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Machine design 2 selection of materials, stressing 1

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aim to introduce the structure of machines, the function, type and load of machine elements and the main methods of designing them Main chapters of this semester: –Fits & tolerances –Basics of stressing –Joints –Drive systems 1. 2

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Requirements 3 hours weekly (mostly 2 lecture, 1 laboratory) Attendance and participation are important! Engineering is not a spectator sport. (allowed absence four occasions) Course requirements: Tests (tolerances, stressing) points Assignments (valve, key joint, coupling) ( ) Written exam 60 point 3

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machine elements 4

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general purpose machine elements static elements joints frames springs (mechanical) drive systems bearings shafts couplings & clutches drives vessel systems pipes valves tanks (technical fluids) 5

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material manufacturing money DESIGN optimum - opportunism time, place 6

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aims no failure (surface or volume) no big deformations enough service time 7

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loads mechanical forces, moments… thermal chemical changing in place/ time … static cyclic dynamic 8

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materials metals –ferrous/ non~ cast iron steel – carbon/ alloy plastics ceramics composite density conductivity … elasticity plasticity ductility brittleness toughness (impact) creep 9

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loads & stresses (Mohr, H-M-H) Stress distribution uniforn/linear 10

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loss of stability Buckling elastic/ plastic 11

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Tensile test stress – strain diagrams Load deformation ductile – carbon steel rigid – cast iron 12

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simplifyed material models elasto-plastic rigid 13

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test UTS R m YS UTS YS R eH eng. real 14

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strain hardening 15

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elastic properties elastic (Young) modulus: E [MPa] (Hook’s law = E ) shear modulus: G [MPa] ( = G ) 16

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main static properties elastic limit R p0.2 yield strengthR eH ultimate tensile strengthR m maximal strainA hardnessH HB … Charpy (V-notch) testK cu 17

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Influencing factor – (not only) static load shape factor α k > 1 18 Stress concentration

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sudden area changes 19

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Influencing factor – dynamic load Tényl = d dynamic load factor 1.1~ 3 Depending on machine type & size (handbooks, measurements, calculations) 20

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Periodic stresses fatique propagating crack – final static break % of all failures!

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Wöhler- curve load cycles fatique limit N > 10 6 RmRm many factors - stohastic phenomenon 22

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types of varying load 23 cycle assymetry ratio cycle mean stress ratio σvσv σrσr Fatique limits

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constructing Smith- diagram

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using Smith-diagram 31 no failure

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Influencing factors – fatique strength 32 size surface roughness

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notch sensitivity η k ( ) 33

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Stress concentration factor β k 34 stress conc. sensitivity shape σ aver.

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Safety factor (SF; n) load, material… all bears uncertainity to be on the safe side use SF n>1 (1.15 ~ 8-10!) importance of element manufacture material testing calculations 35

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That’all, folks! 36

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