Assembly - January 2009 - (Page 13) the cards, calculation based on derived relationships with 35% good data gives better results than picking a nut factor from 30% a reference table. To understand what fac25% tors influence the relationship between torque and tension, 20% let’s review the variables contained in the long-form 15% equations and quantify their relative influence. For a given 10% fastener diameter, the primary influences on the relationship 5% between torque and tension are thread pitch (and pitch 0% 0% diameter which varies with it), the head or nut diameter (whichever is rotated), the diameter of the clearance hole, and the coefficients of friction (COF) at the mating threads and between the head or nut and mating material. Essentially, reaction torques resist the torque input and the long-form equations calculate and sum those resisting torques. How do our design decisions influence Reduction in Tension for a Given Torque Thread Pitch Head/Nut Dia Hole Dia COF Threads COF Under Head 20% 40% 60% 80% Increase in Value of Each Variable torque-tension? The figure above illustrates the effect of doubling the value of each variable. It shows that if the thread COF was doubled while the other variables remained the same, the bolt tension generated for the same installation torque would decrease 29 percent. While COF values occur on a continuous spectrum, the dimensions of standard fasteners and accompanying clearance holes tend to be discrete. Therefore, the graph also contains three points showing the effect of design alternatives for a single fastener size. The three intermediate data points show the effect of replacing a fine-thread, hexhead fastener in a close hole with a coarse-thread, hexflange head fastener in a loose 100% hole. For example, replacing a hex-head fastener with a hexflange head (but not changing thread pitch or hole diameter) increases the bearing diameter by 37 percent, which in turn will cause bolt tension to be reduced by 9 percent for a given torque. While these values vary a bit depending on fastener size and the long-form equation used, the relative importance of each A remains the same. Large Format Dispensing Solutions RG Series Custom • • • • 5 Heads for Multiple Fluid Dispensing On-board Dot Peen Part Marking Handles Large Parts on Custom Carts with Docking Station Laser Surface Sensing & Bar Code Entry Shown 100" x 50" Travel Shown 72" x 36" travel LS Series • Designed for Sealing / Potting Long rails, Wireways, Extrusions, etc. • Unique Design Allows up to a 24 Foot Stroke Length • Modular Design From Stock SG Series • Designed for Large Panel Processing • Continuous Contouring • Available in Custom Sizes DISPENSE WORKS INC For more information call 815-363-3524 Shown 60" x 6" Travel www.dispenseworks.com www.assemblymag.com January 2009 / ASSE M B LY 13 http://www.dispenseworks.com http://www.dispenseworks.com http://www.assemblymag.com
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