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friction coefficient of Loctite 620
5

friction coefficient of Loctite 620

friction coefficient of Loctite 620

(OP)
I have been trying to work out the correct torque for a 12.9 socket cap screw. I have written a spread sheet based on the formula in MIL-HDBK-60, it works well with the friction coefficients supplied in MIL-HDBK-60. The problem is there are no friction coefficients given for any kind of Loctite.
The loctite data sheets don't supply this information. I have contacted loctite directly and have been supplied with nut K factors, this is a very simplistic way of working out the correct bolt torque and does not allow for lubrication under the bolt washer face.
Does any have actual friction coefficients for loctite products.

RE: friction coefficient of Loctite 620

Loctite has several different application approaches it can be used with surface preparation, an activator can be applied and bolt and nut materials and coatings affect the torque applications. Joint conditions also affect the clamp load.
I wonder what the purpose of you recreating this heavily researched area is?

The application of fasteners in a plant situation will not match tightly controlled lab torques as torque tools have percentage of errors, lubrication is typically not used unless the fasteners have a dry film lubricant. The type of torque tool used has an impact. The rigidity of the joint can impact the torque. If you need very precise clamp loads you will have to probably eliminate through empirical testing the various process variables and then contol these variables when you apply the fasteners.

I checked the Loctite website and on the data sheet they spell out the lubricity of the product but there is a note which I give here: "In critical applications, it is necessary to determine the K values independently"

RE: friction coefficient of Loctite 620

(OP)
I should explain further. I am the Technical officer for the Recreational Aircraft Association of NZ and have done a very large amount of research into bolt strengths and tightening over the last year. Lubrication in the threads and under the washer face is a method of improving the accuracy of bolt tightening as it helps to eliminate friction in the threads and under the head and so allowing more control of bolt elongation and clamp pressure.
The application of the bolt in question is a flywheel retaining bolt. They have been breaking and when analysed I have found the bolts were initially tightened to only halve of the bolts proof load. This has allowed decompression of the parts and so allowing the parts to move and break the bolts under cyclic loadings. The accuracy of a torque wrench is only +-25% due to variable friction, if one knows the friction coefficients and uses lubricants it should be possible to improve the accuracy. K factors are empirical factors calculated from experiment but are a bit simplistic for my calculations.

RE: friction coefficient of Loctite 620

Actually, experimentally determined values are far superior to handbook values.  Providing of course that the experiements are done on your application.

If you are using torque control to tighten bolts, then yes, lubrication is better than dry.  But not because it "eliminates" friction.  Because it reduces variation in friction, thus reducing scatter in the torque vs. preload characteristic.

For important joints, like the flywheel on an aircraft powerplant, you really should be measuring elongation rather than using torque control.

Do not rely on handbook values.  You don't know what was really tested.  You don't know how the test was conducted.  You don't generally have access to the actual data to assess scatter and statistical methods used to arrive at "the number".  Do not rely on handbook values.  Really.

RE: friction coefficient of Loctite 620

You could calculate a friction coefficient with the Loctite data.  The procedure they use is ASTM D 5648-01 Standard Test Method for Torque-Tension Relationship of Adhesives Used on Threaded Fasteners (Lubricity).

I had actual data of parts coated with a pre-applied Loctite product that met GM 6175M (sorry, don't know what Loctite product it was).  The mean friction coefficient was 0.15.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: friction coefficient of Loctite 620

Relying on friction maintained just by bolts preload is a risky and unsafe approach. Why not use added anti shearing pins that will take the torque load caused by the flywheel and will free the bolts just to retain the flywheel in position.

Loctite to my opinion should be used for securing the bolts but not one to rely on its friction coefficient which it is not intended too. It his hard to get accurate coefficient friction with a grease and oil that are designed especially for it never the less with a loctite glue which is not specifically formulated for this.

More than that, aerospace bolts should be secured by mechanical means such as steel wires, etc. Then you can use oil or grease to lube the bolts and get a better control of friction. I still believe that the best is to use pins to take the shearing loads and the bolts to retain the flywheel instead of trying to make the bolts do both.



RE: friction coefficient of Loctite 620

Threadlocking compounds should NOT be used/relied on for critical bolting apps. when the joint is "rigid".  It is preload and preload alone that avoids fatigue failures.

Loss of preload can occur even if a bolt does not rotate/back off.  Look at the millions of con-rod bolts that stay tight w/o threadlockers.

All of this was known many years (and tears) ago---.

RE: friction coefficient of Loctite 620

(OP)
Thank you all for your posts. You are of course all correct in what you say. The key point is that it is a very well researched area and loctite is a very popular product. It seems that its friction coefficient is very similar to steel on steel thus making no change to the standard bolt torque.
The main concern was if it had a lower friction coefficient it would allow the bolt to be over torqued past yield, not good for a high strength bolt.
Loctite do make a thread lubrication product which they state gives a higher bolt preload, this could be vary detrimental if the bolt torque is already 90% of yield. 620 does not appear to have this quality.
Regards
Anton

RE: friction coefficient of Loctite 620

I wouldn't be too concerned with exceeding the YS of a high-strength fastener provided it is only used ONCE.  IIRC A-325 structural bolts are tightened/preloaded by the turn-of-the-nut method to just beyond the YS.

Like other comments here, I'd use a more precise method of preloading than torque for critical bolts.  Hot rodders have long used bolt stretch for con-rod bolts-far more accurate than torque methods.

"I'm that dog who saw a rainbow, only none of the other dogs believed me." from "Kate and Leopold"

RE: friction coefficient of Loctite 620

(OP)
Bolt elongation is clearly a more accurate method; it becomes a lot more difficult in a blind hole, unless the bolt has a hollow centre, which would reduce its stress area by an unacceptable margin in my application. I have calculated the turn of the nut from snug tight to be only 54deg but that doesn't take in to account any local relaxation, so I'm forced to use the torque method with lubrication to reduce the margin of error as pointed out by MintJulep. This is why I was after the friction coefficient of Loctite. If loctite helps to give a more consistent friction in the threads and grease under the washer face gives a more consistent friction there also then it should be possible to more accurately torque the bolts.
As previously stated there has been a large amount of research done in this area, getting sensible results is the hard part. Unfortunately Loctite reps have limited engineering background and are there to sell products based on the training they are given, they don't have the technical background which at times would be more helpful, that’s the case in NZ anyway.
Regards
Anton

RE: friction coefficient of Loctite 620

Even if Loctite doesn't reduce friction it is still possible that it reduces variation in friction, and the assoicated scatter.

But again, you need to test your application to find out.

Using grease and loctite on the same fastener just seems like playing with fire.  The only way I would even consider it would be to clean everything, then apply loctite and run the fastener in almost all the way, then brush on some grease under the head.  But there are very few people that I would trust to actually do that correctly.

An ultrasonic extensiometer can be used on fasteners in blind holes, although it may necessitate machining a flat on the head for the transducer (which may remove the grade markings).

Direct tension indicating washers are another option, if the design can tolerate a washer.

RE: friction coefficient of Loctite 620

(OP)
Now we are getting to the guts of the original question, may be I should reword it.
How does loctite affect repetitive torquing of multiple bolts.
I'm not sure the answer is going to come that easily.
The method of product application above would seem appropriate, using an ultrasonic extension meter might be difficult on 5/16th socket cap screws and the availability of
1000Mpa/145000psi tension washers in NZ might be suspect.
Regards
Anton
 

RE: friction coefficient of Loctite 620

I don't know if the economics of the job would stand this answer, but if you need accurate pre-tension measurement you might want to consider the Boltsafe product offered by Scansens (see link below). It's a load sensing washer reading in real time. I know they offer larger sizes (I've previously specified M42) but I don't know how small they go.
http://www.nortechinc.com/index.html

RE: friction coefficient of Loctite 620

kirstant

As far as I know in aerospace for critical parts and for vibrating environment the best bolt securing is to use wire locking. Why are you even trying to use such a risky approach?

RE: friction coefficient of Loctite 620

kirstant,

Repetitive torquing?  Do you mean loosening and retightening?  Loctite is a terrible idea for that kind of application.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: friction coefficient of Loctite 620

israelkk,

Lockwire should only be used if a loose/broken bolt piece could cause damage, such as inside a gearbox, etc.

Lockwire is not generally capable of preventing fatigue failures in rigid connections.

"I'm that dog who saw a rainbow, only none of the other dogs believed me." from "Kate and Leopold"

RE: friction coefficient of Loctite 620

Metalguy

A loose high velocity flywheel definitely can cause great damage. The wire lock should be in addition to the bolt preloaded. My argue is with the concept to rely on the Loctite to secure the bolts and at the same time "to take advantage" of it as a lubricant. I would use an oil/grease to lubricate the bolts to get greater accuracy for bolt tightening and secure the bolts against loosening using mechanical means such as lock wires.

As kirstant explained in its second post the bolt broke after they were loosen. They may become loose because the tightening was too low and this because bolt tightening is not an accurate science. This is why I recommended shear/fixing pins to take the shear load instead of dropping this too on the bolts and the Loctite.

I argue with the idea that the friction caused by the bolts tension will prevent the flywheel from moving/slipping with respect to the bolts. To my opinion and experience, friction should not be relied on for aerospace and/or dangerous situations unless you can use large bolts stronger many times then needed. This is not the case in aerospace where weight is crucial.

RE: friction coefficient of Loctite 620

Lets see what we have here.  The application in question is a flywheel attachment and the observed failure mode to date has been fatigue caused by low preload resulting from undertorquing of the attaching bolts.
OP is looking for a way to reduce preload scatter in the joints.
In my world you aren't going to find a much stiffer joint joint than a flywheel attachment; so that lends itself to one of the more advanced tightening strategies beyond simple torque.  Since it is a blind hole, and low volume, bolt stretch realy isn't going to be practical.  
I wonder what % of YS the target clamp load is, but I would hope that it is right up there, close to or at yield. You don't want to go to a bigger than necessary bolt because then you won't get the needed eongation and you will only increase the potential for loosening as the amount of stretch decreases at a given clamp load.  On a hard joint like this, if the end user has the time to do it, I would go with a manual torque/ angle strategy.  Start out by tightening all of the bolts to a snug-fit torque using a criss-cross pattern and then take them a specified number of degrees further using the same pattern.  the tools are available from people like Snap-on and this would take most of the variability of the friction out of the assembly.
Even then I would not use Locktite on the fasteners but a lockwire set up to resist rotational loosening.  I don't really trust Locktite when used on a single point critical fastener on an airframe.  I'll trust is all day long on the ground, but not in the air.
In the end this is going to have to come down to testing.  This is a deadly critical joint and all the calculations in the world don't substitute for doing a series of tests on actual hardware using an ultrasonic extensometer.
One last thing.  I hope that the bolts in question are not commercial quality PC 12.9 socket head cap screws.  They are not of a quality level that is adequate for an application of this criticality.  You need to make sure that these fasteners are of aerospace quality to assure their consistancy.

RE: friction coefficient of Loctite 620

Depending on the grip length of the joint, it may take precious little rotation to reduce preload by an unacceptable amount.

Safety wire may not be capable of restraining rotation to the degree necessary.

What is the diameter and grip length anyway?

RE: friction coefficient of Loctite 620

(OP)
OK so heres the story.
The engine in question has been built especially for the ultralight/microlight/Light sport aircraft category.
The flywheel has come loose on several aircraft cause instant engine failure as the magneto magnets are also attached to the back of the flywheel. The manufacture has continually denied there is a problem. In New Zealand we have had one engine have two engine out after having the bolts replaced each time, the pilot is not best pleased. The engine was inspected before the third outage happened and it was found to have broken bolts. Subsequently more engines of the same type were inspected and it was discovered that some of them also had broken bolts. The NZ CAA are going to bring out an AD requiring all of these engine types to have the bolts replaced every 100 hours.
An independent engineer has done work on ten of these engines, the work includes drilling the holes deeper into the end of the crank and tapping leaving an untapped length of 10mm to the surface, in other words a far side tapped hole. The fitting of 3 x 1/4inch dowels into the crank and flywheel ( the manufacture does this as well since feb this year). The fitting of new 5/16th high strength (12.9 equivalent) socket cap screws with molybdenum grease in the thread and under the head and tightening to 41Nm or 30ft/lbs. The grip length is 3/4inch with out the far side tapped holes and approximately 1 inch with. There are six of these bolts.
I have checked the torque figure using the formula in MIL-HDBK-60 available from http://assist.daps.dla.mil/online/start/
and come up with a figure of 42Nm based on a proof load of 140600psi (90%ys) a turn of the nut of 47deg and an elongation of 0.0055inch.
The manufacture has specified a torque of only 24Nm 18ft/lbs and the use of loctite 620 and no grease.
So something is definitely wrong in the state of Denmark and I have been trying to get to the bottom of it.
The bolts are breaking because they are loose, there can be no question about that. The dowels may help but if the bolts are only halve tight then they only delay the inevitable.
12.9 bolts are incapable of taking any type of cyclic loading, they will fracture and break.
I can't help but think that the whole problem has come from the fact that the bolts have been specified to be halve tight, reducing the preload clamp force. But as the manufacture has also specified loctite I have been attempting to see how this would affect the torquing process.
As regards repetitive torquing, there are six bolts. There is no question of tightening and loosening and then re tightening, 12.9 bolts should be thrown away once they have been taken to max preload and then released.
I must also admit that with a turn of the nut of only 47deg the use of lockwire does not inspire confidence, but in combination maybe it does.

Anyway its a badly designed joint but with a bit of effort it may be possible to improve it.

Regards
Anton




RE: friction coefficient of Loctite 620

I think you are correct about this joint.  The Loctite is there to prevent bolt rotation, but with only half the preload, the bolt can still fatigue and fracture with the Loctite.

Lockwire will not help retain preload.

More preload (and thread adhesive like Loctite) will help prevent loosening.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: friction coefficient of Loctite 620

Kir,
That torque spec from the mfg of 24 Nm is grossly too low.  I can't imagine where that came from.  Your number of 42 sounds about right, but it might even be a little on the low side.  
This is a tough joint, flywheel attachment always is.  
I am sure that you have already checked it, but make sure that there is a hard enough bearing surface under the fastener head to withstand the additional clamping force without getting embedment.  SHC have a bearing surface designed for hardened steel and on softer material they will embed and lose preload.

RE: friction coefficient of Loctite 620

(OP)
Thank you all for your input, I didn't quite get to the bottom of loctite friction but did none the less confirm my conclusions regarding the joint.
Regards
Anton

RE: friction coefficient of Loctite 620

(OP)
As a mater of interest I have done the calcs with no grease or loctite and come up with figure of 56Nm and that would no doubt put extra torsional loading on the bolt as if it wasn't under enough strain already. So for me lubrication does several things. 1. It helps to give greater consistency. 2. It reduces friction and by so torsional loads on the bolt. 3. It reduces the total torque required to tighten a bolt or nut. 4. It may help the bolt off and loose tension.
Regards
Anton

RE: friction coefficient of Loctite 620

I've seen reports (Bickford, IIRC) where *most* of the friction is the under-the-head friction rather than thread friction.  If true, any "extra" bolt twist may not be a big factor.

For some larger critical applications, the studs are stretched to the correct length w/o any torque.

"I'm that dog who saw a rainbow, only none of the other dogs believed me." from "Kate and Leopold"

RE: friction coefficient of Loctite 620

kirstant,
Could you come back with a little description of where the bolt are failing?
Is it under the head or the first engaged thread?

There is no problem with the reuse of high strength fasteners if properly torqued every time. We have thousands of H-11 SHCS fasteners (220,000 Ts) that are reused multiple times without any problem. Though not having the safety aspects of an aircraft fastener certain failures would cause a $200,000 production outage and overhaul.  

RE: friction coefficient of Loctite 620

(OP)
First engaged thread.
Anton

RE: friction coefficient of Loctite 620

Don't know if they're readily available in small sizes, but you might consider using reduced-section bolts.  They reduce the stress at the first thread and allow the bolt to stretch more.  Try SPS in PA.

"I'm that dog who saw a rainbow, only none of the other dogs believed me." from "Kate and Leopold"

RE: friction coefficient of Loctite 620

(OP)
I think it is cyclic movement which is causing the bolts to break. I know there is movement due to the fretting on the flywheel part. Any change in the type of bolt will only alter the time before failure.
Anton

RE: friction coefficient of Loctite 620

Then do what it takes to stop the fretting.  Larger hardened washers, more torque.  The flywheel bolt surfaces are far better areas to treat with Loctite, et. al., than the threads.

The 620 is "high-temp.", good to ~450 F short time IIRC.  Use 635 high strength if 300 deg F is OK.  There is a strong time-at-temp. effect on the retaining strength.

Given the relatively large shear area, Loctite should be able to resist fretting.

"I'm that dog who saw a rainbow, only none of the other dogs believed me." from "Kate and Leopold"

RE: friction coefficient of Loctite 620

(OP)
More torque is clearly something I have proposed. Are you suggesting I treat the machined surfaces with loctite 635.
I had looked at this idea but couldn't find a product I thought was suitable, also if the product degraded it could lead to decompression of the parts which would get me back to square one. I would look forward to further discussion on this concept.
Regards
Anton

RE: friction coefficient of Loctite 620

You state:
I must also admit that with a turn of the nut of only 47deg the use of lockwire does not inspire confidence, but in combination maybe it does.
--------------------------------------
Have any of the failures used turn of the nut
method for tightening?  It seems to me that turn of the nut
method may overcome the locktite lube uncertainty.
The use of dowels suggest that the problem might be shear?  Are you using hardened washers? or no washers?
I also assume that you are using certified bolts.

RE: friction coefficient of Loctite 620

>"Are you suggesting I treat the machined surfaces with loctite 635."<

Yes, unless they have some newer product with even higher shear strength, AND if the temp. is low enough.  Its purpose would be to resist the tiny movement which is a requirement for fretting.

Here's what I would do--IF you can easily get to the bolts.  Coat the threads and the bolt-side face of the hardened washers with a moly lube that contains a lot of moly--not a "moly chassis lube".  Dow Corning's Moly GP paste would be fine.  Also coat the flywheel surface with Loctite 635/620.  Torque the bolts to at least 45 Nm.  

Go fly a while.  Check the bolt torque.  Do this every so many hours until you see that the bolts are staying tight.  By not using any threadlocking stuff on the threads/underhead you can perform fairly accurate bolt torque tests during service.

I had looked at this idea but couldn't find a product I thought was suitable, also if the product degraded it could lead to decompression of the parts which would get me back to square one. I would look forward to further discussion on this concept.

"I'm that dog who saw a rainbow, only none of the other dogs believed me." from "Kate and Leopold"

RE: friction coefficient of Loctite 620

Oops, wish this forum had an edit-post button.  

Disregard the last 2 sentences where I was re-quoting you, starting with "I had looked at this idea but couldn't find a product I thought---".

"I'm that dog who saw a rainbow, only none of the other dogs believed me." from "Kate and Leopold"

RE: friction coefficient of Loctite 620

(OP)
Dimjim,
As I am trying to resolve a problem which is not directly related to my own equipment I have to make certain assumptions.
1. As the manufacture specifies a torque all be it too low I can only assume turn of the nut has never been applied. Certainly it would overcome lube uncertainty.
2. The cause of failure is without doubt from cyclic shear. I have no idea whether hardened washers are used but will try to find out.
3. The bolts are probably not certified, but I doubt whether the highest quality 12.9 class bolts could withstand this type of cyclic stress in fact I would be bold enough to say they can't.

If I can stop the fretting or movement the problem will be fixed.
I am starting to like Metalguys approach very much.

Regards
Anton

RE: friction coefficient of Loctite 620

Are you sure that your fastener are failing from shear?

Failure in the first thread is usually indicative of fatigue. The usual scenario is that a few fasteners would loosen and fail form fatigue then the remainder fail from shear.
If they are all failing from shear the dowel pins would help carry the load.  

What is the general precision of the mating components?

RE: friction coefficient of Loctite 620

(OP)
unclesyd,
Sorry I meant fatigue do to cyclic loading in the shear plane, other wise you are correct in the mode of failure.
As I understand it the precision of the mating components is perfect although this was given as a problem further checks have shown the fit to be as good as possible.

Regards
Anton

RE: friction coefficient of Loctite 620

(OP)
I have been consulting with my preferred bolt supplier today and he showed me a lock washer I had not seen before. They are called "Nord-Lock" http://www.nord-lock.com
They are a two piece hardened washer which require the preload to increase before they can back off, which clearly would prevent back off. The product looks good the technical specs look good, has anyone tried them out in real life.
Regards
Anton

RE: friction coefficient of Loctite 620

The use of Lock Washers was extensivly and informatively discussed in thread 404-161524 in the Mech Eng Other Topics forum. I recommend it.

RE: friction coefficient of Loctite 620

Wow, what kind of dowels are you using
that would allow that much movement between
the two parts?  Maybe taper pins may be
a better solution to ensure that the parts
stay in position relative to each other.
The trouble with this is that the parts
will not be interchangeable and must be
done as a matched assembly.  What do the
dowel pins look like after the bolt
failures?

RE: friction coefficient of Loctite 620

mog69,

If you omit the spaces when referencing a previous thread it becomes a link with no typing required.

thread404-161524

RE: friction coefficient of Loctite 620

unclesyd
Thanks - I'd exhausted about every permutation of cutting /pasting/ dragging etc, so I went for the "stick with what you know" option !

RE: friction coefficient of Loctite 620

http://www.nord-lock.com/video/nl_uk.wmv

I saw this nord-lock video and am not certain
that the strait line that they show is simply
separation of the parts?  The video shows the
vibration loosening the nut which means that
the clamping load was in turn decreasing.
After you loose the preload who cares which
type of nut stays on the stud.  The video was
impressive but not certain that what you are
concluding is correct.

RE: friction coefficient of Loctite 620

(OP)
dimjim,
I think you need to watch the video in full. Nord lock seemed to have the last say on the other thread mentioned. I have a sample of a Nord Lock washer, even finger tight it works incredibly well, with 45Nm I would be more than confident.
As for the dowels, they have been added after the fact, we have yet to see their worth.
Regards
Anton

RE: friction coefficient of Loctite 620

After a review, I have to agree that the demonstration
is impressive.  Let us know how this works out for you.
I do think the dowels make the most sense but will be
curious to see what they look like after 100 hours in
the field.  From viewing the video, I too would try those
Nord-lock washers.

RE: friction coefficient of Loctite 620

Note that Nord-lock imposes a maximum hardness requirement on the faying surfaces of the joint material next to the washer.

RE: friction coefficient of Loctite 620

We were experiencing fatigue failures in a spindle attachment bolts on a planetary drive axle. The solution in that application was counterboring the thread to allow the bolt to flex.

RE: friction coefficient of Loctite 620

(OP)
unclesyd,
That is another very good approach which would no doubt help.

BillPSU,
Counter boring the holes also increases the effective bolt length which is also a good thing, using the dowels above would also do that. One of the repairs includes counter boring with far side tapped holes.

I have been talking to a local engineer associate, we are both unconvinced that separate dowels was a good idea as they reduce the bearing surface but don't address the problem of clamp pressure and fretting. We think correct bolt tension is a better place to start.

I don't want to modify the existing components to much as we will then be in a very experimental phase, certainly this thread has come up with some very sound ideas.

No one has used or seen loctite on the mating surfaces (a la Metalguy's post) but can't say why it shouldn't work, the only concern is what would happen if the product broke down, would it then form a low friction area with reduced clamp pressure.

Regards to all,
Anton

RE: friction coefficient of Loctite 620

Could you comeback with the hardness of the flywheel proper and the mating crank surface?

Also how thick is the flywheel?

What is the radius under the head of the SHCS?

What is the hole clearance in the flywheel?

What is the call out on the relative surfaces between the flywheel and crank? Things like flatness, surface finish, etc.


I need some food for for thought.

Actually this reverts to several problems that I've encountered in using high strength fasteners and things mentioned in previous posts.

RE: friction coefficient of Loctite 620

One more question, does the flywheel drive the starter as a generator?

You might want to look at this approach as time permits.

http://www.spiralock.com/

RE: friction coefficient of Loctite 620

(OP)
unclesyd,
Unfortunatly I don't have all the answers to tour questions, but here are some.
The radius of SHCS's is approximatly 1.5 X the diameter of the bolt.
The surface of the crank and flywheel have been blued and found to be perfect.
I don't have the exact hole clearance but I imagine it would be the standard size ie 5/16th in this case.
The overall grip length is 3/4" prox.
I have no idea what the hardness is.
No the flywheel does not drive the starter.
Regards
Anton

RE: friction coefficient of Loctite 620

Here is what I was eluding to in respect to the bolt failures in short form general terms.


Hardness:
The hardness of the mating pieces can be concern due several reasons, some of which have been mentioned above like the loss of integrity due say asperities on the mating surface under the fastener head. Another would be that the head of the fastener tends to embed in the mating material. This causes all types of problems like putting a bending moment on the head of the fastener. While embedding there is usually a gall therefore the contact stress and point loading to the head of the fastener,  and friction coefficient under the head can be altered. Another problem if the mating part, crankshaft, has a much lower hardness than the hardened fastener it can actually reform some internal threads corrupting the thread geometry and throwing the friction values off.

Thin Disks.  
I had some problems with flywheels flexing while running if they were too thin. My first encounter with this was in my hot rod days when we attempted to shave  the flywheels. This was solved by leaving a hub in the center for mounting the flywheel. The bolts broke in a similar manner as yours.  In my high speed machinery days we had the same problem with the additional problem with exciting the high speed disks with the resultant vibrations wrecking several machines. I have seen problems where the flywheel was driving a generator and the gears were causing the flywheel to flex.  Displacement transducers and strobe lights reveal strange things.

Radius:
The radius I was referring to is the small radius between the shank of the fastener and head. On some fasteners this radius can be enough to interfere if the hole clearance for the fastener is to close. This happened to me when we had to use hardened ground washers on some H11 SHCS and the forgot the chamfer.

Clearance:
The clearance between the bolt and mating part can become a factor in critical applications. For an application such as yours the tighter the clearance without interference the better. Make sure the under head clearance doesn't interfere, use just enough chamfer to clear. Too much clearance will exasperate any possible relative movement between the 3 components.


RE: friction coefficient of Loctite 620

Loctite to fortify an interference metal-to-metal fit - In the semi modern era Harley factory recommendation for crankshaft assembly was to use high temp, high strength, slow curing loctite (620 ?)on the tapered joints used to lock the mainshafts and crankpin to the flywheels.  Previously nut torque alone was used to lock the tapered shafts to the flywheels.  
General description here on page 4 - http://www.nightrider.com/biketech/download/ssmanuals/flywheel.pdf#search=%22harley%20flywheel%20assembly%20loctite%22

RE: friction coefficient of Loctite 620

You may want to consider using a body fit studs and tapered washers similar to larger truck axles. Your flywheel should not require much clamping force, but may require a lot of torque holding. Another advantage to studs is you can measure elongation with a dial indicator as you tighten them.
Loctite is a plastic, when cured it has a low yield strength, and can be brittle so torque induced impact will turn the material to dust.

RE: friction coefficient of Loctite 620

(OP)
EdDanzer,
Regarding Loctite, that's what I thought also.

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