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SCHOOL OF ENGINEERING SCHOOL OF ENGINEERING

DEPARTMENT OF MECHANICAL ENGINEEERING DEPARTMENT OF MECHANICAL ENGINEEERING

PROJECT REPORT PROJECT REPORT PROJECT TITLE PROJECT TITLE SCREW JACK SCREW JACK

COURS CODE MENG 3161 COURS CODE MENG 3161

PREPARED BY: PREPARED BY: SHUSHAY HAILU SHUSHAY HAILU ID NO 4142/07 ID NO 4142/07 SECTION 2 SECTION 2 SUBMITED TO INSTRACTOR: SUBMITED TO INSTRACTOR: BERIHU BERIHU

SUBMITION DATE 30/08/2009E.C SUBMITION DATE 30/08/2009E.C

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Table

Table of of content content pagepage

Acknowledgment……… Acknowledgment……… 55 Abstract……….… Abstract……….… 66 Nomenclature……….. Nomenclature……….. 7-97-9

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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

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………..1 9 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

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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 would like to acknowledge and appreciate the great guidance from my project I would like to acknowledge and appreciate the great guidance from my project supervisor, instructor berihu

supervisor, instructor berihu I would also like to thank my pare

I would also like to thank my pare nts and classmates for their encouragement,nts and classmates for their encouragement, understanding and support throughout the entire project.

understanding and support throughout the entire project. I would also like to thank the

I would also like to thank the almighty God for bringing me this far almighty God for bringing me this far and giving me the strength to cand giving me the strength to carryarry out the project.

out the project.

I would also like to thank my friend’s

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ABSTRACT ABSTRACT a screw

a screw jack serves jack serves to give to give mechanical advantagmechanical advantage e by chanby changing rotational ging rotational force to force to linear force linear force thusthus allows

allows one to lift one to lift a load a load and support and support it it at a given at a given height. The aiheight. The aim of the pm of the project was to desiroject was to design a screwgn a screw jack that was raised 3500kg mass of car during maintenance and with a desired strength and mechanical jack that was raised 3500kg mass of car during maintenance and with a desired strength and mechanical

properties that was free from any error. properties that was free from any error.

This case study is divided into various sections that describes c

This case study is divided into various sections that describes c lassification of screw threads, designlassification of screw threads, design analysis ,result and dissection ,conclusion and recommendation parts of the screw jack

analysis ,result and dissection ,conclusion and recommendation parts of the screw jack and selection ofand selection of materials used for construction that are in agreement with current industry practice of screw jack materials used for construction that are in agreement with current industry practice of screw jack design. The design procedure adopted here is

design. The design procedure adopted here is from design of machine elements 1 and 2 from design of machine elements 1 and 2 A factor ofA factor of safety of 5 and above should be used in

safety of 5 and above should be used in this design to reduce high chances of failure due this design to reduce high chances of failure due to dynamicto dynamic loadings and impact loadings. Dynamics loading is as a result of external inte

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short time and results into high stresses w

short time and results into high stresses w hich can cause failure hence these hich can cause failure hence these calls for a high factor ofcalls for a high factor of safety.

safety.

Nomenclature

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p - Pitch of screw thread (mm) p - Pitch of screw thread (mm)

n - Number of threads in

n - Number of threads in contact with screwed spindlecontact with screwed spindle

l - Lead of screw thread (mm) l - Lead of screw thread (mm)

t - Thickness of screw t - Thickness of screw

d - Nominal diameter of screw (mm) d - Nominal diameter of screw (mm)

d c - Core diameter of screw (mm) d c - Core diameter of screw (mm)

d m - Mean diameter of screw (mm) d m - Mean diameter of screw (mm)

θ

θ - Friction angle (degree)- Friction angle (degree)

α

α - Helix angle of screw (degree)- Helix angle of screw (degree)

W- Load (kg) W- Load (kg)

N - Normal reaction (Newton, N) N - Normal reaction (Newton, N)

μ − Coefficient of friction μ − Coefficient of friction P - Effort (Newton, N) P - Effort (Newton, N) T - Torque (N. m) T - Torque (N. m) η − Efficiency (%) η − Efficiency (%)

F load - The force the jack exerts on the load. (Newton, N) F load - The force the jack exerts on the load. (Newton, N)

F effort - The rotational force exerted on the handle of the jack. (Newton, N) F effort - The rotational force exerted on the handle of the jack. (Newton, N)

r-the length of the j

r-the length of the j ack handle (mm)ack handle (mm)

M. A

M. A – – Mechanical advantage Mechanical advantage

π =

π = 3.1415926543.141592654

BS

BS – – British standards British standards

σ c

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A c - Cross sectional area of the screw shaft

σ c(max) -Maximum principal stress

τ( max)- Maximum shear stress

J - Polar moments

P b - Bearing pressure on the nut

t 1 - Thickness of nut collar

h - Height of the nut

D 1 - Outer diameter of nut collar

D 2 - Outside diameter of nut collar

σ t - Tearing strength of the nut

σ c - Crushing strength of the nut

τ (screw) -Shearing stress on the screw

τ (nut) - Shearing stress on the nut

Τ-Shearing stress of nut collar

D 3 - Diameter of head on top of screw

D 4 - Diameter of pin

T-Total torque to which the handle is

Subjected

T 1 - Torque required rotating the screw

T 2 –Torque required overcoming

Friction

T- Total torque subjected to handle

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L – Length of the handle

D - Diameter of handle

M - Bending moment

H - The height of head

σ b - Bending stress

L eff - Effective length of screw

H 1 – Lift of screw

W cr - Buckling or Critical load

E – Young’s modulus or modulus of elasticity

C - End fixity coefficient

R- Slenderness ratio

k - The radius of gyration

HB – Hardness number

I − Moment of inertia of the cross section.

D 5 - Diameter of body at the top

t 2 - Thickness of body

t 3 - Thickness of base

D 6 - Inner Diameter at the bottom

D 7 - Outer Diameter at the bottom

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CHAPTER ONE

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1.0 Introduction

Screw jack is also called jack screw in other terms. A screw jack is an example of a power screw and referred to as a mechanical device that can increase the magnitude of an effort force. Screw jacks are used for raising and lowering platforms and they provide a high mechanical advantage in order to move moderately heavy and large weights with m inimum effort. They function byturning the lead screw when raising or lowering of loads. Screw jack is found everywhere is need to lift, position align and hold, to amplify force

1.1 Working principal

A screw jack consists of a screw and a nut. The nut is fixed in a cast iron frame and remains stationary. The rotation of the nut inside the frame is prevented by pressing a set screw against it. The screw is rotated in the nut by means of a handle, which passes through a hole in the head of the screw. The head carries a platform, which supports the load and r emains stationary while the screw is being rot ated. A washer is fixed to the other end of the screw inside the frame, which prevents the screw from being completely turned out of the nut

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1.2 Problem statement

There is one problem that the researcher observes in the environment .during tripe cars may reach the end life time their wheels at that time drivers need maintenance their cares . There for the researchers design is to lift 3500kg of car until the height of 200mm.

1.3 OBJECTIVE

Objective of this design is to overcome the problem of statement.

1 General objective _to design and modal screw jack

2 specific objective _to designs

_to select materials

_to draw 2D and 3D

_to outline dimensions

1.4 methodologies: I use books, like gupta

Ashby,m.f 2005.material selection in mechanical designe.3rd_{ed. New York}

Like bhandari,v.b., 2010.design of machine e lements.

Like strength of material

Like material science

Like internet source

1.5 design concepts: I use the concepts cost, strength, mechanism, mechanical

Properties, creep, fatigue, physical properties, thermal properties.

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1.6 SCOPE AND LIMITION

Based on that design we are lifting only 3500kg .more than this mass is impossible

The development of screw jack is only prototype not ready functioning as commercial.

The developed screw jack is only for normal person.

The developed screw jack is only operated on afloat surface.

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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Collection of input data from research work.

. Study of weight-dimensional parameters

. Study of stresses, deformations in lift

. Study of Vibration and impact resistance.

. Study of Keeping of service life at different loading

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CHAPTER 2

LITERATURE REVIEW 2.0 Introduction

Screw jack is also called jack screw in other terms. A screw jack is an example of a power screw and referred to as a mechanical device that can increase the magnitude of an effort force. Screw jacks are used for raising and lowering platforms and they provide a high mechanical advantage in order to move moderately heavy and large weights with minimum effort. They function by turning the lead screw when raising or lowering of loads.

2.1 Operation

The jack can be raised and lowered with a metal bar that is inserted into the jack. The operator turns the bar with his/her hands in a clockwise direction. This turns the screw inside the jack and makes it go up. The screw lifts the small metal cylinder and platform that are above it. As the j ack goes up, whatever is placed above it will raise as well, o nce the jack makes contact. The bar is turned until the jack is raised to the required level. To lower the jack the bar is turned in the opposite direction.

2.2 Construction of a Screw Jack

A screw jack consists of a screw and a nut. The nut is fixed in a cast iron frame and remains stationary. The rotation of the nut inside the frame is prevented by pressing a set screw against it. The screw is rotated in the nut by means of a handle, which passes through a hole in the head of the screw. The head carries a platform, which supports the load and remains stationary while the screw is being rotated. A washer is fixed to the other end of the screw inside the frame, which prevents the screw from being completely turned out of the nut.

2.3 Advantages and Disadvantages of the Screw Jack

2.3.1 Advantages

The load can be kept in lifted position since the screw jack is se lf-locking. This means it remains motionless where it was left when the rotational force on the screw is withdrawn. It will not rotate backwards regardless of size of the weight. Screw jacks also lift or raise the moderate heavy weights against gravity and uses very small handle force that can be applied manually.

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2.3.2 Disadvantages

The major disadvantage of the screw jack is that chances of dropping, tipping or slipping of the load are high and can cause serious accidents hence the device is termed as not safe fail.

5 Accidents caused by screw jack are due to the following reasons:

(a) Improper securing of load on the jack.

(b) Overloading.

(c) Off center of axis of the jack with respect to center of gravity hence not ideal for side loads.

(d) Placing the jack on a soft ground and unleveled surface.

(e) Using the jack for wrong purpose instead of using it for the purpose for which it is designed.

Precaution: Long lifts should be avoided since they can cause serious overheating and generate a large amount of heat. It should therefore be used under ambient temperatures with the use of the required lubricants. Design and manufacturer’s instructions such as speed, load capacity and recommended temperatures must be followed to avoid accidents. Always keep t he mating surfaces clean after use and check for wear and damage on the surfaces.

2.4 Mechanical Advantage (M.A)

The mechanical advantage of a screw jack can be referred to as the ratio of the force the jack exerts on the load to the input force on the lever, neglecting friction. However, most screw jacks have large

amounts of friction which increase the required input force , so the actual mechanical advantage is often only 30% to 50% of this figure (Bhandari, 2010).

M. A = F load /F effort

Where

F load = The force the jack exerts on the load

F effort = The rotational force exerted on the handle of the jack

2.5 Common Types of Screw Jack

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(a)

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Figure 2.2: Examples of mechanical jacks

(a) Floor Jack (b) Scissor jack

A screw jack is a device that lifts heavy equipment. The most common form is a car jack, floor

Jack or garage jack which lifts vehicles so that maintenance can be per formed. Car jacks usually use mechanical advantage to allow a human to lift a vehicle by manual force alone. Screw jacks are usually rated for maximum lifting capacity. There are several types of mechanical jacks:

Scissor jack, floor jack, scaffolds, bottle jack etc.

Advantages

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ü They are cheap and affordable.

ü They can lifts moderately loads like cars with very less force.

Disadvantages

ü They should always be lubricated.

ü They cannot be used to lift or support very heavy loads.

2.6 Factors to Consider in Selection of the Best Jack for Application Purposes

1. Consider the load carrying capacity of t he lifting screw (column load) when jacks are

Loaded in compression. How high do you need to lift the load? One must choose a jack whose lifting screw is stout enough to handle the load at full rise.

2. Consider the travel speed of the dynamic load. The speed at which the load will be moved is a limiting factor. How fast do you need to move the load? Sometimes double lead machine screw jacks or ball screw jacks are a better choice in a given application.

3. How frequently will the jack need to move the load? Remember that he at builds up between the machine screws and nut during normal operation. Duty cycles for machine screw jacks must include periods of rest to dissipate that heat.

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HAPTER 3

MATERIALS SELECTION 3.0 Introduction

Material selection is an important process in design processes. Sele cting materials is a process that is design-led in that the material selection process uses the design requirements as the input so as to come up with materials that have the desired properties for the part to be designed to function well. 3.1 Engineering Materials for Components

The common engineering materials used in making machine components include; Cast iron,

Steel (all types of steel),

Copper and its alloys, Aluminum and its alloys,

Plastics.

Therefore, the right materials for the design of the screw jack parts should be selected. Selection requires one to consider the following factors which give the best material fit for the design job: a) Specific strength and mass.

It is preferable to select a material of high yield stress with ability to carry external load without failure and low density in order to realize a screw shaft of high strength and low mass. Therefore, the material

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b) Resistance to abrasive wear.

Most of engineering materials in contact with one another are subjected to surface wear due to relative motion. It is therefore desirable to select a material from the candidate materials with low wear rate or capacity to resist abrasive wear at the thread surfaces.

c) Resistance to buckling.

Heavy loads may cause the screw to buckle once the critical load is exceeded. It is preferable to select a material with high resistance to buckling of t he screw, that is, excellent elasticity and deflection behavior in response to application of an external load.

d) Availability, Cost and Affordability.

It is also preferable to choose a material with the highest affordability rating. Relative cost o f the

materials is used in finding or calculating the affordable rates. Therefore, t he availability of the material and the cost of processing the material into the finished product need to be taken into account and considered as supporting information when making the final choice of the material.

e) Heat transmission properties.

As we know there always a relative motion between screw and nut, which cause a friction that generates heat which can cause change in the mechanical properties of the material.

f) Other relevant properties include; resistance to corrosion, electrical and mechanical properties, heat transmission properties etc.

3.2 Steps for Selection of Materials for Components

Selection of materials in engineering design involves the following steps: Translation of design requirements into specifications for a material.

Screening out those materials that do not meet the specifications in order to leave only the viable candidates.

Ranking of the surviving materials to identify those that have the greatest potential. Using supporting information to finally arrive at the choice of material to be used.

The first three steps involve mathematical analysis, use of various charts and graphs of specific property such as specific strength, wear resistance, buckling resistance and affordability. The materials are

compared, ranked as per the indices of merit and available supporting information is used to reach the final decision (Ashby, 2005).

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In this project, information from case studies on previous designs of similar products is used in material selection for the screw jack components/parts. However, other factors such as availability of the

candidate materials, purchase price of the candidate materials, m anufacturing processes and properties, forms and sizes in which the materials are available are also considered.

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3.3 Components and their Specific Materials Selected

The goal of material selection is to c ome up with an appropriate material that best meet s the design requirements. The approach is to identify the connection between functional requirements and the material properties so as to help us reduce the number of candidate materials from which to select from.

The following are components and materials required in the design of a power screw (screw jack):

3.3.1 Frame (Body)

Most of the frames are in conical shape and hollow internally to accommodate both the nut and scre w assembly. The frame works to ensure that the screw jack is safe and has a complete rest on the ground. The purpose of the frame is to support the screw jack and enable it to withstand compressive load exerted on it.

The frame is a bit complex and thus requires casting as a manufacturing process. For this reason, grey cast iron as a material is selected for the frame. This is also evident from the case study on previous design of the same product (Nyangasi, 18 Decem ber, 2006). Cast iron is cheap and it can give any

complex shape without involving costly machining operations. Cast iron has higher compressive strength compared to steel. Therefore, it is technically and economically advantageous to use cast iron for the frame. Graphite flakes cast iron with an ultimate t ensile strength of 220MPa is considered suitable for the design of the frame. The graphite flakes improve the ability to resist compressive load.

Mechanical properties British Standard Specification

Tensile strength (MPa) 220

Compressive strength (MPa) 766

Shear strength (MPa) 284

Endurance limit (MPa) 96

Young’s modulus (GPa) 89 – 114 Modulus of rigidity (GPa) 36 – 45

Hardness number (HB) 196

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3.3.2 Screw

The screw is subjected to tensional moment, compressive force and bending moment. The screw profile is square type because of its higher efficiency and self-locking but not compared to trapezoidal threads. Square threads are usually turned on lathes using a single point cutting tool also square threads are weak at the root and this leads to use of free cutting steel. Screws are usually made of steel where great resistance to weather or corrosion is required. Most fasteners close to 90% use carbon steel because steel has excellent workability, offers abroad range of attainable combinations of strength properties and it is less expensive. Medium plain carbon steel can be heat treated for the purpose of improving properties such as hardness, strength (tensile and yield), the desired results are therefore obtained (Fasteners, 2005). This leads to the use of plain carbon steels.

Table3.2: Mechanical Properties of Plain carbon steel – Appendix B ( Nyanja’s , 18 December, 2006)

3.3.3 Nut

There exists a relative motion between the screw and the nut which causes friction, friction in turn causes wear of the material used for screw and nut. Therefore, it requires one of the two members to be softer. A suitable material for the nut is therefore phosphor bronze which is a copper alloy with small percentage of lead and has the following advantages;

Good corrosion resistance. Low coefficient of friction. High tensile strength.

Bronze has 0.2% phosphor to increase te nsile strength and the yield stresses may be taken as; tension = 125MPa, compression = 150MPa, yield stress in shear = 105MPa with safe bearing pressure of 15MPa, ultimate tensile strength is 190MPa and a coefficient of friction of 0.1.

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Table3..3: Safe Bearing Pressures for Power screws – 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 yield strength 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 and thus re quires casting process. It also has the same properties as in Table3.1. Taking graphite flakes cast iron with an ultimate tensile strength of 200MPa. The graphite flakes

improve the ability to resist compressive load. 3.4.5 Set Screw and Lock nut + Washer

The purpose of the set screw is to resist motion of nut with screw. The lock nut + washer on the other hand is used to provide uniform force by e nlarging the area under the action of the force. We can use plain carbon steel for both and they have the same manufacturing process and properties as in Table 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, it has a square cross section of the thread. It is the most common form used by the screw jack and used especially in high load applications.

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`

Figure 3.1: Nomenclature of square thread 3.1.1.2 Advantages of the Square Thread The advantages of square threads are as follows: (i) They have high efficiency.

(ii) They have lower friction coefficient hence less power loss in lifting the load.

(iii)Motion of the nut is uniform since there is no side thrust and radial pressure on the nut.

3.1.1.3 Disadvantages of Square Thread

The disadvantages of square threads are as follows:

(i) The threads are usually turned on a lathe machine with a single point cutting tool hence e xpensive compared to machining with multi-point cutting tools. This makes them more difficult to manufacture.

(ii) The strength of a screw depends upon the thread thickness at the core diameter. Square threads have less thickness at core diameter than trapezoidal threads. This reduces the load carrying capacity of the screw.

(iii) It is not possible to compensate for wear in square threads since wear of the thread surface

becomes a serious problem in the service life of the power screw. Therefore, replacement of the nut or the screw is required when worn out.

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3.1.2 ISO Metric Trapezoidal Threads

These are threads with trapezoidal outline profile. They are most commonly used for lead screws. They offer high strength and ease 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) They are cheap to manufacture as compared to square threads. Multi-point cutting tools are employed for machining compared to single point cutting tools that ar e used in machining square threads.

(ii) The trapezoidal thread has greater t hickness at core diameter than that of the square thread. Therefore, a screw with trapezoidal threads is stronger than an equivalent screw with square threads. Such a screw has large load carrying capacity.

(iii) The axial wear on the surface of the trapezoidal threads can be compensated by means of a split-type of nut. The nut is cut into two parts along the diameter. As we ar progresses, the looseness is prevented by tightening the two halves of the nut together. The split-type nut c an be used only for trapezoidal threads. It is used in lead-screw o f lathe to compensate wear at periodic intervals by tightening the two halves.

3.1.2.3 Disadvantages of Trapezoidal Threads

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(ii) Trapezoidal threads result in side thrust or radial pressure o n the nut. The radial pressure or bursting pressure on the nut affects its performance.

Application:Trapezoidal and acme threads are used for le ad-screw and other power transmission devices in machine tools.

3.1.3 Buttress Thread

Figure 3.3: Nomenclature of buttress thread 3.1.3.1 Advantages of Buttress Thread

The advantages of buttress threads are as follows:

(i) It has higher efficiency compared to trapezoidal threads.

(ii) It can be economically manufactured on a thre ad milling machine.

(iii) The axial wear at the thread surface can be compensated by means of split-type nut.

(iv) A screw with buttress threads is stronger than equivalent screw with either square threads or trapezoidal threads. This is because of greater t hickness at the base of the thread.

3.1.3.2 Disadvantages of Buttress Thread

The buttress threads have one disadvantage. They can transmit power and motion only in one direction as compared to square and ISO metric trapezoidal threads, which can transmit force and motion in both directions.

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3.2 Thread Series

There are three standard thread series in the unified screw thread system; Fine series

Coarse series

Normal series

Fine thread series have more threads per axial distance and thus have a smaller pitch while coarse thread series have a large pitch (fewer threads per axial distance). This shows that fine series threads are stronger as compared to coarse thread series of the same dimensions (diameter) (Fasteners, 2005). Fine series has advantages over the other series, these are;

They have large stress areas hence are strong in compression.

They have a larger minor diameter which de velops higher tensional and shear strength. They have smaller helix angle therefore pe rmitting closer adjustment accuracy.

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Figure 3.4: Screw Nomenclature (Bhandari, 2010

The terminologies of the screw thread are defined as follows (Gupta, 2005): (i) Pitch ()

The pitch is defined as the distance m easured parallel to the axis of the screw from a point on one thread to the corresponding point on the adjacent thread.

(ii) Lead ()

The lead is defined as the distance measured parallel to the axis of the screw that the nut will advance in one revolution of the screw.

For a single threaded screw = For a double threaded screw =

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It is the largest diameter of the screw. It is also called major diameter. (iv) Core or Minor Diameter ()

It is the smallest diameter of the screw thread. =− (v) Mean Diameter ()

=(+)/2 =−0.5 (vi) Helix Angle()

It is defined as the angle made by the helix of the thread with a plane perpendicular to the axis of the screw. The helix angle is related to the lead and the mean diameter of the screw.

Taking one thread of the screw and unwinding, one complete turn is developed. The thread will become the hypotenuse of a right-angled triangle with the base  and height being equal to the lead .

Figure 3.5: Unwound thread

This right-angled triangle gives the relationship between the helix angle, mean diameter and lead, which can be expressed in the following form: tan= /

Where = ℎ ℎ   ℎ ℎ.

The following conclusions can be drawn on the basis of the development of thread: The screw can be considered as an inclined plane with  as the angle of inclination.

The load  always acts in the vertical downward direction. When the load  is raised, it moves up the inclined plane. When the load  is lowered, it moves down the inclined plane.

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The load  is raised or lowered by means of an imaginary force  acting at the mean radius of the screw. The force  multiplied by the mean radius (/2) gives the torque  required to raise or lower the load. Force  is perpendicular to load .

3.4 Torque Requirement - Lifting Load

The screw is considered as an inclined plane with inclination  when the load is being raised. The following forces act at a point on this inclined plane:

Figure 3.6: Force diagram for lifting load

Load : It always acts in the vertical downward direction.

Normal reaction : It acts perpendicular (normal) to the inclined plane.

Frictional force : Frictional force acts opposite to the motion. Since the load is moving up the inclined plane, frictional force acts along the inclined plane in downward direction.

Effort : The effort  acts in a direction perpendicular to the load . It may act towards the right to overcome the friction and raise the load.

Resolving forces horizontally,

=cos+sin (3.0)

Resolving forces vertically,

=cos−sin (3.1)

Dividing equation (3.0)  (3.1) we get:

=(cos+sin)(cos−sin) (3.2)

Dividing the numerator and denominator of the right hand side of e quation (3.2) by   we get:

=(+tan)(1−tan) (3.3)

The coefficient of friction μ can be expressed as follows:

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Where

 = the friction angle. Substituting (3.4) into equation (3.3),

=(tan+tan)/(1−tantan) (3.5)

=tan(+) (3.6)

The torque  required to raise the load is given by: =×/2 Whence

=[tan(+)]/2 (3.7)

3.5 Torque Requirement - Lowering Load

When the load is being lowered, the following forces act at a point on the inclined plane: Load : It always acts in the vertical downward direction.

Normal reaction : It acts perpendicular (normal) to the inclined plane.

Frictional force : Frictional force acts opposite to the motion. Since the load is moving down the inclined plane, frictional force acts along the inclined plane in the upward direction

Figure 3.7: Force diagram for lowering load

Effort : The effort  acts in a direction perpendicular to the load . It should act towards left to overcome the friction and lower the load.

Resolving horizontally,

 =     −    (3.8)

Resolving vertically,

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Dividing expression (3.8) by (3.9) we get as follows: =(cos−sin)/(cos+sin) (3.10)

Dividing the numerator and denominator of the right hand side of e quation (3.10) by cos α: =(−tan)/(1+tan)

(3.11)

Substituting equation (3.4) into Equation (3.11),

=(tan−tan)/(1+tantan) (3.12)

Whence

 =   ( − ) (3.13)

The torque  required to lower the load is given by, =×/2 Whence

=[tan(−)]/2 (3.14)

3.6 Over Hauling and Self-Locking Screws

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Case 1: When <

The torque required to lower the load becomes negative. This indicates a condition that no force is required to lower the load and the load itself will begin to turn the screw and descend down, unless a restraining torque is applied. This condition is called overhauling of the screw.

Case 2: When >

The torque required to lower the load becomes positive. Under this condition, the load will not turn the screw and will not descend on its own unless effort  is applied. This condition is called self- locking.

The rule for self-locking screw states that: A screw will be self-locking if the coefficient of friction is equal to or greater than the tangent of the helix angle.

For self-locking screw, tan≥tan Or ≥/

Therefore, the following conclusion are made:

(i) Self-locking of the screw is not possible when the coefficient of friction (μ) is low. The coefficient of friction between the surfaces of the screw and the nut is reduced by lubrication. Excessive lubrication may cause the load to descend on its own.

(ii) The self-locking property of the screw is lost when the lead is large. The le ad increases with number of starts. For double-start thread, lead is twice of the pitch and for triple threaded screw, three times of pitch. Therefore, the single threaded screw is better than multiple threaded screws from self-locking considerations. Self-locking condition is essential in applications like screw jack (Naik, Apr 15, 2015).

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3.7 Efficiency of the Square Threaded Screw

Referring to Figure 3.6: Force diagram for lifting the load ,the output consists of raising the load if the load  moves from the lower end to the upper end of the inclined plane.

Therefore,   =      ℎ      =   

The input consists of rotating the screw by means of an effortP.   =      ℎ   

 =   () The efficiency  of the screw is given by,

=   (3.15b)

This equation can also be expressed as:

=l/() (3.15c)

And

tan=/ Therefore

=tanα/ (3.15d)

Substituting for =tan (+) we get;

=tan/tan (+) (3.15e)

From the above equation, it is evident that the efficiency of the square threaded screw depends upon the helix angle  and the friction angle. The following figure shows the variation of thefficiency of the square threaded screw against the helix angle for various values of the coefficient of friction. The graph is applicable when the load is being lifted

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The efficiency of square threaded screw is given by (From equation 3.15e): =tan/tan(+)

For self-locking screw ≥

Substituting the limiting value ( = ) into the equation above ≤tan/tan(+)

(3.16a)

≤tan/tan(2) (3.16b)

And from trigonometric identities

tan2=2tan/1−2 Substituting for tan2 into the above expression,

≤tan/(1−2)tan(2) (3.16c)

Simplifying

≤1/2(1−2) (3.16d)

From the above expression we can deduce that the efficiency of self-locking square threaded power screw is less than 0.5 or 50%. If the efficiency is more than 50%, then the screw is said to be overhauling (Gupta, 2005).

3.9 Coefficient of Friction,



It has been found that the coefficient of friction () at the thread surface depends upon the

workmanship in cutting the threads and on the type of the lubricant used. It is practically independent of the load and dependent on rubbing velocity or materials. An average of 0.1 can be taken for the coefficient of friction when the screw is lubricated with mineral oil (Gupta, 2005

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Chapter 4

Designing procedure for the screw jack

4.0 introduction

The generalized adopted design procedure for screw jack to rise a load of 3500kg, mini height=200mm, max height=400mm

4.1 Design for Screw Shaft

Material specification selected for the screw shaft is plain carbon steel to British Standard specification BS 970 080M30, Hardened and Tempered, whose properties are as shown in Appendix B and the

material yield strength is 700 MPa both in tension and pure compression and 450 MPa in shear. 4.1.1 Core Diameter

The core diameter is determined by considering the screw to be under pure compression. That is;

W=



c



Ac Where



c is pure compression stress=700mpa

Ac is cross sectional area of screw shaft=



/4(dc)



2

Dc is core diameter

8583.75/



700/5)

dc=0.008835m=8.835mm

For square threads of fine series, the following dimensions of screw are selected from Appendix D (Gupta, 2005) hence,,

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 

= tan



=0.1,



=5.71

Section of screw spindle

4.1.2 Torque required to rotate the screw

When the torque required to rotate the screw is the same to torque required 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

Then T1=(8583.75tan(3.31227



5.71)0.011)/2=7.496Nm

4.1.3 Screw Stresses

c)



(



)





max



=121.3068Mpa

And maximum shear stresses as follows:



max



=



c





max



=



/2





4



do



dc



n, where n is

number of treads in contact with screwed spindle

Material specification for the nut is phosphor bronze which has tensile stress=150Mpa, compressive stress 125Mpa,shear stress=105Mpa specific bearing pressure not exceed 17Mpa and



=0.1

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17



=



.



.



.



17



248.39



n



n



14.6

Say n



15

Then height of the nut is as follows;



4.2.3 The outer diameter of Nut

Outer diameter of D1 is found by considering the tearing strength of the nut



t



W/



4



D1



2



do



2



D1



22.097mm,say D1



23mm

4.2.4 The outside diameter of Collar

2



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4.2.5 Thickness of the Nut Collar

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ASSUMING

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The seat for the cup is made equal to the diameter of the head and then chamfered at the top the cup prevents the load from rotating and is fitted with pine of diameter D4



D3/4

D4



5.25mm,say D4



6mm

Section of pin

Take length of pin to be 9mm.

Other dimensions for the cup are taken as:

Diameter at the top of the cup = Diameter of the head = 52mm Height of cup = 9mm

Thickness of cup = 3mm Fillet radii = 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;

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T2



W



D3



D4



D3



2



D4



2

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 to which the handle is subjected is given by

71.396

Activity Professional use Domestic use

Pushing 200N (20.4kg) 119N (12.1kg)

Pulling 145N (14.8kg) 96N (9.8kg)

Table 4.2: Maximal Isometric Force by General European Working Population for Whole Body Work in a Standing Posture

Therefore taking the force of 96N in domestic use (J.J. Fereira, 2004) then the length of the handle required is



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

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M



32



c



D



While



b



t

0.7437m





30/2



215mm

When the screw reaches the maximum lift, it can be regarded as strut whose lower end is fixed and the load end is free. Therefore, buckling or critical load for this given condition is as follows (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. Diameter of the Body at the Top

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2. Thickness of the body

3. t2



0.25do, t2

12mm 6. Height of the Body

Height of the body



max lift



the results I find from my project are listed as follows to design individual parts of 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 set screw 6 to design washer 7 to design screw

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

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4.7 CONCULUTION

From my project I am concluded that from introduction part to design analysis we are seen clearly its working principle of the screw j ack and operation of the screw jack, efficiency of this designed screw jack, methods of increasing efficiency of the screw jack. A screw jack is an example of a power screw and referred to as a mechanical device that can increase the magnitude of an effort force. Screw jacks are used for raising and lowering platforms and they provide a high me chanical advantage in order to move moderately heavy and large weights with minimum effort. Based on my calculations and assumptions the designed values are safe.

4.8 Recommendation

From the case study, I concentrated on design of a simple mechanical screw jack where the nuts fixed in a cast iron frame and remains stationary while the spindle is being rotated by the lever. This design can only work for light loads hence when a screw jack is needed for heavy load application different designs required where the nut is rotated as the spindles moves. I therefore recommend design of a screw jack for the heavy loads. I recommended that the workshops and AutoCAD rooms open in order to practice more.

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)

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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

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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

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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

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