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Exploring Torque and Deflection Response Characteristics to Evaluate the Ergonomics of Dc Torque Tools Via a Tool Test Rig

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Exploring Torque and Deflection Response Characteristics to Evaluate the Ergonomics of Dc Torque Tools Via a Tool Test Rig EXPLORING TORQUE AND DEFLECTION RESPONSE CHARACTERISTICS TO EVALUATE THE ERGONOMICS OF DC TORQUE TOOLS VIA A TOOL TEST RIG A Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Shritama Mukherji, B.E. * * * * * The Ohio State University 2008 Master’s Examination Committee: Dr. Anthony Luscher, Adviser Approved by Dr. Carolyn Sommerich __________________________________ Advisor Graduate Program in Mechanical Engineering ABSTRACT Torque tools used in assembly applications generate impulsive reaction forces during torque build-up that often displace the operator hand and arm, and are associated with an increased risk of muscle damage and injury. Tools are available in a number of handle shapes, sizes, and output capacities and are operated in various working positions and orientations. These factors affect the dynamic interaction between the tool and the operator and the operator’s ability to react against impulsive forces. DC torque tools are controlled fastening tools that are instrumented with sensors for direct measurement of the applied torque and rotation of the threaded fastener during the assembly process. They have several advantages in terms of torque accuracy, error detection, and torque verification over other torquing systems. DC torque tools interface with a tool controller that can be used to set tightening parameters and program different tightening algorithms, making it highly flexible. The objective of this thesis is to quantify the ergonomic impact of various DC torque tool controller settings. This impact was determined by the use of an ergonomic test rig which will capture the interaction between the physical tool, control software, and a model of the human arm. The output of the rig is the reaction force and displacement of ii the tool handle and therefore simulated arm as a function of time. The response curves from the rig were analyzed and a set of metrics were formulated for ergonomic assessment. The work of this thesis will lead to an improved understanding of the interactions between stiffness of the joint to be fastened (joint stiffness), the simulated human arm system, and the tightening algorithms controlling the tool. iii DEDICATION To my parents for their unconditional love and support iv ACKNOWLEDGMENTS With the deepest gratitude, I wish to thank all the people who made this thesis possible. Firstly, I would like to thank my advisor, Dr. Anthony Luscher for his guidance and support throughout this thesis. I am very grateful to him for all that I have learnt during the course of my graduate studies. I would also like to thank Dr. Carolyn Sommerich for co-advising me on this project and for being a part of my thesis defense committee. Her feedback and encouragement have been invaluable for the successful completion of this thesis. I would like to sincerely thank Duane Bookshar, Doug Versele and Jim Steverding from Stanley Assembly Technologies. The technical information provided by them has helped me understand DC torque tools. I would also like to thank various individuals in the Department of Mechanical Engineering, namely, Gary Gardner and Neil Gardner for their help in building the ergonomic assessment rig and Joe West for his assistance with the measurement system. Thanks also belong to all the members of the Fastening lab for their ideas and suggestions during this research. v I have been fortunate to have many friends who have supported me and kept me motivated for the last two years. For this, I am very thankful. Finally I would like to thank my family who made all of this possible. I am grateful to my parents, for having unwavering faith in me and for being a constant source of support and strength. I am also thankful to my sister and brother-in-law, for encouraging me at all times and keeping me in good spirits. vi VITA December 10, 1983……………………. Born – Hyderabad, India June, 2005……………………………... BE, Visveswariah Technological University Bangalore, India January, 2006 - present…………………Graduate Research Associate, The Ohio State University FIELDS OF STUDY Major Field: Mechanical Engineering Design and Manufacturing vii TABLE OF CONTENTS Page Abstract............................................................................................................................... ii Acknowledgement.............................................................................................................. v Vita.................................................................................................................................... vii List of Tables...................................................................................................................... xi List of Figures.................................................................................................................... xii Nomenclature.................................................................................................................... xv Chapters 1. Introduction ................................................................................................................... 1 1.1 Introduction to bolted joints................................................................................ 2 1.2 Torque tool options............................................................................................ 6 1.3 Motivation for current work.............................................................................. 10 1.4 Thesis objectives............................................................................................... 12 2. Literature Review......................................................................................................... 14 2.1 Dynamic models of tool-human operator system.............................................. 14 2.2 Ergonomic injury risk assessment..................................................................... 22 2.3 Effect of work station design, operator posture and position............................ 26 2.4 Design and application of an instrumented tool handle .................................... 29 2.5 Literature summary............................................................................................ 32 3. Design of ergonomic assessment rig........................................................................... 33 3.1 Description of original ergonomic test rig........................................................ 34 3.1.1 Tool and bolted joint assembly.............................................................. 35 3.1.2 Human arm model with measurement system....................................... 37 3.2 Rig improvements............................................................................................. 39 viii 3.2.1 Improved spring design to represent arm stiffness ................................. 39 3.2.2 Design of pneumatic system to drive arm stiffness cylinder................... 43 3.2.3 Improved design of arm mass system .................................................... 45 3.2.4 Load cell modifications.......................................................................... 48 3.2.5 Additional modifications........................................................................ 50 3.3 Final ergonomic test rig .................................................................................... 52 3.4 Repeatability tests ............................................................................................. 54 4. Experimental method ................................................................................................... 58 4.1 Description of the factors .................................................................................. 59 4.1.1 Tightening algorithm .............................................................................. 59 4.1.2 Soft stop feature...................................................................................... 64 4.1.3 Arm mass and stiffness........................................................................... 65 4.1.4 Joint stiffness .......................................................................................... 67 4.1.5 Summary of factors with their levels...................................................... 68 4.2 Measured response and other dependent variables........................................... 69 4.3 Design of experiments (DOE).......................................................................... 69 4.4 Formulation of ergonomic metrics ................................................................... 73 4.4.1 Torque impulse at different percentages of the target torque ................ 73 4.4.2 Deflection - peaks in positive and negative direction, maximum range.. 76 4.4.3 Reaction torques - peaks in positive and negative direction, maximum range ...................................................................................... 77 4.4.4 Latency impulse - torque impulse with muscle latency included........... 78 5. Ergonomic assessment of response curves - results .................................................... 83 5.1 Raw data from screening experiments............................................................... 84 5.2 Assessment of response curves - statistically significant results ................ 90 5.2.1 Torque impulse at different percentages of the target torque ................ 92 5.2.2 Peak deflection negative, peak deflection positive, maximum deflection range .....................................................................................
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