gunlaker wrote on Jun 26
th, 2015 at 9:30pm:
QuestionableMayard, it's an interesting question. I'm no gunsmith but I know that fasteners in critical situations are often used with application specific thread lubricants before torquing.
Chris.
Yes, but that's to reduce the variance in friction, which can throw off the end result. A bolt is nothing but a very stiff tension spring, or as a mentor of mine once said, a rubber band of a harder durometer. It must be stretched (elastically) by some amount to obtain clamping force. Ideally you can measure the stretch directly with a micrometer, as is done with rod bolts in piston engines. Most cases you can't, because there is no access to the other end of the bolt, so you have to rely on controlling the torque. Eliminating friction factors helps do that more accurately.
The case of barrel threads must be considered as a very, very short bolt. Almost all the elastic deformation must takes place in the thread form, and on the interface between the barrel shoulder and the frame face. This is why the final takeup is 10 or 15 degrees, no more.
There should be
some preload on the threads and the shoulder face, because this will make the joint stiffer. Overdo it, however, and under stress the deformation will exceed the elastic limit, and the preload actually drops for the next load cycle. If the preload is too small, the parts move with respect to one another as the stress comes on. This can actually also lead to loosening, as the contact faces become battered past the elastic limit by the repeated impact.
My personal "happy medium" for setting up Model 44 barrels is about 15 degrees. Nominally giving .002" of "stretch", which as I said above is really just preloading the internal and external threads against one another, and the shoulder against the frame face.
Phil
p.s. about now I wish I still had CAD mechanics working for me who could set up a Finite Element Analysis on this problem.