EMM7241-Advanced Machine Design Examination June 2016

7/26/2019 EMM7241-Advanced Machine Design Examination June 2016

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TECHNICAL UNIVERSITY OF KENYA (TUK)

UNIVERSITY EXAMINATIONS JUNE 2016

FIRST YEAR SECOND SEMESTER EXAMINATION FOR

MASTER OF ENGINEERING TECHNOLOGY (MEngTe!) IN MECHANICAL

ENGINEERING TECHNOLOGY

EMMU "2#1 ADVANCED MACHINE DESIGN

DATE$ TIME$ % HOURS

INSTRUCTIONS TO CANDIDATES

You should have the following for this examination.

Answer book-let.

Mathematical tables/calculator.

This paper contains EIGHTquestions.

Answer an FIVE!uestions

All !uestions have equal marks

Maximum marks for each part of a question are as indicated.


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&1 ')"hat is meant b #stress concentration$% &llustrate how the stress concentration in

a component can be reduced.

b'(efine fracture toughness. "hat are the factors that affect fracture toughness%

c' A pulle is keed to a shaft midwa between two bearings. The shaft is made of cold

drawn steel for which the ultimate strength is )** M+/m,and the ield strength is **

M+/m,. The bending moment at the pulle varies from ,** +-m to 0** +-m as the

torque on the shaft varies from 1** +-m to ,** +-m. 2btain the diameter of the

shaft for an indefinite life. The stress concentration factors for the kewa at the pullein bending and in torsion are 1.) and 1.3 respectivel4 the factor of safet 5 1.64 the

load correction factors 5 1., in bending7 and *.) in torsion4 the si8e effect factor 5 *.94

and the surface effect factor 5 *.99. :ind the diameter of the shaft for an indefinite life.

!, a' A steel ring with internal diameter of ,** mm and external diameter 3** mm is

shrunk onto a solid steel shaft. The interference is arranged such that the radial pressure

between the mating surfaces will not fall below 3* M+/m,whilst the assembl rotates

in service. &f the maximum circumferential stress on the inside surface of the ring is

limited to ,* M+/m,

7 determine the maximum speed at which the assembl can berotated. &t ma be assumed that no relative slip occurs between the shaft and the ring.

:or steel7 ;5 6*** kg/m37


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!0 a' "hat do ou understand b surface fatigue failure% Cxplain how surface fatigue

failure can be reduced on machine elements.

b' Eriefl outline the three ke aspects of machine component surfaces to be considered

in machine design.

c' The ball and socket >oint F:igure below' at the end of a rocker arm has a hardened-

steel spherical surface 1* mm in diameter fitting in a hard-bron8e bearing allo

spherical seat 1*.1 mm in diameter. "hat maximum contact stress will result from a

load of ,*** +% =ive our assumptions.

d' "ith the help of neat sketches explain the process of crack initiation and propagation

e' (etermine the crack depth for a thick plate4 a titanium plate is loaded as shown belowto a gross-area stress g of *.9Gy. &f C and < for steel are ,** =+/m, and *.3respectivel7 C and < for bron8e are 11* =+/m, and *.33 respectivel4 t 5 ,mm7 ,w 5

1*mm7 and a/,c 5 *.,7 determine the critical crack depth7 acr7 at which rapid fracture

will occur.

! a' Eriefl explain what ou understand b pressure vessels. (escribe three theories

of failure that are normall used in the design of pressure vessels

b' The cast iron used in the manufacture of an engineering component has tensile and

compressive strengths of 3)*M+/m,and H** M+/m,respectivel.

Fi' &f the maximum value of the tensile principal stress is to be limited to one-quarter of

the tensile strength7 determine the maximum value and nature of the other principal

stress using MohrIs modified ield theor for brittle materials.

Fii' "hat would be the values of the principal stresses associated with a maximum shear

stress of #0 M+/m,according to MohrIs modified theor%

!) a' (efine the :actor of Bafet. Btate the considerations to be taken into account

when selecting factors of safet. "h should the factor of safet be right%


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b' A simpl supported shaft with a span of **mm is sub>ected to a point cclic load at

its mid span. The load varies from a minimum value of ,* D+ to maximum value of *

D+. The load direction is transverse to the shaft axis. (etermine the diameter of the

shaft7 if the Jltimate strength 5 )*+/mm,7 the Yield strength 5 0*+/mm, and the

Cndurance strength 5 3*+/mm,.Assume a factor of safety = 2, a factor of size effect =

0.85 and a factor of surface finish = 0.90.

!6 a'"hat do ou understand b Machine design% Btate and discuss the factors thatinfluence the design of products.

b' "ith the help of sketches derive expressions for the maximum elongation and the

maximum tensile stresses produced b an impact load in a metal bar4 state an

assumptions made.

c' A steel bar of length ,.* m and diameter ,* mm hangs verticall from a rigid support

at its upper end. A sliding collar of 0* kg mass drops from a height of ,** mm onto the

flange at the lower end of the bar without rebounding. &f E for steel is ,** =+/m,7

determine the maximumK

i' change in length of the bar and the corresponding impact factor7) Tensile stress in the bar and the corresponding impact factor.

!9 a' "hat are the factors that affect creep% (escribe the stages of creep.

b' Btate and define the creep properties. Eriefl explain five factors that influence creep.

c' The bending stress in a machine shaft fluctuates between a tensile stress of

3**M+/m,and a compressive stress of$1*M+/m,. &f the ield stress of the shaft is to

be between *L and H*L of the ultimate tensile strength7 determine the minimum

ultimate tensile strength of this shaft to carr this fluctuation indefinitel according toK

i' =erber formula

ii' =oodman$s formula7 andiii' Boderberg formula4

Take endurance strenth = 0.5 !"timate strenth# and factor of safety = 2


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!H. A steel bolt is sub>ecteed to an axial tensile load between two rigid plates so that

the length of the bolt remains constant. The bolt is sub>ected to an elevated temperature

so that creep is produced. "hat must be the initial stress so that 6*L of the stress is

retained after ten ears% Jse the relaxation relation and creep rate given b V=Bon

7

where n5 ).*7 o5 6 Mpa7 $ 5 0., x 1*-9mm/mm per da and%5 ,1* =Na

!1* a' "ith the help of neat sketches explain the process of crack initiation andpropagation

b' (efine crack resistance and discuss the variation of crack resistance with increase in

crack length for ductile and brittles.

c' A pin-on-disk friction testing apparatus7 shown below7 involves the un-lubricated

rounded end of a copper pin of 9* Oickers hardness being pressed with a force of ,* +

against the surface of a rotating steel disk of ,1* Erinell hardness. The rubbing contact

is at a radius of 1) mm4 the disk rotates 9* rpm. After , hours the pin and disk are

weighed. &t is determined that adhesive wear has caused weight losses equivalent to

wear volumes of ,.6 and *.) mm3for the copper and steel7 respectivel. @ompute the

wear coefficients.

!11 a' "hat is meant b endurance strength of a material% Pow do the si8e andsurface condition of a component and tpe of load affect such strength%

b' "rite a note on the influence of various factors of the endurance limit of a ductile

material.

c' A hot rolled steel shaft is sub>ected to a torsional moment that varies from 33* +-m

clockwise to 11* +-m counterclockwise and an applied bending moment at a critical

section varies from 00* +-m to ,,* +-m. The shaft is of uniform cross-section and no

kewa is present at the critical section. (etermine the required shaft diameter. The

material has an ultimate strength of * M+/m,and ield strength of 01* M+/m,. Take


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the endurance limit as half the ultimate strength7 factor of safet of ,7 si8e factor of *.9

and a surface finish factor of *.),.

!1, a' (efine three modes of fracture with the help of neat sketches

b' A )* mm diameter cold drawn steel bar is sub>ected to a completel reversed torque

of 1** +m and an applied bending moment that varies between 0** +m and ,** +m.

The shaft has a machined finish and has a ) mm diameter hole drilled transverselthrough it. &f the ultimate tensile stress tand ield stress yof the material are )** MNa

and 0,* MNa respectivel7 find the factor of safet.

c' At some point in a component principal stresses of 1**M+/m,tensile and 1**M+/m,

compressive are found to be present. Cstimate the safet factor with respect to initial

ield using the maximum principal stress7 maximum shear stress7 distortion energ and

MohrIs modified theories of elastic failure.

!13 a' Cxplain the following terms in connection with design of machine members

sub>ected to variable loadsK

i& Cndurance limit7

ii& Bi8e factor7

iii&Burface finish factor7 and

i'&+otch sensitivit.

b' A cantilever beam made of cold drawn carbon steel of circular cross-section Fas

shown in the figure below' is sub>ected to a load which varies from : to 3 :.

(etermine the maximum load that this member can withstand for an indefinite life using

a factor of safet as ,. The theoretical stress concentration factor is 1.0, and the notch

sensitivit is *.H. Assume the following valuesK

Jltimate stress 5 * MNa4 Yield stress 5 06* MNa4 Cndurance limit 5 ,6 MNa4 Bi8efactor 5 *.94 Burface finish factor 5 *.9H

!10 a' A machine component is sub>ected to a flexural stress which fluctuates between

3** M+/m,and 1* M+/m,4 (etermine the value of minimum ultimate strength

according toK

i' =erber relation4

ii' Modified =oodman relation4 and

iii'Boderberg relation.

Take yie"d strenth = 0.55 !"timate strenth# %ndurance strenth = 0.5 !"timate

strenth# and factor of safety = 2


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b' :ind the maximum stress induced in the following cases taking stress concentration

into accountK

i' A rectangular plate )* mm Q 1* mm with a hole of 1,mm diameter as shown

in :ig Fa'7 and sub>ected to a tensile load of 1, k+.

ii' A stepped shaft as shown in :ig. Fb'7 and carring a tensile load of 1, k+.

!1 a' 2utline at least five tpes of design used in engineering.

b' "hat is Nroduct (esign% "hat is the role of a product designer%c' (efine crack resistance and discuss the variation of crack resistance with increase in

crack length for ductile and brittles.

d' A steel member 3m long is sub>ected to an axial tensile load of 0.0 D+ at an

operating temperature of 0***@.determine the cross-sectional area required for an

estimated life of ,* ears based on an allowable creep rate of *.**1mm per mm per ear

using creep relationC=

t=B(

o

)n

7 where n5 67 *+5 6 Mpa and

(5 ., x 1*-1,mm per mm per da.

!1) a' A hori8ontal barA( of length) is struck at its free end b a heav block of

mass* moving hori8ontall with velocit .(etermine the maximumK

i' @hange in length of the bar and the corresponding impact factor.

ii' @ompressive stressand the corresponding impact factor.

b' A steel rotor disc which is part of a turbine assembl has a uniform thickness of 0*

mm. The disc has an outer diameter of )** mm and a central hole of 1** mm diameter.

&f there are ,** blades each of mass *.13 kg pitched evenl around the peripher of the

disc at an effective radius of 3,* mm7 determine the rotational speed at which ielding

of the disc first occurs according to the maximum shear stress criterion of elastic failure.:or steel7 E 5 ,** =+/m,7


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!19 Fa' (erive expressions for the principal stresses and the maximum shear stress for

a piece of material sub>ected to two perpendicular direct stresses together with

complimentar shear stress in the same planes.

Fb' A hollow shaft 9*mm outside diameter and 0*mm internal diameter7 sub>ected to an

axial end thrust of 0*D+7 transmits **D" at )**rev/min. &f the maximum

compressive principal stress is not to exceed 1**M+/m,7 determineK

i' The maximum bending moment which can be safel applied in con>unction

with the applied torque and end thrust4ii' The value of the other principal stress4

iii' The maximum shear stress.

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EMM7241-Advanced Machine Design Examination June 2016