Transcript
CONTENTS 1. Introduction
1.1 Abstracts 1.2. Scissor jack basic bas icss 1.3. 1.3. History Histor y 1.4. 1.4. Applications Applic ations 1.5. 1.5. Advantages Advan tages 1.6. 1.6. Disadvantag Disad vantages es
3 4 5-7 8 9 10
2. Literature Literature Review Review 2.1. Description of Mechanical Components 2.2. 2.2. Description of Special Mechanism Implemented 2.3. 2.3. Study of Scissor Jack and its Working
12-15 15-20 20-23
3. Design and Fabrication 3.1. Load criteria and assumptions 3.2. 3.2. Material Materi al selection selec tion 3.3. 3D model in CAD
25 26 27-34
4. Problems Faced
35
5. Solutions Adapted
36
6. Conclusion
37
7. Bibliography
38
GOVT. POLYTECHNIC COLLEGE, RAHOGARH
Page 1
Chapter 1: Introduction
1.1. Abstract
1.2. Scissor Jack Basics
1.3. History
1.4. Applications
1.5. Advantages
1.6. Disadvantages
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Chapter 1: Introduction
1.1. Abstract
1.2. Scissor Jack Basics
1.3. History
1.4. Applications
1.5. Advantages
1.6. Disadvantages
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1.1 Abstracts 1.1 Abstracts:: With the increasing levels of technology, the efforts being put to produce any an y kind of work has been continuously decreasing. The efforts required in achieving the desired output can be effectively and economically be decreased by the implementation of better designs. Power screws are used to convert rotary motion into translatory motion. A screw jack is an example of a power screw in which a small force applied in a horizontal plane is used to raise or lower a large load. The principle on which it works is similar to that of an inclined plane. The mechanical advantage of a screw jack is the ratio of the load applied to the effort applied. The screw jack is operated by turning a lead screw. The height of the jack is adjusted by turning a lead screw and this adjustment can be done either manually or by integrating an electric motor. 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 use mechanical advantage to allow a human to lift a vehicle by manual force alone. More powerful jacks use hydraulic 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.
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1.2 Scissor Jack Basics: 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 user to drive the mechanism. Most scissor jacks are similar in design, consisting of four main members 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" 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.
Fig 1.2.1 Conventional Scissor Jack
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1.3 History: 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) were issued the "Jack, Automobile, Screw type, Capacity 1 1/2 ton", Ordnance part number 41-J-66. This jacks, and similar jacks for trucks, were activated by using the lug wrench as a handle for the jack's ratchet action to of the jack. The 41-J-66 jack was carried in the jeep's tool compartment. Screw type jack's continued in use for small capacity requirements due to low cost of production raise or lower it. A control tab is marked up/down and its position determines the direction of movement and almost no maintenance. The virtues of using a screw as a machine, 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, that rotated by the turning of a worm shaft to drive a lifting screw to move the load - instantly recognisable as the principle we use today.
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We can‟t be 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 we have evidence of the product being developed further. With the industrial revolution of the late 18th and 19th ce nturies 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 recognised the need for precision had become as important in industry as the provision of power. 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. 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 Sa muel Duff, proprietor of a local machine shop.
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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.
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. Air Motor Power Jack There was clearly potential for using this technology for other applications and onl y 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 m echanically 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”. Worm Gear
Jack 4
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1.4 Applications:
The main applications of power screws are as follows: (i) To raise the load, e.g. screw-jack. (ii) To obtain accurate motion in machining operations, e.g. lead-screw of lathe. (iii) To clamp a workpiece, e.g. vice. (iv) To load a specimen, e.g. universal testing machine. There are three essential parts of a power screw, viz.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.
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1.5 Advantages: Power screws offer the following advantages: (i) Power screw has large load carrying capacity. (ii) The overall dimensions of the power screw are small, resulting in compact construction. (iii) Power screw is simple to design (iv) 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. (v) Power screw provides large mechanical advantage. A load of 15 kN can be raised by applying an effort as small as 400 N.Therefore, most of the power screws used in various applications like screw-jacks, clamps, valves and vices are usually manually operated. (vi) Power screws provide precisely controlled and highly accurate linear motion required in machine tool applications. (vii) Power screws give smooth and noiseless service without any maintenance. (viii) There are only a few parts in power screw. This reduces cost and increases reliability. (ix) 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.
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1.6 Disadvantages: The disadvantages of power screws are as follows: (i) 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. (ii) 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. (iii) 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 support on uneven surface esp. off-road and no inclination in that jack is tolerable.
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Chapter 2: Literature Review 2.1
Description of Mechanical Components
2.2
Description of Special Mechanism Implemented
2.3
Study of Scissor Jack and its Working
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2.1 Description of Mechanical Components Various Mechanical parts used in Scissor Jack are:
Frame
Power screw
Rivets
Coupling nut
Crank
Frame: The entire frame of the scissor jack consists of links(top and bottom), base frame, support 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. 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, 12 | P a g e
anticlockwise forces are applied to the screw (which would work to undo a right-hand thread), a left-hand-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 terms and figures that need to be understood before designing 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;
Fig 2.1.1 Power screw
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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 support 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 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:
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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
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 joining 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 joining 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.
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Crank:
It 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.
2.2
Description of Special Mechanism Implemented
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 support from the sides, so there is the possibility of the jack tipping (especially on an uneven surface. Ultimately we come up with a design with of scissor jack with side supports.
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Fig 2.2.1 Mechanically Advanced Scissor Jack
We would like to incorporate some type of side support 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 user to operate the jack with tools other than the crank provided.
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)
Design #1 represents the base model of the scissor jack, it is the most
simple.
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(b)
Design #2 has an extended base to prevent tipping when the jack is
under load.
(c)
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Design #3 Also aims to prevent tipping, but also adds stability between
the top and bottom of the jack.
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
Pros
Light weight
Simple design (less places for failure)
Cons
Small base makes tipping a risk.
No added stability
Cheap
between the top risks a
Uses little material
collapse
Easy to store
Extended base makes
1
No added stability
tipping less likely.
between the top and
Simple design
bottom risks a collapse
Shape is not compact, makes storage difficult.
2
Added material, cost weight
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Adds stability to jack.
Extended base makes
creates more areas for
tipping less likely.
problems
Prevents collapse
Reduces to a compact
Added complexity
Added components add cost
shape that is easy to store
3
Stability added with moderate weight increase
Comparing the best out of the three designs:
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 rated by 5, 10, 15 because of its importance as a design goal.
Design #1 3
Design #2 2
Design #3 1
Safety
5
10
15
Weight
3
1
2
Storage
3
1
2
Total
14
14
20
Attribute Cost
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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.
2.3
Study of Scissor Jack and its Working
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 user 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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Fig 2.3.1
Scissor jack • 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.
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. 23 | P a g e
Assembly
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.
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 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.
Product Comparison
Picture
Features
Pros
Cons
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Can lift up
Figure 1
The electric
motor
of the electric
Electric
makes
motor hurts fuel
motor
operating
economy.
powered
the jack
by a 12V
simple and
cost and the
DC power
easy.
increased
Can
complexity of the
Extends
operate
system creates
13”,
jack away
more opportunity
compacts
from the
for failure.
to less than
car.
The motor adds
Need of an
5”.
electrical power
7’ power
source could be a
cord.
hindrance when
Weighs
battery power is
9kgs.
not adequate.
Lifts 1133
The jack’s
Operating the
kg.
simple
crank can be
Extends
design
difficult.
from 3.75”-
minimizes
15.4”.
cost , size
near (practically
Mechanical
and weight,
underneath a
input
so it can be
2,000kg object to
required.
stored
operate.
easily.
Required to be
Like the product
Does not
above, there is no
rely on
stability provided
electricity.
from the sides.
figure 2
The added weight
to 990kg
source.
Tools to raise the jack are not interchangeable.
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Chapter3: Design and Fabrication
3.1. Load criteria and assumptions
3.2. Material selection
3.3. 3D drawing of various parts and assembly in CAD
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3.1
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.
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Assembly 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.
3.2
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 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. Designing 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.
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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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Parts
Base frame
Bottom link
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Bottom packing
Bottom rivet
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Link rivet
Screw shaft
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Coupling nut
Top link
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Support frame
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Assembly
Closed assembly
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Open Assembly
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Assembly with stability arrangement Assembly of mechanically advanced Scissor Jack
Chapter 4:Problems Faced
To fabricate the conventional jack, heavy thickened steel sheet and heavy press is required which was not available in the nearby market.
Arc welding in thin metal sheet completely melts the sheet a nd makes the joint weak.
Using permanent joint with pantograph arrangement makes the problem in working of the
pantograph.
The locking system mechanism is difficult to fabricate. 37 | P a g e
Chapter 5:Solution Adopted
As the press working on heavy metal sheet is difficult as there is no press machine available in nearby market hence the conventional mechanical jack is not fabricated and readymade jack is taken for further mechanically advanced attachment. As arc welding melts the thin sheet, the Amperage rating of the arc welding is set to minimum position but the problem still persists, then the thickness of the sheet is increases. As permanent joint in pantograph arrangement makes the link rigid hence we used pivoted joints for implementing the mechanism.
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Locking system mechanism was difficult to fabricate but with better design and better mechanism using the threaded shaft the problem of slipping is eliminated.
Chapter 6:Conclusion
Scissor jacks are the ideal product to push, pull, lift, lower and position loads of anything from a couple of kilograms to hundreds of tonnes. The need has long existed for an improved portable jack for automotive vehicles. It is highly desirable that a jack become available that can be operated alternatively from inside the vehicle or from a location of safety off the road on which the vehicle is located. Such a jack should desirably be light enough and be compact enough so that it can be stored in an automobile trunk, can be lifted up and carried by most adults to its position of use, and yet be capable of lifting a wheel of a 4,000-5,000 pound vehicle off the 39 | P a g e
ground. Further, it should be stable and easily controllable by a switch so that jacking can be done from a position of safety. It should be easily movable either to a position underneath the axle of the vehicle or some other reinforced support surface designed to be engaged by a mechanically advanced scissor jack.
This project proved to be most valuable in terms of teamwork and management to us. Also we explored new territories in technical creation. We faced new challenges while designing and analyzing scissor jack. The experience gained has provided us confidence in dealing with practical aspects of engineering and will prove to be invaluable for future mechanical advancement.
Chapter 6:Bibliography
1. A text book of Machine Design by R.S.Khurmi, J.K.Gupta 2. Engineering Kinematics by William Griswold Smith
3. Websites
www.google.com
www.xdocs.com 40 | P a g e
