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Machine design 2 selection 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. - PowerPoint PPT Presentation
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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
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
(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
12
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)
16
main static properties
elastic limit Rp0.2
yield strength ReH
ultimate tensile strength Rm
maximal strain A
hardness HHB…
Charpy (V-notch) test Kcu
17
Influencing factor – (not only) static load
shape factor
αk > 1
18
Stress concentration
sudden area changes
19
Influencing factor – dynamic load
Tényl= d
dynamic load factor
1.1~ 3Depending on machine type & size (handbooks, measurements, calculations)
20
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
22
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
32
size
surface roughness
notch sensitivity ηk (0.5-1.0 )
33
Stress concentration factor βk
34
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
35
That’all, folks!
36