Torque Coefficient for Anaerobic Threadlocker on a Steel Fastener

[whitejd]

Does anybody know if the use of a acetate-based spray-on threadlocker primer has any substantial impact on the torque coefficient of a steel fastener on which Loctite 242 threadlocker has been subsequently applied? My concern is in the degreasing effects, removing some of the dry film lube that resides on the fastener in the "as received" condition. I forget the product number of the primer, but it is an anaerobic threadlocker activator also produced by Loctite.

Also, due to the mentioned degreasing effects with such a primer, would one use published lubed or unlubed torque values for the fastener during assembly.

I found data indicating that Loctite 242 applied to a lightly oiled steel fastener produces a Torque coefficient of about 0.14, and on a degreased steel fastener the value would be about 0.20 (same as dry). While the Loctite primer/activator is not a true degreasing step, I don't know if it should be considered as such.

Any input would be much appreciated.

 

[CoryPad]

It is possible but unlikely that the friction coefficient will change for this system. The only way to be sure is to perform testing. What is the procedure for applying the primer and adhesive? Does your screw have a coating/finish?

Regards,

Cory

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

 

[desertfox]

Hi jdwhite

I responded yesterday if you remember and recommended that you use the 0.2 coefficient of friction for your joint and back it up with practical tests if critical as calculating torque figures for fasteners is not very accurate. Further you asked about a specification regarding 2 values of torque's for a given bolt ie one dry and one lubricated as I remember. Over here our british standards tend to quote proof loads for bolts and their corresponding tensile stress. My opinion is to select a torque value to produce a tensile force which satisfies the service conditions ie fatigue etc and doesn't overload the materials being clamped. A very rough estimate I use to find the tensile force in a fastener is:-

F= T/(0.2*d)

where T = torque

d= nominal bolt diameter

0.2 = friction factor

Once F is determined I size my fasteners (with a large safety margin) based on the proof loads mentioned above.

regards desertfox

 

[whitejd]

Desertfox,

Yesterday my posting was removed, and I was subsequently asked to repost in this forum. I did get your input. Thanks.

An update - I found out that the steel fasteners in question do go through a rigorous degreasing process prior to applying the loctite anaerobic threadlocker activator, and subsequently the Loctite 242 threadlocker.

Summarizing, I have conflicting information stating that the fastener should still be treated as lubricated in this case when selecting a torque and defining the applicable torque coefficient (Kt=0.15), and the other source indicates that it should be treated as dry (Kt=0.20).

?

 

[desertfox]

Hi whitjd

How critical is your application and what are the materials
being clamped? have you established a fastener size?
I still think you should use the 0.2 friction factor as this would give you the maximum torque required to achieve the axial load you require for the joint.If you can give more detail about what you are trying to do others or I may be able to help further.

regards desertfox

 

[Metalguy]

I'd call up Loctite and get your answer. They have lots of test data. The coef. will be less than 0.2, and you could overtighten something if it's torqued close to the YS.

 

[nashjp]

Remember that there are two friction surfaces - the underhead surface and the thread surface. If the 242 threadlocker is only applied to the threads after cleaning, the underhead will still be expected to have higher friction due to solvents removing any lubricants. I have seen extremely high coefficients of friction after cleaning (way above K=0.2).

I second Corypad's suggestion that testing will be the best determination.

Regards, Jeff