Design And Analysis Of Scissor Jack k5k1c

MINOR PROJE DESIGN AND ANALYSIS OF

SCISSOR JACK

ADEESH JAIN UE7904 ASHISH SHARMA UE7913 HANISH GUPTA UE7924 PARNEET SINGH UE7939

Contents 1

Introduction.......................................................................................................... 4 1.1

Problem Statement........................................................................................ 5

1.2

Objective..........................................................................................................

1.3 schedule ……………………………………………………………………………………………………………… ………………….……3 2

Research & Decision Making................................................................................ 8 2.1

Product Comparison..................................................................................... 13

2.2

Preliminary Designs....................................................................................... 5

2.3

Decision making............................................................................................ 8

2.4 Final design……………………………………………………………………………………………………… ………………………8 3

4

5

Sketches............................................................................................................... 9 3.1

Jack.............................................................................................................. 10

3.2

s...................................................................................................... 11

PRO-e models..................................................................................................... 13 4.1

Parts............................................................................................................. 13

4.2

Assembly..................................................................................................... 17

Work done.......................................................................................................... 18 5.1

Load criteria and assumptions.....................................................................18

5.2

Material selection......................................................................................... 19

5.3 Stresses acting on various components………………………………………………………………………………….20 5.4 Self locking criteria…………………………………………………………………………………………………… ………..….20 6

Possible failures and errors................................................................................21

7

Work to be done ................................................................................................ 22

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Acknowledgement We would like to express our gratitude to all those who gave us the possibility to complete our projects and we feel really obliged by the and assistance provided by Mr. Amit Chauhan, our project guide. We also feel thankful towards our colleagues for their valuable suggestions and help.

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1 Introduction 1.1Mechanical jack A jack is mechanical device used to lift heavy loads or apply great forces. Jacks employ a screw thread or hydraulic cylinder to apply very high linear forces. A mechanical jack is a device which lifts heavy equipment. The most common form is a car jack, floor jack or garage jack which lifts vehicles so that maintenance can be performed. Car jacks usually use mechanical advantage to allow a human to lift a vehicle by manual force alone. More powerful jacks use hydraulic power to provide more lift over greater distances. Mechanical jacks are usually rated for a maximum lifting capacity. As our area of concern is a screw jack used for lifting the car that is scissor jack, so only the scissor jack and its background is discussed below. 1.2Scissor jack Scissor jacks are simple mechanisms used to drive large loads short distances. The power screw design of a common scissor jack reduces the amount of force required by the to drive the mechanism. Most scissor jacks are similar in design, consisting of four main driven by a power screw. 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" 4 | Page

shaped. The end fits into a ring hole mounted on the end of the screw, which is the object of 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. Power screw in a scissor jack is the foundation of whole mechanism of scissor jack.

1.3Problem Statement: The most common problem encountered while using scissor jack is the instability of jack while giving jerks to loosen the wheel nut. Also the common jack having small base is unable to provide proper on uneven surface esp. off-road and no inclination in that jack is tolerable.

1.4Objective: The project relates to the deg of simple scissor jack and its analysis along with structural improvements to make such a modified jack that is very stable and can take enough load on uneven surfaces and somewhat inclination is also allowed. The project also aims at deg and finding stresses, efficiency, expected life of screw. We endeavor to develop a screw jack such that it is cost effective, having a long life and can be handled roughly. 5 | Page

1.5Roap: Here we outline the timeline for the completion of various aspects for the project. The schedule is set so that the project is completed in phases. Phase I is market research, Phase II consists of the design process, Phase III entails PRO-e modeling of the design and simulation in ANSYS software, and the final aspect of the project is the presentation and the work that went into it.

2 Scissor jack: 2.1Specifications  The term "scissor jack" describes a wide variety of tools that all follow the same principle: using crossed beams to lift something. They do this by acting on the object they are lifting in a diagonal manner; the lift on the right side lifts the object from its left side and vice versa. This allows the to store the jack when it is not in use (with the diagonal beams flat) and to expand it when it is needed.

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 The major specification of scissor lifts is that they are all symmetrical. In order to work, the distance from the loaded point to the cross point must be the same as the distance from the cross point to the ground. This ensures that weight is distributed equally throughout the scissor lift beams.  Since scissor lifts have such a wide variety of use, they also have a wide variety of power sources. Scissor lifts for lifting cars can be powered electrically, hydraulically and of course mechanically. On the other end of the spectrum, industrial scissor lifts that people stand on are often powered by diesel, although electrical options do exist.  Scissor lifts basically fall into two categories: single scissor lifts and multiple scissor lifts. A single scissor lift has just two crossbeams and one "x." This means it can only go so high because the length of the crossbeams restricts the height of the lift, and making them too long would make it unstable. 7 | Page

On the other hand, multiple lifts have beams crossing each other, and then attaching to more beams that go the opposite direction. This allows the scissor lift to rise higher. 2.2Assembly 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. As the screw thread is turned, the jack arms travel across it and collapse or come together, forming a straight line when closed. Then, moving back the other way, they raise and come together. When opened, the four metal arms contract together, coming together at the middle, raising the jack. When closed, the arms spread back apart and the jack closes or flattens out again. 1 Working A scissor jack uses a simple theory of gears 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 8 | Page

not applying force directly to it. If you were to put pressure directly on the crank, or lean your weight against the crank, the person would not be able to turn it, even though your weight is a small percentage of cars. 2.3Components  Frame  Power screw  Rivets  Coupling nut  Crank

 Frame: The entire frame of the scissor jack consists of links(top and bottom), base frame, frame. The frame is manufactured by sheet metal processes and forming by low-medium carbon steel.

 Power screw: Power screws are used to convert rotary motion in to translational motion. It is also called translational screw. They find use in machines such as universal tensile testing machines, machine tools, automotive jacks, vises; aircraft flap extenders, trench braces, linear actuators, adjustable floor posts, micrometers, and C-clamps. A screw thread is formed by cutting a continuous helical groove around the cylinder. These grooves are cut either left hand or right hand. 9 | Page

The majority of screws are tightened by clockwise rotation, which is termed a right-hand thread. Screws with left-hand threads are used in exceptional cases. For example, anticlockwise forces are applied to the screw (which would work to undo a right-hand thread), a lefthand-threaded screw would be an appropriate choice. Power screws are typically made from carbon steel, alloy steel, or stainless steel and they are usually used with bronze, plastic, or steel mating nuts. Bronze and plastic nuts are popular for higher duty applications and they provide low coefficients of friction for minimizing drive torques. There are important and figures that need to be understood before deg power screws: 1. Pitch: is the distance from a point on one thread to the corresponding thread on the next adjacent thread, measured parallel to the axial plane. 2. Lead: is the distance the screw would advance relative to the nut in one rotation. For single thread screw, lead is equal to pitch. 3. Helix Angle: is related to the lead and the mean radius by the equation below;

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Basics of power screws

Power screws provide a compact means for transmitting motion and power. They are ideal for replacing hydraulic and pneumatic drive systems as they require no compressors, pumps, piping, filters, tanks, valves or any other items required by these systems. Also, screws don't leak so there are no problems with seals which are so common to hydraulic and pneumatic systems. And, screw systems are quiet running - no noisy compressors, pumps or exhaust valves. Screw systems are simple, reliable and easy to utilize.

Power screw motions

There are four distinct motion converting actions that can be produced by power screws and nuts. The two most common involve torque conversion to thrust. In Figure 1, the screw is rotated (torqued) and the nut moves linearly producing thrust or the nut is rotated (torqued) and the screw moves linearly. The two less common motions involve thrust conversion to torque. In Figure 2, the nut undergoes a linear force (thrust) and the screw rotates or the screw undergoes a linear force (thrust) and the nut 11 | P a g e

rotates. These two motions are commonly referred to as "back driving", "overhauling", or, improperly, "reversing".

Fig1. Fig2.

Types of power screws

There are 3 types of screw threads used in power screws: 1. Square threads:  Is used for power transmission in either direction  Results in maximum efficiency and minimum  It is employed in screw jacks and clamps 2.Acme threads:  It is a modification of square thread  Efficiency is lower than square threads  The slope increases the area for shear  It is easily manufactured 3.Buttress Thread:  It is used when large forces act along the screw axis in one direction only.  It has higher efficiency like square threads and ease of cutting like acme threads.  It is the strongest thread of all  It has limited use of power transmission 12 | P a g e

 Rivets: A rivet is a permanent mechanical fastener. Before being installed a rivet consists of a smooth cylindrical shaft with a head on one end. The end opposite the head is called the buck-tail. On installation the rivet is placed in a punched or pre-drilled hole, and the tail is upset, or bucked (i.e. deformed), so that it expands to about 1.5 times the original shaft diameter, holding the rivet in place. To distinguish between the two ends of the rivet, the original head is called the factory head and the deformed end is called the shop head or buck-tail.  Coupling nut:

A coupling nut is a threaded fastener for ing two male threads, most commonly threaded rod. The outside of the fastener is usually a hex so a wrench can hold it. Variations include reducing coupling nuts, for ing two different size threads; sight hole coupling nuts, which have a sight hole for observing the amount of engagement; and coupling nuts with left-handed threads.



Crank: is an arm keyed at right angles to the end of a

shaft, by which motion is imparted to the power screw .It mainly suffers from torsional stresses so medium carbon steel is used as it combines merits of malleability and sufficient torsional strength.

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PHASE-I (market research): 3 Research & Decision Making: 3.1 Product Comparison

Below are analyses two other car jacks that are similar to the jack I wish to design. They represent the two primary models of scissor jacks available; those powered by electricity and those that require manual input Picture

Figure 1

Features  Can lift up to 990kg  Electric motor powered by a 12V DC power source.  Extends 13”, compact s to less than 5”.  7’ power cord.  Weighs 9kgs.

Pros  The electric motor makes operatin g the jack simple and easy.  Can operate jack away from the car.

Cons The added weight of the electric motor hurts fuel economy. The motor adds cost and the increased complexity of the system creates more opportunity for failure. Need of an electrical power source could be a hindrance when battery power is not adequate.

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 



Lifts  1133 kg. Extends from 3.75”15.4”. Mechanic al input required.

 figure 2

The  jack’s simple  design minimize s cost , size and weight, so it can be stored  easily. Does not rely on electricit y. 

Operating the crank can be difficult. Required to be near (practically underneath a 2,000kg object to operate. Like the product above, there is no stability provided from the sides. Tools to raise the jack are not interchangeab le.

From Error: Reference source not found it can be seen that the overall concept of the scissor jack is constant and that any new product will be based on that concept. The products above lack from the sides, so there is the possibility of the jack tipping (especially on an uneven surface. During further investigation, I stumbled upon a video showcasing a scissor jack design with side s.

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We would like to incorporate some type of side in my jack because it enhances safety and redistributes stress, enhancing product life and functionality. The designs above also lacked interchangeability. In my design I would like the make it possible for the to operate the jack with tools other than the crank provided. 3.2 Preliminary Designs

As stated before, the basic design and mechanics of the scissor jack are simplistic and lend little room for drastic change, so any change will be a modification on this base model. Below are three preliminary design concepts sketched:

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(a)

(b)

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(c)

Design #1 represents the base model of the scissor jack, it is the most simple. Design #2 has an extended base to prevent tipping when the jack is under load. Design #3 also aims to prevent tipping, but also adds stability between the top and bottom of the jack (much like Error: Reference source not found). The stabilizing arms on design #3 raise and lower with the jack, lock into place while rising, and, when the jack is lowered, rotate to compact its shape and make storage easier. Design

1

Pros  Light weight  Simple design (less places for failure)  Cheap  Uses little material  Easy to store

Cons  Small base makes tipping a risk.  No added stability between the top risks a collapse

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



Extended base makes tipping less likely. Simple design



 2

  

3

 



Adds stability to jack. Extended base makes tipping less likely. Prevents collapse Reduces to a compact shape that is easy to store Stability added with moderate weight increase





No added stability between the top and bottom risks a collapse Shape is not compact, makes storage difficult. Added material, cost weight Added complexity creates more areas for problems Added components add cost

3.3 Decision Making

To help make a decision for the final design, the table below weighs the attributes of each design. The designs are ranked on their performance for each category, the best performance receives a 3 and the worst a 1, the values are then totaled to determine the overall best design. The designs will be assigned values based on their cost, safety, weight and storage (functionality has been omitted from this table because all three designs operate in the same basic manner and are capable of being used with a ratchet). The values for safety will be

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rated by 5, 10, 15 because of its importance as a design goal.

Attribute Cost

Design #1

Design #2

Design #3

3

2

1

Safety

5

10

15

Weight

3

1

2

Storage

3

1

2

14

14

20

Total

Design #1 uses the least amount of material, so it scored high in cost, weight and storage, but, because of the small amount of material, it is not as safe as the other designs. Design #2 adds safety but also weight, cost and poor storage. Design #3 adds safety without compromising on weight and storage, but adds cost because it has the most parts. 3.4 Final Design

Design #3 scored the highest in the analysis of alternative designs because safety is extremely important when deg a product that is to lift a 2,000 kg object from the ground. I have also decided to lengthen the base of design #3 so that it is the same length as the fully compacted jack. Design #3 also adds stability with modest increase in weight and no compromise in ease of storage, and its hexagonal screw head lends it to operation by other tools like a power drill or wrench (this product is to come equipped with a ratcheting crank).

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3.5 Load criteria and assumptions:

The load for which the jack is to be employed has to be considered first. For very heavy loads we have to deal with heavy duty jacks and in those situations scissor jacks do not work efficiently and most probably fail. While in case of low and medium intensity loads, scissor jack works efficiently and smoothly without much effort. Also the jack is handy enough to carry in the vehicle. So considering the above situation, making a scissor jack for low and moderate dead loads will be a good idea. Estimated vehicle weight: 1105kg/2440 kgs(weight of swift in unloaded condition. Weight on one side: 2440/4: 610kgs. Factor of safety: 4 Weight for which is designed: 2440kgs. 3.6 Material selection:

Secondly, the problem of material selection is solved by selecting some materials on the basis of their strength and modulus of elasticity. We here compared mild steel , aluminum , plain carbon steels and alloy 21 | P a g e

steel, stainless steel and got an overall result for the best fit material to be low-medium carbon steel . (comparison on basis of data given in MATERIALS AND HEAT TREATMENT PROCESSES by O.P. KHANNA) The material will be designed completely using plain carbon steel. Deg a scissor jack using plain carbon steel is a work of sheet metal shop. To overestimate the safety we will use calculations of strength using the plain carbon steel in its undisturbed, solid form.

LOW-MEDIUM CARBON STEEL will be used 0.29% to 0.54% carbon –e.g. AISI 1040 steel Medium carbon steels can be heat treated to have a good balance of ductility and strength. These steels are typically used in large parts, forgings and machined components. MATERIAL PROPERTIES at 25c : low-medium carbon steel Density = 7845kg/m3 Young’s modulus (E)=200 GPa Poisson’s Ratio(v)-0.3 Ultimate shear strength= 57420 PSI=342.4 MPa approx. 66% of the UTS(87000 PSI=518.8 Mpa) Yield strength= 52500 PSI =353.4 MPa

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PHASE-II (Design phase): 4 Sketches Error: Reference source not found through Error: Reference source not found show the original hand sketches used to

begin the 3D modeling phase of the project. Many of the dimensions and some design aspects of the sketches shown below were changed as necessary by the design during the solid modeling phase. 4.1 Jack 4.2 s

4.3Stresses acting on various components

1.Torsional stress acting on power screw. 23 | P a g e

4

τ max ≥

T πd j= J 32

= max torsional stress T =¿ Torque d = screw diameter τ max

2.Buckling load acting on lifting frame. W A ≤Cπ2 A L K

2

( )

W=axial load on frames L=length of frame C=1for long columns K= radius of gyration

3.Yielding stress acting on lifting frame. σT ≤

= yielding stress S = endurance limit n=¿ Factor of safety σ

Sy n

T

y

4.Bearing stress acting on rivets. σp≤

Sy n

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σp= yielding stress S = endurance limit n=¿ Factor of safety y

5.Shear stress acting on rivets τ ≤.577 S y /n

= endurance limit n=¿ Factor of safety Sy

τ =shear stress

6.Bending stress acting on coupling ts. m∗d 2 σ= Ix

I x=

π ( R 4−r 4 ) 4

= polar moment of inertia R = outer radius r = inner radius m = bending moment d = average diameter = (r+R)/2 σ = bending stress Ix

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4.4Self -locking criteria πfd ≥ l

f = coefficient of friction d = diameter of screw

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PHASE-III 5 PRO-e Models 5.1 Parts

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Baserivet Link Coupling Bottom nut packing

Screw Bottom link Top link shaft Bottom rivet frame

5.2 Assembly

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Closed assembly

Open Assembly

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ANALYSIS Following are the images of stress concentration during analysis of scissor jack. The bar on the side of the images shows the value of stress relative to its colour as colour progresses from blue to red, stress conc. Increases thereby making it prone to failure. So as we see there is a scope of improvement. Now we will try to design a modified jack making it safer.

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Bottom frame

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Bottom link

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frame

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Top rivet

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Top rivet 2

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Top packing

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Top link 1

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Top link 2

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Power screw

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Bottom rivet 1

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Bottom rivet 2

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Coupling nut

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Bottom link 2

6 Possible failures and errors: A. Unstable center of gravity 46 | P a g e

(Remedy: Weighted rear brace for balance and lengthened front ¼ floor plates extending under car.) B. Jack failure due to excess mass being lifted(>2440kgs) C. Failure of primary bolts due to bending moments and shear stresses.

7 References 7.1Books referred  Materials and heat treatment by O.P. khanna  Design of machine elements by khurmi  Strength of materials by R.K. rajput  A textbook of machine design by P.C.Sharma and D.K.Agarwal, S.K.Kataria and sons, 2006.  A text book of machine drawing by R.S Khurmi, S. Chand and Co. Ltd., 2005.  Mechanical engineering design by Joseph E. Shigley, McGraw Hill, 1986.

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7.2Websites referred  youtube.com (Davison Design: Jack 'N Stand Animated Video) (1 oct 10)

 google.com

 idoub.com(30 sep 10)  Wikipedia.com(4 oct 10)  Howstuffworks.com(23 sep 10)  Sciencedirect.com(2 oct 10)  B2bhydrualicjacks.com(1 oct 10)

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Thus we conclude “Impossible is nothing its just the mind perception and based on root analysis of data” minor project proved to be most valuable in of teamwork and management to us. Also we explored new territories in technical creation. We faced new challenges while deg and analyzing scissor jack by proengineer and inventor. The experience gained has provided us confidence in dealing with practical aspects of engineering and will prove to be invaluable as we go into placement season.

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