On 4/5/2013 12:39 PM, dave wrote: > Hi all, > > The wiki for buttress threads states the buttress threads tend to fail > because only the first 4 threads are load bearing. > > In contrast to that is: > > www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA483296 > > Which uses them in a munition where failure us not usually an > option. ;-) > > Does anyone have any experience or comments on either US or ISO buttress > (pull) threading.
No practical experience here, Dave, but books.google.com turned up "Ballistics: Theory and Design of Guns and Ammunition", By Donald E. Carlucci, Sidney S. Jacobson, CRC Press 2007(ish). Of course, Google gives me only its usual teaser-view but I think you want Section 4.1.1 "Buttress Thread Design" which begins on p.129. To whet your appetite, here are three consecutive paragraphs from p.130. "We use buttress threads for several reasons: most important is to improve the directional loading characteristics of the thread; also to allow for a more repeatable, controllable shear during an expulsion event, i.e., if we want the thread to intentionally and controllably fail allowing separation of the components; and to prevent thread slip in joints with fine threads or threads on thin shell walls. If thread slip occurs, the threads can either dilate or contract elastically and the joint can pop apart with little or no apparent damage to the threads. "When we design for strength, we typically calculate the strength based on the shear area at the pitch diameter in the weaker material. This, of course, translates to half the length of engagement of the threads. This is acceptable because we usually use conservative properties and add a safety factor to account for material variations and tolerances. We must always based our calculations on the weaker material if the design is to be robust. When designing to actually fail the threads, however, we need to be more exact in our analysis and take everything such as actual material property variation and tolerancing into account or our answers will be wrong. "We will proceed in this analysis in meticulous detail, initially, as a cantilevered beam subjected to compressive and tensile stresses caused by contact forces and bending moments. This technique was first developed during the U.S. Army's sense and destroy armor (SADARM) program by Dan Pangburn of Aerojet Corporation [cites personal communication with Pangburn] and has been used by the U.S. Army." What follows is five pages of mathematical analysis of the strength of the buttress thread. It ends with a statement about partial threads which probably will make more sense to you than it does to me. This analysis appears in Chapter 4 "Ammunition Design Practice" rather than Chapter 5 "Weapon Design Practice." Perhaps that explains why they consider designing for thread failure ?!? If "expulsion event" doesn't refer to the outgoing round, then I want nothing to do with it :-) New, the book is roughly US$160. Google sez there is an ebook edition going for US$115. The cheapest used-copy listed on abebooks.com currently is $107. Knowledge doesn't come cheap but experience is an expensive teacher too. Regards, Kent ------------------------------------------------------------------------------ Minimize network downtime and maximize team effectiveness. Reduce network management and security costs.Learn how to hire the most talented Cisco Certified professionals. Visit the Employer Resources Portal http://www.cisco.com/web/learning/employer_resources/index.html _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
