Design And Fabrication Of A Hydraulic Scissor Lift Engineering Essay
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Published: Mon, 5 Dec 2016
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. 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. 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 hydraulic platform is hydraulically powered and is similar in appearance to a crane consisting of separate jointed sections which allow for ‘up and over’ applications. Although, a scissor lift can move only vertically due to different unfolding system of elevation whereby a criss-cross structure elongates and compresses itself to alter the height. A scissor lift can be mechanical, pneumatic or hydraulic. In pneumatic systems, lowering, requires no energy and simply requires that pressure be released – this means collapsing quickly is impossible making them slightly safer, though less maneuverable, than articulated designs. Their powerfully supported structure also means that they are capable of bearing greater weight and using a larger platform.
1.2 Objectives of study
The objective of this project work is to design and fabricate a portable work platform elevated by two hydraulic cylinders that will be used to give access to workers in areas that would previously be out of reach, mostly on buildings or building sites. Others objectives are as follows:
To make a complete mechanical device: The idea is to make a device which does not use any electrical power so that it is wholly independent of its own.
To make a device this is suitable economically for small Scale industries: taking in to consideration the cost factor this device is suitable for small scale as well as big scale industries.
Taking safety as prime consideration: This device is safer in all respects.
To build a device which cuts the bolt without applying greater force
To develop the abilities such as working in groups, sharing responsibilities, initiative, and perseverance.
1.3 Scope and limitation of study
The scope and limitations of this project work is to design and fabricate a portable work platform elevated by two hydraulic cylinders. The design of a hydraulic lift (Elevated work platform) is known as its architecture and the space it is meant to operate is called its working envelope. Therefore, the study is limited to designing an elevated work platform using two hydraulic cylinders and depends on hydraulic actuation and also known for its portability.
Limitations: This project is not up to industrial standard due to efficiency and safety measures which is the absence of halting pin for the wheels.
The requisite design equations for a portable work platform will be modeled. A pseudo code algorithm which was implemented in MATLAB environment and Maple15 programming software was used for the simulations, and a detail design analysis and specifications of the project work were performed. These analyses, simulations and specifications coupled with the modeled equations were used to fabricate a prototype model of this work.
2.1 HISTORICAL BACKGROUND
Man has always devised ways of raising and lowering loads from one level to the next. Counterweighted levers were used in Ancient Egypt to carry water to irrigation ditches for agricultural use. The 2000 columns of the temple of Diana in Ephesus were raised to the top by using a ramp made of sandbags. Archimedes invented the Archimedean screw to lift buckets of water and other types of heavy material. In the early 13th century, the monks of the Abbey of Mont St. Michel on the coast of France used a treadmill-hoisting machine that was pulled by donkeys.
In 1743, King Louis XV commissioned the building of a counterweighted personal lift between the palace and the apartment of his mistress. However, it was the invention of the steam engine and the development of electromagnetism that paved the way for the creation of the first lifts. In 1823, paying visitors could stand on a platform in an ascending room to have a panoramic view of the city of London. In 1833, miners in the Harz Mountains were lowered into and raised from the mine by means of reciprocating rods that extended 200 feet into the mineshaft.
Frost and Strutt developed the first belt-driven, counterweighted steam lift in England in 1835. Sir William Thompson came up with the idea for the first hydraulic crane in 1845 and his device was used to unload cargo at the docks of Newcastle. The first hydraulic lift driven by water pressure was invented in 1846.
It was Elisah Grave Otis who demonstrated the first lift in New York City in 1854 and the Otis name is still synonymous with lift manufacturing. The company received its first order for a passenger lift in 185, even though they had been making freight lifts prior to that. This lift was installed in a five-story department store Broadway.
After this date many inventors came up with ingenious improvements to the lift and its uses. Escalators, or moving stairways, came on the scene in the late 1800’s. Today these devices are required in all buildings with more than one story.
The aerial work platform invention is widely credited to John L. Grove, who was an American inventor and industrialist. However, even before JLG’s first model, a company called Selma Manlift introduced a model in 1966.
As for John L. Grove, after selling his previous business, Grove Manufacturing, in 1967 he and his wife headed out on a road trip. During a stop at the Hoover Dam, Grove witnessed two workers electrocuted while working on scaffolding. Through this “tragic event” John Grove saw a large untapped market for a product that could put workers in the air more safely to perform construction and maintenance tasks.
When Grove returned home from his trip, he formed a partnership with two friends, bought a small metal fabrication business, and began designing concepts for the aerial work platform. The company was named JLG Industries Inc., and with the aid of 20 employees it released its first aerial work platform in 1970.
Aerial work platforms eventually began being designed with a variety of additional features. Many are now equipped with electrical outlets, compressed air connectors, and various other adaptations for tools.
JLG introduced its line of Workstation in the Sky models, which were designed with features for specific trades.
“Tradesmen are using AWPs to lift more than people. They need to put tools and materials up in the air, so why not give them an AWP with some tools they need-a rack for securing glass or handling duct, or a platform that has electrical outlets, a welder, or even a masonry saw-built right in ?”
2.2 Recent Advances
In recent years more modifications has been made on the Scissors type hydraulic lift table, on its design, durability, cost of production, function ability, etc. We also intend to contribute to these works of great inventors of our time by modifying their work in order to make it popular in a country like Nigeria, where such device is not much in use domestically. And to achieve the aim, portability, reduced cost of production, material used that do not affect its quality and functions, we also introduced hydraulic ports for the pressure instead of foot pedals which requires lots of energy in the up and down mode of operation of the device, thus saving energy.
2.3 Expected Contribution
Nothing much though, our contribution is to try and enhance the speed of the rate of the ascent and descent in the hydraulic circuit. Also we used rugged materials in building this project for high life span and its reliability in operations.
THEORITICAL BACKGROUND AND DESIGN ANALYSIS
3.1 Design considerations
In this section all design concepts developed are discussed and based on evaluation criteria and process developed, and a final design was chosen. However, some features of the selected design were modified to further enhance the functionality of the design. The modifications are also discussed. Detailed drawings and CAD presentations were made.
Considerations made during the design and fabrication of a portable work platform being elevated by two hydraulic cylinders is as follows:
Functionality of the design
Economic availability, that is general cost of materials and fabrication techniques employed.
3.2 System analysis
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. To ease design effort, the system is divided into the following subsystems which include:
The frame is a carriage (the skeleton of the system) which is the scissor legs and serves as a support for the occupant and the other components to be added on. The frame is made of stainless steel. The only modification to be carried out on this frame is the relocation of the bar with supports the hydraulic cylinders, and the base of the platform will be wheeled and manually hinged for easy access and transportability.
3.4 Hydraulic cylinder
The hydraulic cylinder (or the hydraulic actuator) is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in engineering vehicles.
Figure 3.1: Main components of the hydraulic cylinder
3.4.1 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 spring located inside the cylinder. For single acting cylinders with no spring, some external force acting on the piston rod causes its return. Most applications require a single acting cylinder with the spring pushing the piston and rod to the in stroked position. For other applications sprung out stroked versions can be selected. Fig 2 shows both types of single acting cylinder.
Figure 3.4.2: Single Acting Cylinder
The spring in a single acting cylinder is designed to provide sufficient force to return the piston and rod only. This allows for the optimum efficiency from the available pressure. Most single acting cylinders are in the small bore and light duty model ranges and are available in a fixed range of stroke sizes. It is not practical to have long stroke or large bore single acting cylinders because of the size and cost of the springs needed. Single acting cylinders with no spring have the full thrust or pull available for performing work. These are often double acting cylinders fitted with a breather filter in the port open to atmosphere. The cylinder can be arranged to have a powered outstroke or a powered in stroke as shown in Figure 3.3.
Figure 3.3: Single Acting Cylinder with No Spring, Push.
3.4.2 Double acting cylinders
Double acting cylinders use compressed air to power both the outstroke and in stroke. This makes them ideal for pushing and pulling within the same application. Superior speed control is possible with a double acting cylinder, achieved by controlling the exhausting back pressure. Non cushioned cylinders will make metal to metal contact between the piston and end covers at the extreme ends of stroke. They are suitable for full stroke working only at slow speeds which result in gentle contact at the ends of stroke as shown in Figure 3.4. For faster speed, external stops with shock absorption are required. These should be positioned to prevent internal contact between the piston and end covers.
Figure3.4: Double Acting Cylinder
The force exerted by double acting pneumatic cylinder on outstroke can be expressed as (3.1). The force exerted on in-stroke can be expressed as
where d1 = full bore piston diameter [m]; d2 = piston rod diameter [m].
3.4.3 Rod less cylinder
For some applications it is desirable to contain the movement produced by a cylinder within the same overall length taken up by the cylinder body. For example, action across a conveyor belt or for vertical lifting in spaces with confined headroom. The novel design of a rod less cylinder is ideal in these circumstances. The object to be moved is attached to a carriage running on the side of the cylinder barrel. A slot, the full length of the barrel, allows the carriage to be connected to the piston. Long sealing strips on the inside and outside of the cylinder tube prevent loss of air and ingress of dust. The slot is unsealed only between the lip seals on the piston as it moves backwards and forwards as shown in Figure 3.5. Direction and speed control is by the same techniques as applied to conventional cylinders.
Figure 3.5: Rodless Cylinder
3.5 Cylinder Sizing For Thrust
Theoretical thrust (outstroke) or pull (in stroke) of a cylinder is calculated by multiplying the effective area of the piston by the working pressure. The effective area for thrust is the full area of the cylinder bore. The effective area for pull is reduced by the cross section area of the piston rod as shown in Figure 3.6.
Figure 3.6: Rod and piston diameters
From the figure 3.6, B is the cylinder bore in millimeters, d is the piston rod diameter in millimeters and P is the working pressure (P) in Newton per square millimeter. The theoretical thrust or pull (F) is given by
3.5.1 Usable thrust
When selecting a cylinder size and suitable operating pressure, estimation must be made of the actual thrust required. This is then taken as a percentage of the theoretical thrust of a suitably sized cylinder. The percentage chosen will depend on whether the thrust is required at the end of movement as in a clamping application or during movement such as when lifting a load.
3.6 Clamping applications
In a clamping application the force is developed as the cylinder stops. This is when the pressure differential across the piston reaches a maximum. The only losses from the theoretical thrust will be those caused by friction. These can be assumed to be acting even after the piston has stopped. As a general rule, make an allowance of 10% for friction. This may be more for very small bore cylinders and less for very large ones. If the cylinder is operating vertically up or down the mass of any clamping plates will diminish or augment the clamping force.
3.7 Speed control
For many applications, cylinders can be allowed to run at their own maximum natural speed, but, this results in rapid mechanism movement and quick overall machine cycle times. However, there will be applications where uncontrolled cylinder speed can give rise to shock fatigue, noise and extra wear and tear to the machine components. The factors governing natural piston speed and the techniques for controlling it are covered in this section. The maximum natural speed of a cylinder is determined by:
â€¢ Cylinder size
â€¢ Port size
â€¢ Inlet and exhaust valve flow
â€¢ Oil pressure
â€¢ Bore and length of the hoses
â€¢ Load against which the cylinder is working.
From this natural speed it is possible to either increase speed or as is more often the requirement, reduce it. First we will look at how the natural speed for any given load can be changed by valve selection, hence, the smaller the selected valve, the slower the cylinder movement. When selecting for a higher speed however, the limiting factor will be the aperture in the cylinder ports as shown in figure 3.7.
Figure 3.7: Full & restricted port aperture
Valves with flow in excess of this limitation will give little or no improvement in cylinder speed. The aperture in the cylinder ports is determined by the design. Robustly constructed cylinders will often be designed full bore ports. This means that the most restrictive part of the flow path will be the pipe fitting. These cylinders are the type to specify for fast speed applications and would be used with a valve having at least the same size ports as the cylinder. Lighter duty designs, particularly small bore sizes, will have the port aperture much smaller than the port’s nominal thread size. This has the desired effect of limiting the speed of the cylinder to prevent it from self destructing through repeated high velocity stroking. The maximum natural speed of these cylinders can often be achieved with a valve that is one or two sizes down from the cylinder port size. Larger bore cylinders are designed with port sizes large enough to allow fast maximum speeds, (Khurmi and Gupta, 2006).
In many applications however they are required to operate at relatively low speeds. For an application like this, a cylinder can be driven from a valve with smaller sized ports than those of the cylinder. Once a cylinder/valve combination has been chosen, and the load is known, the natural maximum speed will be dependent on pressure. For an installed cylinder and load, an experiment can be carried out. Connect a control valve that will cause the cylinder to self reciprocate. Then start the system running at low pressure and gradually increase it. The cylinder will cycle faster and faster until a limiting speed is reached. This is the optimum pressure for that application. Increase the pressure further and the cylinder starts to slow down. This is caused by too much air entering the cylinder on each stroke. More time is therefore taken to exhaust it and results in a slower cylinder speed with any fixed combination of valve, cylinder, pressure and load, it is usually necessary to have adjustable control over the cylinder speed. This is affected with flow regulators, and allows speed to be tuned to the application. For the majority of applications, best controllability results from unidirectional flow regulators fitted to restrict the flow out of the cylinder and allow free flow in. The regulator fitted to the front port controls the outstroke speed and the one fitted to the rear port controls the in stroke speed. Speed is regulated by controlling the flow of air to exhaust which maintains a higher back pressure. The higher the back pressure the more constant the velocity against variations in load, friction and driving force. On the other side of the piston full power driving pressure is quickly reached. Many flow regulators are designed specifically for this convention.
There are a variety of seals required within a hydraulic cylinders Single acting non cushioned cylinders use the least, double acting adjustable cushioned cylinders use the most key:
Figure 3.8: Types of seals
Where 1=cushion screw seal; 2= Cushion seal; 3= Wear ring; 4= Piston seal; 5=Barrel seal; 6=piston rod/ wiper seal.
A sliding seal such as fitted to a piston, has to push outwards against the sliding surface with enough force to prevent compressed air from escaping, but keep that force as low as possible to minimize the frictional resistance. This is a difficult trick to perform, since the seal is expected to be pressure tight from zero pressure to 10 bars or more. There is a large difference between static and dynamic friction. Static friction or break-out friction as it is sometimes called builds up when the piston stops moving. Seals inherently need to exert a force radially outward to maintain a seal. This force gradually squeezes out any lubricants between the seal and the barrel wall and allows the seal to settle in to the fine surface texture. After the piston has been standing for a while, the pressure required to start movement is therefore higher than it would be if it is moved again immediately after stopping. To minimize this effect, seals should have a low radial force and high compliance. High compliance allows the seal to accommodate differences in tolerance of the seal molding and machined parts without affecting the radial force by a great degree.
3.9 Hose Pipe
Hose is flexible pipe used to transmit flow from one point to another it completes, the hydraulic circuit. The advantages of using hose is to allow relative motion between components at either end of the hose assembly; simplify routing and installation.
3.10 Hydraulic Oil
Hydraulic oil servo 68grade mineral oil is used of Viscosity range = 16 to 32 [kg/ms]
Temperature range -10 to 80degree Celsius
Wheels are made up of mild steel having diameters of Î¦180mm and shaft diameter of Î¦25 mm. the wheels are chosen on the base on the design load criteria which can sustain the external load and well as the equipment load during transpiration in industrial line. The main function of using wheels for this equipment is that machine can be moved from corner to the other corner of the industry premises as per the requirement to lift the load.
3.12 Material selection
Most engineering works are involved with materials on a daily basis in manufacturing, processing, design and construction of components and structures. The materials selected to be incorporated into any design must withstand failure analysis, (Hedge, 1995). Assumptions were made in the selection of materials which includes:
Whether the material can consistently be machined to dimensional tolerance and maintain form in use.
Ease of joining with other parts of an assembly.
Whether material can be machined economically to save costs.
The materials used in this project includes: Mild steel for the lift mechanism, the hydraulic cylinder, the piston and rod, the platform and the base, and other parts of this project will all use this material.
Figure 3.10: Side View of the Design Concept.
This concept employs a hydraulic scissor lift device which raises and lifts the caged platform. The design operates solely on electric power, but I’d rather go with the hydraulic power system due to ease of maintenance schedule and no power failure during operation.
3.13 Design analysis
Figure 3.10: standards for single acting cylinders.
Pressure = 315bar; Material – structure steel st-42 hollow tube; Tensile strength = 42kgf/mm2
= 412.02 N/mm2; FOS = 4 (Gupta, 2006)
Hoop stress induced can be found by
Where to = stress imparted on the tube. But the standard size is Î¦75; therefore a cylinder of 75 / 50 is used; since the available size is Î¦75mm then Thickness t,
3.13.2 Design of Piston Rod
For piston rod material of mild steel EN – 8, Ïƒt = 541.9856 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
3.13.3 Design of End Cover
Material used Mild steel; Based on strength basis
The thickness is found by industrial formula
where Ïƒw = working stress
3.13.4 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 the 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; Ïƒy= yield point; L = length of rod; I = radius of gyration; K = Minimum radius of gyration and is given by
Critical load using Euler’s Formula
Where the Slenderness ratio, L / K is 73.75,
The base structure is built up of C – channels & hollow bars are usually used in engineering applications due to their high rigidity, 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 & load kept on it is taken by the two cylinders & four supports which are made up of C – Channels.
3.14 Hydraulic System
The benefits of using the hydraulic system is highlighted, (Barsel, 1998)
It is easily movable and easy in operation.
It is capable of handling greater loads, reducing in labor stress
Transmission of high forces with a small space and High energy density
Energy storage capability and Easy monitoring of force
Steeples variation in motive quantities such as speed, forces and torques
Rapid reversal due to low component masses (low inertia)
Fast operating response and Uniform motion (free from shock)
Wide transmission ratio and Long service life
Design freedom in the arrangement components and Overload protection
Easy usage of standing components and sub -assemblies
Lifting of loads at particular height
Accuracy of device is higher as it works on hydraulic system.
Some demerits of using the hydraulic system is stated below, but its benefits out ways its demerits.
Pressure and flow losses in pipes and control devices
Fluid viscosity sensitive to temperature and pressure
Leakage problems and Compressibility of the hydraulic fluid
Initial costs are more and it’s Prone to oil condition.
The input data for the design of the project work is tabulated below.
Table 3.1 Input data
Outer diameter, Do
End cover stress
Factor of safety
Tensile stress on piston rod
Piston head diameter
Yield point stress
The density to be used for the calculation is constant since all the components are fabricated with the same material (steel). Also, the load acting on the top base is taken as the weight of the system.
Results and discussions
4.1.1 The simulated program
Figure 4.1: 3-D model of Hydraulic scissors Lift
Figure 4.2: The 2-D Schematic of the Hydraulic Circuit of the Scissor Lift
Figure 4.3: Pressure against time (between port A and the port to the check valve)
fluid flow rate
Figure 4.4: volumetric flow rate against time (probe is located at the same place with the pressure probe)
cylinder force stroke
Figure 4.5a: force against time Figure 4.5b: stroke (length) against time
Figure 4.5: both force and stroke (length) against time (the probe is located at the connecting line between the hydraulic cylinder and the box geometry of the model).
The values of the results obtained from MATLAB computations are tabulated in table 4.2 by inputting the parameters in table 3.1 using the equations in chapter 3.
4.1.2 Results from the fabrication
Table 4.2 Calculations and results
Stress at the cylinder, stc
Load at piston rod,L
Diameter of piston rod, Dp
Minimum thickness of
End cover, Tc
Stress on each support arms, sts
Diameter of the centre pin ( rod)
From the results obtained above, it can be seen that work is safe because the hoop stress being induced is less than tensile stress under certain conditions. This result agrees with the objective of the project because the hydraulic cylinders and the scissors arms have the strength to actuate both its maximum weight and the top base. Table 4.3 below summarizes the cost analysis of this project.
4.2.1 The simulation program:
MapleSim software was used to simulate Figure 4.1 and 4.2 which presents the 3-D model and 2-D hydraulic circuit of an elevated hydraulic cylinder work platform. This is a prototype model comprising of various components which include: probes, hydraulic cylinder, spool and check valve, atmospheric pressure source, orifice, and the Boolean step (associated with the switch logic). We had challenges with the actual hydraulic cylinder and the arms though. They were several hit and trials in making the motion between the arms joint and the hydraulic cylinders which with precise selection of the center rod which the cylinder will operate.<
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