Fastener Technology International - December 2007/January 2008 - (Page 31)

Average Error Across All Loads load cell and the ultrasonic channel were then recorded. The procedure was then repeated in tensioning the bolt to the next level. Three tension readings were taken for each of the bolts at 30 kN, 35 kN and 40 kN. One of the reasons these values were chosen is that the 40 kN reading is beyond the maximum tension at which the bolts were calibrated, providing the opportunity to compare the accuracy of that reading to those within the calibration range. All bolts were calibrated to 36 kN, 75% of the bolt’s proof load. The results of measuring the difference between ultrasonic tension readings and those from a load cell are summarized in Figure 3. Self-calibration refers to using the individual calibration of each bolt as the basis for its conversion of elongation to tension. Average calibration means that all bolts within each group in Table 1 would use the same calibration, one that is the average of all the bolts within the same group that were calibrated individually. For example, in Group 4 in Table 1, five bolts were calibrated using the tensile method. Five additional bolts were sensored but never calibrated. They converted elongation to tension based on a calibration file created by averaging the results of the five bolts that were calibrated. The result that stands out the most is the high error of the short grip length relative to the long grip, particularly for the tensile calibrated groups. This is an area where it is possible that the test method influenced the results. As grip length decreases it is increasingly important that the grip established during calibration matches the actual grip in the joint. To illustrate, Group 4 bolts were retested after placing a 2 mm washer behind the nut to increase grip length. The ultrasonic readings went from averaging about 3% low to about 5% high, quite an impact for a relatively small change. A likely reason for this is the short grip length only exposed a few threads, so the 2 mm increase in grip increased the number of exposed threads by a large percentage. Since the cross-sectional area of the threaded portion is smaller than the unthreaded shank a disproportional level of elongation occurs in the threaded portion. But the behavior of tensile versus torque-based tensioning relative to grip length should be examined in a more fundamental way. When calibration is performed through tensile elongation, the distance between the bolt head and nut member increases with applied tension, while in torque-based calibration that distance remains essentially constant while the mating threads advance. This shortens the bolt, so all else equal, the bolt calibrated by being elongated with torque will calibrate as being a bit stiffer than that calibrated with tension. This effect is somewhat normalized relative to grip length because although the amount which the tensile grip length will “change” increases with increasing grip length, the effect of that change is decreased as it is a smaller proportion of total grip. 7% 6% 5% 4% 3% 2% 1% 0% Short Torque Short Long Short Tensile Tensile-Disc Torque Error Self Cal. Ave. Cal. Long Long Tensile Tensile-Disc Configuration Fig. 3 — Average error of all three tension readings by group. Comments on Test Results As this test was based on only a single bolt style and sample sizes that are smaller than ideal, the applicability of results should be viewed narrowly and as guidance only. The fact that in 33 of 36 instances the average calibration files (applied to bolts that were never calibrated) resulted in smaller errors that bolts that used their own individual calibration file may seem surprising. However, whether that should be surprising depends on the source of calibration variation from bolt to bolt. If the variation were mainly due to differences in the inherent acoustic properties within each bolt, self calibration should be superior. However, by simply taking the same bolt and calibrating it multiple times in the same setup (beyond the initial “exercise” run) will result in small variations on each calibration. This indicates that the manner in which the ultrasonic pulse travels through the bolt varies a small amount each time it is tensioned. Add to this the fact that the way each successive bolt fits the tooling is not identical, and the conclusion is that more often than not the tooling and test signal variation is larger than the inherent variation in material properties so that the average calibration file better accounts for this variation. The outcome of the Pullers versus Twisters debate is not clear-cut. The torque-based calibrations showed a fairly significant reduction in Standard Error, while the tensile tension readings were slightly more accurate. To complicate things, use of the load cell gave the torque calibrated groups a bit of a home-field advantage in tension readings as both the calibration and readings were performed using the same standard. There was no evidence in this test that the adding torsional stress to the bolt during calibration had any benefit for torque calibration. However, the ability to accurately duplicate actual grip length during calibration clearly has potentially significant impact. In this regard torque-based calibration is more similar to the test condition. Tensile calibrations can attempt to account for this by setting grip length artificially short by an amount equal to half the expected advance of the bolt threads during test. Ultimately, the best answer will be determined by the setup employed. With standard tooling, varying grip length with a load cell is much more difficult than on a UTM. However applying torque to a UTM setup is troublesome without special tooling to better hold the load string in-line and to fixture the torque tool so it imparts torsional loads only to the bolt not to the load string. So at least conceptually, perhaps cross-over setups like a load cell with infinitely adjustable screw collar to adjust grip length or a UTM with a lower fixed platen and tooling that is similar to the input side of a torque-tension bench-top cell commonly used for friction factor testing would provide the best of both worlds. But in the end, the differences between these methods is small and not conclusive. Ultimately, this report will probably not change any experienced practitioner’s mind in this regard, and that is probably the right outcome for most. FTI Company Profile: Archetype Joint, LLC, provides joint design, test and validation services for OEMs and suppliers of assembly equipment and fasteners. Located in Orion, MI, USA, the firm has a complete independent test lab accredited to ISO 17025 by A2LA. Archetype Joint takes pride in using the diversity of its professional experience and its customer base to provide clients with interpretation and recommendations, not just test data. www.archetypejoint.com About the Author David Archer is President/Founder of Archetype Joint. He started the company after over 20 years of broad experience in design, manufacturing and consulting, focusing on enhancing product value through reducing design and process complexity. The realization that joint design was the most widely ignored barrier to achieving product cost, performance and quality goals was the genesis for Archetype Joint. Archer holds a Bachelor of Science Degree in Mechanical Engineering and Applied Mechanics and a Master of Science degree in Manufacturing Engineering from the University of Rhode Island. December 2007/Fastener Technology International 31 http://www.archetypejoint.com

Table of Contents Feed for the Digital Edition of Fastener Technology International - December 2007/January 2008

Fastener Technology International - December 2007/January 2008 Contents Editorial Calendar Industry News 5S – Turbocharged Lean House Cleaning: Principles and Practices for Creating an Efficient Workspace Corporate News Mechanical Properties – Part 7: Strength & Ductility in Bending FTI Technical Achievement Awards — Class of 2007 The Heat Treatment of Fasteners — Practice Melt Shop Provides More Efficient Titanium Wire & Rod Production Spotlight On Simulation Comparing Preparation and Calibration Techniques for Ultrasonic Bolt Tension Measurement – Part 2 Future-Oriented Testing Equipment Wire Handling Roundup What’s Happening at the IFI? Product Showcases Machinery Product Showcases Quality Education & Training in 2008 Fastener Focus Quality Checks on Retaining Rings: Your Recipe for Success Straight From The Show: NIFS/WEST Spotlight on Plastic Fasteners Competence in Stainless Steel Pick More Orders in Less Time With Slotting New Fastener, Drive Systems & Insert Join a Proven Product Line ASTM Update on Fastener Test Methods – The Bolt Doctor Column Advertisers’ Index Classified Advertisements Editorial Index

Fastener Technology International - December 2007/January 2008

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