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Fabrication of Hydraulic Ladder
A Project
submitted in partial fulfillment for the award of the Degree of Bachelor Bachelor of Technology Technology i n Departme Department nt of M echanical En gin eer i ng
Guided by:
Submitted by:
Mr. Dayal Singh Rathore
Pradeep Attal (33)
Department of Mechanical Engineering
Shubham Saxena (52) Sunil Kumar Dhakar (55) Vinod Kumar Sharma (59) Vishal Bajpai (61)
Department of Mechanical Engineering
Jaipur Engineering College and Research Centre, Shri Ram Ki Nangal via Vatika, Tonk Road ,Jaipur
Annexure - 1
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Candidate Declaration
We Pradeep, Shubham, Sunil, Vinod and Vishal hereby declare that the work presented in this Project
entitle “Fabrication of Hydraulic Ladder ” in partial
fulfillment of the requirements for the award of Degree of Bachelor of Technology, submitted in the Department of Mechanical Engineering
of
JECRC, Jaipur, is an authentic record of my own work under the supervision of Mr. Dayal Singh Rathore.
I also declare that the work embodied in the present thesis is my original work/extension of the existing work and has not been copied from any Journal/thesis/book, and has not been submitted by me for any other Degree/Diploma.
(Name & Signature of Candidate) Enrolment No.:…………………... Date:………………………………
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Candidate Declaration
We Pradeep, Shubham, Sunil, Vinod and Vishal hereby declare that the work presented in this Project
entitle “Fabrication of Hydraulic Ladder ” in partial
fulfillment of the requirements for the award of Degree of Bachelor of Technology, submitted in the Department of Mechanical Engineering
of
JECRC, Jaipur, is an authentic record of my own work under the supervision of Mr. Dayal Singh Rathore.
I also declare that the work embodied in the present thesis is my original work/extension of the existing work and has not been copied from any Journal/thesis/book, and has not been submitted by me for any other Degree/Diploma.
(Name & Signature of Candidate) Enrolment No.:…………………... Date:………………………………
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Annexure - II
Certificate of the Supervisor(s)
This is to certify that the Project
entitled “ Fabrication Of Hydraulic Ladder
”
submitted by Pradeep, Sunil, Shubham, Vinod and Vishal for the award of Degree of Bachelor of Technology in the Department of Mechanical Engineering of JECRC, Jaipur, is a record of authentic work carried out by him/her under my/our supervision.
The matter embodied in this Project is the original work of the candidate and h as not been submitted for the award of any other degree or diploma. It is further certified that he/she has worked with me/us for the required period in the Department of Mechanical Engineering at JECRC, Jaipur.
(Name and Signature of Supervisor) Date:………………………………….
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Acknowledgements
I would like to express my sincere gratitude to my project guide “Mr. Dayal Singh Rathore” for giving me the opportunity to work on this topic. It would never be possible for us to take this project to this level without his innovative ideas and his relentless support and encouragement.
Pradeep (33), Sunil (55), Shubham (52), Vinod (59) and Vishal (61)
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Abstract The following paper describes the design as well as analysis of a hydraulic scissor lift. Conventionally a scissor lift or jack is used for lifting a vehicle to change a tire, to gain access to go to the underside of the vehicle, to lift the body to appreciable height, and many other applications also such lifts can be used for various purposes like maintenance and many material handling operations. It can be of mechanical, pneumatic or hydraulic t ype. The design described in the paper is developed keeping in mind that the lift can be operated by mechanical means by using pantograph so that the overall cost of the scissor lift is reduced. The upward motion is achieved by the application of pressure to outside of the lowest set of support elongating the crossing pattern and propelling the work platform verticall y. In our case our lift was needed to be designed a portable and also work without consuming any electric power so we decided to use a hydraulic hand pump to power the cylinder also such design can make the lift more compact and much suitable for medium scale s cale work. This paper describes the complete study of components (hydraulic cylinder, scissor arms, spacing shaft and platform), selection of materials and analyzes the dimensions of components. Further fabrication of all the parts and assembly is carried out. Keywords — Hydraulic Hydraulic scissor lift, pantograph, hand pump, vonmisses stresses
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TABLE OF CONTENT
CHAPTER 1: INTRODUCTION
1.1 OBJECTIVE 1.2 HISTORY OF HYDRAULIC LIFT CHAPTER 2: LITERATURE REVIEW
2.1 UPRIGHT’S SCISSORS LIFT 2.2 SCAFFOLD 2.3 MECHANICAL SCISSOR LIFT 2.4 DESIGN THEORY 2.5 DESIGN ANALYSIS CHAPTER 3: CONSTRUCTION
3.1 TYPES OF LIFTS 3.2 HYDRAULIC SCISSOR LIFT 3.3 COMPONENTS OF SCISSOR LIFT 3.4 MATERIAL SELECTION 3.5 CONSTRUCTION OPERATION AND TOOLS CHAPTER 4: WORKING
4.1 WORKING PRINCIPLE 4.2 PRINCIPLE OF OPERATION OF A HYDRAULIC LIFT 4.3 CYLINDER SELECTION 4.4 DESCRIPTION AND FUNCTIONING 4.5 WORKING PROCESS 4.6 FUTURE IMPROVEMENT CHAPTER 5: NEW DEVELOPMENT (Experiment)
5.1. SCOPE OF THE STUDY
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5.2 IMPORTANCE OF THE STUDY 5.3 ANALYSIS OF MECHANICAL PROPERTY REQUIRMENTS OF MACHINE COMPONENTS 5.4 DEFLECTIONS IN SCISSOR LIFT 5.5. WHAT CAUSES DEFLECTION 5.6 WHAT CAN BE DONE TO LIMIT DEFLECTION 5.7 SUMMARY ON DEFLECTION 5.8 TYPICAL APPLICATIONS 5.9 COMMON INDUSTRIES SERVED 5.10 CUSTOM DESIGN CHAPTER 6: CONCLUSION
6.1 CONLUSION 6.2 RECOMMENDATIONS CHAPTER 7: REFERENCES
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List of TablesTable 4.1- Standards for single ac ting cylinder ……………………………………………...35 Table 4.2- Input variables and values associ ated with
the design…………………................36
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List of Figures-
Fig 2.1 Upright Scissor Lift …………………………………………………………………...16 Fig 2.2 Scaffold Tower ……………………………………………………………………….17 Fig 2.3 Mechanical Scissor Lift………………………………………………………………18 Fig 3.1 Hydraulic Scissor Lift ………………………………………………………………...24 Fig 3.2 Schematic diagram of hydraulic scissor lift …………………………………………...24 Fig 3.3 Scissor Arms. ………………………………………………………………………...26 Fig 3.4 Hydraulic Scissor Lift Schematic Diagram …………………………………………...27 Fig 4.1 Pascal Law…………………………………………………………….. …………….32 Fig 4.2 Hydraulic Multiplication …………………………………………………….……….33 Fig 4.3 Scissor jack: Loading applied at the bottom …………………………………...…......37 Fig. 4.4 Scissor Jack: Free body diagram …………………………………………………….37 Fig. 5.1 Foot control for vertical or horizontal travel. ……………………………………….46 Fig. 5.2 Accordion skirt/bellows ……………………………………………………………...46 Fig. 5.3 Explosion proof lights and switch (TK only) ………………………………..………47 Fig. 5.4 Platform roll-outs/extensions …………………………………………...……………47 Fig. 5.5 Electric/hydraulic power unit ……………………………………………………….. 47 Fig. 5.6 Rubber bumper stops (rail guided lifts only) ………………………………………..48 Fig. 5.7 Large Work Platform ………………………………………………………………..49 Fig. 5.8 Dual Mast Work Platform ………………………………………………….………..49 Fig. 5.9 Overhead Lift System ……………………………………………………………….49 Fig. 5.10 Multi-Axis Paint Booth Lift ……………………………………………………….50 Fig. 5.11 Multi-Axis Extended Reach Lift …………………………………………………...50 Fig. 5.12 Multi-Axis Blast Booth Lift.. ………………………………………………………50 Fig. 5.13 Lifting car through Scissor Lift. ……………………………………………………51
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CHAPTER 1 INTRODUCTION 1.1 OBJECTIVE-
The project is aimed at designing and constructing a hydraulically powered scissors lift to lift and lowers worker and his working equipment with ease and in the most economical way. The lift is expected to work with minimal technical challenges and greater comfort due to its wide range of application. The device can easily be handled to the site to be used with a tow-van and then powered by a generator. Between the heights of lift (i.e. the maximum height) the device can be used in any height within this range and can be descend immediately in case of emergency, and can be operated independent of a second party.
A hydraulic pallet lift is a mechanical device used for various applications for lifting of the loads to a height or level. A lift table is defined as a scissor lift used to stack, raise or lower, convey and/or transfer material between two or more elevations. The main objective of the devices used for lifting purposes is to make the table adjustable to a desired height. A scissor lift provides most economic dependable & versatile methods of lifting loads; it has few moving parts which may only require lubrication. This lift table raises load smoothly to any desired height. The scissor lift can be used in combination with any of applications such as pneumatic, hydraulic, mechanical, etc. Lift tables ma y incorporate rotating platforms (manual or powered); tilt platforms etc. as a part of the design. Scissor lift design is used because of its ergonomics as compared to other heavy lifting devices available in the market. The frame is very sturdy & strong enough with increase in structural integrity. A multiple height scissor lift is made up of two or more leg sets. These types of lifts are used to achieve high travel with relatively short platform. Industrial scissor lifts & tilters are used for a wide variety of applications in many industries which include manufacturing, warehousing, schools, grocery distribution, military, hospitals and printing. The scissor lift contains multiple stages of cross bars which can convert a linear displacement between any two points on the series of cross bars into a vertical displacement multiplied by a mechanical advantage fa ctor. This factor depends on the position of the points chosen to connect an actuator and the number of cross bar st ages. The amount of force required from the actuator is also amplified, and can result in very large forces required to begin lifting even a moderate amount of weight if the actuator is not in an optimal position. Actuator force is not constant, since the load factor decreases as a function of lift height. Elevated work platforms are mechanical devices that are used to give acces s to areas that would previously be out of reach, mostly on buildings or building sites. They are also known as Aerial Work Platforms (AWPs). They usually consist of the work platform itself – often a small metal base surrounded by a cage or railings and a mechanical arm used to raise the platform. The user then stands on the platform and controls their ascent or descent via a control deck situated there.
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Some forms of aerial work platform also have separate controls at the bottom to move the actual AWP itself while others are controlled entirely on the platform or towed by other vehicles. Most are powered either pneumatically or hydraulically. This then allows workers to work on areas that don’t include public walkways, such as top -story outdoor windows or gutters to provide maintenance. Other uses include use by fire brigade and emergency services to access people trapped inside buildings, or other dangerous heights. Some can be fitted with specialist equipment, for example allowing them to hold pieces of glass to install window planes. They are temporary measures and usually mobile, making them highly flexible as opposed to things such as lifts or elevators. However generally they are designed to lift fairly light loads and so cannot be used to elevate vehicles, generators or pieces of architecture for which a crane would more likely be used. In some cases however elevated work platforms can be designed to allow for heavier loads. Depending on the precise task there are various different types of aerial work platform which utilize separate mechanisms and fuel sources. The most common type is the
articulated Elevated Work Platform, (EWP) or ‘hydraulic platforms’ (and also known as boom lifts or cherry picker). A pantograph is connected in a manner based on parallelograms so that the movement of one pen, in tracing an image, produces identical movements in a second pen. If the first point traces a line drawing, an identical, enlarged, or a pen will draw miniaturized copy fixed to the other. Using the same principle, different kinds of pantographs are used for other forms of duplication in areas such as sculpture, minting, engraving and milling. Scissor lifts (Aerial work platforms in general) are generally used for temporary, exile access purposes such as maintenance and construction work for emergency access, which distinguishes them from permanent access equipment such as elevators. They are designed to lift limited weights. The contraction of the scissor action can be hydraulic, pneumatic or mechanical (via a lead screw or rack and pinion system). 1.2 HISTORY OF HYDRAULIC LIFT1.2.1 Hydraulic Lifting Platform:
The hydraulic lift makes use of fluid pressure to produce smooth movement during lifting. It has some benefits when compared to other lifting device; firstly, its dependency on power supply is eliminated. Secondly, it allows smooth movement without jerking due to steady increase in fluid pressure, majority of lift platform in market make use of hydraulic. Above all it has a high capacity in terms of load lifting.
In conclusion, hydraulic lifts are heavier because of the amount of fluid in circulation in the system, in extreme cold conditions, the fluid can falter or get frozen which might leads to leakage in hydraulic lines or pipes. The challenges of this system are;
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a) It is not economical to the common technician or artesian. b ) I t r e q u i r e s t r a i n e d p e r s o n n e l t o o p e r a t e i t . c) Since it make use of oil, it require a t emperature range for it proper storage. d) It is very difficult to move from place to place due to its complex design. e) Studies have shown that hydraulic lift that operates on two cylinders at most time experience delay in one of the cylinder to actuate due to poor cross feeding between cylinders. f) Sometime debris from improperly preserved oil block oil tubes and at times disrupts proper functioning of the system. g) There is always problem of valve failure. h) Hydraulic system requires too many accessories t o function efficiently. i) There is risk of slipping while working with hydraulic system, due to le ak ag e th at might emanate from the system. j) Hydraulic system is not flexible for usage because its component parts are not fully attach as a whole. k) There is frequent problem of se al leakage. l) Aging problem of oil leads to failure in valves and shorter life of pumps. There is problem of accumulation of debris in oil tank.
Fig 1.1 - Hydraulic Lifting Platform
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1.2.2 ELECTRICLIFTING PLATFORM
These lift devices make use of electromagnetic power to raise or lower through the use of electric motor. The device could be very expensive and there is high probability of jerking during startup of the device through the torque cre ated by the electric motor. The device could be put to stand still during electricity/power outage, and there is potential of electrocution when electric cables are exposed. The challenges of this system are; a) Due to frequent raising and lowering of the lift, there is possibility of snapping in the electric cable which could lead to exposure of the cable and could lead to electrocution. b) It requi res ot he r acce ssor ies to be op erat ed . c) Electric lift cannot be used where the electric power is fluctuation. d) It requires trained personnel to operate it successfully. e) It requires regular maintenance. f) The electrical control unit must not be exposed to water or higher temperature. g) Electrically operated solenoid valve could easily get damage during operation with irregular voltage supply. h) Over heating in electrical coil could damage the system. i) Fuses easily blow-out when they are used as safety device. j) Dirt in electrical system could also lead to malfunctioning of the system. k) It is expensive to acquire.
Fig 1.2 .- Electric Lifting Platform
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Hydraulic lift is a device for carrying persons and loads from one floor to another, in a multi storey building. The hydraulic lifts are of the following types.
Direct acting hydraulic lift-
The direct acting hydraulic lift consist of a ram sliding in a cylinder. A platform or a cage is fitted to the top end of ram on which goods may be placed or the persons may stand. As the liquid under pressure is admitted to the cylinder, the ram moves up and the cage is lifted. The lift of the cage is equal to the stroke of the ram. The cage moves in the downward direction when the liquid from the fixed cylinder is removed. Suspended hydraulic lift-
The suspended hydraulic lift is a modified form of the direct acting hydraulic lift. It is fitted with a jigger which is exactly, same as in the case of a hydraulic crane. The cage is suspended by ropes. It runs between guides of hard wood round steel. In order to balance the weight of the cage sliding balance weights are provided.
1.2.3 PNEUMATIC SYSTEM/AIRBAG
This device also operates like the hydraulic device, but it acquires its driving force/pressure from the air. Testimonies from operators of this device show that the failure rate of the device is very high due to frequent air leakage during operation as a result of failure in valves.
Fig 1.3.- Pneum atic Sys tem The challenges of this system are; a) There is high risk of air leakage .
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b) Pn eu mat ic sys tems ar e freq ue nt wi th val ve lea ka ge . c) The air bag is not flexible during usage.
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Chapter-2 REVIEW OF RELATED LITERATURE
Man’s quest for improvement has never been satisfied. The drive towards better and greater scientific and technological outcome has made the world dynamic. Before now, several scientist and engineers have done a lot of work as regards the scissors lift in general. A review of some of that work gives the design and construction of a hydraulic scissors lift a platform. 2.1 UPRIGHT’S SCISSORS LIFT
In Selma California, there is a manufacturer of aerial platforms by name “UPRIGHT”, this world – wide company was founded in 1946, and now it manufactures and distributes its product. According to Wikipedia article, upright was founded by an engineer, Walkce Johnson who
created and sold the first platform which was called a “scissors lift” due to the steel cross bricking that supported the platform giving it the product name “magic carpet”. The magic carpet was able to provide instant revenue for the young company due to its quick popularity among its companies. Wikipedia further explained that the company constructed innovating and by early 1930s their product included the X – series scissors lift. By 1986, they had introduced their first sigma arm lift, model SL20. In 1990, they improved upon their product line by introducing the sigma arm speed level. This feature continued to be unique to be upright product and allow self-leveling of the platform on rough terrains.
Fig 2.1.- Upright Scissor Lift
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Upright introduced an equal innovative family of boom lift in 1990s. In 1995 they produced their first trailer mounted boom. The 8P37 (known as AS38) in 1996. This truly innovated company has left their mark with the other products including compact scissors design and modular alloy bridging, as well as expanding the versatility of instant span towers with aircraft docking and faced system, you will find upright products, especially the scissors lift, as standard equipment for a variety of application it is now a visual application in numerous fields and locations. 2.2 SCAFFOLD
Scaffold allows workers to transport themselves and their materials to elevat ed heights, usually up and down in an unfinished building. Scaffolds are designed to allow workers get to elevated heights; they are used in building sites and construction sites but used mainly in building sites. According to Google internet search machine, scaffold is cr oss section of pipes, irons or woods which are arranged in such a way that workers or operators can climb on the arranged pipes to get to elevated heights. Scaffolds cannot be adjusted automatically and they only can remain fixed the way it is arranged unless rearranged. The tubes are either steel or aluminum, although composite scaffolding using filament wound tubes of glass fiber in a nylon or polyester matrix. If steel,
they are either “black” or galvanized. The tubes come in a variety of length and a standard diameter of 48.3mm. The basic difference between the two types of tubes is the lower weight of aluminum tubes (1.7kg/m as opposed to 4.4kg/m) and also a greater flexibility and so less resistance to force. Tubes are generally bought in 6.3m length and can be cut down to certain typical sizes. Boards provide a working surface for users of the scaffold. They are seasoned wood and are very strong. Scaffolds for increased height are preferably made of hardened materials l ike metal pipes. After arranging the pipes, a flat materials usually made of wood is placed on top so that the worker can stand comfortable on top.
Fig 2.2.- Scaffold Tower
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2.3 MECHANICAL SCISSOR LIFT
The mechanical scissors lift is used for lifting materials especially on construction sites. This is one of the most recent advancement on scissors lift. There, the lift utilizes a belt drive system
connected to a load screw which constructs the “X” pattern on tightening and expands it on loosening. The lead screw actually does the work, since the applied force from the wheel is converted to linear motion of the lift by help of the lead screw. This can be used to lift the working and equipment to a height.
Fig 2.3.- Mechanical Scissor Lift A general knowledge however, regarding screws will reveal the loss due to friction in the screw threats. Therefore, the efficiency of this device is low due to losses in friction. Also, the power needed to drive the machine is manual, and much energy is expanded to achieve a desired result. Its suitability however, cannot be overemphasized as it can be used in almost every part of the country whether there is availability of electricity or not. The scissor lift has a unique mechanism which uses worm and worm wheel. This mechanism provides a self-locking system which makes the scissor lift completely safe for use. Unlike the hydraulic systems, this mechanism has to be driven to bring the platform back down. This gives us the opportunity to use this lift as a machine part for accurate elevation. We have calibrated the lift w.r.t. each rotation of the angle.
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2.4 DESIGN THEORY
In this chapter, mathematical relationships are developed for the various parameters necessary for the implementation of this design and arranged in sections below corresponding to the sequence of their implementation. Hydraulic systems are used to control and transmit power. A pump driven by a prime mover such as an electric motor creates a flow of fluid, in which the pressure, direction and rate of flow are controlled by values. An actuator is used to convert the energy of the fluid back into mechanical power. The amount of output power developed depends upon the flow rate, the pressure drop a cross the actuator and its overall efficiency. Most lifting devices are powered by either electricity, pneumatic or mechanical means. Although these methods are efficient and satisfactory, they exist lots of limitations and complexity of design of such lifts as well as high cost of electricity, maintenance and repairs does not allow these lifts to exist in common places. The idea of a hydraulically powered scissors lift is based on Pascal’s law employed in car jacks
and hydraulic rams which states that “pressure exerted anywhere in a conformed incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure ratio remains the same” In this section all design concepts developed are discussed and based on evaluation cr iteria and process developed, and a final here modified to further enhance the functionality of the design. Considerations made during the design and fabrication of a s ingle acting cylinder is as follows: a. Functionality of the design. b. Manufacturability. c. Economic availability. General cost of material and fabrication techniques employed Hydraulic cylinder: The hydraulic cylinder is mounted in inclined position. The total load acting on the cylinder consists of: Mass to be put on lift: 500 kg Taking FOS = 1.5 for mass in pallet 500 x 1.5 = 750 kg rounding the mass to 800kg
Mass of top frame= 22.5 kg
Mass of each link: 5 kg (5*8) = 40kg
Mass of links of cylinder mounting = 4kg
Mass of cylinder=8.150kg
Total Mass: 22.5+40+8.150+4+800 = 874.65 kg
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Total load = 874.65x 9.81 = 8580.316N Scissors lift calculations: For a scissor lift Force required to lift the load is dependent on, Angle of link with horizontal Mounting of cylinder on the links Length of link. Formula used-
Where W = Load to be lifted S= a2 + L2 -2aL*cos α S = Distance between end points of cylinder. L= length of link = 0.6 m
α = angle of cylinder with horizont al. Now the maximum force will act on the cylinder when the cylinder is in shut down position i.e
when the scissor links are closed. For calculations we will consider α=300 Thus substituting α=300 in eqn (1), We get F=8580.316N Selecting 63mm diameter cylinder Area of the cylinder= force/pressure Area = (3.14*632)/2 =3117.24mm2 Pressure = (Force/Area) =(8580.316/3117.24*10-6 ) =27.52bar 2.4.1 Design Of Link-
Now Let Hy0 =Mass applied on the lift=800kg B=Mass of the lit which the cylinder needs to lift=74.65kg Hyi=Total weight =8580.316N Only two forces are calculated here
1. Forces at the end of link: as forces at ends of link are same in magnitude. 2. Force at middle of link. In our case, the levels are numbered from the top.
For level 1 X1 = XBi-1 for level 2 X 2= XBi-2
The angle of cylinder with horizontal is θ=200. Hyi=8580.316N X2=Hyi*i =8580.316*1*0.5*(cot20/2) =11787.112N Resultant of X 2 & HYi/4
R1 = √(11787.112)2 +(8580.316/4)2 R1 = 119 80.708N. Above force will act on all the joints at end of each link. Now force acting on the intermediate point of link is given by, Xmi=(2i-1)*Hyo* +(2i2 -2i+1)*by * =Hyo* +(2i2 -2i+1)*by * = (7848 X 0.5 X cot 200 )+(732.316 X 0.25 X cot200 ) =11512.48N
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2.5 DESIGN ANALYSIS-
Design Considerations made during the design and fabrication of a portable work platform being elevated by two hydraulic cylinders is as follows: (a) Functionality of the design (b) Manufacturability Economic availability, that is general cost of materials and fabrication techniques employed.
Design Analysis 1. Cylinder Bore = φ80 Standards for single acting cylindersMaterial – structure steel st-42 hollow tube; Tensile strength = 42kgf/mm2 = 412.02 N/mm2; FOS = 4. Hoop stress induced can be found by t = di/2 × {√st + (1– 2μ)p / st-(1+μ)p – 1} 1. Outer Diameter = d + 2 to
2. Where to = stress imparted on the tube. But the standard size is Φ75; therefore a cylinder of 75 / 50 is used; since the available size is Φ75mm then Thickness t, t = (D – d) 3. Design of Piston Rod For piston rod material of mild steel EN – 8, σt = 54 1.9856 N/mm2 . But the piston rod diameter is rounded off to 32 mm in order to sustain buckling load. The internal resistance of piston is given by; Force F= Area × Stress 4. Design of End Cover Material used Mild steel; Based on strength basis F = d × t c × σt
5. The thickness is found by industrial formula tc = d√(3 × σw / 16 × P), Where σw = working stress Piston Head Piston head diameter is 49.794 – 49.970 mm the clearance is given as the piston is used to slide forward and backward. The piston head length is chosen based on piston seals to fox and width also no of seals to fix. To check the piston rod for column action when a structure is subjected to compression it undergoes visibly large displacements transverse to t he load then it is said to buckle, for small lengths the process is elastic since the buckling displacements disappear when the load is removed.
For one end fixed and other end free C = 0.25 Let Fcr = Critical buckling load; σy= yield point; L = length of rod; I = radius of gyration; K = Minimum radius of gyration and is given by K =
√I/A 6. Critical load using Euler’s Formula Fcr = C × π2 × E / (L / K) 2 7. Fcr = π 2 × E I / 4 L2 8. Where the Slenderness ratio, L / K is 73.75, 5. Base The base structure is built-up of C – channels and hollow bars are usually used in engineering applicati ons due to their high rigidity,
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strength as compared to the other bars, the chosen C channel is ISMC (Indian standard medium weight channel). The supports and the two cylinders are flexibly coupled to the base there by not transmitting the full load on to the base. The total load on the platform and load kept on it is taken by the two cylinders and four supports which are made up of C – Channels.
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CHAPTER- 3 CONSTRUCTION 3.1 TYPES OF LIFTS-
Classification based on the type of energy used(a) Hydraulic lifts (b) Pneumatic lifts (c) Mechanical lifts Classification based on their usage(a) Scissor lifts (b) Boom lifts (c) Vehicle lifts 3.2 HYDRAULIC SCISSOR LIFT-
A scissor lift or mechanism is a device used to extend or position a platform by mechanical
means. The term “scissor” comes from the mechanic which has folding supports in criss cross “X” pattern. The extension or displacement motion is achieved by the application of force to one or more supports, resulting in an elongation of the cross pattern. The force applied to extend the scissors mechanism may by hydraulic, pneumatic or mechanical (via a lead screw or rack and pinion system). The need for the use of lift is very paramount and it runs across labs, workshops, factories, residential/commercial buildings to repair street lights, fixing of bill boards, electric bulbs etc. expanded and less-efficient, the engineers may run into one or more problems when in use. The name scissors lift originated from the ability of the device to open (expand) and close (contract) just like a scissors. Considering the need for this kind of mechanism, estimating as well the cost of expanding energy more that result gotten as well the maintenance etc. it is better to adopt this design concept to the production of the machine. The initial idea of design considered was the design of a single hydraulic ram for heavy duty vehicles and putting it underneath, but this has limitations as to the height and stability, and someone will be beneath controlling it. It was rather found out that; t here is a possibility of the individual ascending/descending, to be controlling the device himself. Therefore further research was made to see how to achieve this aim.
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Fig 3.1- Hydraulic Scissor Lift Before this time scissors lift existing use mechanical or hydraulic system powered by batteries for its operations. Several challenges were encountered in this very design. Some amongst many include; low efficiency, risk of having the batteries discharged during an emergency, extended time of operation, dependent operation, as well as maintenance cost. It is the consideration of these factors that initiated the idea of producing this hydraulically powered scissors lift with independent operator. The idea is geared towards producing a scissors lift using one hydraulic ram placed across flat, in between two cross frames and powered by a pump connected to a motor wheel may be powered by a pump generator. Also, the indivi dual ascending / descending is still the same person controlling it. I.e. the control station will be located on the top frame.
Fig 3.2.- Schematic diagram of hydraulic scissor lift
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A scissors lift is attached to a piece of equipment having a work station known as scissors lift table that houses the pump, the reservoir, the generator, control valves and connections and the motor. A scissors lift does not go as high as a boom lift; it sacrifices heights for a large work station. Where more height is needed, a boom lift can be used. A scissor lift is a device used for lifting purposes, its objectives is to make the table adjustable to desirable height. A scissor lift provide the most economic dependable and versatile methods of lifting loads, it has few moving parts, which may only require lubrication. This lift table raises the load smoothly to any desired height. The scissor lift can be used in combination w i t h a n y o f t h e p r e v i o u s l y m e n t i o n e d a p p l i c a t i o n , i . e . hydraulic, pneumatic or electrical. In order to reduce the inadequacies of the devices mentioned above, a scissors mechanism is proposed. This mechanism is incorporated with a power screw and the top o f t he sc is sor s is att ach ed a t abl e pla tf orm . Thi s dev ic e wil l ma ke use of the pow er generated from a power screw to raise or lower a platform manually.
3.3 COMPONENTS OF SCISSOR LIFT1. Scissors Arms- Leg deflection due to bending is a result of stress, which is driven by total weight supported by the legs, scissors leg length, and available leg cross section. The longer the scissors legs are, the more difficult it is to control bending under load. Increased leg strength via increased leg material height does improve resistance to deflection, but can create a potentially undesirable increased collapsed height of the lift. 2. Platform Structure - Platform bending will increase as the load’s center of gravity moves from the center (evenly distributed) to any edge (eccentrically loaded) of the platform. Also, as the scissors open during rising of the lift, the rollers roll back towards the platform hinges and create an increasingly unsupported, overhung portion of the platform assem bly. Eccentric loads applied to this unsupported end of the platform can greatly impact bending of the platform. Increased platform strength via increased support structure material height does improve resistance to deflection, but also contributes to an increased collapsed height of the lift. 3. Base Frame- Normally, the lift’s base frame is mounted to the floor and should not experience deflection. For those cases where the scissors lift is mounted to an elevated or portable frame, the base frame must be rigidly supported from beneath to support the point loading created by the two scissors leg rollers and the two scissors leg hinges. 4. Pinned Joints- Scissors lifts are pinned at all hinge points, and each pin has a running clearance between the O.D. of the pin and the I.D. of its clearance hole or bushing. The more scissors pairs, or pantographs, that are stacked on top of each other, the more pinned connections there are to accumulate movement, or deflection, when compressing these designed clearances. 5. Hydraulic Circuit – Air Entrapment- All entrapped air must be removed from the hydraulic circuit through approved “bleeding” procedures – air is very compressible and is
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often the culprit when a scissors lift over-compresses under load, or otherwise bounces (like a spring) during operation.
Fig 3.3.- Scissor Arms
6. Hydraulic Circuit – Fluid Compressibility- Oil or hydraulic fluid will compress slightly under pressure. And because there is an approximate 5:1 ratio of lift travel to cylinder stroke for most scissors lift designs (with the cylinders mounted horizontally in the legs), there is a resulting 5:1 ratio of scissors lift compression to cylinder compression. 7. Hydraulic Circuit – Hose Swell- All high pressure, flexible hosing is susceptible to a degree of hose swell when the system pressure is incre ased. System pressure drops slightly because of this increased hose volume, and the scissors table compresses under load until the maximum system pressure is reestablished. And, as with compressibility, the resulting lift movement is 5 times the change in oil column height in the hose. 8. Cylinder Thrust- Resistance Cylinders lay nearly flat inside the scissors legs when the lift is fully lowered and must generate initial horizontal forces up to 10 times the amount of the load on the scissors lift due to the mechanical disadvantage of their lifting geometry. As a result, there are tremendous stresses (and resulting deflection) placed on the scissors inner leg member(s) that are designed to resist these cylinder forces. And, as already mentioned above with any change in column length of the lifting actuator/cylinder, resulting vertical lift movement is 5 times that amount of change. 9. Load Placements- Load placement also plays a large part in scissors lift deflection. Offcentered loads because the scissors lift to deflect differently than with centered, or evenly distributed, loads. End loads (inline with the scissors) are usually shared well between the two scissors leg pairs. Side loads (perpendicular to the scissors), however, are not shared well
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between the scissors leg pairs and must be kept within acceptable design limits to prevent leg twist (unequal scissors leg pair deflection) – which often results in poor roller tracking, unequal axle pin wear, and misalignment of cylinder mounts. 10. Lift Elevations- during Transfer As mentioned above, degree of deflection is directly related to change in system pressure and change in component stress as a result of loading and unloading. Scissors lifts typically experience their highest s ystem pressure and highest stresses (and therefore the highest potential for deflection) within the first 20% of total avail able vertical travel (from the fully lowered position). Wheels Scissors Arms: this component is subjected to buckling load and bending load tending to break or cause bending of the components. Hence based on strength, stiffness, plasticity and hardness. A recommended material is stainless steel. Hydraulic Cylinder: this component is considered as a strut with both ends pinned. It is subjected to direct compressive force which imposes a bending stress which may cause buckling of the component. It is also subjected to internal compressive pressure which generates circumferential and longitudinal stresses all around the wall thickness. Hence necessary material property must include strength, ductility, toughness and hardness. The recommended material is mild steel. Top Platform: this component is subjected to the weight of the workman and his equipment, hence strength is required, the frame of the plat form is mild steel and the base is wood. Base Platform: this component is subjected to the weight of the top plat form and the scissors arms. It is also responsible for the stability of the whole assembly, therefore strength. Hardness and stiffness are needed mechanical properties. Mild steel is used.
Fig 3.4.- Hydraulic Scissor Lift Schematic Diagram
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3.4 MATERIAL SELECTION 3.4.1 Material Selection-
Material selection plays a very important role in machine design. For example, the cost of materials in any machine is a good determinant of the cost of the machi8ne. More than the cost is the fact that materials are always a very decisive factor for a good design. The choice of the particular material for the machine depends on the particular purpose and the material for the machine depends on the particular purpose and the mode of operation of the machine components. Also, it depends on the expected mode of failure of the components. Engineering materials are mainly classified as: Metal and their alloys, such as iron, steel, copper, aluminum etc. Non-metals such as glass, rubber, plastic etc. metals are further classified as ferrous metals and non-ferrous metals. Ferrous metals are those metals which have iron as their main constituent, such as cast iron, wrought iron and steels. Non-ferrous metals are those which have a metal other than iron as there main constituent, such as copper, aluminum, brass, tin, zinc etc. For the purpose of this project, based on the particular working conditions machine component were designed for only the ferrous metals have been considered. Also, certain mechanical properties of metals have greatly influenced our decisions. These properties include: Strength: it is the ability of a material to resist the externally applied force without break down or yielding the internal resistance offered without break down or yielding the internally applied force is called stress. Stiffness: it is the ability of a material to resist deformation under stress. Elasticity: it is the property of a material to regain its original shape after deformation when the external force are removed. Plasticity: it is property of a material which retains the deformation produced under load, permanently. Ductility: a very important property of the material enabling it to be drawn into wire with the application of a tensile force. A ductile material is both strong and plastic. Ductile materials
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commonly used in engineering practical (in order of diminishing ductility) are mold steel, copper, aluminum, nickel, zinc tin and lead. Malleability: it is a special case of ductility which permit materials to be rolled or hammered into thin sheets. A malleable material is plastic but not 80 essentially strong. Examples include; lead soft steel, wrought iron, wrought iron, copper and aluminum in order of diminishing malleability. Toughness: it is the property of a material to resist fracture due to high impact loads like hammer blows, when heated. This property decreases. Brittleness: it is the properties of a material opposite to ductility, it is the property of breaking of a material with little permanent deformation when subjected to tensile load, brittle materials snap off without giving any sensible elongation. Cast iron is a brittle material.
Hardness it embraces difference properties such as resistance to water, scr atching, deformation and machinability etc. it also measure of the ability of a metal to cut another metal.
3.4.2 Choice of Stainless and Mild Steel Mild Steel contains 0.05 to 0.3 percent carbons it has for almost all purpose replaced wrought iron, its greater strength giving it under viable advantages. Mild steel can be rolled, wielded and down. It can even be cast, though not very successfully. Among its application are plates for ship building, bicycle frame tubes, mesh work, bolts, nuts, studs etc. solid and hollow constructional sections, sheet metal parts and steel castings such as flywheels and locomotive wheel centers. Stainless Steel: these are steel with high rust and corrosion resistance to meet specific application requirements. They also have high strength and toughness. It is an alloy of iron with about 11% chromium and other metals l ike nickel, molybdenum etc. the properties of rust and corrosion resistance, toughness and strength, aesthetics and how coefficient of friction were considered to meet all requirements and the choice of stainless steel for the scissors members.
3.5 CONSTRUCTION OPERATION AND TOOLS
In the design and construction of the hydraulic scissors lift, the procedures followed to achieve a positive result are laid down in the preceding text. But first, a look at the operations and tools involved.
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Operations
1. 2. 3. 4.
Marking out Cutting Drilling Joining (welding and bolt and nut)
Tools
1. Engineers rule 2. Scriber 3. Hack saw 4. Hand file 5. Drilling machine 6. Welding machine 7. Pliers 8. spanner 9. Try square 10. Electric grinder 3.5.1 Construction Procedure-
Base Platform- the material used for this purpose is mild steel angle bar. (3×3 inch) thickness 3mm. this is used because the base frame is responsible for the stability of the platform. The basic dimensions were marked out using on Engr’s rule and scriber and then cut with the use of hack saw after being welded firmly clamped to the vice. They are then joined together by welding to give the base frame. Scissors Arms- these include the members that are arranged in a cross-cross ‘X’ pattern and whose construction is responsible for lifting the platform and extension and lowering of the platform.
It is usually made up of pipes with rectangular cross-section and have high resistance to bending. The material is stainless steel for corrosion and rust resistance to give high strength. After marking out, they were cut to the required sizes holes of appropriate diameter were drilled at both ends and the middle of each member. Hollow pins of external diameter corresponding to the drilled holes we then fit into holes and welded in order to strengthen the position then
joined together to give the “X” pattern using bolts and nuts. The scissors arms were brazed to increase the strength and bending resistance.
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Top Platform-
The material used for the construction of this component is mild steel angle bar for the frame and timber for the base of the platform. The angle bar is cut into the required sizes and welded to form a rigid platform. The timber was equally into the required dimensions, drilled at the edges and fastened using bolt and nut to secure it i n position at the base of the platform. Assembling of various components of the hydraulic scissors lift-
The scissors assemblage was mounted on the base frame with one end hinged and the other fitted with roller (bearing) to produce the needed motion of rolling along the rail to cause lifting and lowering of the scissors lift. The scissors arm connected to the platform is also connected with one end hinged and the other fitted with roller to effect extension and contraction of the lit. The hydraulic cylinder is connected to the first arm of the scissors lift with both ends hinged. This cylinder provides the force needed to lift the load on the platform. The force is as a result of the pressure of the hydraulic supplied to the cylinder by the pump from the reservoir. The lift is fitted with wheels to aid mobility from one location to another. Painting of the entire unit is done to improve it aesthetics and increase the corrosion and resistance to rust.
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CHAPTER- 4 WORKING 4.1 WORKING PRINCIPLE-
Hydraulic lift is a system where a liquid, usually crude oil, is pumped down hole under high pressure to operate a reciprocating pump or a jet pump. This is a very flexible pumping system and can be used to produce low- to high-volume wells. This system is capable of producing a higher volume of fluid than the mechanical lift pump. Hydraulic lift uses a pump and pumps oil very high pressure. The pump pressure is usually between 300-400 pounds per square inch (PSI) and pushes the liquid to the bottom of the piston to lift it from its seat which relatively lifts the load connected to the head of the piston-cylinder assembl y. The required power oil or produced water is reclaimed and reused to continue operating the wells. The pump produces oil on both the upstroke and the down stroke. The pump stroke speed is not easily adjustable due to varying load.
Fig 4.1.- Pascal Law Hydraulic Lift basically works on the principle of Pascal Law. Pascal Law Pascal's law or the principle of transmission of fluid-pressure is a principle in fluid mechanics that states that pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure variations (initial differences) remain the same. The basic idea behind any hydraulic system is very simple: Force that is applied at one point is transmitted to another point using an incompressible fl uid. The fluid is almost alwa ys an oil of some sort. The force is almost always multiplied in the process. The picture below
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shows the simplest possible hydraulic system: A Simple hydraulic system consisting of two pistons and an oil-filled pipe connecting them.
Fig 4.2.- Hydraulic Multiplication
Because of the shape of the original device, a pantograph also refers to a kind of structure that can compress or extend like an accordion, forming a characteristic rhomboidal pattern. This can be found in extension arms for wall-mounted mirrors, temporary fences, scissor lifts, and other scissor mechanisms such as the pantograph used in electric locomotives and trams. A Scissors lifts provide the most economical, dependable, and versatile method of lifting heavy loads. Scissors lifts have few moving parts, are well lubricated, and provide many years of trouble free operation. These lift tables raise the loads smoothly to any desired height, and can be easily configured to meet the specific speed, capacity, and foot print requirement of any hydraulic lifting application. And is by far the most popular and efficient of all styles of scissors tables used in material handling applications. In this drawing, two pistons (red) fit into two glass cylinders filled with oil (light blue) and connected to one another with an oil-filled pipe. If you apply a downward force to one piston (the left one in this drawing), then the force is transmitted to the second piston through the oil in the pipe. Since oil is incompressible, the efficiency is very good -- almost all of the applied force appears at the second piston. The great thing about hydraulic systems is that the pipe connecting the two cylinders can be any length and shape, allowing it to snake through all sorts of things separating the two pistons. The pipe can also fork, so that one master cylinder can drive more than one slave cylinder if desired. The neat thing about hydraulic systems is that it is very easy to add force multiplication (or division) to the system. In a hydraulic system, all you do is change the size of one piston and cylinder relative to the other, as shown here: The piston on the right has a surface area nine times greater than the piston on the left. When force is applied to the left piston, it will move nine units for every one unit that the right piston
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moves, and the force is multiplied by nine on the right-hand piston. Click the red arrow to see the animation. To determine the multiplication factor, start by looking at the siz e of the pistons. Assume that the piston on the left is 2 inches in diameter (1-inch radius), while the piston on 2
the right is 6 inches in diameter (3-inch radius). The area of the two pistons is Pi * r . The area of the left piston is therefore 3.14, while the area of the piston on the right is 28.26. The piston on the right is 9 times larger than the piston on the left. What that means is that any force applied to the left-hand piston will appear 9 times greater on the right-hand piston. So if you apply a 100-pound downward force to the left piston, a 900-pound upward force will appear on the right. The only catch is that you will have to depress the left piston 9 inches to raise the right piston 1 inch. The brakes of a car are a good example of a basic piston-driven hydraulic system. When you depress the brake pedal in your car, it is pushing on the piston in the brake's master cylinder. Four slave pistons, one at each wheel, actuate to press the brake pads against the brake rotor to stop the car. (Actually, in almost all cars on the road today two master cylinders are driving two slave cylinders each. That way if one of the master cylinders has a problem or springs a leak, you can still stop the car.) In most other hydraulic systems, hydraulic cylinders and pistons are connected through valves to a pump supplying high-pressure oil. Scissors lifts has developed overtime, and at each stage of its development, critical problems are solved. The hydraulic type, but this time, the load screw is replaced by a hydraulic ram powered by a pump and on electric motor and generator. One outstanding feature about this design however. Is its independent operation and increased efficiency. Fluid power is one of the greater form of power where small input results in a very large output. This scissors lift can be handled by one person to a place of use, and power the generator. The lift does not lifting immediately, the operators climbs on the platform and switches open the hydraulic circuit thereby leading to an upward extension. When the required height is reached the circuit is closed, and lifting stops the control panel or station is located on the top frame. When work is done, the scissors lift is folded by hydraulic means and handled back to the point of collection. 4.2 PRINCIPLE OF OPERATION OF A HYDRAULIC LIFT (EXTENSION AND CONTRACTION)
A scissors lift is a type of platform that can usually only move vertically. The mechanism to achieve this is the use of linked, folding supports in a criss- cross “X” pattern, known as a scissors mechanism. The upward motion is achieved by the applicati on of pressure to the outer side of the lowest set of supports, elongating the crossing pattern and propelling the work platform vertically. The platform may also have extending “bridge” to allow closer access to the work area, because of the inherent limits of vertical – only movement. The contraction of the scissor action can be hydraulic, pneumatic or mechanical (via a lead screw or rack and pinion system), but in this case, it is hydraulic. Depending on the power system employed on
the lift; it may require no power to enter “desert” mode, but rather a simple release of hydraulic or pneumatic pressure. This is the main reason that these methods of powering the lift
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(hydraulic) is preferred, as it allows a fail – safe option of returning the platform to the ground by release of a manual valve. 4.3 CYLINDER SELECTION
The hydraulic cylinder (or the hydraulic actuator) is a mechanical actuator that is used to give a unidirectional stroke. It has many applications, notably in engineering. Single Acting Cylinders
Single acting cylinders use hydraulic oil for a power stroke in one direction only. The return stroke is affected by a mechanical in one direction only. The return stroke is affected by a mechanical spring located inside the c ylinder. For single acting cylinders with no spring, some external actin force on the piston rod causes its return.
Table 4.1- Standards for single acting cylinder Double Acting Cylinders
Double acting cylinder uses compressed air or hydraulic fluid to pour both the forward and return strokes. This makes them ideal for bushing and pulling and pulling within the same application they are suitable for full stroke working only at slow speed which results in gentle contact at the ends of stroke.
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Table 4.2- Input variables and values associated wit h the design 4.4 DESCRIPTION AND FUNCTIONING-
The machine consists of a worm and worm wheel, rack and pinion, shaft in a slot and the lift scissors. Rotation of handle attached to worm/worm wheel drives the system. The worm wheel and pinion are attached to a common shaft and thus, the pinion gets driven by the rotating worm wheel. The rack moves forward and this drives the main scissor mechanism. The physics is explained below. Equations:
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Fig.- 4.3 Scissor jack: Loading applied at the bottom
Fig. 4.4- Scissor Jack: Free body diagram
4.5 WORKING PROCESS- It consist of the work platform itself – often a small metal base surrounded by a cage or railings and a mechanical arm used to raise the platform. The user then stands on the platform and controls their ascent or descent via a control deck situated there. This then allows workers to work on areas that don’t include public walkways, such as top story outdoor windows or gutters to provide maintenance. Other uses include use by fire brigade and emergency services to access people trapped inside buildings, or other dangerous heights. Some can be fitted with specialist equipment, for example allowing them to hold pieces of glass to install window planes. They are temporary measures and usually mobile, making them highly flexible as opposed to things such as lifts or elevators. Depending on the precise task there are various different types of aerial work platform which utilize separate mechanisms and fuel sources.
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It is necessary to evaluate the particular type of forces imposed on components with a view to determining the exact mechanical properties and necessa ry material for each equipment. A very brief analysis of each component follows thus: I. Scissors arms II. Hydraulic cylinder III. Top plat form IV. Base plat form V.
4.6 FUTURE IMPROVEMENTS:
We can increase contact force between shaft and pinion so as to prevent slipping and allow lifting of larger weights. Number of plates in the scissors can be increased to improve the height to number of rotations ratio.
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Chapter-5 NEW DEVELOPMENT
5.1 SCOPE OF THE STUDY
The design and construction of the hydraulic scissors lift is to lift up to a height of 3.2m and carrying capacity of less than 500kg (500 kilograms) with the available engineering materials. However, there is for academic purpose, a similar project for general carrying – capacity with a selection of better engineering materials. 5.2 IMPORTANCE OF THE STUDY
The design and construction of a hydraulic scissors lift is to lift a worker together with the working equipment comfortably and safely to a required working height not easil y accessible. It may be used without a necessary external assistance or assistance from a second party due to the concept of the design. This project will be an important engineering tool or device used in maintenance jobs. Changing of street lights, painting of high buildings and walls around the school environment. 5.3 ANALYSIS OF MECHANICAL PROPERTY REQUIREMENT OF ESSENTIAL MACHINE COMPONENTS.
It is necessary to evaluate the particular type of forces imposed on components with a view to determining the exact mechanical properties and necessa ry material for each equipment. A very brief analysis of each component follows thus: Scissors arms Hydraulic cylinder Top plat form Base plat form Wheels Scissors Arms: this component is subjected to buckling load and bending load tending to break or cause bending of the components. Hence based on strength, stiffness, plastici ty an hardness. A recommended material is stainless steel. Hydraulic Cylinder: this component is considered as a strut with both ends pinned. It is subjected to direct compressive force which imposes a bending stress which may cause buckling of the component. It is also subjected to internal compressive pressure which generates circumferential and longitudinal stresses all around the wall thickness. Hence
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necessary material property must include strength, ductility, toughness and hardness. The recommended material is mild steel. Top Platform: this component is subjected to the weight of the workman and his equipment, hence strength is required, the frame of the plat form is mild steel and the base is wood. Base Platform: this component is subjected to the weight of the top plat form and the scissors arms. It is also responsible for the stability of the whole assembly, therefore s trength. Hardness and stiffness are needed mechanical properties. Mild steel is used. Wheels: the wheels are position at the base part of the scissors lift and enable the lift to move from one place to the other without necessar y employment of external equipment like car.
5.4 DEFLECTIONS IN SCISSORS LIFT 5.4.1 LIMITING DEFLECTION IN SCISSOR LIFT-
Selecting a lift with design capacity greater than required for the application; most scissor lift design for duty at higher capacities will experience less stress in all structural components as well as lower system pressures, at lower, working capacities. Reduced stresses and pressure always result in reduced deflection. Avoid transfer of load within the first 20% of lift travel: To minimize stress and deflection, at transfer elevations, it is critical to design the conveyor or transfer system to ensure that these elevations are above the scissor lift’s “critical zone” of the first 20% of the lift available travel. Transfer load over fixed end of the lift platform: First if possible, load should not be transfer over the sides of a raised scissor lift is much more difficult to control deflection when the load is not shared equally between the two scissor legs pairs. Load transfer should be made over the hinge or fixed end of the lift platform to avoid placing concentrated load on the l ess supported, over hung end of the platform, provided the platform is equipped with “trapped” roller or is otherwise capable of withstanding this edge loading without risk of platform tipping up or losing contact with the rollers. Ensure that the based frame is lagged down and fully supported: First, base frame should be adequately attached to the surface on which they are mounted. Base frame that are not bolted, welded or otherwise attached to withstand the upward force created by the eccentric loading of the platform will contribute to deflection by bending or moving while resisting such forces. Bases must also be rigidly supported beneath the entire perimeter of the frame in order to withstand without deflection on the four point loads imposed upon the frame from above by the four scissor-legs. Two moving roller and two fixed hinge points.
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5.4.2 Deflection Defined-
Deflection in scissors lifts can be defined as the resulting change in ele vation of all or part of a scissors lift assembly, typically measured from the floor to the top of platform deck, whenever loads are applied to or removed from the lift. ANSI MH29.1 - Safety Requirements for Industrial Scissors Lifts states that. all industrial
scissors lifts will deflect under load”. The industry standard goes on to outline the maximum allowable deflection based on platform size and number of scissors mechanisms within the lift design. 5.5 What Causes Deflection?
Before attempting to discuss how to limit scissors lift deflection, it is important to understand
the contributing factors to a lift’s total deflection. An open, or raised, scissors lift acts very much like a spring would – apply a load and it compresses, remove a load and it expands. Each component within the scissors lift has the potential to store or release energy when loaded and unloaded (and therefore deflect). There are also application-specific characteristics that may promote deflection. Understanding these Top 10 root causes helps to pinpoint and apply effective measures to limit deflection. Although, there are few literatures on the design of scissors lift, this chapter has highlighted the features and constraints in the design and fabrication of a prototype unit. ANSI MH29.1 accurately points out that “it is the responsibility of the user/purchaser to advise the manufacturer where deflection may be critical to the application”. It has been noted that there are industrial best practices which can be applied to reduce the impact or amount of deflection being experienced. Scissors Legs-
Leg deflection due to bending is a result of stress, which is driven by total weight supported by the legs, scissors leg length, and available leg cross section. The longer the scissors legs are, the more difficult it is to control bending under load. Increased leg strength via increased leg material height does improve resistance to deflection, but can create a potentially undesirable increased collapsed height of the lift.
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Platform Structure
Platform bending will increase as the load’s center of gravity moves from the center (evenly distributed) to any edge (eccentrically loaded) of the platform. Also, as the scissors open during raising of the lift, the rollers roll back towards the platform hinges and create an increasingly unsupported, overhung portion of the platform assembly. Eccentric loads applied to this unsupported end of the platform can greatly impact bending of the platform. Increased platform strength via increased support structure material height does improve resistance to deflection, but also contributes to an increased collapsed height of the lift. Base Frame
Normally, the lift’s base frame is mounted to the floor and should not experience deflection. For those cases where the scissors lift is mounted to an elevated or portable frame, the potential for deflection increases. To effectively resist deflection, the base frame must be rigidly supported from beneath to support the point loading created by the two sci ssors leg rollers and the two scissors leg hinges. Pinned Joints
Scissors lifts are pinned at all hinge points, and each pin has a running clearance between the O.D. of the pin and the I.D. of its clearance hole or bushing. The more scissors pairs, or pantographs, that are stacked on top of each other, the more pinned connections there are to accumulate movement, or deflection, when compressing these running clearances under load. Hydraulic Circuit – Air Entrapment
All entrapped air must be removed from the hydraulic circuit through approved “bleeding” procedures – air is very compressible and is often the culprit when a scissors lift overcompresses under load, or otherwise bounces (like a s pring) during operation. Hydraulic Circuit – Fluid Compressibility
Oil or hydraulic fluid will compress slightly under pressure. And because there is an approximate 5:1 ratio of lift travel to cylinder stroke for most scissors lift designs (with the cylinders mounted horizontally in the legs), there is a resulting 5:1 ratio of scissors lift
compression to cylinder compression. For example: 1/16” of fluid compressibility in the cylinder(s) translates into 5/16” of vertical lift movement. Hydraulic Circuit – Hose Swell
All high pressure, flexible hosing is susceptible to a degree of hose swell when the system pressure is increased. System pressure drops slightly because of this increased hose volume, and the scissors table compresses under load until the maximum system pressure is re-
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established. And, as with compressibility, the resulting lift movement (deflection) is 5 times the change in oil column height in the hosing. Cylinder Thrust Resistance
Cylinders lay nearly flat inside the scissors legs when the lift is fully lowered and must generate initial horizontal forces up to 10 times the amount of the load on the scissors lift due to the mechanical disadvantage of their lifting geometry. As a result, there are tremendous stresses (and resulting deflection) placed on the scissors inner leg member(s) that are designed to resist these cylinder forces. And, as already mentioned above with any changes in column length along the line of the lifting actuator(s)/cylinder(s), the resulting vertical lift movement is 5 times the amount of deflection or movement of cylinder hinge points mounted to leg cross members. Load Placement
Load placement also plays a large part in scissors lift deflection. Off-centered loads cause the scissors lift to deflect differently than with centered or evenly distributed, loads. End loads (inline with the scissors) are usually shared well between the two scissors leg pairs. Side loads (perpendicular to the scissors), however, are not shared as well between the scissors leg pairs and must be kept within acceptable design limits to prevent leg t wist (unequal scissors leg pair deflection) – which, in addition to platform movement due to deflection, often results in poor roller tracking, unequal axle pin wear, and misalignment of cylinder mounts. Lift Elevation During Transfer
As mentioned above, degree of deflection is directly related to change in system pressure and change in component stress as a result of loading and unloading. Scissors lifts typically experience their highest system pressure and highest stresses (and therefore the highest potential for deflection) within the first 20% of total available vertical travel (from the fully lowered position). 5.6 What Can Be Done To Limit Deflection?
There are a variety of proven methods to reduce scissors lift deflection, with varying design and cost impacts to accomplish each. Listed below are the most common of these methods, in no particular order, to provide the reader an understanding of where to begin when attempting to reduce or eliminate deflection during load transfer (i.e. applying a load, or removing a load). Select a Lift with a Design Capacity Greater Than Required for the Application
Most scissors lifts designed for duty at higher capacities will experience less stress in all structural components, as well as lower system pressures, at lower, or de-rated, working capacities. Reduced stresses & pressures always result in reduced deflection. The amount of
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this reduction varies depending on the lift’s design, so consult the manufacturer to obtain a more specific estimate of reduction in deflection. Minimize Potential for Air Entrapment
Scissors lift manufacturers provide an approved method of “bleeding” entrapped air from a new or repaired hydraulic system which may have had air introduced. This usually involves operating an empty lift through multiple cycles, and then safely cracking open fittings near high spots in the system where air accumulates. R efer to the O&M manual for this procedure. Limit or Eliminate Hosing
Flexible hose lengths should be limited wherever possible and replaced with pipe or mechanical tubing as practicable to minimize or eliminate swell as the system pressure fluctuates. Use Mechanical Actuators in lieu of Hydraulic Actuators
Although it is more difficult, and more expensive, to achieve high vertical lifting forces with mechanical actuators, they do eliminate the issue of fluid compressibility and provide a more accurate and repeatable means of achieving – and holding – a desired transfer elevation. Avoid Transfer of Loads within First 20% of Lift’s Travel
To minimize stresses and deflection at transfer elevations, it is critical to design the conveyor
or transfer system to ensure that these elevations are above the scissors lift’s “critical zone” of the first 20% of the lift’s available travel. Transfer Loads Over Fixed End of the Platform
First, if possible, loads should not be transferred over the sides of a raised scissors lift. It is much more difficult to control deflection when the load is not shared equally between the two scissors leg pairs. Make it rule to only transfer over the ends of the lift – in line with the scissors legs. Second, load transfer should be made over the hinged, or fixed, end of the lift platform to avoid placing concentrated loads on the less supported, overhung end of the platform –
provided the platform is equipped with “trapped” rollers, or is otherwise capable of withstanding this edge loading without risk of the platform tipping up or losing contact with the rollers. Ensure that the Base Frame is Lagged Down and Fully Supported
First, base frames should be adequately attached to the surface on which they are mounted. Base frames that are not bolted, welded, or otherwise attached to withstand the upward forces created by eccentric loading of the platform will contribute to deflection by bending or moving while resisting such forces. Next, bases must be rigidly supported beneath the entire perimeter
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of the frame in order to withstand without deflection the four point loads imposed upon the frame from above by the four scissors legs – (2) moving roller points and (2) fixed hinge points. Platform Locking Pins
When there is no alternative to transferring loads over the sides of a lift, or whenever lift deflection must be held to near zero in any transfer orientation, consider using platform locking pins. These pins can be manual or powered, and mounted beneath the scissors lift deck or an adjoining fixed landing. The pins are extended into receivers located in the mating elevated structure during load transfer, and then retracted before the lift can be operated again. Use Vertical Acting Actuators in lieu of Horizontal Mounts Some permanent installations may accommodate actuators which are mounted vertically beneath the lift instead of horizontally inside the lift structure. Vertical orientation of the actuators provide a 1:1 ratio of lift travel to actuator stroke instead of the 5:1 ratio normal with horizontal mounting of the actuators inside the scissors. This means a 1:1 rat io of lift deflection to actuator compression, 80% less than the 5:1 ratio experienced normally. Vertical mounting and pushing upward against underneath side of the platform to raise the lift also eliminates the high stresses usually exerted at the actuator thrust inner leg member(s). 5.7 Summary On Deflection-
Deflection is a normal and expected characteristic of industrial scissors lifts. And though odds are that most scissors lift users have not had to concern themselves with this issue because their lifting application is fairly immune to the effects of deflection, there is always a chance that it
matters greatly. ANSI MH29.1accurately points out that “It is the responsibility of the user/purchaser to advise the manufacturer where deflection may be critical to the application.” Though deflection is easier to qualify than it is to quantify, there are industry best practices which can be applied to reduce the impact or amount of deflecti on being experienced. 5.8 TYPICAL APPLICATIONS:
A hydraulic pallet lift is a mechanical device used for various applications for lifting of the loads to a height or level. Elevated work platforms are mechanical devices that are used to give access to areas that would previously be out of reach, mostly on buildings or building sites. Most are powered either pneumatically or hydraulically. Generally they are designed to lift fairly light loads and so cannot be used to elevate vehicles, generators or pieces of architecture for which a crane would more likely be used. In some cases however elevated work platforms can be designed to allow for heavier loads. The most common type is the articulated Elevated Work Platform,
(EWP) or ‘hydraulic platforms’ (and also known as boom lifts or cherry picker).
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• Paint/Powder/Coating Application • Blast Booth/Room (Stripping & Surface Prep) • Welding/Fabrication/Assembly • Wash/Rinse (Prep) • Fall Protection/Safety/Ergonomics • Maintenance/Inspection • Material Handling/Loading Bays • Clean Room
5.8.1 STANDARD LIFT OPTIONS :
Fig. 5.1 Foot control for vertical or horizontal travel
Fig. 5.2 Accordion skirt/bellows
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Fig.- 5.3 Explosion proof lights and switch (TK only)
Fig.- 5.4 Platform roll-outs/extensions
Fig.- 5.5 Electric/hydraulic power unit
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Fig- 5.6 Rubber bumper stops (rail guided lifts only) 5.9 COMMON INDUSTRIES SERVED:
• Aircraft & Aerospace • Agricultural & Construction Equipment • Electrical Power and Distribution • Light Rail and Bus Manufacturing/Maintenance • Military Equipment • Rail Car and Locomotive Manufacturing/Maintenance • Rail Systems • Transit Authorities • Trucks and Recreational Vehicles
5.10 CUSTOM DESIGN- Custom lift systems for specific applications are as shown below-
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\
Fig.- 5.7 Large Work Platform
Fig.- 5.8 Dual Mast Work Platform
Fig. 5.9 Overhead Lift System
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Multi-Axis Lifts- Standard and custom multi-axis personnel lifts.
Fig.- 5.10 Multi-Axis Paint Booth Lift
Fig.- 5.11 Multi-Axis Extended Reach Lift
Fig.- 5.12 Multi-Axis Blast Booth Lift
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Scissor Lifts & Work Platforms:
Scissor lifts typically operate in two axis of movement and are designed for applications where people and material need only up and down travel (stationary lift), where the lift needs to be moved around to perform work (manually positioned lift), or to acces s work along a fixed area of travel (rail guided lift). Utilized in applications from wash, prep and paint, to metal blasting, welding, and assembly; scissor lifts offer a safer, more ergonomic and cost effective solution for positioning workers. Systems utilize pneumatic/hydraulic power units for hazardous areas such as paint applications or can be configured with electric/hydraulic power units for nonhazardous environments. CASE STUDY AND DESIGN METHODOLOGY-
In a typical production plant, it is possible to observe, along the l ine of handling, the platforms on which the operator can stay and proceed to assembl y, with times established by the product manufacturing. Those platforms are called skillet and, thanks to their versatility and modularity, they may be used in different quantities according to the production rate. A skillet has a tubular structure that supports any elements needed to move the platform and to make the operator able to carry out properly the assembly operations.
Fig. 5.13.- Lifting car through Scissor Lift The aim of this work is to design a new lifting table with the cheapest actuation commercially available, simple and able to respond to the functional requirements, in order to replace two commercial lifting tables actually in use on the skillets along the handling line.
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CHAPTER- 6 CONCLUSION 6.1 CONCLUSION
The design and fabrication of a portable work platform elevated by a hydraulic cylinder was carried out meeting the required design standards. The portable work platform is operated by hydraulic cylinder which is operated by a motor. The scissor lift can be design for high load also if a suitable high capacity hydraulic cylinder is used. The hydraulic scissor lift is simple in use and does not required routine maintenance. It can also lift heavier loads. The main constraint of this device is its high initial cost, but has a low operating cost. The shearing tool should be heat treated to have high strength. Savings resulting from the use of this device will make it pay for itself with in short period of time and it can be a great companion in any engineering industry dealing with rusted and unused metals.
The scissor lift can be design for high load also if a suitable high capacity hydraulic cylinder is used. The hydraulic scissor lift is simple in use and does not required routine maintenance. It can also lift heavier loads. The main constraint of this device is its high initial cost, but has a low operating cost. The shearing tool should be heat treated to have high strength. Savings resulting from the use of this device will make it pay for itself with in short period of time and it can be a great companion in any engineering industry dealing with rusted and unused metals. 1. The design and fabrication of a portable work platform elevate d by a two hydraulic cylinders was carried out successfully meeting the required design standards. 2. The portable work platform is operated by hydraulic cylinder which is operated by the hand pump. 3. The scissor lift can be designed for high load also if a suitable high capacit y hydraulic cylinder is used. 4. The hydraulic scissor lift is simple in use and does not required routine maintenance. It can also lift heavier loads. For the present dimension we get a lift of 5 ft, the scissor lift can lift a load of 1.5 – 2 tons. 5. The main constraint of this device is its high initial cost, but has a low operating cost. The shearing tool should be heat treated to have high strength. Savings resulting from the use of this device will make it pay for itself with in short period of time and it can be a great companion in any engineering industry dealing with rusted and unused metals.
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6. The device affords plenty of scope for modifications f or further improvements and operational efficiency, which should make it commercially available and attractive. Hence, its wide application in industries, hydraulic pressure system, for lifting of vehicle in garages, maintenance of huge machines, and for staking purpose. Thus, it is recommended for the engineering industry and for commercial production.
6.1.1 Performance Analysis-
Maximum height attained = 46.8 cm (21.8 cm increase) Maximum load successfully tested = 5-6 kgs 6.1.2 Apparatus Budget-
Cylinder- 5000 INR L beam- 500 INR Rectangular bar- 5000 INR Screwed rod- 200 INR U Clip- 70 INR L coupler- 200 INR Furniture- 300 INR Funner blade- 20 INR Nut, bolt, screw washer- 100 INR Stationary and measurement- 300 INR Traveling expenditure- 600 INR Other expenditure- 1000 INR Total- Approx. 9000 INR
6.2 RECOMMENDATIONS-
It is recommended that the screw and thread should be lubricated frequently so as to reduce the amount of effort required to operate the system. This also reduces the amount of wear between the screw and the nut. It is also suggested that the spindle and 40 nut should not be exposed to moisture so that it would not be susceptible to corrosion thereby reducing its strength and toughness.
