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Re: Stripping Treads

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If the condition is sufficiently accessible to determine the exposed threads, then you can simulate the connection. Multiple tests are necessary for any statistical significance, but 5 or 6 should either tell you the spread is too wide to predict the behavior, or has a reasonable range you can use to determine if the connection can be made soundly without going to extremes. I've done the same for questionable blind rivet connections which were made on spaceflight hardware with the wrong grip length (the material thickness straddled two grip ranges). We shot five rivets of the same batch into sample coupons which were in .005 increments, with the middle of five coupons being the outside tolerance for the parts mated. We also shot rivets into the nominal and minimum grip length. Standard visual inspection for all rivets (the last two coupons for control) revealed all but one rivet in the thickest (max tol buildup + .010) had proper head formation. It saved a good deal of reanalysis and re-work for a drill and replace operation on a piece of hardware otherwise ready for service. Sure, if we were being anal about it we could have riveted two pieces and performed shear tests, both pre - and post-vibe testing under a simulated load cycle for X mission cycles, but in this case we just wanted to know if the extra 0.004 out of tolerance was a real issue with poor head formation. Less than two man-days of time, total, which is pretty small for a flight part.

You bring up a good point, though: why not just preload these bolts? You know what you need, and a bolt interaction analysis can give you a preload which should result in most of the potential forces in the plate. Lube it good, torque it up, and if it breaks then you need to replace it. ;-)

Jordan



Christopher Wright wrote:

On Oct 3, 2006, at 6:33 AM, Jordan Truesdell, PE wrote:

This brings up a reasonable question: Is it worth running five or six pull tests on a similar condition in the lab? You certainly could spend a good amount of dollars in time running down a theoretical solution, and still not being confident that the math matches the real world.
Sounds like classic paralysis by analysis. What's your criteria? What do you do if some of the tests pass and others don't? How do you really know what the engagement will be? Why 5 tests? or 6? WIll tests on new hardware match tests on hardware that's been in service for a while? What if the tests don't match the calculations? (If the tests come out with greater loads, do you dare push it? If they come out low does that mean that the rest of the anchors are too weak?) Most important, has your contractor always done it that way and never had any problems? Tests are only good if you know your test matches the reality, but I'd suspect you never really know that for anchor bolts.

What you have is a situation where the incomplete engagement is eating into your reserve. You don't actually know what your reserve is, only that there's usually enough, either in the specified loading or the threaded connection so that a properly made-up connection usually holds. This is really a feel-good calculation. Do it using the estimated engagement less 2 threads--one each for the imperfect threads on the nut and the anchor. Don't try to cut it too fine, because it really isn't all that accurate.

I've had people ask me similar questions about improper engagement, but for machinery, it's usually a no brainer, thank god. Fasteners are almost always preloaded and full threads are required. If it doesn't engage, you replace it on the spot, because fatigue is important. Moreover tolerances are tight enough, so that if the specified fastener doesn't engage, it's a sigh that there's been a screw-up somewhere else that needs fixing. When that's done, the poor engagement is usually not a problem.

 Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)skypoint.com   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw/


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