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
DESIGN
manufacturing
money
optimum - opportunism
time, place
6
aims
• no failure (surface or volume)
• no big deformations
• enough service time
7
loads
•
•
•
•
mechanical  forces, moments…
thermal

chemical
changing in place/ time
…
static
cyclic
dynamic
8
materials
• metals
– ferrous/ non~
• cast iron
• steel
– carbon/ alloy
• plastics
• ceramics
• composite
•
•
•
•
•
•
•
•
•
density
conductivity
…
elasticity
plasticity
ductility
brittleness
toughness (impact)
creep
9
loads & stresses
Stress distribution uniforn/linear
(Mohr, H-M-H)
10
loss of stability
Buckling
elastic/ plastic
11
Tensile test
Load  deformation
stress – strain diagrams
ductile – carbon steel
rigid – cast iron
12
simplifyed material models
elasto-plastic
rigid
13
test
YS
UTS
ReH
Rm
UTS
YS
real
eng.
14
strain hardening
15
elastic properties
elastic (Young) modulus: E
(Hook’s law s = Ee)
shear modulus: G [MPa]
[MPa]
(t = Gg)
16
main static properties
elastic limit
yield strength
ultimate tensile strength
maximal strain
hardness
Charpy (V-notch) test
Rp0.2
ReH
Rm
A
HHB…
Kcu
17
Influencing factor – (not only) static load
Stress concentration
shape factor
αk > 1
18
sudden area changes
19
Influencing factor – dynamic load
s Tényl= d s
dynamic load factor
1.1~ 3
Depending on machine type &
size (handbooks,
measurements, calculations)
20
Periodic stresses
fatique
propagating crack – final static break
90-95% of all failures!
21
Wöhler- curve
Rm


fatique limit
N > 106
load cycles
many factors - stohastic phenomenon
22
types of varying load
Fatique limits
σv
σr
s min
r
s max
cycle assymetry ratio
sm
v
s max
cycle mean stress ratio
23
constructing Smith- diagram
constructing Smith- diagram
constructing Smith- diagram
constructing Smith- diagram
constructing Smith- diagram
constructing Smith- diagram
constructing Smith- diagram
using Smith-diagram
no failure
31
Influencing factors – fatique
strength
size
surface roughness
32
notch sensitivity ηk (0.5-1.0 )
33
Stress concentration factor βk
k  1
k 
k  1
σaver.
stress conc.
sensitivity
shape
34
Safety factor (SF; n)
• load, material…
all bears uncertainity
to be on the safe side
use SF
• importance of
element
• manufacture
• material testing
• calculations
n>1 (1.15 ~ 8-10!)
35
That’all, folks!
36