SPRING 2015

8 THE DISTRIBUTOR’S

8 THE DISTRIBUTOR’S LINK Bengt Blendulf Bengt Blendulf was educated in Sweden and moved to the US in 1974 to start a subsidiary for a Swedish fastener manufacturer. After working as a technical consultant on the faculty of the College of Engineering and Science at Clemson University, he established EduPro US in 1997 to teach highly rated courses in Fastening Technology in the US, Canada, Mexico, Europe and Asia. Being one of the founders, Bengt served as the chairman of ASTM F16.96 Bolting Technology from 1996 to 2006. In 2006 he received the Fred F. Weingruber award from ASTM for “his efforts to promote and develop standards for the fastener industry.” In 2013 he also received IFI’s Soaring Eagle Award for “significant contributions to the technological advancement of the fastener industry”. Bengt is the author of an extensive lecture book, well over 110 articles and “Mechanical Fastening and Joining”, a book published in 2013 by the Industrial Fasteners Institute. He can be contacted through www.edupro.us or by email bblendulf@yahoo.com. FASTENER TOLERANCES - JUST RIGHT OR TOO TIGHT? High strength fasteners are subjected to a very large number of demands both for dimensional accuracy and physical strength. These are specified in standards issued by various engineering societies, both domestic (ASME, ASTM, SAE, etc.) and international (ISO). Most of the time, the fastener standards are very well reflective of the practical needs of both end users and fastener manufacturers. Sometimes, however, it seems like we just want to write in as many choices as can possibly be considered by the various “players” in this field. One such example is the mind-boggling number of diameter/ pitch combinations for inch fasteners. ISO 263 is listing no less than 316 (yes, I have counted) possible thread choices between the smallest (No. 0- 80) to the 4” diameter which has as many as 6 various pitch combinations! If we then also take into account the 3 thread tolerance classes (1, 2 and 3) we must realize that this is a practical and commercial nightmare and pure nonsense. The effectiveness of the thread tolerance classes was actually investigated in the 1940’s by a professor Buckingham from MIT (I believe IFI initiated the study). He found that there were no practical differences in strength in fasteners with thread classes 1, 2 or 3. Of course, CONTRIBUTOR ARTICLE thread class 3 required more expensive thread gauging and quality control due to much tighter tolerances. Class 3 is still “preferred” for aerospace fasteners and socket screws. But, just because the thread tolerances are much tighter does not necessarily mean that we are gaining any corresponding strength advantages. Later studies by IFI and some independent research companies have also confirmed this. It would be great if our standards organizations could expand on this and conduct more up-to-date testing to actually prove or disprove the benefits of the tighter thread tolerances. Socket screw standards (inch) use 3A with no allowance between maximum condition and nominal size. The ISO (metric) version of socket screws use a 5g6g thread tolerance which actually have an allowance to aid in assembly. The ASME B1M standard cut this down to the much tighter 4g6g where at least 90% of the tolerance is the same as 5g6g. As far as I know, there has been no test protocol to justify why we had to have a tighter tolerance than used in ISO. Personally and professionally, I believe the 6g thread tolerance should be used for ALL screw threads! Why make a threaded fastener more expensive if there are no performance benefits? CONTINUED ON PAGE 124