2 76 Multi scale System Design Manufacturing 2 76 2 760 Lecture 13 Interfaces forces Pictures removed for copyright reasons 2 76 Multi scale System Design Manufacturing Purpose of today Bolted joints pointers Kinematic coupling experiment Repeatability Stiffness Metrology issues Perspective for use in STM Project questions design help 2 76 Multi scale System Design Manufacturing Bolted stiffness and stability Qualitatively determine What is good What is bad Think about Static loads Dynamic loads Bending moments Mass 2 76 Multi scale System Design Manufacturing Minimize variation System Environment changes People Desired output Material properties Stress Actual output ForcesVibrationProcedureFriction Measured output Perceived Error Real error 2 76 Multi scale System Design Manufacturing Bolted joint topology A In plane Moment stiffness Over constraint Singular B B A A B 2 76 Multi scale System Design Manufacturing Bolt physics Rough estimate generatedstoredoutin EEEE 0 stretchheadfthreadsfapplied ETTT 0 stretchheadfthreadsfapplied ETTT 0 21 dxFFrCFrCT boltboltheadfboltboltthreadsfboltapplied FS AF yield crossbolt 2 76 Multi scale System Design Manufacturing Interface variation On average get about 30 50 of contact at mechanical interfaces These little springs reduce joint stiffness Stiffness important for dynamic disturbances Use potting to stiffen up the joint interface Calculate joint stiffness Shigley Mischke Machine Design 2 76 Multi scale System Design Manufacturing KC Experiment Study Sensitivity due to mechanical interface Step 1 Setup pay attention up front Step 2 Test repeatability Non lubricated 35 mates Lubricated 35 mates Step 3 Test stiffness Preload levels 1 2 and 3 note moment arms Step 4 Compare theory spread sheet and measured E 30 Mpsi 0 28Ri measured Questions to think about How do not should lateral axial stiffness scale with preload Plate stiffness sweet spot to prevent metrology errors How repeatable is your bio unit preload Who has the best repeatability What are the implications for your STM