Module 1 - Design Considerations DME

8/12/2019 Module 1 - Design Considerations DME

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MODULE 1- DESIGN OFMACHINE ELEMENTS

MANUFACTURING CONSIDERATIONSIN DESIGN, STRESS CONCENTRATION,

THEORIES OF FAILURE


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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


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CONSIDERATION FOR THE SELECTION

Material of the component

Cost of manufacture Geometric shape of the component

Surface finish and tolerance required

Volume of production


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Metal Castings

Types of casting

Design rules

Drafting practices


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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


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Types of casting:

Sand mould casting

Shell mould casting


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Types of casting:

Plaster mould casting

Permanent mouldcasting


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Types of casting:

Investment mould casting (Formerly called

lost wax casting)

Centrifugal casting


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Types of casting:

Die casting


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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


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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


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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


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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).


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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.


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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.


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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.


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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.


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Casting of Wheels

Use curved spokes

Use an odd number ofspokes

Consider wall thickness

Select parting lines

Drill holes in castings

Meehanite Metal Corp.


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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


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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.


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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


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FORGING


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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


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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


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DISADVANTAGES

Costly method

Useful only in the large scale production


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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


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MACHINING
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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


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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


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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


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Powder Metallurgy.

Design considerations


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Definition:

Powder metallurgy

The process of making parts by

compressing and sintering powders intoshape


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Design considerations in powder

metallurgy:

Ejection from the die

Axial variations

Reverse tapers

Corner reliefs


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metallurgy:

Holes at right

angles to the

direction of

pressing

Undercuts

Knurls


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metallurgy:

Blind holes

Flanges

Corners


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metallurgy:

Wall thickness

Chamfers

Changes in crosssection


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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


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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


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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


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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.


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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)


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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


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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


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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


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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 (


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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


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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)


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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.


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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


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FACTORS

Number of cycles

Mean stress

Stress amplitude

Stress concentration

Residual stresses

Corrosion

Creep


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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


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SODERBERG AND GOODMAN LINES


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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..


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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.


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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.


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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.


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RESIDUAL STRESSES

Reasons for residual stresses can be

Manufacturing processes

Machining methods

Cold working processes

Chemical processes

Heat treatment Assembly operations


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END OF MODULE 1

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Module 1 - Design Considerations DME