Machine design 2

selection of materials, stressing

1

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

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

3

machine elements

4

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

material

manufacturing money

DESIGN

optimum - opportunism

time, place6

aims• no failure (surface or volume)

• no big deformations

• enough service time

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loads

• mechanical forces, moments…

• thermal • chemical changing in place/ time

• …static cyclic dynamic

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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

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loads & stresses

(Mohr, H-M-H)

Stress distribution uniforn/linear

10

loss of stability

Buckling

elastic/ plastic

11

Tensile test

stress – strain diagrams

Load deformation

ductile – carbon steel rigid – cast iron

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simplifyed material models

elasto-plastic rigid

13

testUTS Rm

YS

UTS

YS ReH

eng.

real

14

strain hardening

15

elastic properties

elastic (Young) modulus: E [MPa]

(Hook’s law = E)

shear modulus: G [MPa] (= G)

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main static properties

elastic limit Rp0.2

yield strength ReH

ultimate tensile strength Rm

maximal strain A

hardness HHB…

Charpy (V-notch) test Kcu

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Influencing factor – (not only) static load

shape factor

αk > 1

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Stress concentration

sudden area changes

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Influencing factor – dynamic load

Tényl= d

dynamic load factor

1.1~ 3Depending on machine type & size (handbooks, measurements, calculations)

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Periodic stresses

fatique

propagating crack – final static break21

90-95% of all failures!

Wöhler- curve

load cycles

fatique limit N > 106

Rm

many factors - stohastic phenomenon

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types of varying load

r min

max

23v m

max

cycle assymetry ratio

cycle mean stress ratio

σv σrFatique limits

constructing Smith- diagram

constructing Smith- diagram

constructing Smith- diagram

constructing Smith- diagram

constructing Smith- diagram

constructing Smith- diagram

constructing Smith- diagram

using Smith-diagram

31no failure

Influencing factors – fatique strength

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size

surface roughness

notch sensitivity ηk (0.5-1.0 )

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Stress concentration factor βk

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kk

k

1

1

stress conc.

sensitivity

shape

σaver.

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

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That’all, folks!

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