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18 fasteners - ME250 Design and Manufacturing I Fasteners...

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Fasteners and joints ME250 Design and Manufacturing I
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2 What? Why? Threaded fasteners Welded Joints Summary Outline
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3 Fasteners are mechanical elements that join two or more objects together nonpermanently What?
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4 Welding is a fabrication that joins metal members permanently What?
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5 Disassembly for inspection and repair Modular design Why?
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6 The use of fasteners increase the manufacturing cost 50% of production time is spent on assembly Selection of proper fastener is an important part of product design Why? Boeing’s 747 require as many as 2.5 million fasteners
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7 Features Joins two or more members together Can disassemble Comes in standard sizes and materials Threaded fasteners Bolt head Nut Joint members
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8 Threaded fasteners Nomenclature :mean diameter 2 r m d d d p: pitch : major diameter d : minor diameter r d : tensile-stress area t A
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9 Specification example Threads standards and definitions 5 18 8 8 UNF SAE d[in] 1/p thread series grade 12 1.75 5.8 M ISO “metric” d[mm] pitch class
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10 UN threads (US customary units) Threaded fasteners
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11 Metric threads Threaded fasteners
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12 Bolt strength - SAE Threaded fasteners
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13 Bolt strength - ISO Threaded fasteners
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14 Bolt strength ASTM (American society for testing and materials) Threaded fasteners
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15 Tensile loads on fasteners Preload F i due to tightening the nut is applied Tensile force to fastener Compressive force to members Stress concentration areas 1-3 (will fail at these places) Threaded fasteners i F member 1 member 2 2 3 1
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16 Resultant loads on bolt and members View bolt & members as springs with k b and k m Resulting bolt load = stiffness of the bolt = extensions of the bolt Resulting load on members = stiffness of the members = extensions of the members (note d m < 0: compression) Threaded fasteners b b b k F d m m m k F d b k b d m k m d
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17 Force analysis If no external load, Threaded fasteners ; b i m i F F F F   slope of ) ( m b k k slope of m k F d compression extension i F i F m d b d
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18 Force analysis External tensile load P to pull the members apart C = fraction of P that acts upon the bolt Threaded fasteners P C F F CP F F i m i b ) 1 ( ; slope of m k F d compression extension i F i F slope of ) ( m b k k P C F i ) 1 ( CP F i b k CP b / m k P C m / ) 1 (
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19 Stiffness constant of the joint For F m < 0, joints are intact (no separation of members) Threaded fasteners (1 ) b m CP C P m b k k   b b m k C k k “Joint constant”
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