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.

 

[CoryPad]

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.

 

[Screwman]

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.

 

[kirstant]

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

 

[kirstant]

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

 

[Metalguy]

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"

 

[unclesyd]

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.

 

[kirstant]

First engaged thread.
Anton

 

[Metalguy]

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"

 

[kirstant]

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

 

[Metalguy]

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"

 

[kirstant]

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

 

[dimjim]

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.

 

[Metalguy]

>"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"

 

[Metalguy]

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"

 

[kirstant]

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

 

[unclesyd]

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?

 

[kirstant]

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

 

[kirstant]

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

 

[mog69]

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

 

[dimjim]

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?