
near to the average so that it is not so bad.
Of course this fastener is not of use for applications with only a
few bolts as in automotive, but could be of interest in airborne
assemblies where the redundancy is also high imposed by the thin
walls with limited local carnying capacity. To make a comparison,
for a car in general an assembly consists of about 3 to 4 bolts
working together (one side of suspension), for a small plane one
wing is fastened with over 150 bolts to the main body and for
A380 there are 2500 bolts on one wing side.
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bwireGuru
Join Date: Dec 2007
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Re: Bolt Length01/09/2010 1:17 AM
Lug nuts etc. are torqued and re-torqued many times but many
applications require replacement of the fasteners each time.
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nilayveeraActive C ontributor
Join Date: Jan 2010Posts: 16
Re: Bolt Length01/12/2010 1:34 AM
Hi There
There is indeed need for that extra threaded length, because there could
be vibrations in the assembly, Vibrations can b caused due to thermal
contactions and expansions due to c limatic changes. That extra lengthcan prvent the nut from falling off and thus save your assembly from the
failure.
drbobwooleryCommentator
Join Date: Feb 2010Location: Vallejo, CAPosts: 90
Good Answers: 6
Re: Bolt Length04/18/2010 8:45 AM
From a retired Union Steamfitter's perspective, you guys are straining at
gnats. This applies to the 1/2 to 1 inch or so B7 studs and Heavy hex nuts
that hold together most of the flanges in US refineries. Only at nuclear
sites have I seen torque wrenches used for nuts smaller than about 3
inches. Elongation measurement is obviously best. One obvious reason to
leave extra length is to allow for a slip blind and its extra gasket whenisolating parts of systems. On 150# to 600 # series flanges, the flange
nuts get pulled up in stages until final tightening with two combination
wrenches linked together for leverage. If it leaks on hydrotest, go round
and round with a knocker wrench (what one poster called a thrashing
spanner) until it tightens up. Most of the time this extends to the largest
pipe sizes. Make the contractor see that the studs are buttered with
appropriate compound, Coppr cote, Ni antisieze or whatever from one end
to the other. Demolition of old worn out units often reveals studs with
intact threads near the nuts, and rotted away in the area that didn't get
any bolt dope.
JohnDGGuru
Join Date: May 2006
Re: Bolt Length04/18/2010 8:54 AM
"you guys are straining at gnats ..."- that's as maybe, but it doesn't
wash with a customer's inspector when you're trying to get a new piece
of gear passed for payment.
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drbobwoolery Re: Bolt Length04/18/2010 10:14 AM
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Comments rated to be Good Answers:
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#4 "Re: Bolt Length" by TVP45 on 01/05/2010 10:35 AM (score 2)
#5 "Re: Bolt Length" by BoltIntegrity on 01/05/2010 11:50 AM (score 2)
#21 "Re: Bolt Length" by BoltIntegrity on 01/06/2010 9:43 AM (score 2)
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#13 "Re: Bolt Length" by Trevor Walden on 01/06/2010 4:47 AM (score 1)
#14 "Re: Bolt Length" by Randall on 01/06/2010 5:55 AM (score 1)
#19 "Re: Bolt Length" by Hooker on 01/06/2010 9:21 AM (score 1)
#26 "Re: Bolt Length" by redfred on 01/06/2010 11:23 AM (score 1)
#29 "Re: Bolt Length" by BoltIntegrity on 01/06/2010 11:39 AM (score 1)
#36 "Re: Bolt Length" by nick name on 01/07/2010 4:20 AM (score 1)
#68 "Re: Bolt Length" by nick name on 03/02/2010 10:02 AM (score 1)
Users who posted comments:
Anonymous Poster (6); Anthony@ALNO (3); Aviator (1); BoltIntegrity (8); bwire (9);
CaptMoosie (1); crimich13 (3); Dedalus (3); drbobwoolery (2); FJDomingues (2); Hooker (4);
JohnDG (2); nick name (11); nilayveera (1); omw7 (3); ozzb (1); Randall (6); redfred (4);
Trevor Walden (1); TVP45 (1)
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