CHAPTER 1 INTRODUCTION 1.1 Mechanical 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 beperformed. Car jacks usually use mechanical advantageto allow a human to lift a vehicle by manual force alone. More powerful jacks use hydraulic power to provide more lift overgreater distances. Mechanical jacks are usually rated for a maximum lifting capacity.
1.2 Scissor Jacks Scissors jacks are also mechanical and have been in use at least since the 1930s. A scissor jack is a device constructed with a cross-hatch mechanism, much like a scissor, to lift up a vehicle for repair or storage. It typically works in just 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 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 thousands of pounds.
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CHAPTER 2 LITERATURE SURVEY Scissor 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.
Fig. 2.1
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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.
Fig. 2.2 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 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 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. Whitworth‟s tools had 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 Coaticook, a small town near Quebec, a 24year-old inventor named Frank Henry Sleeper designed a lifting jack. Like da Vinci ‟s jack, it 3
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. Local Coaticook industrialist, Arthur Osmore Norton, spotted the potential for Sleeper‟s design and in 1886 hired the young man and purchased the patent. The „Norton‟ jack was born. Over the coming years the famous „Norton‟ jacks were manufactured at plants in Boston, Coaticook 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 local 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. 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 DuffNorton 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 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 4
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”. Since then the product has evolved to push, pull, lift, lower and position loads of anything from a few kilos to hundreds of tonnes. One of the biggest single screw jacks made to date is a special Power Jacks E-Series unit that is rated for 350 tonnes –even in earthquake conditions for the nuclear industry. More recent developments have concentrated on improved efficiency and durability, resulting in changes in both lead screw and gearbox design options for scissor jacks. Today, scissor jacks can be linked mechanically or electronically and with the advances in motion-control, loads can be positioned to within microns. Improvements in gear technology together with the addition of precision ball screws and roller screws mean the applications for scissor jacks today are endless and a real alternative to hydraulics in terms of duty cycles and speed at a time when industry demands cleaner, quieter and more reliable solutions.
CHAPTER 3 PROBLEM DEFINITION 5
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.
CHAPTER 4 OBJECTIVE 6
The project relates to the designing 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 designing and finding stresses, efficiency, expected life of screw. We endeavor to develop a scissor jack such that it is cost effective, having a long life and can be handled roughly.
CHAPTER 5 SCISSOR JACK 7
5.1 Specifications •
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.
Fig. 5.1 •
Fig. 5.2
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.
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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.
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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. 8
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.
5.2 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.
5.3Working 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 asmall percentage of cars.
5.4 Components •
Frame
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Power screw 9
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Rivets
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Coupling nut
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Crank
5.4.1 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.
5.4.2 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, anticlockwise forces are applied to the screw (which would work to undo a right-hand thread), a left-handthreaded 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. 10
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. 5.3
5.4.2.1 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.
5.4.2.2 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 11
linear force (thrust) and the nut rotates. These two motions are commonly referred to as "back driving", "overhauling", or, improperly, "reversing".
Fig. 5.4
Fig. 5.5
5.4.2.3 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
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Results in maximum efficiency and minimum
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It is employed in screw jacks and clamps
2. Acme threads: •
It is a modification of square thread
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Efficiency is lower than square threads
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The slope increases the area for shear
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It is easily manufactured
3. Buttress Thread: •
It is used when large forces act along the screw axis in one direction only.
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It has higher efficiency like square threads and ease of cutting like acme threads.
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It is the strongest thread of all 12
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It has limited use of power transmission
5.4.3 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 bucktail. 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. 5.4.4 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 lefthanded threads.
5.4.5 Crank A 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.
CHAPTER 6 MARKET RESEARCH 6.1 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.
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Picture
Features • Can lift up to 990kg •
• Fig 6.1
Fig. 6.2
Electric motor powered by a 12V DC power source. Extends 13”, compacts to less than 5”.
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7’ power cord.
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Weighs 9kgs.
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Lifts 1133 kg.
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Extends from 3.75”15.4”.
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•
•
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Mechanic al input required. •
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Pros The electric motor makes operating the jack simple and easy. Can operate jack away from the car.
The jack’s simple design minimizes cost , size and weight, so it can be stored easily.
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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Operating the crank can be difficult.
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Required to be near (practically underneath a 2,000kg object to operate.
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Like the product above, there is no stability provided from the sides.
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Tools to raise the jack are not
Does not rely on electricity.
interchangeable.
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.
CHAPTER 7 MATERIAL SELECTION 7.1 Load criteria and assumptions: 15
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.
7.2 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. 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. 16
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
CHAPTER 8 3-D MODELS 8.1 Parts 17
Base frame Fig 8.1
Bottom link Fig 8.2
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Bottom packing Fig 8.3
Bottom rivet
Fig 8.4
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Link Rivet
Fig 8.5
Screw
Fig 8.6
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Coupling nut Fig 8.7
Top Link
Fig 8.8 21
Support frame Fig 8.9 8.2 Assembly
Closed Fig 8.10 22
Expanded Fig 8.11
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CHAPTER 9 PICTURES OF FABRICATED SCISSOR JACK
Fig 9.1
Fig 9.2 24
CHAPTER 10 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 tones. 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 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 jack. Thus, the product has been developed considering all the above requirements. This particular design of the motorized scissor jack will prove to be beneficial in lifting and lowering of loads.
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CHAPTER 11 REFERENCES 11.1 Books referred •
Materials and heat treatment by O.P. khanna
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Design of machine elements by khurmi
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Strength of materials by R.K. rajput
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A textbook of machine design by P.C.Sharma and D.K.Agarwal, S.K.Kataria and sons, 2006.
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A text book of machine drawing by R.S Khurmi, S. Chand and Co. Ltd., 2005.
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Mechanical engineering design by Joseph E. Shigley, McGraw Hill, 1986.
11.2 Websites referred • youtube.com(Davison Design: Jack 'N Stand Animated Video) •
google.com
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scribd.com
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Wikipedia.com
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Howstuffworks.com
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Sciencedirect.com
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B2bhydrualicjacks.com
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