module 1 - design considerations dme
TRANSCRIPT
-
8/12/2019 Module 1 - Design Considerations DME
1/59
MODULE 1- DESIGN OFMACHINE ELEMENTS
MANUFACTURING CONSIDERATIONSIN DESIGN, STRESS CONCENTRATION,
THEORIES OF FAILURE
-
8/12/2019 Module 1 - Design Considerations DME
2/59
SELECTION OF METHOD
The selection of method of manufacturing is oneof the most complicated area which a designerhas to come across.
The manufacturing processes can be broadly intofour classes
Casting processes
Deformation processes
Material removal or cutting processes
Powder metallurgy
-
8/12/2019 Module 1 - Design Considerations DME
3/59
CONSIDERATION FOR THE SELECTION
Material of the component
Cost of manufacture Geometric shape of the component
Surface finish and tolerance required
Volume of production
-
8/12/2019 Module 1 - Design Considerations DME
4/59
Metal Castings
Types of casting
Design rules
Drafting practices
-
8/12/2019 Module 1 - Design Considerations DME
5/59
DISADVANTAGES
Simple and inexpensive tooling
Almost all metals can be cast
Complex shapes can be easily handled
ADVANTAGES
Not possible to achieve close tolerance
Rough surface finish
Long and thin sections are difficult
-
8/12/2019 Module 1 - Design Considerations DME
6/59
Types of casting:
Sand mould casting
Shell mould casting
-
8/12/2019 Module 1 - Design Considerations DME
7/59
Types of casting:
Plaster mould casting
Permanent mouldcasting
-
8/12/2019 Module 1 - Design Considerations DME
8/59
Types of casting:
Investment mould casting (Formerly called
lost wax casting)
Centrifugal casting
-
8/12/2019 Module 1 - Design Considerations DME
9/59
Types of casting:
Die casting
-
8/12/2019 Module 1 - Design Considerations DME
10/59
Selection of a casting method is based
on:
Type of metal
Number of castings needed
Size and shape of part
Level of accuracy required
Casting finish
-
8/12/2019 Module 1 - Design Considerations DME
11/59
Design for Soundness
Most metals and alloys shrink when
they solidify.
Design components so that all members
of the parts increase in dimension
progressively to one or more suitable
areas where feeder heads (risers) can
be placed to offset liquid shrinkage
-
8/12/2019 Module 1 - Design Considerations DME
12/59
Fillet or round all sharp edges
Solidification of molten metalalways proceeds from themold face,
A simple section presentsuniform cooling and greatestfreedom from mechanical
weakness.When two or more sectionsconjoin, mechanical weaknessis induced at the junction andfree cooling is interrupted,
creating a hotspot,the mostcommon defect in castingdesign
-
8/12/2019 Module 1 - Design Considerations DME
13/59
Minimize the Number of SectionsA well-designed casting brings the
minimum number of sections together
at one point. A simple wall section will
cool freely from all surfaces, but by
adding a section (forming a T), a hot spot
is created at the junction, and it will cool
like a wall that is 50% larger.
To prevent uneven cooling, bring the
minimum number of sections together
or stagger them so that no more than
two sections conjoin.
When this is not possible, a circular webwith adjoining sections is the preferred
way to design structures that must
intersect (4b).
-
8/12/2019 Module 1 - Design Considerations DME
14/59
Employ Uniform Sections
Thicker walls will solidify more slowly, sothey will feed thinner walls, resulting inshrinkage voids. The goal is to designuniform sections that solidify evenly. Ifthis is not possible, all heavy sectionsshould be accessible to feeding from
risers.
This hydraulic coupling was originallydesigned with a core that caused
localized porosity. By redesigning thecomponent with uniform walls, theweight of the casting was reduced,lowering the manufacturing cost andremedying the shrinkage problem.
-
8/12/2019 Module 1 - Design Considerations DME
15/59
Correctly Proportion Inner Walls
Inner sections of castings(resulting from complex cores) cool
much slower than outer sectionsand cause variations in strengthproperties. A good rule is to reduceinner sections to 0.9 of the thicknessof the outer wall.
The inside diameter of cylindersand bushings should exceed the wallthickness of castings. When theinside diameter of a cylinder is lessthan the wall thickness, it is better tocast the section solid, as holes can
be produced by cheaper (and safer)methods than with extremely thincores.
-
8/12/2019 Module 1 - Design Considerations DME
16/59
Fillet All Sharp Angles
Fillets (rounded corners) have three
functional purposes:
to reduce the stress concentration in
a casting in service;
to eliminate cracks, tears and draws
at reentry angles; to make corners more moldable by
eliminating hot spots
To avoid a section size that is too large at
an "L" junction, round an outside corner to
match the fillet on the inside wall. Where
this is not possible, consideration must be
given to which is more vital: the
engineered design or the possible casting
defect.
-
8/12/2019 Module 1 - Design Considerations DME
17/59
Avoid Abrupt Section Changes
The difference in relativethickness of adjoining sectionsshould not exceed a ratio of2:1. If a greater difference isunavoidable, consider a designwith detachable parts, likemachine tool beds that can bebolted.
When a change in thickness isless than 2:1, it may take the
form of a fillet. When thedifference is greater, therecommended shift is in theform of a wedge.
However, wedgeshapedchanges in wall thicknessshould not taper more than 1
in 4. Where a combination oflight and heavy sections isunavoidable, use fillets andtapered sections to temperthe shifts.
-
8/12/2019 Module 1 - Design Considerations DME
18/59
Casting of Wheels
Use curved spokes
Use an odd number ofspokes
Consider wall thickness
Select parting lines
Drill holes in castings
Meehanite Metal Corp.
-
8/12/2019 Module 1 - Design Considerations DME
19/59
Avoid Using Bosses and Pads
Bosses and pads increase metalthickness, create hot spots
Bosses should not be used in castingdesign when the surface to supportbolts may be obtained by milling orcountersinking.
The thickness of bosses and padspreferably should be less than thethickness of the casting section theyadjoin but thick enough to permitmachining without touching the castingwall.
In large castings, pouring a metalsection that is too heavy at the bossesis difficult to feed. A better design is tomake the walls of the boss at uniformthickness to the casting walls
-
8/12/2019 Module 1 - Design Considerations DME
20/59
Maximize Design of Ribs
Ribs have two functions: to increase stiffness and to
reduce weight. If they are too shallow or too widelyspaced, they can be ineffective.
The thickness of ribs should approximate 80% of theadjoining thickness and should be rounded at the edge.
The design preference is for the ribs to be deeper thanthey are thick
In general, ribs in compression offer a greater safety factorthan ribs in tension.
Avoid cross ribs or ribbing on both sides of a casting. Cross
ribbing creates hot spots and makes feeding difficult. Instead, design cross-coupled ribs in a staggered double
"T" form.
-
8/12/2019 Module 1 - Design Considerations DME
21/59
On a casting drawing, the primary datum surface should be:
Able to be used for mounting the part and as a basis formeasurement
Not machined
Parallel with the top of the mold or parting line
Integral with the main body of the casting
Able to be clamped without distortion
A surface that will provide locating points as far apart aspossible
-
8/12/2019 Module 1 - Design Considerations DME
22/59
FORGING
-
8/12/2019 Module 1 - Design Considerations DME
23/59
FORGING
In forging, metal is taken to its plastic stage and
forced to flow into desired shape.
Different types of forgings are
Hand forging
Drop forging
Press forging
Upset forging
-
8/12/2019 Module 1 - Design Considerations DME
24/59
ADVANTAGES
Fibrelines can be arranged in a predetermined
way
Good utilization of materials
Can be provided with thin sections without
affecting the strength
Closer tolerance can be achieved
High production rate and reproductivity
-
8/12/2019 Module 1 - Design Considerations DME
25/59
DISADVANTAGES
Costly method
Useful only in the large scale production
-
8/12/2019 Module 1 - Design Considerations DME
26/59
DESIGN CONSIDERATIONS FOR
FORGINGS
Orientation of the fibres
Forging should be provided with adequate
draft
Should fix the parting line sensibly
Should have adequate fillet and corner radii
Try to avoid very thin sections
-
8/12/2019 Module 1 - Design Considerations DME
27/59
MACHINING
http://www.google.co.in/imgres?imgurl=http://news.uns.purdue.edu/UNS/images/shin.ceramic2.jpeg&imgrefurl=http://news.uns.purdue.edu/html4ever/0004.Shin.ceramics.html&usg=__t2be1vUI5JBr4pCli8K3T2Ki-0M=&h=800&w=1200&sz=379&hl=en&start=2&itbs=1&tbnid=ao9gky0MzxbpBM:&tbnh=100&tbnw=150&prev=/images?q=%22laser+assisted+machining%22&hl=en&safe=off&sa=G&gbv=2&tbs=isch:1 -
8/12/2019 Module 1 - Design Considerations DME
28/59
MACHINING
Material removal or cutting process is the most
versatile and common production methodology.
Finishing operation always require a machining
process
It is classified into
Metal cutting process
Grinding process
Unconventional machining process
-
8/12/2019 Module 1 - Design Considerations DME
29/59
ADVANTAGES
Any material can be machined
Best tolerances
Good surface finishDISADVANTAGES
Costly and low rate of production
Difficult to machine thin sections
Wastage of material
-
8/12/2019 Module 1 - Design Considerations DME
30/59
DESIGN CONSIDERATION
To the maximum extent, avoid machining
Sensibly provide the tolerances
Avoid sharp corners Use stock dimensions
Design rigid parts
Avoid shoulders and undercuts
Avoid hard materials
-
8/12/2019 Module 1 - Design Considerations DME
31/59
Powder Metallurgy.
Design considerations
-
8/12/2019 Module 1 - Design Considerations DME
32/59
Definition:
Powder metallurgy
The process of making parts by
compressing and sintering powders intoshape
-
8/12/2019 Module 1 - Design Considerations DME
33/59
Design considerations in powder
metallurgy:
Ejection from the die
Axial variations
Reverse tapers
Corner reliefs
-
8/12/2019 Module 1 - Design Considerations DME
34/59
Design considerations in powder
metallurgy:
Holes at right
angles to the
direction of
pressing
Undercuts
Knurls
-
8/12/2019 Module 1 - Design Considerations DME
35/59
Design considerations in powder
metallurgy:
Blind holes
Flanges
Corners
-
8/12/2019 Module 1 - Design Considerations DME
36/59
Design considerations in powder
metallurgy:
Wall thickness
Chamfers
Changes in crosssection
-
8/12/2019 Module 1 - Design Considerations DME
37/59
STRESS CONCENTRATION
Localisation of the stresses due to irregularities present in the
component or abrupt changes in the cross-section.
It can be due to the following reasons in manufacturing
Internal cracks or flaws
Cavities in welds
Air holes
Foreign inclusions
It can also be due to faulty designs like
Abrupt changes in sections
Discontinuities in the component
Machining scratches
-
8/12/2019 Module 1 - Design Considerations DME
38/59
STRESS CONCENTRATION FACTOR
The stress concentration is considered in a
design by applying stress concentration factor Kt
in the design process. It is determined by two
methods
Mathematical methods- Using FEA analysis
Experimental methods- Photo elasticity
-
8/12/2019 Module 1 - Design Considerations DME
39/59
REASON FOR STRESS CONCENTRATION
Abrupt changes in cross sections without
understanding the flow anology
Not providing fillet radius
undercutting and notching for members in
tension
Drilling additional holes for shafts
Thread cutting
-
8/12/2019 Module 1 - Design Considerations DME
40/59
REDUCTION OF STRESS
CONCENTRATION
Additional notches and holes in Tension member
Fillet radius, undercutting and notching for members inbending
Drilling additional properly designed holes around a
keyway of a shaft For threaded component, the reduction in the stress
concentration can be achieved by
providing an undercut just before the staring of
thread Make the base shaft diameter slightly less than the
root diameter of the thread.
-
8/12/2019 Module 1 - Design Considerations DME
41/59
THEORIES OF FAILURE
Maximum principal stress theory ( RankinesTheory)
Maximum Principal Strain Theory ( St. VenantsTheory)
Maximum shear stress theory (Guests or Trescas
Theory)
Maximum total strain energy theory ( HaighsTheory)
Shear strain energy Theory (Von Mises Theory)
-
8/12/2019 Module 1 - Design Considerations DME
42/59
Maximum principal stress theory
( RankinesTheory)
Based on the work done on brittle materials
Failure of the component acted upon by bi-
axial or tri-axial loads occurs when the
maximum principal stress reaches the yield
point or ultimate tensile stress of the material
Gives good results for brittle materials
1 m
-
8/12/2019 Module 1 - Design Considerations DME
43/59
Maximum Principal Strain Theory ( St.
VenantsTheory)
The elastic failure occurs when maximum
principal strain on the material crosses the
strain at the limit in the simple tension test
Not practically used now due to the absence
of experimental basis
]21 1[1 mE
-
8/12/2019 Module 1 - Design Considerations DME
44/59
Maximum shear stress theory (Guests
or TrescasTheory)
Failure of the component acted upon by bi-
axial or tri-axial loads occurs when the
maximum shear stress reaches the maximum
shear stress of the material
Gives good results for ductile materials
21 m
-
8/12/2019 Module 1 - Design Considerations DME
45/59
Maximum total strain energy theory
( HaighsTheory)
According to this theory, elastic failure occurs
when the energy per unit volume in the
strained material reaches the value of the
strain energy per unit volume at the elasticlimit.
E
e
mE 2
22[
21
]21
2
2
2
1
h h (
-
8/12/2019 Module 1 - Design Considerations DME
46/59
Shear strain energy Theory (Von Mises
Theory)
This theory states that the elastic failure
occurs when the shear strain energy per unit
volume in the stressed material reaches a
value equal to the shear strain energy per unitvolume at elastic limit
22
1
2
2
2
21 2)( e
-
8/12/2019 Module 1 - Design Considerations DME
47/59
SHOCK AND IMPACT LOADS
The shock and impact loads are sudden loads
like the load that comes to the suspension
system and adjoining brackets when the
vehicle negotiates a rough patch in the road.
This is usually taken as double the normal load
coming to the part. ( Derivation)
-
8/12/2019 Module 1 - Design Considerations DME
48/59
CYCLIC STRESSES
The condition of static loads in a component isvery rare
In most of the cases, components are subjected
to loads which are varying in nature and havingvarying magnitudes and frequencies
Fourier series is employed for finding out thevariation in complicated cases
The material fails at a very low stress whencompared to the static load conditions when weapply the fluctuating loads.
-
8/12/2019 Module 1 - Design Considerations DME
49/59
FATIGUE FAILURE
The failure of a component subjected to cyclic
stresses is termed as fatigue failure
There are three mathematical models for
cyclic stresses (graphs)
Fluctuating or alternating stresses
Repeated stresses
Reversed stresses
-
8/12/2019 Module 1 - Design Considerations DME
50/59
-
8/12/2019 Module 1 - Design Considerations DME
51/59
FACTORS
Number of cycles
Mean stress
Stress amplitude
Stress concentration
Residual stresses
Corrosion
Creep
-
8/12/2019 Module 1 - Design Considerations DME
52/59
ENDURANCE LIMIT
Fatigue or endurance limit of a material is defined
as the maximum amplitude of completely
reversed stresses that the standard specimen can
sustain for an unlimited number of cycles withoutfatigue failure
Since the fatigue test cannot be conducted for
infinite cycles 106 cycles is considered as
sufficient number of cycles to define endurance
strength
-
8/12/2019 Module 1 - Design Considerations DME
53/59
SODERBERG AND GOODMAN LINES
-
8/12/2019 Module 1 - Design Considerations DME
54/59
FACTOR OF SAFETY
The designer should be able to foresee thedefects that can be expected in thecomponent during manufacturing phase and
during the final usage stage by the customer. To account these expectations, the designer
multiplies a factor called factor of safety to thedesigned material to overcome these
deficiencies. This factor is known as Factor ofSafety..
-
8/12/2019 Module 1 - Design Considerations DME
55/59
CREEP
When a component is constantly subjected to
a load, it may undergo a constant plastic
deformation over a period of time. This time
dependant strain is called creep.
It is a function of stress level and temperature.
-
8/12/2019 Module 1 - Design Considerations DME
56/59
THERMAL STRESSES
When a fixed component in a machine is
subjected to change in temperature it tries to
expand or contract according to the variation
in temperature.
If there is no room for the component to
accommodate this variation in temperature,
thermal stresses are induced in the material.
-
8/12/2019 Module 1 - Design Considerations DME
57/59
RESIDUAL STRESSES
Stresses are classified into load stresses and
residual stresses or internal stresses or locked-in
stresses.
These stresses are induced as a result ofmanufacturing processes and assembly methods.
When a material with residual stresses is used as
a part of an assembly, the stress acting on thecomponent will be the sum of the two stresses.
-
8/12/2019 Module 1 - Design Considerations DME
58/59
RESIDUAL STRESSES
Reasons for residual stresses can be
Manufacturing processes
Machining methods
Cold working processes
Chemical processes
Heat treatment Assembly operations
-
8/12/2019 Module 1 - Design Considerations DME
59/59
END OF MODULE 1