13336a79-4966-47bc-b30e-39a9cce06742-150908165005-lva1-app6892.pdf

DESIGN AND FABRICATION OF POWER SCISSOR JACK A Project Report submitted in partial fulfillment of the requirements for the award of the degree of

BACHELOR OF TECHNOLOGY (Mechanical Engineering) submitted to

KL UNIVERSITY by S.SASANK BABU BH.BALA CHANDRA REDDY M.BALA VAMSI T.BHARGAV A.KRISHNA CHAITANYA

11007065 11007291 11007292 11007294 11007296

Under the guidance of

Dr.K.V.Ramana Professor

KL UNIVERSITY Green Fields, VADDESWARAM – 522 502 Guntur District, A.P., INDIA. 2014-2015

KL UNIVERSITY Green fields, VADDESWARAM

CERTIFICATE This is to certify that the project report entitled “DESIGN AND FABRICATION OF POWER SCISSOR JACK” submitted by

S.SASANK BABU BH.BALA CHANDRA REDDY M.BALA VAMSI T.BHARGAV A.KRISHNA CHAITANYA

11007065 11007291 11007292 11007294 11007296

in partial fulfillment of the requirements for the award of the Degree Bachelor of Technology in “MECHANICAL ENGINEERING” is a bonafide record of the work carried out under my guidance and supervision at KL University during the academic year 2014-2015.

Signature of Project guide Date:

Dr.K.V.Ramana Professor

Head Mechanical Engineering Department

Internal Examiner

External Examiner

TABLE OF CONTENTS Page No.

Acknowledgement

i

Abstract

ii

List of Tables

iii

List of Figures

iii

List of graphs

iv

List of Nomenclature

v

Chapter 1: INTRODUCTION 1.1 Jack

1

1.2 Problem Statement

1

1.3 Objectives

1

Chapter 2: LITERATURE REVIEW 2.1 Various Developments in Lifting Devices

4

2.1.1 Levers

4

2.1.2 Screw threads

4

2.1.3 Gears

5

2.2 Necessity of Jack

5

2.3Types of load lifting devices

6

2.3.1 Artificial Lifting Devices

6

2.3.2 Portable Automotive Lifting Devices

6

2.4Types of Jacks Used Today

7

2.4.1 Scissor Jack

7

2.4.2Bottle (Cylinder) Jack

9

2.4.3 Hydraulic jack

9

2.5 Operational Considerations of a screw jack

10

Chapter 3: POWER SCREWS 3.1 Applications

12

3.2 Advantages

12

3.3 Disadvantages

13

3.4 Forms of Threads

13

3.4.1 Square Thread

13

3.4.1.1Advantages of square threads

14

3.4.1.2Disadvantages of square threads

14

3.4.2 Trapezoidal Threads

14

3.4.2.1 Advantages of Trapezoidal Threads

15

3.4.2.2 Disadvantages of Trapezoidal Threads

15

3.4.3 ACME Thread

16

3.5 Designation of Threads

16

3.5.1 Multiple Threaded Power Screws

17

3.6 Terminology of Power Screw

18

3.7 Self Locking Screw

19

3.8 Efficiency of Self-Locking Screw

20

Chapter 4: DESIGN 4.1Power Screw

22

4.1.1 Material Used

22

4.1.2 Assumptions

22

4.1.3 Design Calculations

23

4.2 Nut

24

4.2.1 Material Used

24

4.2.2 Design Calculations

24

4.3 Pins in Nut

25

4.3.1 Material Used

25

4.3.2 Design calculations

25

4.4 Top Arm

25

4.4.1 Material Used

25

4.4.2 Design Calculations

25

4.5 Bottom Arm

26

4.5.1 Material Used

26

4.5.2 Design Calculations

26

4.6 Top Plate

27

4.6.1 Material Used

27

4.6.2 Design Calculations

27

4.7 Bottom Plate

27

4.7.1 Material Used

27

Chapter 5: DRAWINGS 5.1 Part Drawings

28

5.1.1 Power Screw

28

5.1.2 Trunion

29

5.1.3 Top Arm

30

5.1.4 Bottom Arm

31

5.1.5 Top Plate

32

5.1.6 Bottom Plate

33

5.2 Assembly Drawing

34

5.2.1 Scissor Jack

34

5.2.2 Exploded Front View

35

5.2.3 Exploded Side View

36

5.2.4 Bill of Materials

37

Chapter 6: MANUFACTURING METHODS 6.1 Production of Screw Threads – Possible Methods and Their Characteristics

38

6.1.1 Casting

38

6.1.2 Forming

38

6.1.3 Removal process

38

6.1.4 Semi finishing and finishing

38

6.1.5 Precision forming to near – net – shape

39

6.1.6 Non-conventional process

39

6.2 Processes, Machines and Tools Used For Producing Screw Threads

39

6.2.1 Production of screw threads by machining

39

6.2.1.1 Thread cutting by hand operated tools

39

6.2.1.2 Machining screw threads in machine tools

43

6.3 Production of screw threads by thread rolling

48

6.4 Finishing and production of screw threads by grinding

51

Chapter 7: FABRICATION 7.1 Top Arms and Bottom Arms

53

7.2 Power Screw

54

7.3 Trunions

55

7.4 Top and Bottom Plates

56

7.5 Power Gun

57

7.6 Light Source

57

7.7 Power Source

57

Chapter 8: CONCLUSION AND SCOPE

58

References

59

Acknowledgment I wish to express my sincere thanks to project guide Dr.K.V.Ramana for his cordial support able guidance and support in every step of this project and constant encouragement throughout the course of this project, help and guidance given by him time to time shall carry me a long way in the journey of life on which I am about to embark.

I take this opportunity to express a deep sense of our gratitude to our Head of the Department Dr.P.V.CHALAPATHI for his support, Valuable information and guidance, which helped me in completing this task through various stages. Many thanks go to the lab technician Mr.YesuDas for his help in fabrication of the prototype. Nevertheless, we express our gratitude toward our families and colleagues for their kind co-operation and encouragement which help us in completion of this project.

i

Abstract

A Scissor Jack is a mechanical device used to easily lift a vehicle off the ground, to gain access to sections underneath vehicles or to change the wheel. The most important fact of a jack is that it gives the user a mechanical advantage by changing the rotational force on power screw into linear motion, allowing user to lift a heavy car to the required height. It is called a scissor jack as the structure consists of diagonal metal components that expand and contract in the same way as a pair of scissors. In this project an attempt has been made to design and fabricate a power scissor jack to lift and support a load of 4.5kN, for typical use in four wheeler. The entire work has been divided into eight chapters. 1st chapter deals with Introduction, explaining a jack. The objectives of the project work have been also presented in this chapter. 2nd chapter presents the Literature Review. This chapter covers various developments in screw jack, other types of load lifting mechanisms and present status of screw jacks in use. 3rd chapter highlights Power Screws. In this chapter application of power screws, their advantages and dis advantages, forms of power screws, terminology and efficiency of power screw are discussed. 4th chapter deals with Design of various elements of proposed jack, with due consideration regarding selection of materials, their working stress along with necessary assumptions made. 5th chapter presents Drawings, which includes part drawings, assembly drawings, exploded views and bill of materials. 6th chapter emphasize on the different Manufacturing Methods which are in practice to get the elements that are designed. 7th chapter highlights Fabrication and assembly details of the proposed power scissor jack along with the pictorial views of the fabricated unit. 8th chapter deals with the Conclusions which are drawn based on the present work. A few suggestions are given as a Scope for further development.

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List of Tables Table 4.1 Nut Parameters………………………………………………………………...24 Table 7.1 Sequence of operations on top and bottom arms……………………………...54 Table 7.2 Sequence of operations on power screw……………………………………....54 Table 7.3 Sequence of operations on trunions……………………………………………56 Table 7.4 Sequence of operations on top and bottom plates………………………………57

List of Figures Fig 2.1 Scissor Jack………………………………………………………………………08 Fig 2.2 Bottle Jack………………………………………………………………………..09 Fig 2.3 Hydraulic Jack…………………………………………………………………...10 Fig 3.1Nomenclature of Square Thread………………………………………………….14 Fig 3.2Nomenclature of Trapezoidal Thread…………………………………………….15 Fig 3.3 ACME thread…………………………………………………………………….16 Fig 3.4(a) Single Start (b) Double Start (c) Triple Start…………………………………17 Fig 3.5 Nomenclature of a Power Screw………………………………………………...18 Fig 4.1 Layout of Scissor Jack…………………………………………………………...22 Fig 5.1 Power Screw……………………………………………………………………..28 Fig 5.2 Trunion…………………………………………………………………………...29 Fig 5.3 Top Arm………………………………………………………………………….30 Fig 5.4 Bottom Arm……………………………………………………………………...31 Fig 5.5 Top Plate…………………………………………………………………………32 Fig 5.6 Bottom Plate……………………………………………………………………..33 Fig 5.7 Assembly Drawing for Scissor Jack……………………………………………..34 Fig 5.8 Exploded Front View…………………………………………………………….35 Fig 5.9 Exploded Side View……………………………………………………………..36 Fig 5.10 Bill of Materials………………………………………………………………...37 Fig6.1 Different types of thread cutting dies…………………………………………….40 Fig 6.2 Hand operated taps for cutting internal threads………………………………….41 Fig6.3 Hand operated tapping in center lathe……………………………………………41 Fig 6.4 External threading in lathe by chasing…………………………………………...42 iii

Fig 6.5 Thread milling by attachment in center lathes…………………………………...43 Fig 6.6 Thread cutting in center lathe by rotating tools…………………………….........44 Fig 6.7(a) single point tool,(b) solid tap and (c) milling cutter for internal threading in center lathe……………………………………………………………………………….45 Fig 6.8 Cutting (a) external and (b) internal threads in capstan and turret lathes……….46 Fig 6.9 Tapping attachment for machining internal threads in drilling machines……….47 Fig 6.10 Threading of nuts in drilling machine by special tapping attachment…………48 Fig 6.11Principle of thread rolling by flat dies…………………………………………..49 Fig 6.12 Principle of thread rolling by circular die with plunge feed……………………49 Fig 6.13Thread rolling by spiral feed circular die……………………………………….50 Fig 6.14 Thread rolling in the annular space between two circular dies………………...50 Fig 6.15Thread rolling by sector circular die……………………………………………51 Fig 6.16Grinding of external screw threads……………………………………………..52 Fig 7.1 Arm of Scissor Jack……………………………………………………………...53 Fig 7.2Fabrication of power screw on lathe……………………………………………..54 Fig 7.3 Trunions with internal threading………………………………………………...55 Fig 7.4Top Plate and Bottom plate………………………………………………………56

List of Graphs Graph3.1 Graph Between coefficient of friction and lead angle……………………......19 Graph3.2 Graph between Efficiency and Helix angle…………………………………..21

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List of Nomenclature P - Pitch of screw thread (mm) L - Lead of screw thread (mm) d0 - Nominal or outer diameter of screw (mm) dc - Core diameter of screw (mm) d - Mean diameter of screw α - Helix angle of screw (degree) W - Load (kg) N - Normal reaction (N) μ - Coefficient of friction p - Effort (N) θ - Angle made by link with horizontal (degree)

 - Friction Angle (degree) σc - crippling stress (N/mm2) F.S- Factor of safety T- Tension (N) a- Rankine’s constant η - Efficiency (%)

t - Tensile stress  - Shear Stress (N/mm2) t max - Maximum Principal stress (N/mm2) max - Maximum Shear Stress (N/mm2) σyt - Yield Stress (N/mm2) b – Width of nut (mm) n - Number of threads in engagement with the nut. Pb - Unit bearing pressure (N/mm²) R - Radius of gyration of the cross-section about its axis (mm) I - Moment of inertia of the cross-section (mm4) A - Area of the cross-section (mm2) Pcr - critical load (N). E - Modulus of elasticity (N/mm²) Z- Section Modulus (N/mm3)

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CHAPTER-1 INTRODUCTION 1.1 Jack Jack is a mechanical device used to lift heavy loads or apply great forces. A mechanical jack employ a square thread for lifting heavy equipment. The most common form is a car jack, floor jack or garage jack which lifts vehicles so that maintenance can be performed. Mechanical jacks are usually rated for a maximum lifting capacity (for example, 1.5 tons to 3 tons). More powerful jacks use hydraulic power to provide greater lift. 1.2 Problem Statement Available jacks present difficulties for the elderly people and women and are especially disadvantageous under adverse weather conditions. Presently available jacks further require the operator to remain in prolonged bent or squatting position to operate the jack which is not ergonomic to human body. It will give physical problems in course of time. Moreover, the safety features are also not enough for operator to operate the present jack. Furthermore, available jacks are typically large, heavy and also difficult to store, transport, carry or move into the proper position under an automobile. The purpose of this project is to overcome these problems. An electric car jack which has a frame type of design by using electricity from the car will be developed. Operator only needs to press the button from the controller without working in a bent or squatting position for a long period of time to change the tire.

1.3 Objectives

1. To design a power scissor jack which is safe and reliable to raise and lower the load easily. 2. Use of double start square thread in power screw. 3. Pins in bearings. 4. To fabricate the prototype of a scissor jack which is operated by a gun powered by the car battery. 1

CHAPTER-2 LITERATURE REVIEW Screw type mechanical jacks were very common for jeeps and trucks of World War II vintage. For example, the World War II jeeps (Willys MB and Ford GPW) issued the "Jack, Automobile, Screw type, Capacity 1 1/2 ton", Ordinance part number 41-J-66. These jacks, and similar jacks for trucks, were activated by using the lug wrench as a handle for the ratchet action to the jack. The 41-J-66 jack was carried in the jeep's tool compartment. Screw type jacks continued in use for small capacity requirements due to low cost of production to raise or lower the load. A control tab is marked up/down and its position determines the direction of movement and with no maintenance. The virtues of using a screw as a machine element, which is essentially an inclined plane wound round a cylinder, was first demonstrated by Archimedes in 200BC with his device used for pumping water. There is evidence of the use of screws in the Ancient Roman world but it was the great Leonardo da Vinci, in the late 1400s, who first demonstrated the use of a screw jack for lifting loads. Leonardo’s design used a threaded worm gear, supported on bearings, rotated by the turning of a worm shaft to drive a lifting screw to move the load. People were not sure of the intended application of his invention, but it seems to have been relegated to the history books, along with the helicopter and tank, for almost four centuries. It is not until the late 1800s that people have evidence of the product being developed further [3]. With the industrial revolution of the late 18th and 19th centuries, came the first use of screws in machine tools, via English inventors such as John Wilkinson and Henry Maudsley. The most notable inventor in mechanical engineering from the early 1800s was undoubtedly the mechanical genius Joseph Whitworth, who recognized the need for precision as important in industry. While he would eventually have over 50 British patents with titles ranging from knitting machines to rifles, it was Whitworth’s work on screw cutting machines, accurate measuring instruments and standards covering the angle and pitch of screw threads that would most influence our industry today.

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Whitworth’s tools have become internationally famous for their precision and quality and dominated the market from the 1850s. Inspired young engineers began to put Whitworth’s machine tools to new uses. During the early 1880s in Coati cook, a small town near Quebec, a 24- year-old inventor named Frank Henry Sleeper designed a lifting jack. Like da Vinci’s jack, it was a technological innovation because it was based on the principle of the ball bearing for supporting a load and transferred rotary motion, through gearing and a screw, into linear motion for moving the load. The device was efficient, reliable and easy to operate. It was used in the construction of bridges, but mostly by the railroad industry, where it was able to lift locomotives and railway cars. Arthur Osmore Norton, spotted the potential for Sleeper’s design and in 1886 hired the young man and purchased the patent and then Norton jack was born. Over the coming years the famous Norton jacks were manufactured at plants in Boston, Coati cook and Moline, Illinois. Meanwhile, in Alleghany County near Pittsburgh in 1883, an enterprising Mississippi river boat captain named Josiah Barrett had an idea for a ratchet jack that would pull barges together to form a tow. The idea was based on the familiar lever and fulcrum principle and he needed someone to manufacture it. That person was Samuel Duff, proprietor of a machine shop. Together, they created the Duff Manufacturing Company, which by 1890 had developed new applications for the original Barrett Jack and extended the product line to seven models in varying capacities [10]. Over the next 30 years the Duff Manufacturing Company became the largest manufacturer of lifting jacks in the world, developing many new types of jack for various applications including its own version of the ball bearing screw jack. It was only natural that in 1928, The Duff Manufacturing Company Inc. merged with A.O. Norton to create the Duff-Norton Manufacturing Company. Both companies had offered manually operated screw jacks but the first new product manufactured under the joint venture was the air motor-operated power jack that appeared in 1929. With the aid of the relatively new portable compressor technology, users now could move and position loads without manual effort. The jack, used predominantly in the railway industry, incorporated an air motor manufactured by The Chicago Pneumatic Tool Company. 3

There was a clear potential for using this technology for other applications and only 10 years later, in 1940, the first worm gear screw jack, that is instantly recognizable today, was offered by Duff-Norton, for adjusting the heights of truck loading platforms and mill tables. With the ability to be used individually or linked mechanically and driven by either air or electric motors or even manually, the first model had a lifting capacity of 10 tons with raises of 2′′ or 4′′ [3]. 2.1 Various Developments in Lifting Devices 1. Levers 2. Screw threads 3. Gears 4. Wheels and axles 5. Hydraulics

2.1.1 Levers Use of the lever gives the operator much greater lifting force than that available to a person who tried to lift with only the strength of his or her own body. Types of levers are first, second and third order. 2.1.2 Screw thread A screw is a mechanism that converts rotational motion to linear motion, and a torque to a linear force. The most common form consists of a cylindrical shaft with helical grooves or ridges called threads around the outside. The screw passes through a hole in another object or medium, with threads on the inside of the hole that mesh with the screw's threads. When the screw is rotated relative to the stationary threads, the screw moves along its axis relative to the medium surrounding it for example rotating a wood screw forces it into wood. In screw mechanisms, either the screw can rotate through a threaded hole in a stationary object, or a threaded collar such as a nut can rotate around a stationary screw. Geometrically, a screw can be viewed as a narrow inclined plane wrapped around a cylinder [7].

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2.1.3 Gears The jack will lift a load in contact with the load platform when the power screw is rotated through its connecting gear with the pinion gear when connected to the motor, plugged to the automobile 12V battery source to generate power for the prime mover (motor), which transmits its rotating speed to the pinion gear meshing with the bigger gear connected to the power screw to be rotated with required speed reduction and increased torque to drive the power screw. The power screw rotates within the threaded hole of its connecting members in the clockwise direction that will cause the connecting members to be drawn along the threaded portion towards each other during a typical load-raising process. During the typical loadraising process, the jack will first be positioned beneath the load to be lifted such that at least a small clearance space will exist between the load platform and the object to be raised. Next, power screw will be turned so that the load platform makes contact with the object and the clearance space is eliminated. As contact is made, load from the object will be increasingly shifted to the load platform and cause forces to be developed in and transmitted through lifting members and connecting members. The force transmitted through the connecting members will be transferred at the threaded bore to the lead Acme threads, there within. A switch button connected to the motor is used to regulate the lifting and lowering process. 2.2 Necessity of Jack In the repair and maintenance of automobiles (car), it is often necessary to raise an automobile to change a tire or access the underside of the automobile. Accordingly, a variety of car jacks have been developed for lifting an automobile from a ground surface. Available car jacks, however, are typically manually operated and therefore require substantial laborious physical effort on the part of the user. Such jacks present difficulties for the elderly and handicapped and are especially disadvantageous under adverse weather conditions. Furthermore, available jacks are typically large, heavy and also difficult to store, transport, carry or move into the proper position under an automobile. In addition, to the difficulties in assembling and setting up jacks, such jacks are generally not adapted to be readily disassembled and stored after automobile repairs have been completed. Car jacks must be easy to use for women or whoever had problem with the tire in the middle of nowhere.

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In light of such inherent disadvantages, commercial automobile repair and service stations are commonly equipped with large and hi-tech car lift, wherein such lifts are raised and lowered via electrically-powered systems. However, due to their size and high costs of purchasing and maintaining electrically-powered car lifts, such lifts are not available to the average car owner. Engineering is about making things simpler or improving and effective. Such electricalpowered portable jacks not only remove the arduous task of lifting an automobile via manuallyoperated jacks, but further decrease the time needed to repair the automobile. Such a feature can be especially advantageous when it is necessary to repair an automobile on the side of a roadway or under other hazardous conditions. There also reports on car jacks which lead to a serious number of accidents [7] [12] [13]. A specified jack purposed to hold up to 1000 kilograms, but tests undertaken by Consumer Affairs has revealed that is fails to work after lifting 250 kilograms and may physically break when it has a weight close to its 1000 kilograms capacity. Whilst no injuries have been reported to date, Ms. Rankine has expressed concerned about the dangers associated with the use of a vehicle jack that does not carry the weight it is promoted to hold. Tests have proven that the jack has the property to buckle well under the weight it is promoted to withstand, and it doesn’t meet the labeling or performance requirements of the Australian Standard for vehicle jacks. 2.3 Types of load lifting devices [11] 1. Artificial Lifting Devices (ALD) 2. Portable Automotive Lifting Devices (PALD) 2.3.1 Artificial Lifting Devices 1. Hydraulic pumping system 2. Electric Submersible Pumps 3. Gas lifts 4. Hybrid gas lifts 2.3.2 Portable Automotive Lifting Devices 1. Hydraulic hand jacks 2. Transmission jacks 3. Engine Stands 6

4. Vehicle support stands 5. Upright type mobile lifts 6. Service jacks 7. Wheel dollies 8. Swing type mobile lifts 9. Scissor type mobile lifts 10. Auxiliary stands 11. Automotive ramps 12. High rich supplementary stands 13. Fork lift jacks 14. High reach fixed stands 15. Vehicle transport lifts 16. Cranes 17. Lever 18. Hydraulic ram 19. Block and tackle 20. Wedge 21. Escalator 2.4 Types of Jacks Used Today 2.4.1 Scissor Jack Scissor jacks are mechanical devices and have been in use since 1930s. A scissor jack is a device constructed with a cross-hatch mechanism, much like a scissor, to lift up a vehicle for repair. It typically works in a vertical manner. The jack opens and folds closed, applying pressure to the bottom supports along the crossed pattern to move the lift. When closed, they have a diamond shape. Scissor jacks are simple mechanisms used to handle large loads over short distances. The power screw design of a common scissor jack reduces the amount of force required by the user to drive the mechanism. Most scissor jacks are similar in design, consisting of four main members driven by a power screw [7] [9]. A scissor jack is operated simply by turning a small crank that is inserted into one end of the scissor jack. This crank is usually "Z" shaped. The end fits into a ring hole mounted on the end of the screw, which is the object of 7

force on the scissor jack. When this crank is turned, the screw turns, and this raises the jack. The screw acts like a gear mechanism. It has teeth (the screw thread), which turn and move the two arms, producing work. Just by turning this screw thread, the scissor jack can lift a vehicle that is several thousand pounds. A scissor jack has four main pieces of metal and two base ends. The four metal pieces are all connected at the corners with a bolt that allows the corners to swivel. A screw thread runs across this assembly and through the corners. When opened, the four metal arms contract together, coming together at the middle, raising the jack.

Fig 2.1 Scissor Jack When closed, the arms spread back apart and the jack closes or flattens out again. A scissor jack uses a simple gear drive to get its power. As the screw section is turned, two ends of the jack move closer together. Because the gears of the screw are pushing up the arms, the amount of force being applied is multiplied. It takes a very small amount of force to turn the crank handle, yet that action causes the brace arms to slide across and together. As this happens the arms extend upward. The car's gravitational weight is not enough to prevent the jack from opening or to stop the screw from turning, since it is not applying force directly to it. If a person applies pressure directly on the crank, or lean his weight against the crank, the person would not be able to turn it, even though his weight is a small percentage of the cars.

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2.4.2 Bottle (Cylinder) Jack Bottle screws may be operated by either rotating the screw when the nut is fixed or by rotating the nut and preventing rotation of the screw. Bottle jacks mainly consist of a screw, a nut, thrust bearings, and a body. A stationary platform is attached to the top of the screw. This platform acts as a support for the load and also assists it in lifting or lowering of the load. These jacks are sturdier than the scissor jacks and can lift heavier loads. In a bottle jack the piston is vertical and directly supports a bearing pad that contacts the object being lifted. With a single action piston the lift is somewhat less than twice the collapsed height of the jack, making it suitable only for vehicles with a relatively high clearance.

Fig 2.2 Bottle Jack

2.4.3 Hydraulic Jacks Hydraulic jacks are typically used for shop work, rather than as an emergency jack to be carried with the vehicle. Use of jacks not designed for a specific vehicle requires more than the usual care in selecting ground conditions, the jacking point on the vehicle, and to ensure stability when the jack is extended. Hydraulic jacks are often used to lift elevators in low and medium rise buildings. A hydraulic jack uses a fluid, which is incompressible. Oil is used since it is self-lubricating and stable. When the plunger pulls back, it draws oil out of the reservoir through a suction 9

check valve into the pump chamber. When the plunger moves forward, it pushes the oil through a discharge check valve into the cylinder. The suction valve ball is within the chamber and opens with each draw of the plunger. The discharge valve ball is outside the chamber and opens when the oil is pushed into the cylinder [9]. At this point the suction ball within the chamber is forced to shut and oil pressure builds in the cylinder. For lifting structures such as houses the hydraulic interconnection of multiple vertical jacks through valves enables the even distribution of forces while enabling close control of the lift [6] In a floor jack a horizontal piston pushes on the short end of a bell crank, with the long arm providing the vertical motion to a lifting pad, kept horizontal with a horizontal linkage. Floor jacks usually include castors and wheels, allowing compensation for the arc taken by the lifting pad. This mechanism provide a low profile when collapsed, for easy maneuvering underneath the vehicle, while allowing considerable extension.

Fig 2.3 Hydraulic Jacks 2.5 Operational Considerations of a screw jack [11] 1. Maintain low surface contact pressure Increasing the screw size and nut size will reduce thread contact pressure for the same working load. The higher the unit pressure and the higher the surface speed, the more rapid the wear will be. 2. Maintain low surface speed .Increasing the screw head will reduce the surface speed for the same linear speed. 3. Keep the mating surfaces well lubricated .The better the lubrication, the longer is the service life. Grease fittings or other lubrication means must be provided for the power screw and nut. 10

4. Keep the mating surfaces clean Dirt can easily embed itself in the soft nut material. It will act as a file and abrade the mating screw surface. The soft nut material backs away during contact leaving the hard dirt particles to scrap away the mating screw material. 5. Keep heat away. When the mating surfaces heat up, they become much softer and are more easily worn away. Means to remove the heat such as limited duty cycles or heat sinks must be provided so that rapid wear of over-heated materials can be avoided.

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CHAPTER-3 POWER SCREWS A power screw is a mechanical device used for converting rotary motion into linear motion and transmitting power. A power screw is also called translation screw. It uses helical translatory motion of the screw thread in transmitting power rather than clamping the machine components. 3.1 Applications The main applications of power screws are as follows: 1. To raise the load, e.g. screw-jack, scissor jack, 2. To obtain accurate motion in machining operations, e.g. lead-screw of lathe, 3. To clamp a work piece, e.g. vice, and 4. To load a specimen, e.g. universal testing machine. There are three essential parts of a power screw i.e., screw, nut and a part to hold either the screw or the nut in its place. Depending upon the holding arrangement, power screws operate in two different ways. In some cases, the screw rotates in its bearing, while the nut has axial motion. The lead screw of the lathe is an example of this category. In other applications, the nut is kept stationary and the screw moves in axial direction. Screw-jack and machine vice are the examples of this category. 3.2 Advantages Power screws offer the following advantages: 1. Power screw has large load carrying capacity. 2. The overall dimensions of the power screw are small, resulting in compact construction. 3. Power screw is simple to design 4. The manufacturing of power screw is easy without requiring specialized machinery. Square threads are turned on lathe. Trapezoidal threads are manufactured on thread milling machine.

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5. Power screw provides large mechanical advantage. A load of 15 kN can be raised by applying an effort as small as 400N. Therefore, most of the power screws used in various applications like screw-jacks, clamps, valves and vices are usually manually operated. 6. Power screws provide precisely controlled and highly accurate linear motion required in machine tool applications. 7. Power screws give smooth and noiseless service without any maintenance. 8. There are only a few parts in power screw. This reduces cost and increases reliability. 9. Power screw can be designed with self-locking property. In screw-jack application,

self-locking characteristic is required to prevent the load from descending on its own 3.3 Disadvantages The disadvantages of power screws are as follows: 1. Power screws have very poor efficiency; as low as 40%.Therefore, it is not used in continuous power transmission in machine tools, with the exception of the lead screw. Power screws are mainly used for intermittent motion that is occasionally required for lifting the load or actuating the mechanism. 2. High friction in threads causes rapid wear of the screw or the nut. In case of square threads, the nut is usually made of soft material and replaced when worn out. In trapezoidal threads, a split- type of nut is used to compensate for the wear. Therefore, wear is a serious problem in power screws. 3.4 Forms of Threads There are two popular types of threads used for power screws viz. Square, I.S.O metric trapezoidal and Acme threads. 3.4.1 Square Thread The square thread form is a common screw thread form, used in high load applications such as lead screws and jackscrews. It gets its name from the square cross-section of the thread. It is the lowest friction and most efficient thread form.

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Fig 3.1 Nomenclature of Square Thread 3.4.1.1 Advantages of square threads The advantages of square threads over trapezoidal threads are as follows: 1. The efficiency of square threads is more than that of trapezoidal threads. 2. There is no radial pressure on the nut. Since there is no side thrust, the motion of the nut is uniform. The life of the nut is also increased. 3.4.1.2 Disadvantages of square threads The disadvantages of square threads are as follows: 1. Square threads are difficult to manufacture. They are usually turned on lathe with single-point cutting tool. Machining with single-point cutting tool is an expensive operation compared to machining with multi-point cutting tool. 2. 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. 3. The wear of the thread surface becomes a serious problem in the service life of the power screw. It is not possible to compensate for wear in square threads. Therefore, when worn out, the nut or the screw requires replacement. 3.4.2 Trapezoidal Threads Trapezoidal thread forms are screw thread profiles with trapezoidal outlines. They are the most common forms used for lead screws. They offer high strength and ease of manufacture. 14

Fig 3.2 Nomenclature of Trapezoidal Thread 3.4.2.1 Advantages of Trapezoidal Threads The advantages of trapezoidal threads over square threads are as follows: 1. Trapezoidal threads are manufactured on thread milling machine. It employs multipoint cutting tool. Machining with multi-point cutting tool is an economic operation compared to machining with single point-cutting tool. Therefore, trapezoidal threads are economical to manufacture. 2. Trapezoidal thread has more thickness at core diameter than that of square thread. Therefore a screw with trapezoidal threads is stronger than equivalent screw with square threads. Such a screw has large load carrying capacity. 3. 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 wear progresses, the looseness is prevented by tightening the two halves of the nut together, the split-type nut can be used only for trapezoidal threads. It is used in lead-screw of lathe to compensate wear at periodic intervals by tightening the two halves. 3.4.2.2 Disadvantages of Trapezoidal Threads The disadvantages of trapezoidal threads are as follows 1. The efficiency of trapezoidal threads is less than that of square threads. 2. Trapezoidal threads result in side thrust or radial pressure on the nut. The radial pressure or bursting pressure on nut affects its performance.

15

3.4.3 ACME Thread There is a special type of thread called acme thread as shown in Fig 3.3. Trapezoidal and acme threads are identical in all respects except the thread angle. In acme thread, the thread angle is 29° instead of 30°.The relative advantages and disadvantages of acme threads are same as those of trapezoidal threads. There is another type of thread called buttress thread. It combines the advantages of square and trapezoidal threads. Buttress threads are used where heavy axial force acts along the screw axis in one direction only.

Fig 3.3 ACME thread

3.5 Designation of Threads There is a particular method of designation for square and trapezoidal threads. A power screw with single-start square threads is designated by the letters ‘Sq’ followed by the nominal diameter and the pitch expressed in millimeters and separated by the sign ‘x’. For example, Sq 30 x 6 It indicates single-start square threads with 30mm nominal diameter and 6mm pitch. Similarly single-start I.S.O metric trapezoidal threads are designated by letters ‘Tr’ followed by the nominal diameter and the pitch expressed in millimeters and separated by the sign ‘x’. For example, Tr 40x7 It indicates single-start trapezoidal threads with 40mm nominal diameter and 7mm pitch.

16

3.5.1 Multiple Threaded Power Screws Multiple threaded power screws as shown in Fig 3.4 are used in certain applications where higher travelling speed is required. They are also called multiple start screws such as doublestart or triple-start screws. These screws have two or more threads cut side by side, around the rod.

(a)

(b)

(c)

Fig 3.4(a) Single Start (b) Double Start (c) Triple Start Multiple-start trapezoidal threads are designated by letters ‘Tr’ followed by the nominal diameter and the lead, separated by sign ‘x’ and in brackets the letter „P‟ followed by the pitch expressed in millimeters. For example, Tr 40 x 14 (P7) In above designation, Lead=14mm pitch=7mm Therefore, No. of starts =14/7=2 It indicates two-start trapezoidal thread with 40mm nominal diameter and 7mm pitch. In case of left handed threads. The letters ‘LH’ are added to thread designation. For example, Tr 40 x 14 (P7) LH

17

3.6 Terminology of Power Screw The terminology of the screw thread is given in Fig 3.5:

Fig 3.5 Nomenclature of a Power Screw 1. Pitch: The pitch is defined as the distance, measured parallel to the axis of the screw, from a point on one thread to the corresponding point on the adjacent thread. It is denoted by the letter ‘p’. 2. 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. It is denoted by the letter ‘L’. For a single-threaded screw, the lead is same as the pitch, for a double-threaded screw, the lead is twice that of the pitch, and so on. 3. Nominal diameter: It is the largest diameter of the screw. It is also called major diameter. It is denoted by the letter ‘do’. 4. Core diameter: It is the smallest diameter of the screw thread. It is also called minor diameter. It is denoted by the letters ‘dc’. 5. 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. Helix angle is related to the lead and the mean diameter of the screw. It is also called lead angle. It is denoted by α.

18

3.7 Self Locking Screw It can be seen that when    , the torque required to lower the load is negative. It indicates a condition that no force is required to lower the load. The load itself will begin to turn the screw and descend down, unless a restraining torque is applied. This condition is called “overhauling” of screw.

When    , a positive torque is required to lower the load. Under this condition, the load will not turn the screw and will not descend on its own unless effort P is applied. In this case, the screw is said to be “self-locking”. The rule for self-locking screw is as follows: “A screw will be self-locking if the coefficient of friction is equal to or greater than the tangent of the helix angle”.

Graph 3.1 Graph Between coefficient of friction and lead angle

19

Therefore, for a self-locking screw the following conclusions can be made 1. Self-locking of 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. 2. Self-locking property of the screw is lost when the lead is large. The lead 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, single threaded is better than multiple threaded screw from self-locking considerations. Self-locking condition is essential in applications like scissor jack. 3.8 Efficiency of Self-Locking Screw The output consists of raising the load. Therefore, Work output = force x distance travelled in the direction of force =WxL The input consists of rotating the screw by means of an effort P. Work input = force x distance travelled in the direction of force = P x (π d) The efficiency η of the screw is given by, η = Work output/ Work input = W x L/ P x (π d) = (W/P)* tan () = tan ()/tan (+) From the above equation, it is evident that the efficiency of the square threaded screw depends upon the helix angle α and the friction angle.The following figure shows the variation of the efficiency of square threaded screw against the helix angle for various values of coefficient of friction. The graph is applicable when the load is lifted.

20

Graph 3.2 Graph between Efficiency and Helix angle Following conclusions can be derived from the observation of these graphs, 1. The efficiency of square threaded screw increase rapidly up to helix angle of 20°. 2. The efficiency is maximum when the helix angle between 40 to 45°. 3. The efficiency decreases after the maximum value is reached. 4. The efficiency decreases rapidly when the helix angle exceeds 60° 5. The efficiency decreases as the coefficient of friction increases. There are two ways to increase the efficiency of square threaded screws. They are as follows: 1. Reduce the coefficient of friction between the screw and the nut by proper lubrication 2. Increase the helix angle up to 40 to 45° by using multiple start threads. However, a screw with such helix angle has other disadvantages like loss of self-locking property.

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CHAPTER-4 DESIGN W

T 1

T 1

Fig 4.1 Layout of Scissor Jack 4.1 Power Screw 4.1.1 Material selected Mild Steel 4.1.2 Assumptions The weight of the car is considered as 1.5 ton. The weight acting on front and rear axle is 60% and 40% of total weight respectively, hence the weight acting on front axle i.e.; 900 kg is considered for designing the jack. A weight of 450 kg acts on each wheel. And the maximum load on screw act when jack is at its lowest position. We assumed the thread on screw be a Double Start Square thread and coefficient of friction between threads is 0.20.

22

4.1.3 Design Calculations Length of each arm = L1 =L2 =L3 =L4 =160mm Length of the power screw = (w1+w2+w3) = 350mm w1 = w3 = 150 mm w2 = 50 mm Maximum lift of the jack = (h1+h2) = 300 mm

 is the angle made by link with horizontal when jack is at its lowest position. cos () = (175-25)/160 = 20.36˚ W = (load * g) = (450*10) = 4500 N = 4.5 kN The tension T acting on the power screw is shown in the above Fig 4.1. Tension, T = W/2*tan () Total tension = 2*T = W/tan () For a power screw under tension we can take t = 124 N/mm2 for mild steel Let dc be the core diameter of the screw. But load on the screw is Load = (π/4)* dc2 * t So, 2*T = W/tan () = (π/4)* dc2 * t 2*T = 4.5 kN/tan (20.36˚) = 12123.44 N dc2 = (W/tan ())*(4/ (π* t)) Hence, dc = 11.34 mm Since the screw is subjected to torsional shear stress we adopt, dc = 14 mm Taking pitch, P = 2 mm Outer diameter, do = dc + P = (14+2) = 16 mm Mean diameter, d = do – P/2 = 16-2/2 = 15 mm Check for self-locking tan () = Lead/π*d;  = helix angle Lead L = 2*P; since the screw has a double start square thread. tan () = 2*p/π*d = 2*2/ π*15 = 0.084 Helix angle;  = 4.85˚ Coefficient of friction; μ = tan () = 0.20; friction angle;  = 11.3˚ 23

 >  hence the screw is self-locking Effort required to support the load = 2*T tan (+) = 12123.44 (tan () + tan ())/ (1- (tan () * tan ())) = 3510.715 N Torque required to rotate the screw = effort *d/2 = 3510.715 * 15/2 = 26330.36 N-mm Shear stress in the screw due to torque  = 16*T/ (π* dc3) = 16*26330.36/ (π*143) = 48.87N/mm2 But tensile stress t = 2*T/ (π/4) * dc2 = 12123.44/ (π/4) * 142 = 78.755 N Maximum principal stress t max = t/2 + (t2 + 2)/2 = 102.13 N/mm2 Maximum shear stress max = (t2 +2)/2 = 62.76 N/mm2 Since the maximum stresses t max and max within the safe limits, the design of double started square threaded screw is satisfactory. 4.2 Nut 4.2.1 Material Selected Bronze 4.2.2 Design Calculations Let n be the number of threads in contact with the screw assumed that load is Uniformly Distributed over the cross section area of the nut. Allowable Bearing pressure between the threads (Pb) are Table 4.1 Nut Parameters Material

Safe Bearing pressure

Rubbing speed at thread pitch diameter

Screw

Nut

(N/mm2)

Steel

Bronze

12.6 - 17.5

Low speed < 2.4 m/min

Steel

C.I

11.2 - 17.5

Low speed < 3 m/min

24

Bearing pressure is assumed as 15 N/mm2 Pb = (2*T)/((π/4)*(do2-dc2)*n) 15 = (12123.44)/ ((π/4)*(162-142)*n) Number of threads, n = 10.6 ≈ 11 In order to have good stability let n=11 Thickness of Nut = n*p = 11*2=22 mm Width of Nut b =1.5*do =1.5*16=24 mm To control the movement of nuts beyond 300 mm the rings of 8 mm thickness are fitted on the screw with the help of set screw The length of screw portion = 300 + (8*2) + 22 = 338 mm ≈ 350 mm Total length of screw is 350 mm. 4.3 Pins in Nut 4.3.1 Material selected Mild Steel 4.3.2 Design calculations Let d1 = diameter of pins in the nuts Since Pins are in double shear stress Load on pins = W/2 = 2*(π/4)*d12* =12123.44/2

 = Shear stress = 50 MPa for steel Hence d1 = 8.78 mm ≈ say 10 mm Diameter of pins head is taken as 1.5*d1 = 15 mm and thickness be 4 mm 4.4 Top Arm 4.4.1 Material selected Mild Steel 4.4.2 Design calculations σyt for mild steel = 248 N/mm2 Factor of safety (F.S) = 2.5 σt = σyt/F.S=248/2.5=99.2 N/mm2 σc = 1.25*σt = 1.25*99.2 = 124 N/mm2 25

Cross section area (A) = (40*3) + (24*3) + (40*3) = 312 mm2 Moment of Inertia Ixx = 47376 mm4, Iyy= 51009.38 mm4 Radius of Gyration Rx= 12.323 mm, Ry= 12.786 mm Rankine’s constant (a) =1/7500 Ends are hinged (Leff = L) Pcr in vertical plane σc= crippling stress = 330 N/mm2 Pcr = (σc*A)/(1+a*(L/ Ry)2)= (330*312)/(1+(1/7500)*(160/12.786)2) = 100854.26 N Pcr in horizontal plane σc= crippling stress = 330 N/mm2 Pcr = (σc*A)/(1+a*(L/2*Rx)2)= (330*160*40)/(1+(1/7500)*(160/2*12.323)2) = 2100198.258 N Since Buckling load is more than Design load the dimensions of the link safe. 4.5 Bottom Arm 4.5.1 Material selected Mild Steel 4.5.2 Design calculations σyt for mild steel = 248 N/mm2 Factor of safety (F.S) = 2.5 σt = σyt/F.S=248/2.5=99.2 N/mm2 σc = 1.25*σt = 1.25*99.2 = 124 N/mm2 Cross section area (A) = (40*3) + (30*3) + (40*3) = 330 mm2 Moment of Inertia Ixx = 72270 mm4, Iyy= 54469.31 mm4 Radius of Gyration Rx= 14.79 mm, Ry= 12.84 mm Rankine’s constant (a) =1/7500 Ends are hinged (Leff = L) Pcr in vertical plane σc= crippling stress = 330 N/mm2 Pcr = (σc*A)/(1+a*(L/ Ry)2)= (330*330)/(1+(1/7500)*(160/12.84)2) 26

= 106691.09.09 N Pcr in horizontal plane σc= crippling stress = 330 N/mm2 Pcr = (σc*A)/(1+a*(L/2*Rx)2)= (330*160*40)/(1+(1/7500)*(160/2*14.8)2) = 2103804.02 N Since Buckling load is more than Design load the dimensions of the link safe. 4.6 Top Plate (Loading Platform) 4.6.1 Material used Mild Steel 4.6.2 Design calculations Moment, M = (p*l)/4 p = 5000 N l = 50 mm M = (5000*50)/4 = 250000/4 = 62500 N-mm Z = (b*h2)/6 = (36*402)/6 = 9600 mm3 b = 36 mm, h = 40 mm σb = M/Z = 62500/9600 = 6.51 N/mm2 Conclusion The permissible stress for mild steel is 124 N/mm2 and it is greater than σb = 6.51 N/mm2 The top plate design is safe. 4.7 Bottom Plate (Support) 4.7.1 Material used Mild Steel The size and shape of the bottom plate have been selected to provide the stability to the power Scissor Jack. Fixing the dimensions of bottom plate as 120*70*3 all in mm.

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CHAPTER-5 DRAWINGS 5.1 Part Drawings 5.1.1 Power Screw

Fig 5.1 Power Screw

28

5.1.2 Trunion

Fig 5.2 Trunion

29

5.1.3 Top Arm

Fig 5.3 Top Arm

30

5.1.4 Bottom Arm

Fig 5.4 Bottom Arm

31

5.1.5 Top Plate

Fig 5.5 Top Plate

32

5.1.6 Bottom Plate

Fig 5.6 Bottom Plate

33

5.2 Assembly Drawing 5.2.1 Scissor jack

Fig 5.7 Assembly Drawing for Scissor Jack

34

5.2.2 Exploded Front View

Fig 5.8 Exploded Front View

35

5.2.3Exploded Side View

Fig 5.9 Exploded Side View

36

5.2.4 Bill of Materials

4 3 9

7 6 8 2 5 10 1 Fig 5.10 Bill of Materials

37

Chapter-6 MANUFACTURING METHODS 6.1 Production of Screw Threads – Possible Methods and Their Characteristics The various methods, which are more or less widely employed for producing screw threads are: 6.1.1 Casting Characteristics 1. Only a few threads over short length 2. Less accuracy and poor finish 3. Example – threads at the mouth of glass bottles, spun cast iron pipes etc. 6.1.2 Forming (Rolling) Characteristics 1. Blanks of strong ductile metals like steels are rolled between threaded dies 2. Large threads are hot rolled followed by finishing and smaller threads are straight cold rolled to desired finish 3. Cold rolling attributes more strength and toughness to the threaded parts 4. Widely used for mass production of fasteners like bolts, screws etc. 6.1.3 Removal process (Machining) 1. Accomplished by various cutting tools in different machine tools like lathes, milling machines, drilling machines (with tapping attachment) etc. 2. Widely used for high accuracy and finish 3. Employed for wide ranges of threads and volume of production; from piece to mass production. 6.1.4 Semi finishing and finishing (Grinding) Characteristics 1. Usually done for finishing (accuracy and surface) after performing by machining or hot rolling but are often employed for direct threading on rods 2. Precision threads on hard or surface hardened components are finished or directly produced by grinding only 3. Employed for wide ranges of type and size of threads and volume of production 38

6.1.5 Precision forming to near – net – shape Characteristics 1. No machining is required, slight grinding is often done, if needed for high accuracy and finish 2. Application – Investment casting for job order or batch production – Injection molding (polymer) for batch or mass production 6.1.6 Non-conventional process (EDM, ECM etc.) Characteristics 1. When conventional methods are not feasible 2. High precision and micro threads are needed 3. Material is as such difficult – to – process 6.2 Processes, Machines and Tools Used For Producing Screw Threads By (a) Machining (b) Rolling (c) Grinding 6.2.1 Production of screw threads by machining Machining is basically a removal process where jobs of desired size and shape are produced by gradually removing the excess material in the form of chips with the help of sharp cutting edges or tools. Screw threads can be produced by such removal process both manually using taps and dies as well as in machine tools of different types and degree of automation. In respect of process, machine and tool, machining of screw threads are done by several ways: 6.2.1.1 Thread cutting by hand operated tools Usually small threads in few pieces of relatively soft ductile materials, if required, are made manually in fitting, repair or maintenance shops. (a) External screw threads Machine screws, bolts and studs are made by different types of dies which look and apparently behave like nuts but made of hardened tool steel and having sharp internal cutting edges. Fig 6.1.1 shows the hand operated dies of common use, which are coaxially rotated around the pre machined rod like blank with the help of handle or die stock.

39

1. Solid or button die: used for making threads of usually small pitch and diameter in one pass. 2. Spring die: the die ring is provided with a slit, the width of which is adjustable by a screw to enable elastically slight reduction in the bore and thus cut the thread in number of passes with lesser force on hands. 3. Split die: the die is made in two pieces, one fixed and one movable (adjustable) within the cavity of the handle or wrench to enable cut relatively larger threads or fine threads on harder blanks easily in number of passes, the die pieces can be replaced by another pair for cutting different threads within small range of variation in size and pitch. 4. Pipe die: pipe threads of large diameter but smaller pitch are cut by manually rotating the large wrench (stock) in which the die is fitted through a guide bush as shown in Fig. 6.1.

(a) Solid die

(c) Split die

(b) Spring die

(d) Pipe die

Fig 6.1 Different types of thread cutting dies (b) Internal screw threads: Internal screw threads of usually small size are cut manually, if needed, in plates, blocks, machine parts etc. by using taps which look and behave like a screw but made of tool steel or HSS and have sharp cutting edges produced by axial grooving over the threads as shown in Fig 6.2. Three taps namely, taper tap, plug tap and bottoming tap are used consecutively after drilling a tap size hole through which the taps are axially pushed helically with the help of a handle or wrench.

40

Fig 6.2 Hand operated taps for cutting internal threads Threads are often tapped by manually rotating and feeding the taps through the drilled hole in the blank held in lathe spindle as shown in Fig 6.3. The quality of such external and internal threads will depend upon the perfection of the taps or dies and skill of the operator.

Fig 6.3 Hand operated tapping in center lathe 41

6.2.1.2 Machining screw threads in machine tools Threads of fasteners in large quantity and precision threads in batches or lots are produced in different machine tools mainly lathes, by various cutting tools made of HSS or often cemented carbide tools. (a)Machining screw threads in lathes Screw threads in wide ranges of size, form, precision and volume are produced in lathes ranging from center lathes to single spindle automats. Threads are also produced in special purpose lathes and CNC lathes including turning centers. • In center lathes o External threads External threads are produced in center lathes by various methods Δ Single point and multipoint chasing, as schematically shown in Fig 6.4 This process is slow but can provide high quality. Multipoint chasing gives more productivity but at the cost of quality to some extent

(a) Single point

(b) Multipoint Fig 6.4 External threading in lathe by chasing

42

Δ Thread milling: This process gives quite fast production by using suitable thread milling cutters in centre lathes as indicated in Fig 6.5. The milling attachment is mounted on the saddle of the lathe. Thread milling is of two types

(a) Long thread milling

(b) Short thread milling

Fig 6.5 Thread milling by attachment in center lathes ο Long thread milling Long and large diameter screws like machine lead screws are reasonably accurately made by using a large disc type form milling cutter as shown in Fig 6.5. ο Short thread milling Threads of shorter length and fine pitch are machined at high production rate by using a HSS milling cutter having a number of annular threads with axial grooves cut on it for generating cutting edges. Each job requires only around 1.25 revolution of the blank and very short axial (1.25 pitch) and radial (1.5 pitch) travel of the rotating tool Δ Rotating tool Often it becomes necessary to machine large threads on one or very few pieces of heavy blanks of irregular size and shape like heavy casting or forging of odd size and shape. In such cases, the blank is mounted on face plate in a center lathe with proper alignment. The deep and wide threads are produced by intermittent cutting action by a rotating tool. A separate attachment 43

carrying the rotating tool is mounted on the saddle and fed as usual by the lead screw of the center lathe. Fig 6.6 shows schematically the principles of threading by rotary tools. The tool is rotated fast but the blank much slowly. This intermittent cut enables more effective lubrication and cooling of the tool.

Fig 6.6 Thread cutting in center lathe by rotating tools oInternal threads Internal threads are produced in center lathes at slow rate by using; Δ Single point tool Δ Machine taps Δ Internal thread milling Δ Internal threading by single point chasing Internal threads in parts of wide ranges of diameter and pitch are accurately done in centre lathes by single point tool, as in boring, as shown in Fig 6.7 (a). Multipoint flat chaser is often used for faster production.

44

(a)

(b)

(c) Fig 6.7 (a) single point tool, (b) solid tap and (c) milling cutter for internal threading in center lathe Δ Internal threading by taps Internal threads of small length and diameter are cut in drilled holes by different types of taps; Δ Straight solid tap (Fig 6.7 (b) – used for small jobs Δ Taps with adjustable blades – usually for large diameter jobs Δ Taper or nut taps – used for cutting threads in nuts. Δ Internal thread milling cutter Such solid cutter, shown in Fig 6.7 (c) produces internal threads very rapidly, as in external short thread milling, in lathes or special purpose thread milling machine.

(b) Machining threads in semiautomatic lathes Both external and internal threads are cut, for batch or small lot production, in capstan and turret lathes using different types of thread cutting tools.

45

Δ External threads in capstan lathe by self-opening die and single or multipoint chaser in turret lathe Δ Internal threads of varying size by collapsible tap. The self-opening die, typically shown in Fig 6.8 (a), is mounted in the turret and moved forward towards the rotating blank. At the end point, when the turret slows down and is about to stop or reverse, the front position of the die gets pulled and open automatically to enable free return of the die without stopping the job – rotation. The thread chasers may be flat or circular type as shown. In a collapsible tap, shown in Fig 6.8 (b), the radially raised blades collapse (move radially inward) and the tap returns (along with the turret or saddle) freely from the threaded hole after completing the internal thread in one stroke.

(a)

(b)

Fig 6.8 Cutting (a) external and (b) internal threads in capstan and turret lathes

(c) Machining threads in automatic lathes Small external threads for mass production of fasteners are produced by machining in single spindle automatic lathes or similar but special purpose (threading) lathes using solid die. The die is mounted on the coaxially moving turret or sliding attachment in turret lathes and SPM respectively. In turret lathe, the solid die is returned by reversing the job rotation, and in the special purpose machine, the die is freely returned by rotating the die slightly faster than the job and in the same direction.

46

(d) Machining screw threads in drilling machine Drilling machines are used basically for originating cylindrical holes but are also used, if needed, for enlarging drilled holes by larger drills, counter boring, countersinking etc. Internal threads of relatively smaller diameter, length and pitch are also often produced in drilling machines by using tapping attachment with its taper shank fitted axially in the spindle bore. Fig 6.9 typically shows one such tapping attachment.

Fig 6.9 Tapping attachment for machining internal threads in drilling machines The tapping attachment is pushed slowly inside the drilled hole at low speed for cutting threads and at the end of this stroke, it is withdrawn slowly by rotating in reversed direction. Just at the point of start of return, the lower part of the attachments momentarily gets delinked from the upper part and is then up and rotated respectively by the spring and the clutch as shown in Fig 6.9 to move at per with the upper part fitted into the spindle. This is necessary for the safe return of the tap without damaging the through or blind hole. Threading of small identical components like nuts for its mass production is also possible and done in general purpose drilling machines by using special attachment as shown in Fig 6.10. The taper tap is connected with a bent rod which is made to rotate at high speed along with the spindle causing rotation of the tap at the same speed. The blanks are automatically pushed intermittently under the tap and after threading the tap returns but along with the threaded nut. Finally the accumulated nuts are thrown out form the rod by centrifugal force to come out from the hopper as shown.

47

Fig 6.10 Threading of nuts in drilling machine by special tapping attachment

6.3 Production of screw threads by thread rolling In production of screw threads, compared to machining thread rolling, • is generally cold working process • provides higher strength to the threads • does not cause any material loss • does not require that high accuracy and finish of the blank • requires simpler machines and tools • applicable for threads of smaller diameter, shorter length and finer pitch • enables much faster production of small products like screws, bolts, studs etc. • cannot provide that high accuracy • is applicable for relatively softer metals • is used mostly for making external screw threads • needs separate dies for different threads Thread rolling is accomplished by shifting work material by plastic deformation, instead of cutting or separation, with the help of a pair of dies having same threads desired.. Different types of dies and methods are used for thread rolling which include, • Thread rolling between two flat dies • Thread rolling between a pair of circular dies • Thread rolling by sector dies

48

o Rolling of external screw threads by flat dies The basic principle is schematically shown in Fig 6.11. Flat dies; one fixed and the other moving parallel, are used in three configurations Δ Horizontal: most convenient and common Δ Vertical: occupies less space and facilitates cleaning and lubrication under gravity Δ Inclined: derives benefit of both horizontal and vertical features All the flat dies are made of hardened cold die steel and provided with linear parallel threads like grooves of geometry as that of the desired thread.

Fig 6.11 Principle of thread rolling by flat dies

o Thread rolling by circular dies Circular die sets occupy less space and are simpler in design, construction, operation and maintenance. The different types of thread rolling circular dies of common use and their working methods are Δ Circular dies with plunge (radial) feed The two identical circular dies with parallel axis are rotated in the same direction and speed as indicated in Fig 6.12. One stays fixed in a position the other is moved radially desirably depending upon the thread depth

Fig 6.12 Principle of thread rolling by circular die with plunge feed

49

Δ Circular die with inherent radial feed Here the forced penetration of the threads in the blank is accomplished not by radial shifting of one of the dies but gradual projection of the thread in Archimedean spiral over an angle on one of the dies as indicated in Fig 6.13. This makes the system simpler by eliminating a linear motion.

Fig 6.13 Thread rolling by spiral feed circular die Δ Thread rolling in the annular space between two dies In this simpler system and process the outer die remains fixed and the inner one rotates as shown in Fig 6.14. Because of simple construction and motions, this method is more productive but limited to smaller jobs.

Fig 6.14 Thread rolling in the annular space between two circular dies Δ Thread rolling by circular die sector This method, schematically shown in Fig 6.15, is the simplest and fastest way of thread rolling enabling easy auto-feed of the blanks. blank feed spiral sector circular sector die product die work rest collecting pocket loading zone release zone working zone Fine internal threads on large diameter and soft metals may also be done, if needed, by using a screw like threaded tool which will be rotated and pressed parallel against the inner cylindrical wall of the product.

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Fig 6.15 Thread rolling by sector circular die 6.4 Finishing and production of screw threads by grinding In production of screw threads, grinding is employed for two purposes • Finishing the threads after machining or even rolling when o High dimensional and form accuracy as well as surface finish are required, e.g., screw threads of precision machines and measuring instruments o The threaded parts are essentially hardened and cannot be machined or rolled further, e.g., lead screws of machine tools, press – screws etc. • Directly originating (cutting) and simultaneously finishing threads in any hard or soft preformed blanks. This is employed generally for finer threads of small pitch on large and rigid blanks However screw threads are ground in several methods which include Δ External and internal thread grinding by single ribbed formed grinding wheel as schematically shown in Fig 6.16 (a). Such grinding is usually done in cylindrical grinding machine but is also occasionally done in rigid center lathes by mounting a grinding attachment like thread milling attachment, on the lathe’s saddle. Δ Multi-ribbed wheels save grinding time by reducing the length of travel of the wheel but raises wheel cost. Fig 6.16 (b) shows such thread grinding with both fully covered and alternate ribbing.

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(a) Single rib wheel

(b) Multiple rib wheel

Fig 6.16 Grinding of external screw threads Δ External threads by centerless grinding Like centerless grinding of short and long rods by plunge feed and through feed respectively, centerless thread grinding is also done by ribbed grinding wheel using respectively parallel and desirably inclined plain guide wheels. Centerless grinding, if feasible, is more productive but at the cost of accuracy to some extent.

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CHAPTER-7 FABRICATION The fabrication process started with identification of suitable materials which are used for prototype and designing of various parts. 7.1 Top Arms and Bottom Arms As per the calculations and the drawings the cross section of arms is selected as Channel Section. The availability of the Channel section of the required dimensions was low in our locality.so, we have grinded the edges of arms to obtain the required curvatures and drilled two holes to each of the arms as per the dimensions as shown in the Fig 7.1

Fig 7.1 Arm of Scissor Jack

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Table 7.1 Sequence of operations on top and bottom arms S no. Machine

Operation

Tools

Time (min)

1

Stores

Check the raw material

Try square, steel rule,

20

and dot punch 2

Welding shop

Welding of a flat plate

Welding gun, Files

to the angular to obtain

and Emery paper

120

channel section. 3

4

Grinding

Grinding the plate in

Grinding wheel

60

machine

vice

Radial Drilling Drilling 10 mm holes at

Drill bit, dot punch ,

40

machine

hammer and steel rule

both the ends of the plate

7.2 Power Screw A circular rod was turned to the required dimensions in a lathe machine and then we have adjusted the lathe machine in order to obtain external square threads and thus the external square threads of required dimensions were obtained as shown in the Fig 7.2.

Fig 7.2 Fabrication of power screw on lathe 54

Table 7.2 Sequence of operations on power screw S no.

Machine

Operation

Tools

Time (min)

1

Stores

Check the raw material

Outer calipers,

5

steel rule 2

Sawing machine

Cutting the length of the rod

Hack saw

25

Turning the diameter to

Single point

35

16 mm

cutting tool

as per requirement 3

Lathe machine

4

Lathe machine

Threading of square thread

Threading tool

60

5

Shop Floor

Inspection

Vernier calipers

5

7.3 Trunions A circular rod was drilled to form a through hole. Then the hole has been finished to form internal square threads corresponding to the external threads of the power screw so that the internal square threads of the trunions mate with the external threads of the power screw as shown in the Fig 7.3.

Fig 7.3 Trunions with internal threading

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Table 7.3 Sequence of operations on Trunion S no.

Machine

Operation

Tools

Time (min)

1

Stores

Check the raw material

Inner calipers,

5

steel rule 2

Sawing machine

Cutting the length of the rod

Hack saw

25

Turning the outer diameter to

Single point

35

24 mm

cutting tool

Boring the Trunions to

Boring tool

15

Internal

60

as per requirement 3

4

Lathe machine

Lathe machine

16mm diameter 5

Lathe machine

Threading of square thread

Threading tool 6

Shop Floor

Inspection

Vernier calipers

5

7.4 Top and Bottom Plates The left out pieces of the channel sections of the arms have been used for the top plate and then holes were drilled to the plate for fasteners connecting top plate and the arms. The top plate is fabricated in order to act as a loading platform as shown on the Fig 7.4. The bottom plate was fabricated by welding two L-angles so that the bottom arms fit into the bottom plate. The bottom plate is fabricated in order to obtain maximum stability to the Power Scissor Jack.

Fig 7.4 Top Plate and Bottom plate

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Table 7.4 Sequence of operations on top and bottom plates S no.

Machine

Operation

Tools

Time (min)

1

Stores

Check the raw material

Try square, steel

15

rule, dot punch 2

Welding shop

3

Grinding machine

4

Radial Drilling machine

Welding of a flat plate to the

Welding gun,

120

angular to obtain channel

Files and Emery

section.

paper

Grinding the plate in vice

Grinding wheel

90

Drilling 10 mm holes at both

Drill bit, dot

60

the ends of the plate

punch , hammer and steel rule

5

Shop Floor

Inspection

Vernier calipers

10

7.5 Power Gun It has been purchased to drive the power screw by providing a suitable slot in the head of the power screw.

7.6 Light Source A LED bulb is fitted to the base plate to facilitate the repairs during the night times.

7.7 Power Source Both power gun and light source can be activated through the battery of the automobile.

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Chapter- 8 Conclusion and Scope Conclusion In this project a prototype of power scissor jack which can be operated by a power gun has been designed and fabricated. The jack has been designed to a pay load of 4.5kN. The salient features of the present fabrication are elimination of human effort to operate the jack, through a simple electrical device which can be actuated by a 12 V battery and provision of a light source to facilitate convenient operation during night time. All the elements of the jack are fabricated in the machine shop. The assembly of the component can be achieved in 100 minutes. Another feature of the unit is provision of two trunions on both the sides of the jack to ensure jerk free operation. The elements which are useful are readily available commercially for each and early replacement of failed components if required.

Scope for future work As a development the web part of the arms can be replaced by stiffening ribs to reduce the overall weight .the top and base plates can be made foldable to make the unit more compact. Permanently mounted jacks on the vehicle can be developed so that tire change can be completely automated.

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References [1] http://powerjacks.com/about-us/powerjacks-what-we-do.php [2] RS Khurmi, A text book of Machine Design, Eurasia publishing house [3] Msmillar.hubpages.com/hub/The-Hydraulic-Jack [4]Powerjacks.com/downloads/Design%20Guides/PJLMPT-02/S1-Screw-Jacks PJLMPTDG-02.pdf [5] Scholarsresearchlibrary.com/EJAESR-vol1-iss4/EJAESR-2012-1-4-167-172.pdf [6] INPRESSCO-GERNAL ARTICLE; E-ISSN2277-4106, AUTOMATED CAR JACK. [7] Academia.edu/6167889/Modification_of_the_Existing_Design_of_a_Car_Jack. [8] http://en.wikipedia.org/wiki/Jackscrew [9]http://scholarsresearchlibrary.com/EJAESR-vol1-iss4/EJAESR-2012-1-4-167172.pdf [10] http://www.duffnorton.com/productmenu.aspx?id=7898 [11] http://www.ehs.utoronto.ca/Assets/ehs+Digital+Assets/ehs3/documents/Lifting+ Devices+Standard.pdf [12] Design and fabrication of motorized automated object lifting jack; IOSRJEN.ISSN (e):2250-3021. [13] http://www.ijceronline.com/papers/Vol4_issue07/Version-1/A0470101011.pdf [14] Module 7 Screw threads and Gear Manufacturing Methods, http://nptel.ac.in/courses/112105127/31 [15] IOSR Journal of Engineering (IOSRJEN) www.iosrjen.org, ISSN (e): 2250-3021, ISSN (p): 2278-8719, Vol. 04, Issue 07 (July. 2014), ||V1|| PP 15-28

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