Disassembly torque required for a torqued and glued bolt 2

[Surfex]

Hi everyone,

I have a question here concerning the interaction of different locking devices while used together in an assembly. For example, what would be the required torque to disassemble a bolt if it was torqued at assembly to a value of 500 lb-ft and if we added a threadlock glue (loctite) that requires a torque of 1000 lf-ft to disassemble, if used by itself. Does these two values add to make a total of 1500 lb-ft? Or does the larger value (here 1000lb-ft) rules the disassembly?

I need help to understand the behavior of this assembly. Thanks.

 

[Metalguy]

You'll be closer if you add them together. There are 2 places where friction/adhesives must be overcome to loosen a fastener-the threads and the underside (contact face). The 1000 ft.lb. will be required for the threads, and the major part of the 500 for the contact face. I've seen reports (Bickford-?) where 90% of the torque can be from the contact face.

This assumes that the Loctite didn't get on the faces.

 

[CoryPad]

I concur with Metalguy. Torque components (bearing face friction, thread pitch/preload, thread friction) are additive. However, removal torque does not necesarily equal tightening torque (due to friction, shank winding, etc.), so it could be somewhat less or somewhat more than the sum.

Regards,

Cory

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

 

[Surfex]

Thanks Metalguy,

So if I understand, there is no direct relation between the two locking methods (like 1+1=2). On the other hand, both locking methods must be overcome in order to disassemble the part, so that the overall torque required to disassemble the part will be over 1000 lb-ft. Is that right?

Similarly, if we torque a bolt to a certain amount of torque (500 lf-ft) and than drill a hole through the part and the bolt and than insert a pin to prevent rotation, would the same theory apply (if the torque required to shear the pin is 1000 lb-ft)?

 

[Metalguy]

Yes. Whatever torque would be required to overcome the thread and bearing face friction would have to be added to the shear pin force. I hope you understand the fundamentals of preload in keeping fasteners tight. *IF* there is any relative movement because the preload isn't high enough, you can lose ALL your preload as the bearing surfaces fret away and end up with a broken bolt-despite shear pins and Loctite. The fracture will start via fatigue cracking if you have varying stresses on the bolt.

Had a case once where a big 2" dia. nut (thread size) kept working loose because of insufficient preload. The heavy nut would fall 30-40', and could've killed someone. Maintenance decided to fix it without telling us in engineering. They WELDED the far end of the nut to the threads after cutting the bolt off flush. Sure enough, the bolt broke via fatigue, and when it dropped it made a bigger dent in the floor because it weighed more with some of the bolt still inside! TWICE!

Also, in vibration service your preload MUST be higher than the service loads, or you can get fatigue cracking without any fretting-related loss of preload.