friction coefficient of Loctite 620 5

[kirstant]

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.

 

[BillPSU]

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"

 

[kirstant]

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.

 

[MintJulep]

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.

 

[CoryPad]

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.

 

[israelkk]

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.

 

[Metalguy]

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---.

 

[kirstant]

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

 

[Metalguy]

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"

 

[kirstant]

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

 

[MintJulep]

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.

 

[kirstant]

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

 

[mog69]

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

 

[israelkk]

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?

 

[CoryPad]

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.

 

[Metalguy]

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"

 

[israelkk]

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.

 

[Screwman]

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.

 

[MintJulep]

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?

 

[kirstant]

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