This document provides information about screw jacks. It discusses the working principle, construction, advantages, disadvantages and common types of screw jacks. Specifically, it notes that a screw jack consists of a screw and nut, where the nut remains stationary and the screw is rotated using a handle to lift a load. It provides mechanical advantage by converting rotational force to linear force. Advantages include self-locking ability to hold loads, while disadvantages include potential for accidents if not used properly. Common types of screw jacks include floor jacks and scissor jacks used to lift vehicles. Mechanical advantage is defined as the ratio of load force to input force, neglecting friction.

1. 1
SCHOOL OFENGINEERING
DEPARTMENT OF MECHANICALENGINEEERING
PROJECT REPORT
PROJECT TITLE
SCREW JACK
COURS CODE MENG 3161
PREPAREDBY:
SHUSHAY HAILU
ID NO4142/07
SECTION 2
SUBMITED TO INSTRACTOR:
BERIHU
SUBMITION DATE 30/08/2009E.C

2. 2
Table of content page
Acknowledgment……………………………………………………………………………………………………5
Abstract……………………………………………………………………………………………………………….…6
Nomenclature………………………………………………………………………………………………………..7-9
Chapter1
Screw jack ………………………………………………………………………………………………………….….10
1.0 introduction……………………………………………………………………………………………………..10
1.1Working principal ……………………………………………………………………………………......10-11
1.2 Problem statement ........................................................................................11-12
1.3 OBJECTIVE……………………………………………………………………………………………….…….12
1.4 methodology………………………………………………………………………………………………….12
1.5 design concepts……………………………………………………………………………………………...12-13
1.6 SCOPE AND LIMITION……………………………………………………………………………….……. 13-14
1.7 SCOPE OF THE PROJECT………………………………………………………………………………………14
CHAPTER TWO…………………………………………………………………………………………………………. 15
LITERATURE REVIEW………………………………………………………………………………………………...15
2.0 Introduction …………………………………………………………………………………………………...15

3. 3
2.1 Operation …………………………………………………………………………………………….15
2.2 Construction of a Screw Jack ………………………………………………………….…...15
2.3 Advantages and Disadvantages of the Screw Jack ………………………………. 15
2.4 Mechanical Advantage (M.A) ………………………………………………………….…...16
2.5 Common Types of Screw Jack…………………………………………………………….….16
CHAPTER 3…………………………………………………………………………………………………………………..19
MATERIALS SELECTION……………………………………………………………………………………………19
3.0 Introduction…………………………………………………………………………………………………...…19
3.1 Engineering Materials for Components ………………………………………………...19
3.2 Steps for Selection of Materials for Components…………………………….….…19-20

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3.3 Components and their Specific Materials Selected ………………………….………….21
CLASSIFICATION OF SCREW THREADS ……………………………………………………….……..23
3.1 Introduction…………………………………………………………………………………………………..….23
3.1.1 Square Thread ……………………………………………………………………………………..……..…23
3.1.1.1 Nomenclature of Square Thread ………………………………………………………….23
3.1.1.2 Advantages of the Square Thread ……………………………….........................23
3.1.1.3 Disadvantages of Square Thread ……………………………………………………...…23
3.1.2 ISO Metric Trapezoidal Threads………………………………………………….............24
3.1.2.1 Nomenclature of ISO Metric Trapezoidal Thread ………………………...….…24
3.1.2.2 Advantages of the Trapezoidal Thread ………………………………….….…..........24
3.1.2.3 Disadvantages of Trapezoidal Threads……………………………….…………….…24
3.3 Definition of Screw Thread Basic Terms ………………………………………………..…26-27
3.4 Torque requirement lifting load………………………………………………………….......28
3.5 Torque requirement lowering the load…………………………………………….……...30
3.6 Over haling and self locking screw………………………………………………..............31
3.7 Efficiency of square treaded screw………………………………………………………..….33
3.8 Efficiency of self locking screws………………………………………………………………...35
3.9 Coefficient of friction………………………………………………………………………………...35
3.10 Buckling of columns………………………………………………………………………………...36
Chapter 4………………………………………………………………………………………………………..38
Designing procedure for the screw jack……………………………………………………….…38
4.0 Introduction………………………………………………………………………………………………….38
4.1 Design for Screw Shaft…………………………………………………………………..38
4.1.1 Core Diameter……………………………………………………………….38
4.1.2 Torque required to rotate the screw…………………………….….39
4.1.3 Screw Stresses………………………………………………………………..39
4.1.4 Principal Stresses…………………………………………………………….40
4.2 Design for Nut………………………………………………………………………………...40

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4.2.2 Stresses in the Screw and Nut…………………………………………………..41
4.2.3 The outer diameter of Nut………………………………………………………..41
4.2.4 the outside diameter of Collar………………………………………………….41
4.2.5 Thickness of the Nut Collar…………………………………………………….…43
4.3 Designs for Head and Cup…………………………………………………………………….43
4.3.2 Torque Required to Overcome Friction……………………………………………….45
4.3.3 Total Torque Subjected to the Handle………………………………………………..45
4.3.4 Diameter of Handle/Lever…………………………………………………………………..45
4.3.5 Height of Head……………………………………………………………………………………….46
4.3.6 Design Check against Instability/Buckling………………………………………………………47
4.4 Design of Body ………………………………………………………………………………………….47
4.5 Dimensions for the body of the screw ……………………………………………………….47
4.6 Efficiency of the Screw Jack………………………………………………....51
4.7 Result and dissection ……………………………………………………….....52
4.8CONCULUTION……………………………………………………………...53
4.9 Recommendation……………………………………………………………..………53
4.10 2D-drawing…………………………………………………………………54-55

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Acknowledgement
I wouldlike toacknowledge andappreciate the greatguidance frommyproject
supervisor,instructorberihu
I wouldalsolike tothankmy parentsandclassmatesfortheirencouragement,
understandingandsupportthroughoutthe entireproject.
I wouldalsolike tothankthe almightyGodfor bringingme thisfarand givingme the strengthtocarry
out the project.
I wouldalsolike tothankmy friend’szemical andtoweled.

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ABSTRACT
a screw jackservestogive mechanical advantage by changingrotational force tolinearforce thus
allows one tolifta load andsupportit at a givenheight.The aimof the projectwasto designa screw
jack that wasraised3500kg mass of car duringmaintenance andwithadesiredstrengthandmechanical
propertiesthatwasfree fromanyerror.
Thiscase studyisdivided intovarioussectionsthatdescribesclassificationof screw threads,design
analysis,resultand dissection ,conclusionandrecommendationpartsof the screw jackand selectionof
materialsusedforconstructionthatare in agreementwithcurrentindustry practice of screw jack
design.The designprocedure adoptedhere isfromdesignof machineelements1and 2 A factor of
safetyof 5 andabove shouldbe usedinthisdesigntoreduce highchancesof failure due todynamic
loadingsandimpactloadings.Dynamicsloadingisasa resultof external interferencessuchaswhirl
wind,earthtremorsandexternal forceswhileimpactloadingissuchas loadisappliedsuddenlywitha

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short time andresultsintohighstresseswhichcancause failure hence these calls forahighfactor of
safety.
Nomenclature

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p - Pitchof screw thread(mm)
n - Numberof threadsincontact withscrewedspindle
l - Lead of screwthread(mm)
t - Thicknessof screw
d - Nominal diameterof screw(mm)
d c - Core diameterof screw(mm)
d m - Mean diameterof screw(mm)
θ - Frictionangle (degree)
α - Helix angle of screw(degree)
W- Load (kg)
N - Normal reaction(Newton,N)
μ − Coefficientof friction
P - Effort(Newton,N)
T - Torque (N.m)
η − Efficiency(%)
F load- The force the jack exertsonthe load.(Newton,N)
F effort- The rotational force exertedonthe handle of the jack.(Newton,N)
r-the lengthof the jack handle (mm)
M. A – Mechanical advantage
π = 3.141592654
BS – Britishstandards
σ c - Pure compressionstress

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A c - Cross sectional areaof the screwshaft
σ c(max) -Maximumprincipal stress
τ( max)- Maximumshearstress
J - Polarmoments
P b - Bearingpressure onthe nut
t 1 - Thicknessof nutcollar
h - Heightof the nut
D 1 - Outerdiameterof nutcollar
D 2 - Outside diameterof nutcollar
σ t - Tearingstrengthof the nut
σ c - Crushingstrengthof the nut
τ (screw) -Shearingstressonthe screw
τ (nut) - Shearingstressonthe nut
Τ-Shearingstressof nutcollar
D 3 - Diameterof headontop of screw
D 4 - Diameterof pin
T-Total torque to whichthe handle is
Subjected
T 1 - Torque required rotatingthe screw
T 2 –Torque required overcoming
Friction
T- Total torque subjectedtohandle
σ y -Yieldstress

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L – Lengthof the handle
D - Diameterof handle
M - Bendingmoment
H - The heightof head
σ b - Bendingstress
L eff - Effective lengthof screw
H 1 – Liftof screw
W cr - BucklingorCritical load
E – Young’s modulusormodulusof elasticity
C - End fixitycoefficient
R- Slendernessratio
k - The radiusof gyration
HB – Hardnessnumber
I − Momentof inertiaof the cross section.
D 5 - Diameterof bodyat the top
t 2 - Thicknessof body
t 3 - Thicknessof base
D 6 - InnerDiameteratthe bottom
D 7 - OuterDiameteratthe bottom
H b - Heightof the body

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CHAPTER ONE
SCREW JACK

13. 13
1.0 Introduction
Screwjack isalso calledjackscrewinotherterms.A screw jack isan example of apowerscrew and
referredtoas a mechanical device thatcanincrease the magnitude of aneffortforce.Screw jacksare
usedforraisingand loweringplatformsandtheyprovide ahighmechanical advantage inordertomove
moderatelyheavyandlarge weightswithminimumeffort.Theyfunctionbyturningthe leadscrewwhen
raisingor loweringof loads.Screw jackisfoundeverywhereisneedtolift,positionalignandhold,to
amplifyforce
1.1 Working principal
A screwjack consists of a screwand a nut. The nut isfixedina cast ironframe and remainsstationary.
The rotationof the nut inside the frame ispreventedbypressingasetscrew againstit. The screw is
rotatedinthe nutby meansof a handle,whichpassesthroughahole inthe headof the screw.The head
carriesa platform,whichsupportsthe loadandremainsstationarywhile the screw isbeingrotated.A
washerisfixedtothe otherendof the screw inside the frame,whichpreventsthe screw frombeing
completelyturned outof the nut

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15. 15
1.2 Problem statement
There isone problemthatthe researcherobserves inthe environment.duringtripe carsmayreach the
endlife time theirwheelsatthat time driversneedmaintenance theircares . There for the researchers
designistolift3500kg of car until the heightof 200mm.
1.3 OBJECTIVE
Objective of thisdesign istoovercome the problemof statement.
1 General objective _to designandmodal screw jack
2 specific objective _to designs
_to selectmaterials
_to draw 2D and 3D
_to outline dimensions
1.4 methodologies: I use books,like gupta
Ashby,m.f 2005.material selectioninmechanical designe.3rd
ed. New York
Like bhandari,v.b.,2010.design of machine elements.
Like strengthof material
Like material science
Like internetsource
1.5 design concepts: I use the conceptscost,strength,mechanism, mechanical
Properties,creep, fatigue,physical properties,thermal properties.
There are differenttypesof screwjackasshownblew

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but my design ismostpreferable one lookthe image blow

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1.6 SCOPE AND LIMITION
Basedon that designwe are liftingonly 3500kg.more than thismass isimpossible
The developmentof screwjackisonly prototype notreadyfunctioningas commercial.
The developedscrew jackisonlyfornormal person.
The developedscrew jackisonly operatedon afloatsurface.
1.7 SCOPE OF THE PROJECT
The scope of the project is starting from acknowledgment, abstract, nomenclature, introduction
to screw, litracher review, material selection, force analysis, design analyses, result and diction,
conculition, recommendation.

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Collectionof inputdatafromresearchwork.
. Studyof weight-dimensional parameters
. Studyof stresses,deformationsinlift
. Studyof Vibrationandimpactresistance.
. Studyof Keepingof servicelifeatdifferentloading
. Studyof Reliableoperation.

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CHAPTER 2
LITERATURE REVIEW
2.0 Introduction
Screwjack isalso calledjackscrewinotherterms.A screw jack isan example of apowerscrew and
referredtoas a mechanical device thatcanincrease the magnitude of aneffortforce.Screw jacksare
usedforraisingand loweringplatformsandtheyprovide ahighmechanical advantage inordertomove
moderatelyheavyandlarge weightswithminimumeffort.Theyfunctionbyturningthe leadscrewwhen
raisingor loweringof loads.
2.1 Operation
The jack can be raisedandloweredwithametal barthat is insertedintothe jack.The operatorturnsthe
bar withhis/herhandsina clockwise direction.Thisturnsthe screw insidethe jackandmakesit go up.
The screw liftsthe small metal cylinderandplatformthatare above it.Asthe jackgoesup, whateveris
placedabove itwill raise aswell,once the jackmakescontact.The bar isturneduntil the jackis raisedto
the requiredlevel.Tolowerthe jackthe bar isturnedinthe opposite direction.
2.2 Construction of a Screw Jack
A screwjack consistsof a screwand a nut. The nut isfixedina cast ironframe and remainsstationary.
The rotationof the nut inside the frame ispreventedbypressinga setscrew againstit. The screw is
rotatedinthe nutby meansof a handle,whichpassesthroughahole inthe headof the screw.The head
carriesa platform,whichsupportsthe loadandremainsstationarywhile the screw isbeingrotated.A
washerisfixedtothe otherendof the screw inside the frame,whichpreventsthe screw frombeing
completelyturnedoutof the nut.
2.3 Advantages and Disadvantages of the Screw Jack
2.3.1 Advantages
The load can be keptin liftedpositionsince the screw jackisself-locking.Thismeansitremains
motionlesswhere itwasleftwhenthe rotationalforce onthe screw iswithdrawn.Itwill notrotate
backwardsregardlessof size of the weight.Screw jacks alsoliftorraise the moderate heavyweights
againstgravityand usesverysmall handle force thatcanbe appliedmanually.

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2.3.2 Disadvantages
The major disadvantage of the screwjackisthat chances of dropping,tippingorslippingof the loadare
highand can cause seriousaccidentshence the deviceistermedasnotsafe fail.
5 Accidentscausedbyscrewjackare due to the followingreasons:
(a) Impropersecuringof loadon the jack.
(b) Overloading.
(c) Off centerof axisof the jack withrespecttocenterof gravityhence notideal forside loads.
(d) Placingthe jackon a soft groundand unleveledsurface.
(e) Usingthe jack for wrongpurpose insteadof usingitforthe purpose forwhichitis designed.
Precaution:Longliftsshouldbe avoidedsince theycancause seriousoverheatingandgenerate alarge
amountof heat.It shouldtherefore be usedunderambienttemperatureswiththe use of the required
lubricants.Designandmanufacturer’sinstructionssuchasspeed,loadcapacityandrecommended
temperaturesmustbe followedtoavoidaccidents.Alwayskeepthe matingsurfacescleanafteruse and
checkfor wearand damage on the surfaces.
2.4 Mechanical Advantage (M.A)
The mechanical advantage of a screwjack can be referredtoas the ratio of the force the jack exertson
the loadto the inputforce onthe lever,neglectingfriction.However,mostscrew jackshave large
amountsof frictionwhichincrease the requiredinputforce,sothe actual mechanical advantage isoften
only30% to 50% of thisfigure (Bhandari,2010).
M. A = F load/F effort
Where
F load= The force the jackexertsonthe load
F effort= The rotational force exertedonthe handle of the jack
2.5 Common Types of Screw Jack
Commonly used screw jacks are as shown below

21. 21
(a)
(b)

22. 22
Figure 2.2: Examples of mechanical jacks
(a) Floor Jack (b) Scissor jack
A screwjack isa device thatliftsheavyequipment.The mostcommonformisa car jack, floor
Jack or garage jackwhichliftsvehiclessothatmaintenance canbe performed.Carjacksusuallyuse
mechanical advantage toallowahumanto lifta vehicle by manual force alone.Screw jacksare usually
ratedfor maximumliftingcapacity.There are several typesof mechanical jacks:
Scissorjack,floorjack, scaffolds,bottlejacketc.
Advantages
ü they are self-locking.
ü Theyare simple todesign.

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ü They are cheapand affordable.
ü Theycan liftsmoderatelyloadslike carswithverylessforce.
Disadvantages
ü Theyshouldalwaysbe lubricated.
ü Theycannot be usedto liftorsupportveryheavyloads.
2.6 Factors to ConsiderinSelectionof the BestJackforApplicationPurposes
1. Considerthe loadcarryingcapacity of the liftingscrew (columnload) whenjacksare
Loadedincompression.Howhighdoyouneedto liftthe load?One mustchoose a jack whose lifting
screwis stoutenoughtohandle the loadat full rise.
2. Considerthe travel speedof the dynamicload.The speedatwhichthe loadwill be movedisalimiting
factor. Howfast doyou needtomove the load?Sometimesdouble leadmachine screw jacksorball
screwjacks are a betterchoice ina givenapplication.
3. How frequentlywill the jackneedtomove the load?Rememberthatheatbuildsupbetweenthe
machine screwsandnut duringnormal operation. Dutycyclesformachine screw jacksmustinclude
periodsof restto dissipate thatheat.

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HAPTER 3
MATERIALS SELECTION
3.0 Introduction
Material selectionisanimportantprocessindesignprocesses.Selectingmaterialsisaprocessthatis
design-ledinthatthe material selectionprocessusesthe designrequirementsasthe inputsoas to
come up withmaterialsthathave the desiredpropertiesforthe partto be designedtofunctionwell.
3.1 Engineering Materials for Components
The common engineering materials used in making machine components include;
Cast iron,
Steel (all typesof steel),
Copperand itsalloys,
Aluminumandits alloys,
Plastics.
Therefore,the rightmaterialsforthe designof the screw jackpartsshouldbe selected.Selection
requiresone toconsiderthe followingfactorswhichgive the bestmaterial fitforthe designjob:
a) Specificstrengthandmass.
It ispreferable toselectamaterial of highyieldstresswithabilitytocarry external loadwithoutfailure
and lowdensityinordertorealize ascrew shaftof highstrengthandlow mass.Therefore,the material
selectionprocessshouldaimtomaximize the quantitytermedasthe specificstrength.

25. 25
b) Resistance toabrasive wear.
Most of engineeringmaterialsincontactwithone anotherare subjectedtosurface weardue torelative
motion.Itis therefore desirable toselectamaterial fromthe candidate materialswithlow wearrate or
capacityto resistabrasive wearat the threadsurfaces.
c) Resistance tobuckling.
Heavyloadsmay cause the screwto buckle once the critical loadis exceeded.Itispreferable toselecta
material withhighresistance tobucklingof the screw,thatis,excellentelasticityanddeflectionbehavior
inresponse toapplicationof an external load.
d) Availability,CostandAffordability.
It isalso preferabletochoose amaterial withthe highestaffordabilityrating. Relative costof the
materialsisusedinfindingorcalculatingthe affordable rates.Therefore,the availabilityof the material
and the cost of processingthe material intothe finishedproductneedtobe takenintoaccount and
consideredassupporting informationwhenmakingthe final choice of the material.
e) Heat transmissionproperties.
As we knowthere alwaysarelative motionbetweenscrew andnut,whichcause a frictionthat
generatesheatwhichcancause change in the mechanical propertiesof the material.
f) Otherrelevantpropertiesinclude;resistance tocorrosion,electrical andmechanical properties,heat
transmissionpropertiesetc.
3.2 Steps for Selection of Materials for Components
Selectionof materialsinengineeringdesigninvolvesthe following steps:
Translationof designrequirementsintospecificationsforamaterial.
Screeningoutthose materialsthatdonot meetthe specificationsinordertoleave onlythe viable
candidates.
Rankingof the survivingmaterialstoidentifythose thathave the greatestpotential.
Usingsupportinginformationtofinallyarrive at the choice of material tobe used.
The firstthree stepsinvolve mathematicalanalysis,use of variouschartsandgraphs of specificproperty
such as specificstrength,wearresistance,bucklingresistance andaffordability.The materialsare
compared,rankedasper the indicesof meritandavailable supportinginformationisusedtoreachthe
final decision(Ashby,2005).

26. 26
In thisproject,informationfromcase studiesonpreviousdesignsof similarproductsisusedinmaterial
selectionforthe screwjackcomponents/parts.However,otherfactorssuchas availabilityof the
candidate materials,purchase price of the candidate materials,manufacturingprocessesandproperties,
formsand sizesinwhichthe materialsare availableare alsoconsidered.

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3.3 Components and their Specific Materials Selected
The goal of material selectionistocome up withan appropriate material thatbestmeetsthe design
requirements.The approachistoidentifythe connectionbetweenfunctional requirementsandthe
material propertiessoasto helpus reduce the numberof candidate materialsfromwhichtoselect
from.
The followingare componentsandmaterialsrequiredinthe designof apowerscrew (screw jack):
3.3.1 Frame (Body)
Most of the framesare in conical shape andhollow internallytoaccommodate boththe nutand screw
assembly.The frame workstoensure thatthe screw jack issafe and has a complete restonthe ground.
The purpose of the frame isto supportthe screw jack and enable ittowithstandcompressive load
exertedonit.
The frame is a bit complex andthusrequirescastingasa manufacturingprocess.Forthisreason,grey
cast ironas a material isselectedforthe frame.Thisisalsoevidentfromthe case study onprevious
designof the same product(Nyangasi,18 December,2006). Cast ironischeap andit can give any
complex shape withoutinvolvingcostlymachiningoperations.Castironhashighercompressive strength
comparedto steel.Therefore,itistechnicallyandeconomicallyadvantageoustouse castiron forthe
frame.Graphite flakescastironwithan ultimate tensilestrengthof 220MPa is consideredsuitablefor
the designof the frame.The graphite flakesimprovethe abilitytoresistcompressive load.
Mechanical properties BritishStandardSpecification
Tensile strength(MPa) 220
Compressive strength(MPa) 766
Shearstrength(MPa) 284
Endurance limit(MPa) 96
Young’smodulus(GPa) 89 – 114
Modulusof rigidity(GPa) 36 – 45
Hardnessnumber(HB) 196
Table3.1: MechanicalPropertiesof Castiron – Appendix A (Marshek,2012

28. 28

29. 29
3.3.2 Screw
The screw issubjectedto tensional moment,compressive force andbendingmoment.The screw profile
issquare type because of itshigherefficiencyandself-lockingbutnotcomparedto trapezoidal threads.
Square threadsare usuallyturnedonlathesusingasingle pointcuttingtool alsosquare threadsare
weakat the root and thisleadstouse of free cuttingsteel.Screwsare usuallymade of steelwhere great
resistance toweatherorcorrosionisrequired.Mostfastenersclose to90% use carbon steel because
steel hasexcellentworkability,offersabroadrange of attainable combinationsof strengthproperties
and itis lessexpensive.Mediumplaincarbonsteel canbe heattreatedforthe purpose of improving
propertiessuchashardness,strength(tensileandyield),the desiredresultsare therefore obtained
(Fasteners,2005).This leadstothe use of plaincarbonsteels.
Table3.2: MechanicalPropertiesof Plain carbon steel – Appendix B(Nyanja’s,18December, 2006)
3.3.3 Nut
There existsarelative motionbetweenthe screw andthe nutwhichcausesfriction,frictioninturn
causeswearof the material usedforscrew and nut.Therefore,itrequiresone of the twomembersto
be softer.A suitable material forthe nutistherefore phosphorbronze whichisacopperalloywithsmall
percentage of leadandhas the followingadvantages;
Goodcorrosionresistance.
Lowcoefficientof friction.
Hightensile strength.
Bronze has 0.2% phosphorto increase tensile strengthandthe yieldstressesmaybe takenas;tension=
125MPa, compression=150MPa, yieldstressinshear= 105MPa withsafe bearingpressure of 15MPa,
ultimate tensile strengthis190MPa anda coefficient of frictionof 0.1.

30. 30
Table3..3: SafeBearing PressuresforPowerscrews – Appendix C(Nyangasi,18 December,2006) &
(Gupta,2005
3.3.4 Handle
The handle is subjected to bending moments so plain carbon steel of BS 080M30 with yieldstrength
of 385MPa can also be used. It has the same mechanical properties and process as in Table 3.2.
3.4.4 Cup
Shape of cup is complex andthusrequirescastingprocess.Italsohas the same propertiesasinTable3.1.
Takinggraphite flakescastironwithan ultimate tensilestrengthof 200MPa. The graphite flakes
improve the abilitytoresistcompressiveload.
3.4.5 Set Screw and Lock nut + Washer
The purpose of the setscrewisto resistmotionof nut withscrew.The locknut + washeron the other
handis usedto provide uniformforce byenlargingthe areaunderthe actionof the force.We can use
plaincarbonsteel forbothand theyhave the same manufacturingprocessandpropertiesasinTable 3.2
CLASSIFICATION OF SCREW THREADS
3.1 Introduction
Screw jacks commonly use various forms of threads, namely; square threads, ISO metric trapezoidal
threads and buttress thread.
3.1.1 Square Thread
As the name suggest,ithasa square cross sectionof the thread.Itis the mostcommon formusedby
the screwjack and usedespeciallyinhighloadapplications.
3.1.1.1 Nomenclature of Square Thread

31. 31
`
Figure 3.1: Nomenclatureof squarethread
3.1.1.2 Advantages of the Square Thread
The advantagesof square threadsare as follows:
(i) Theyhave highefficiency.
(ii) Theyhave lowerfrictioncoefficienthence lesspowerlossinliftingthe load.
(iii)Motionof the nutisuniformsince there isnoside thrustandradial pressure onthe nut.
3.1.1.3 Disadvantages of Square Thread
The disadvantagesof square threadsare as follows:
(i) The threadsare usuallyturnedonalathe machine witha single pointcuttingtool hence expensive
comparedto machiningwithmulti-pointcuttingtools.Thismakesthemmore difficulttomanufacture.
(ii) The strengthof a screwdependsuponthe threadthicknessatthe core diameter.Square threads
have lessthicknessatcore diameterthantrapezoidal threads.Thisreducesthe loadcarryingcapacityof
the screw.
(iii) Itisnotpossible tocompensate forwearinsquare threadssince wearof the threadsurface
becomesaseriousprobleminthe service life of the powerscrew.Therefore,replacementof the nutor
the screwis requiredwhenwornout.
Applications: Square threads are used for screw-jacks and presses.

32. 32
3.1.2 ISO Metric Trapezoidal Threads
These are threadswithtrapezoidal outline profile.Theyare mostcommonlyusedforleadscrews.They
offerhighstrengthandease of manufacture.
3.1.2.1 Nomenclature of ISO Metric Trapezoidal Thread
Figure 3.2: Nomenclature of ISO metric trapezoidal thread
3.1.2.2 Advantages of the Trapezoidal Thread
(i) Theyare cheap to manufacture ascomparedto square threads.Multi-pointcuttingtoolsare
employedformachiningcomparedtosingle pointcuttingtoolsthatare usedinmachiningsquare
threads.
(ii) The trapezoidal threadhasgreaterthicknessatcore diameterthanthat of the square thread.
Therefore,ascrewwithtrapezoidal threadsisstrongerthananequivalentscrew withsquare threads.
Such a screwhas large loadcarrying capacity.
(iii) The axial wearonthe surface of the trapezoidal threads canbe compensatedbymeansof a split-
type of nut. The nut iscut into twoparts alongthe diameter.Aswearprogresses,the loosenessis
preventedbytighteningthe twohalvesof the nuttogether.The split-type nutcanbe usedonlyfor
trapezoidal threads.Itisusedinlead-screw of lathe tocompensatewearatperiodicintervalsby
tighteningthe twohalves.
3.1.2.3 Disadvantages of Trapezoidal Threads
The disadvantagesof trapezoidal threadsare asfollows:
(i) The efficiencyof trapezoidalthreadsislessthanthatof square threads.

33. 33
(ii) Trapezoidal threadsresultinside thrustorradial pressure onthe nut.The radial pressure orbursting
pressure onthe nut affectsitsperformance.
Application: Trapezoidal andacme threadsare usedfor lead-screw andotherpowertransmission
devicesinmachine tools.
3.1.3 Buttress Thread
Figure 3.3: Nomenclature of buttress thread
3.1.3.1 Advantages of Buttress Thread
The advantagesof buttressthreadsare as follows:
(i) It hashigherefficiencycomparedtotrapezoidalthreads.
(ii) Itcan be economicallymanufacturedonathreadmillingmachine.
(iii) The axial wearatthe threadsurface can be compensatedbymeans of split-typenut.
(iv) A screwwithbuttressthreadsisstrongerthanequivalentscrew witheithersquare threadsor
trapezoidal threads.Thisisbecause of greaterthicknessatthe base of the thread.
3.1.3.2 Disadvantages of Buttress Thread
The buttressthreadshave one disadvantage.Theycantransmitpowerandmotiononlyinone direction
as comparedto square and ISOmetrictrapezoidal threads,whichcantransmitforce and motioninboth
directions.
Application:Buttressthreadsare usedinvices,where force isappliedonlyinone direction. 12

34. 34
3.2 Thread Series
There are three standardthreadseriesinthe unifiedscrew threadsystem;
Fine series
Coarse series
Normal series
Fine threadserieshave more threadsperaxial distance andthushave asmallerpitchwhile coarse
threadserieshave alarge pitch (fewerthreadsperaxial distance).Thisshowsthatfine seriesthreads
are strongeras comparedto coarse threadseriesof the same dimensions(diameter) (Fasteners,2005).
Fine serieshasadvantagesoverthe otherseries,these are;
Theyhave large stressareashence are strong incompression.
Theyhave a largerminordiameterwhichdevelopshigher tensionalandshearstrength.
Theyhave smallerhelix angle therefore permittingcloseradjustmentaccuracy.
3.3 Definition of Screw Thread Basic Terms

35. 35
Figure 3.4: Screw Nomenclature(Bhandari,2010
The terminologiesof the screwthreadare definedasfollows(Gupta,2005):
(i) Pitch(𝒑)
The pitch isdefinedasthe distance measuredparalleltothe axisof the screw from a pointonone
threadto the correspondingpointonthe adjacentthread.
(ii) Lead(𝒍)
The leadis definedasthe distance measuredparallel tothe axisof the screw that the nutwill advance in
one revolutionof the screw.
For a single threadedscrew 𝒍=𝒑
For a double threadedscrew 𝒍=𝟐𝒑
(iii) Nominal orOutside Diameter (𝒅𝒐)

36. 36
It isthe largestdiameterof the screw.Itisalsocalledmajordiameter.
(iv) Core or MinorDiameter(𝒅𝒄)
It isthe smallestdiameterof the screwthread. 𝑑𝑐=𝑑𝑜−𝑝
(v) Mean Diameter(𝒅𝒎)
𝑑𝑚=(𝑑𝑜+𝑑𝑐)/2 𝑑𝑚=𝑑𝑜−0.5𝑝
(vi) Helix Angle(𝜶)
It isdefinedasthe angle made bythe helix of the threadwitha plane perpendiculartothe axisof the
screw.The helix angle isrelatedtothe leadandthe meandiameterof the screw.
Takingone threadof the screwand unwinding,one completeturnisdeveloped.The threadwill become
the hypotenuse of aright-angledtriangle withthe base 𝜋𝑑𝑚 andheightbeingequaltothe lead 𝑙.
Figure3.5: Unwound thread
Thisright-angledtrianglegivesthe relationshipbetweenthe helix angle,meandiameterandlead,which
can be expressedinthe followingform: tan𝛼= 𝑙/𝜋𝑑𝑚
Where 𝛼= 𝑇ℎ𝑒 ℎ𝑒𝑙𝑖𝑥 𝑎𝑛𝑔𝑙𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑡ℎ𝑟𝑒𝑎𝑑.
The followingconclusionscanbe drawnon the basisof the developmentof thread:
𝛼asthe angle of inclination.
𝑊 alwaysactsin the vertical downward direction.Whenthe load 𝑊israised,itmovesupthe
inclinedplane.Whenthe load 𝑊islowered,itmovesdownthe inclinedplane.

37. 37
𝑊 israisedor loweredbymeansof animaginaryforce 𝑃 actingat the meanradiusof the
screw.The force 𝑃 multipliedbythe meanradius(𝑑𝑚/2) givesthe torque 𝑇requiredtoraise orlower
the load.Force 𝑃 isperpendiculartoload 𝑊.
3.4 Torque Requirement - LiftingLoad
The screw isconsideredasaninclinedplane withinclination 𝛼whenthe loadisbeingraised.The
followingforcesactat a pointon thisinclinedplane:
Figure3.6: Force diagramforlifting load
Load 𝑾: It alwaysactsin the vertical downwarddirection.
Normal reaction 𝑵: It acts perpendicular(normal) tothe inclinedplane.
Frictional force 𝝁𝑵:Frictional force actsopposite tothe motion.Since the loadismovingupthe inclined
plane,frictional force actsalongthe inclinedplane indownwarddirection.
Effort 𝑷: The effort 𝑃 acts ina directionperpendiculartothe load 𝑊.It may act towardsthe rightto
overcome the frictionandraise the load.
Resolvingforceshorizontally,
𝑃=𝜇𝑁cos𝛼+𝑁sin𝛼 (3.0)
Resolvingforcesvertically,
𝑊=𝑁cos𝛼−𝜇𝑁sin𝛼 (3.1)
Dividingequation(3.0) 𝑏𝑦(3.1) we get:
𝑃=(𝜇cos𝛼+sin𝛼)(cos𝛼−𝜇sin𝛼) (3.2)
Dividingthe numeratoranddenominatorof the righthandside of equation(3.2) by 𝑐𝑜𝑠 𝛼 we get:
𝑃=(𝜇+tan𝛼)(1−𝜇tan𝛼) (3.3)
The coefficientof frictionμcanbe expressedasfollows:
𝜇=tan𝜃 (3.4)

38. 38
Where
𝜃 = the frictionangle.
Substituting(3.4) intoequation (3.3),
𝑃=(tan𝜃+tan𝛼)/(1−tan𝜃tan𝛼) (3.5)
𝑃=𝑊tan(𝛼+𝜃) (3.6)
The torque 𝑇 requiredtoraise the loadis givenby: 𝑇=𝑃×𝑑𝑚/2
Whence
𝑇=[𝑊tan(𝛼+𝜃)]𝑑𝑚/2 (3.7)
3.5 Torque Requirement - LoweringLoad
Whenthe loadis beinglowered,the following forcesactat a pointonthe inclinedplane:
Load 𝑾: It alwaysactsin the vertical downwarddirection.
Normal reaction 𝑵: It acts perpendicular(normal) tothe inclinedplane.
Frictional force 𝝁𝑵:Frictional force actsopposite tothe motion.Since the loadismovingdownthe
inclinedplane,frictional force actsalongthe inclinedplane inthe upwarddirection
Figure 3.7: Force diagramforlowering load
Effort 𝑷: The effort 𝑃 acts ina directionperpendiculartothe load 𝑊.It shouldact towardsleftto
overcome the frictionandlowerthe load.
Resolvinghorizontally,
𝑃 = 𝜇 𝑁 𝑐𝑜𝑠 𝛼 − 𝑁 𝑠𝑖𝑛 𝛼 (3.8)
Resolvingvertically,
𝑊 = 𝑁 𝑐𝑜𝑠 𝛼 + 𝜇 𝑁 𝑠𝑖𝑛 𝛼 (3.9)

39. 39
Dividingexpression(3.8) by(3.9) we get as follows:
𝑃=(𝜇cos𝛼−sin𝛼)/(cos𝛼+𝜇sin𝛼)
(3.10)
Dividingthe numeratoranddenominatorof the righthandside of equation(3.10) by cos α:
𝑃=(𝜇−tan𝛼)/(1+𝜇tan𝛼)
(3.11)
Substitutingequation(3.4) intoEquation(3.11),
𝑃=(tan𝜃−tan𝛼)/(1+tan𝜃tan𝛼)
(3.12)
Whence
𝑃 = 𝑊 𝑡𝑎𝑛 (𝜃 − 𝛼) (3.13)
The torque 𝑇 requiredtolowerthe loadisgivenby, 𝑇=𝑃×𝑑𝑚/2
Whence
𝑇=[𝑊tan(𝜃−𝛼)]𝑑𝑚/2
(3.14)
3.6 OverHaulingandSelf-LockingScrews
From equation(3.14),we knowtorque requiredtolowerloadisgivenby: 𝑇=[𝑊tan(𝜃−𝛼)]𝑑𝑚/218

40. 40
Case 1: When 𝜽<𝛼
The torque requiredtolowerthe loadbecomesnegative.Thisindicatesaconditionthatnoforce is
requiredtolowerthe loadandthe load itself will begintoturnthe screw and descenddown,unlessa
restrainingtorque isapplied.Thisconditioniscalledoverhauling of thescrew.
Case 2: When 𝜽>𝛼
The torque requiredtolowerthe loadbecomespositive.Underthiscondition,the loadwill notturnthe
screwand will notdescendonitsownunlesseffort 𝑃isapplied.Thisconditioniscalled self- locking.
The rule for self-lockingscrewstatesthat:A screw will be self-locking if the coefficientof friction is equal
to or greaterthan the tangentof thehelix angle.
For self-lockingscrew,tan𝜃≥tan𝛼
Or 𝜇≥𝑙/𝜋𝑑𝑚
Therefore,the followingconclusionare made:
(i) Self-lockingof the screwisnotpossible whenthe coefficientof friction(μ) islow.The coefficientof
frictionbetweenthe surfacesof the screw andthe nut isreducedbylubrication.Excessive lubrication
may cause the loadto descendonitsown.
(ii) The self-lockingpropertyof the screw islostwhenthe leadislarge.The leadincreaseswithnumber
of starts.For double-startthread,leadistwice of the pitchandfor triple threadedscrew,three timesof
pitch.Therefore,the single threadedscrew isbetterthanmultiple threadedscrewsfromself-locking
considerations.Self-lockingconditionisessential inapplicationslike screw jack(Naik,Apr15,2015).

41. 41
3.7 Efficiencyof the Square ThreadedScrew
ReferringtoFigure 3.6: Force diagramfor liftingthe load,the outputconsistsof raisingthe loadif the
load 𝑊 movesfromthe lowerendto the upperendof the inclinedplane.
Therefore, 𝑊𝑜𝑟𝑘 𝑜𝑢𝑡𝑝𝑢𝑡= 𝐹𝑜𝑟𝑐𝑒 𝑥 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑒𝑑 𝑖𝑛 𝑡ℎ𝑒 𝑑𝑖𝑟𝑒𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑓𝑜𝑟𝑐𝑒
𝑊𝑜𝑟𝑘 𝑜𝑢𝑡𝑝𝑢𝑡= 𝑊 𝑥 𝑙
The inputconsistsof rotatingthe screw bymeansof an effort P. 𝑊𝑜𝑟𝑘 𝑖𝑛𝑝𝑢𝑡= 𝐹𝑜𝑟𝑐𝑒 𝑥 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑒𝑑 𝑖𝑛 𝑡ℎ𝑒 𝑑𝑖𝑟𝑒𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑓𝑜𝑟𝑐𝑒
𝑊𝑜𝑟𝑘 𝑖𝑛𝑝𝑢𝑡= 𝑃 𝑥 (𝜋𝑑𝑚)
The efficiency 𝜂of the screwisgivenby,
𝜂=𝑤𝑜𝑟𝑘 𝑜𝑢𝑡𝑝𝑢𝑡𝑤𝑜𝑟𝑘 𝑖𝑛𝑝𝑢𝑡 (3.15b)
Thisequationcanalso be expressedas:
𝜂=𝑤l/(𝜋𝑑𝑚) (3.15c)
And
tan𝛼=𝑙/𝜋𝑑𝑚
Therefore
𝜂=𝑊tanα/𝑃 (3.15d)
Substitutingfor 𝑃=𝑊tan(𝛼+𝜃) we get;
𝜂=tan𝛼/tan(𝛼+𝜃) (3.15e)
From the above equation,itisevidentthatthe efficiencyof the square threadedscrew dependsupon
the helix angle 𝛼andthe frictionangle𝜃.The followingfigureshowsthe variationof thefficiencyof the
square threadedscrewagainstthe helix angle forvariousvaluesof the coefficientof friction.The graph
isapplicable whenthe loadisbeinglifted

42. 42

43. 43
3.8 Efficiency of Self-Locking Screw

44. 44
The efficiencyof square threadedscrewisgivenby(Fromequation3.15e):
𝜂=tan𝛼/tan(𝛼+𝜃)
For self-lockingscrew 𝜃≥𝛼
Substitutingthe limitingvalue (𝜃= 𝛼) intothe equationabove
𝜂≤tan𝜃/tan(𝜃+𝜃)
(3.16a)
𝜂≤tan𝜃/tan(2𝜃)
(3.16b)
Andfrom trigonometricidentities
tan2𝜃=2tan𝜃/1−𝑡𝑎𝑛2𝜃
Substitutingfortan2𝜃intothe above expression,
𝜂≤tan𝜃/(1−𝑡𝑎𝑛2𝜃)tan(2𝜃)
(3.16c)
Simplifying
𝜂≤1/2(1−𝑡𝑎𝑛2𝜃) (3.16d)
From the above expressionwe candeduce thatthe efficiencyof self-lockingsquare threadedpower
screwis lessthan0.5 or 50%. If the efficiencyismore than50%, thenthe screw is saidto be overhauling
(Gupta,2005).
3.9 Coefficient of Friction, 𝝁
It has beenfoundthatthe coefficientof friction(𝜇) atthe threadsurface dependsuponthe
workmanshipincuttingthe threadsandonthe type of the lubricantused.Itis practicallyindependent
of the loadand dependentonrubbingvelocityormaterials.Anaverage of 0.1 can be takenfor the
coefficientof frictionwhenthe screwislubricatedwithmineral oil (Gupta,2005

45. 45

46. 46

47. 47
Chapter 4
Designing procedure for the screw jack
4.0 introduction
The generalizedadopteddesignprocedureforscrew jackto rise a loadof 3500kg, mini height=200mm,
max height=400mm
4.1 Design for Screw Shaft
Material specificationselectedforthe screw shaftisplaincarbonsteel toBritishStandard specification
BS 970 080M30, HardenedandTempered,whose propertiesare asshownin Appendix Band the
material yieldstrengthis700 MPa bothin tensionandpure compression and450 MPa inshear.
4.1.1 Core Diameter
The core diameterisdeterminedbyconsideringthe screw tobe underpure compression.Thatis;
W=cAc
Where c ispure compressionstress=700mpa
Ac is crosssectional areaof screw shaft=/4(dc)2
Dc is core diameter
There for W=c/4(dc)2dc=(4W/c/)
Takingfactor of safetyf.s=5
dc=(4W/c/5)dc=48583.75/700/5)
dc=0.008835m=8.835mm
For square threadsof fine series,the followingdimensionsof screw are selectedfrom Appendix D
(Gupta,2005) hence,,
The core diameterdc=10mm,do=12mm,pitchp=l=2mm

48. 48
 = tan=0.1, =5.71
Section of screw spindle
4.1.2 Torque required to rotate the screw
When the torquerequired to rotate thescrew is the sameto torquerequired lift the load is given by
T1PdmWtandm
dmdodc121011mm
tanldm2110.05787
ThenT1=(8583.75tan(3.312275.71)0.011)/2=7.496Nm
4.1.3 Screw Stresses
Compressive stressesduotoaxial loadsusingthe new core diameteris
c=W/Ac=W/(dc2/4)=48583.75/0.012=109.29Mpa

49. 49
The shear stressdue to thistorque usingthe new core diameterisgiven
=T1dc/2J, where Jis polarmoments=dc4/32
=16T1/dc3=167.496/0.013=38.177Mpa
4.1.4 Principal Stresses
Maximumprincipal stressisasfollows:
c max=c(c)()
c max=..43.7=
cmax=121.3068Mpa
And maximum shear stresses as follows:
max=c
max=/2..=66.66Mpa
Designvalue of  is450=90Mpa
Cheek;those maximumshearandcompressive stressesare lessthanthe permissiblestresseswhichis
safe design.
4.2 Design for Nut
4.2.1 Height of the Nut
We findthe heightof the nut (h) by consideringthe bearingpbonthe nut
Pb=W/4dodcn, where n is
numberof treadsin contact withscrewedspindle
Material specificationforthe nutisphosphorbronze whichhastensile stress=150Mpa,compressive
stress125Mpa,shear stress=105Mpa specificbearingpressurenotexceed17Mpa and =0.1

50. 50
17=...
17248.39nn14.6
Sayn15
Thenheightof the nut isas follows;
hnp152mm30mm
Check:For a safe nut height ℎ ≤ 4dc40mm
4.2.2 Stresses in the Screw and Nut
Shearstressinthe screwisas follows
Screww/ndct where t isthicknessof screw p/22/21mm
Screw8583.75/150.010.00118.215Mpa
4.2.2 Stresses in the Screw and Nut
Shearstressinthe screwisas follow
NutW/ndot ,8583.75/150.0120.00115.179Mpa
The givenvalue of  is 105/521Mpa
Check: These stressesare withinpermissiblelimit,hence,designforthe nutissafe.
4.2.3 The outer diameter of Nut
Outerdiameterof D1 is foundbyconsideringthe tearingstrengthof the nut
tW/4D12do2
t is tearingstrengththe nut Tensile stresst/f.s150/530Mpa
Thenwe get D1 as follows
308583.75//4D12122

51. 51
D122.097mm,say D123mm
4.2.4 The outside diameter of Collar
Outside diameterD2is foundbyconsideringthe crashingstrengthof the nutcollar
cW/4D22D12 where c iscrushingstrengthof the nutcompressive strength
c125/525Mpa
Thenwe get D2 as follow
258583.75/4D22232
D2 31.083mm,sayD232mm

52. 52
4.2.5 Thickness of the Nut Collar
The thicknessof nutcollar t1 is foundbyconsideringshearingstrengthof the nutcolor

53. 53
T1 W/D1
 isshearingstrengthof nut collar105/521Mpa
T18583.75/23 215.656mm,say t16mm
4.3 Designs for Head and Cup
4.3.1 Dimensions of Diameter of Head on Top of Screw and for the Cup D3
ASSUMING
D31.75do1.7512mm21mm

54. 54
The seat forthe cup ismade equal to the diameterof the headandthenchamferedat the topthe cup
preventsthe load fromrotating and isfittedwithpine of diameterD4D3/4
D45.25mm,say D46mm
Section of pin
Take lengthof pinto be 9mm.
Otherdimensionsforthe cupare takenas:
Diameteratthe topof the cup = Diameterof the head= 52mm
Heightof cup = 9mm
Thicknessof cup= 3mm
Filletradii =1mm
Figure 5.4: Section of Cup
4.3.2 Torque Required to Overcome Friction
We know that by assuming uniform pressure condition torque required to overcome friction is
given as follows;

55. 55
T2WD3D4D32D42
Where D3diameterof head21mm
D4diameterof pin6mm
T20.18583.750.0210.0060.02120.006263.90Nm
4.3.3 Total Torque Subjected to the Handle
Total torque towhichthe handle issubjectedisgivenby
TT1T2
T7.496Nm63.90Nm71.396
Activity Professional use Domestic use
Pushing 200N (20.4kg) 119N (12.1kg)
Pulling 145N (14.8kg) 96N (9.8kg)
Table 4.2: MaximalIsometricForceby General European Working Population forWhole
Body Work in a Standing Posture
Therefore takingthe force of 96N indomesticuse (J.J.Fereira,2004) thenthe lengthof the
handle required is
TFLLTF
LT/F71.396Nm/96N0.7437m743.7mm, say 744mm
The length of the handle may be fixed by giving some allowance for gripping 70mm
Therefore, the length of the handle/lever is 814mm
Section of Lever
4.3.4 Diameter of Handle/Lever
The diameterof the handle/lever,Dmaybe obtainedbendingeffects

56. 56
M32cD
Whilebtc700/5140Mpa
MForce appliedlengthof lever
M96N0.7437m71.395Nm
71.39532140106D
D0.0173m17.3mm,say D18mm
4.3.5 Height of Head
The heightof headis usuallytakenastwice the diameterof handle.
H2D
H218mm36mm

57. 57
4.3.6 Design Check against Instability/Buckling
Effective lengthof screw, 𝐿𝑒𝑓𝑓=𝐿𝑖𝑓𝑡 𝑜𝑓 𝑠𝑐𝑟𝑒𝑤+ 1/2 𝑜𝑓 ℎ𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑛𝑢𝑡
𝐿𝑒𝑓𝑓=𝐻1+ℎ/2 Leff20030/2215mm
Whenthe screw reachesthe maximumlift,itcanbe regardedasstrut whose lowerendisfixedandthe
loadendis free.Therefore,bucklingorcritical loadforthisgivenconditionisasfollows(Gupta,2005

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WcrAc.yy4cELeffk
 Wcr13199.04N
W8583.75N
4.4 Design of Body
4.4.1 Dimensions for the body of the screw
The dimension of the body may be fixed and given as in shown in the figure above (Gupta,
2005)
1. Diameterof the Bodyat the Top
D51.5D2
D51.532mm48mm

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2. Thicknessof the body
3. t20.25do, t20.2512mm3mm
3Inside Diameteratthe Bottom
D62.25D2
D62.253272mm
4. OuterDiameteratthe Bottom
D71.75D6
D71.7572mm126
5. Thicknessof Base
t32t1, t326mm12mm
6. Heightof the Body
Heightof the bodymax liftheightof nutextra50mm
200mm30mm70mm300mm
Finally,the bodyistaperedinordertoachieve stabilityof the jack.

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4.5 Efficiency of the Screw Jack
Efficiencyof screwjackisgivenasfollows:
torque requiredtorotate screw withnofrictiontorquerequiredoutput
ToT
But ToWtandm/2
To8583.750.057870.0112
To2.732Nm
And
T71.396Nm
To/T2.732/71.3960.0383.8

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4.6 result and dissection
the resultsIfindfrommy projectare listed as follows
to designindividual partsof the screw jack
1 to design body(frame)
2 to design nut
3 to design handl (Tommy bar)
4 to designs the cup
5 to design setscrew
6 to design washer
7 to designscrew
Dissection

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RESULTES OF NUMERICAL VALUE OFTHE DESIGNE
Dc 10mm LPIN 9mm D5 =48mm
Do
12mm D head 52mm t2 =3mm
H
30mm Hcup 9mm D6 =72mm
D1
23mm tcup 3mm D7 =126mm
D2
32mm Filitraduis 1mm t3 =12mm
t1
6mm Lhandl 814mm Hbody =300mm
D3
21mm Dhandl 18mm
D4
6mm Hhead 36mm

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4.7 CONCULUTION
From myprojectI am concludedthatfromintroductionpartto design analysiswe are seenclearly
itsworkingprinciple of the screwjackandoperationof the screw jack, efficiency of this designed
screw jack, methods of increasingefficiencyof the screw jack. A screw jackis an example of a
powerscrewand referredtoasa mechanical device thatcanincrease the magnitude of aneffort
force. Screwjacks are usedfor raisingandloweringplatformsandtheyprovide ahighmechanical
advantage inorderto move moderatelyheavyandlarge weightswithminimum effort.Basedon
my calculationsandassumptionsthe designedvaluesare safe.
4.8 Recommendation
From the case study,I concentratedondesignof a simple mechanical screw jackwhere the nutsfixedin
a cast ironframe and remainsstationarywhile the spindleisbeingrotatedbythe lever.Thisdesigncan
onlyworkfor lightloadshence whenascrew jack isneededforheavyloadapplicationdifferentdesigns
requiredwhere the nutisrotatedasthe spindlesmoves.Itherefore recommenddesignof ascrewjack
for the heavyloads.Irecommendedthatthe workshopsandAutoCADroomsopeninordertopractice
more.

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)
.1 Appendix A: Mechanical Properties of Cast Iron (Nyangasi, 18 December, 2006)

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6.2 Appendix B: Mechanical Properties of Steels (Nyangasi, 18 December, 2006)
Materials
British
standards
Production
process
Maximum
section
size,
mm
Yield
Strength
MPa
Tensile
Strength,
MPa
Elongation
%
Hardness
number,
HB
0.20C 070M20 HR 152 215 430 22
126 –
179
254 200 400 20
116 –
170
CD 13 385 530 12 154
76 340 430 14 125
0.30C 080M30 HR 152 245 490 20
143 –
192
254 230 460 19
134 –
183
CD 13 470 600 10 174
63 385 530 12 154
H&T 63 385 550 - 700 13
152 –
207
0.40C 080M40 HR 150 280 550 16
152 –
207
CD 63 430 570 10 165
H&T 63 385 625 - 775 16
179 –
229
0.50C 080M50 HR 150 310 620 14
179 –
229

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CD 63 510 650 10
202 –
255
H&T 150 430 625 – 775 11
248 –
302
1Cr 530M40 H&T 100 525
700 –
850
17
202 –
255
29 680 850 - 1000 13
248 –
302
1.5MnMo 605M36 H&T 150 525
700 –
850
17
202 –
255
29 755 925 - 1075 12
269 –
331
1.25NiCr 640M40 H&T 152 525
700 –
850
17
202 –
255
102 585 770 – 930 15
223 –
277
64 680 850 - 1000 13
248 –
302
29 755 930 - 1080 12
269 –
331
3NiCr 653M31 H&T 64 755
930 -
1080
12
269 –
331
- 680 850 – 000 12
248 –
302
Key: HR - Hot- Hot rolled and normalized
CD - Cold drawn
H&T - Hardened and tempered

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Appendix C: Safe Bearing Pressure for Power Screws (Gupta, 2005)
Type of power screw Material Safe bearing
pressure,MPa
Rubbing speed
m/s
screw Nut
Hand press Steel Bronze 17.0-24.1 Low speed
,well
lubricated
Screw jack Steel Cast Iron 12.0-17.0 Low speed <
2.5
Screw jack Steel Bronze 11.0-17.0 Low speed < 3
Hoisting screw Steel Cast Iron 4.0-7.0 Medium speed
(6-12)
Hoisting screw Steel Bronze 5.5-10.0 Medium speed
(6-12)

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References
1. Ashby, M. F., 2005. Material Selection in Mechanical Design. 3rd ed. New York: Pergamon Press
.
2. Bhandari, V. B., 2010. Design of Machine Elements. Third Edition ed. New Delhi: Tata McGraw-
Hill Education.
3. Collection, J., 2015. hubpages.com› Autos › Automobile History. [Online] Available at:
https://www.history of screw jacks.com [Accessed 11 November 2015].
4. Fasteners, C. o., 2005. Technical Reference Guide. Ninth Edition ed. Winona, Minnesota: Fastenal
Industrial & Construction Supplies.
5. Gupta, R. K. &. J., 2005. Theory of Machines. Revised Edition ed. Punjab, India: S. Chand and
Company

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“IF MECHANICAL AT REST
WORLED BECOMS RUST”

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“MECHANICAL ENGINERING DEPARTMENT IS THE
POWER OF THE WORLED”
“ THE WORLED IS NULL WITH OUT MECHANICAL
ENGINEER”

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