AdvancedBoltTorqueAuditYields Bolt Tension Data

1 Gin and analysis of the torque-angle signature to de-termine the elastic tightening angle of turn. The angulardisplacement (turn) of the fastener stretches the boltand compresses the parts being clamped. The elasticclamping region of the torque-angle signature is theregion where clamping force is directly proportional tothe angle of turn from the elastic origin. The elastic origin is located by projection of thestraight-line portion of the signature curve backwardsto zero torque, or the prevailing torque level in the caseof thread locking interference type of principal can be observed when viewing a clampload-angle curve (see Figure 1).

2 Once the curve be-comes linear, the clamp load will increase at a consis-tent rate with each angle of rotation. This remains trueuntil the curve stops being linear due to yielding of thefastener or the amount of preload in the joint determines howwell the joint will function when it experiences workingloads. Insufficient preload is a major cause of fastenerfailures, leading to expensive machinery failures, tragicaccidents, personal injuries, and even loss of torque-angle signature does not directly pro-vide a measure of the fastener Tension .

3 However, ithas been verified that for a given bolted joint, properlyqualified, this signature can be used to directly esti-mate the clamping force on the bolt, as seen in Figure2. For example, strain gages on a bolt or stud can beused to simultaneously measure the installation torqueand the bolt Tension . By scaling and plotting the mea-About the Author/RALPH SHOBERGR alph S. Shoberg is the Technical Director of PCB Load & Torque, Inc., a manufacturer of load cells,torque transducers, and fastening technology measurement has 30 years expereince in test, measurement, and control of the threaded fastenertightening process.

4 He has been granted numerous and international patents for transducer andinstrumentation inventions. Shoberg helped create the first electronic strain-gage transducer basedmonitor and control system for automated fastener assembly. He is a frequent lecturer on the designand analysis of bolted joints, has contributed to a number of threaded fastener handbooks, and is afrequent contributor to fastener industry-based Bolt Torque Audit YieldsBolt Tension DataTorque-Angle Signature Analysis & M-Alpha Audit MethodAuditing the performance of tightened bolted joints haslong been a priority of quality and assemblypersonnel in the manufacturing and maintenance have been many methods developed over thepast 40 years to determine the residual torque on atightened fastener and this value is used as a mea-sure of joint integrity.

5 Each of these techniques is apractical method of determining the torque remainingin the joint or the capability of the installation tool; how-ever the true object of tightening a fastener is to pro-duce a clamping load in the joint that will create a reli-able assembly. The object of tightening a fastener isto achieve a Tension load on the fastener that clampsthe bolted components with sufficient force to create areliable is needed is a method of estimating the re-sidual Tension achieved by a fastener in a bolted as-sembly (not torque).

6 RS Technologies (a division ofPCB Load & Torque, Inc.) developed and patented anauditing method known as the M-Alpha Torque-Ten-sion Audit to estimate residual clamping load withouthaving to rely on strain gaging a bolt, using ultrasonicmethods, or using force washers. The ability to esti-mate both residual torque and residual Tension on abolted joint is the reason that the M-Alpha auditingprocess is considered to be a significant technologicalbreakthrough for fastener tightness the M-Alpha audit method can permit theuser to obtain bolt Tension related measurements, itrepresents a paradigm shift in the way torque audits can be conducted.

7 Reliable bolted joints must havethe proper clamp force holding the components to-gether. Tension on the fastener is what is needed tobe verified, not installation or residual torque. Theory and Application of the M-Alpha JointAuditing MethodThe patented M-Alpha joint auditing method is a pro-cess of estimating the amount of clamp load on athreaded fastener using torque-angle signature analy-sis. The method is based on locating the Elastic Ori-sured torque and Tension , shown in Figure 3, it is ap-parent that both are in direct proportion to each other.

8 Step 4: Determine the Elastic Clamping Angle,measured from the Elastic Origin, as in Figure torque and clamp load plots match up preciselywhen the ordinates are scaled. This relationship forthe linear portion of the torque-angle curve can be usedto directly calculate the clamp load in the joint. This isa fundamental characteristic of bolted joints tightenedbelow the yield point of the bolt or components the Elastic Clamp Angle from the ElasticOrigin to obtain the final torque value. The clampingforce achieved by the fastener is directly proportionalto the measured Elastic Clamping Angle.

9 Knowing howthe torque-angle signature looks when tightening a fas-tener helps to understand the M-Alpha Audit the M-Alpha Joint Auditing MethodThe objective of the M-Alpha Audit process is thedetermine the Elastic Tightening Angle for a preciouslytightened fastener. The amount of clamping load in thejoint is directly proportional to the elastic tighteningangle. The process of the torque-angle curve analysisis shown in Figures 7 through 10 to illustrate the con-cepts of the Elastic Origin and how the Elastic Tight-ening Angle is determined for a given bolted joint.

10 Step 1: Tighten the fastener with the specifiedtorque (as seen in Figure 2, previously) stopping in theelastic clamping region. Step 2: Locate two points on the linear section ofthe torque-angle signature curve (Figure 4).M-Alpha Torque- Tension AuditsTorque audits typically involve tightening a pre-assembled joint a small additional angle, usually aslittle as five degrees or less and rarely more than tendegrees. Step 3: Project a line through the two points to thezero torque level, locating the Elastic Origin, as de-picted in Figure audits simply measured the peak torquevalues or used a calculation algorithm to more accu-rately locate a residual torque value as a measure ofthe joint integrity.