Machine design 2 selection of materials, stressing

Machine design 2selection of materials, stressing

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.

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) 10+10 points Assignments (valve, key joint, coupling) (10+15+15) Written exam 60 point

machine elements

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

material DESIGN money manufacturing optimum - opportunism time, place

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

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

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

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

loss of stability Buckling elastic/ plastic

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

simplifyed material models elasto-plastic rigid

test YS ReH UTS Rm UTS YS real eng.

strain hardening

elastic properties elastic (Young) modulus: E [MPa] (Hook’s law s = Ee) shear modulus: G [MPa] (t = Gg)

main static properties elastic limit Rp0.2 yield strength ReH ultimate tensile strength Rm maximal strain A hardness HHB… Charpy (V-notch) test Kcu

Influencing factor – (not only) static load Stress concentration shape factor αk > 1

sudden area changes

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

Periodic stresses fatique propagating crack – final static break 90-95% of all failures!

Wöhler- curve Rm • fatique limit • N > 106 load cycles many factors - stohastic phenomenon

types of varying load σv σr Fatique limits cycle assymetry ratio cycle mean stress ratio

constructing Smith- diagram

using Smith-diagram no failure

Influencing factors – fatique strength size surface roughness

notch sensitivity ηk (0.5-1.0 )

Stress concentration factor βk σaver. stress conc. sensitivity shape

Safety factor (SF; n) • load, material… allbearsuncertainity to be onthesafesideuse SF n>1 (1.15 ~ 8-10!) • importance of element • manufacture • material testing • calculations

That’all, folks!

Machine design 2 selection of materials, stressing