ALL AMERICAN FASTENER SHOW Branson, MO – May 12-13, 2011
THE DISTRIBUTOR’S LINK 109 A SYSTEMATIC APPROACH TO BOLTED/SCREWED JOINT DESIGN continued from page 107 use tools with less precision – larger tension scatter. Therefore, a designer should also be taking this into account so as to avoid broken fasteners or too low preloads at the field service level. 7. Putting things in order Once we have advanced to a point where we have collected enough information, and given appropriate thoughts to these six areas, it is time to do some mathematics. One of the absolute best ways to proceed from here is to get hold of a graph paper, the calculator and start developing a Joint Diagram. A faster way may be to use a computer software (like SR1 which is based on the VDI 2230 guide line), but you will be able to do just fine (but slower) also with the graph paper and your calculator. In my FTB class (Fastening Technology and Bolted/Screwed Joint Design) the method for developing a joint diagram is covered. From the diagram we are able to find, for example, how much of the external force is absorbed in the compressed joint parts and how much Guess-work is dangerous and we can no longer have the approach –“let’s drill some holes and put some bolts there” (or little, if we do things right) is added to the fastener in addition to the preload force. If the external force is cyclic/alternating, it is also from the diagram data we can determine if we will likely have fatigue failures or not. A very useful tool, indeed! Conclusion Well designed bolted/screwed joints should have a high level of safety to avoid failures that can cause personal injuries or property damages. Not too long ago it was OK to do some “over-design” to add to safety factors, but in today’s competitive environment we are forced to cut things closer. Guess-work is dangerous and we can no longer have the approach –“let’s drill some holes and put some bolts there”. The better we understand how highly stressed joints should be designed, assembled and serviced, the more successful we will be in our designs. Unfortunately, the newly graduated mechanical engineer from a typical engineering college is not well equipped in this area and need some solid continuing education in fastening technology.
THE DISTRIBUTOR’S IN THE SUMMER 2
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THE DISTRIBUTOR’S LINK 183 FASTEN
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