Design and Fabrication of Hydraulic Scissor Lift

Transcription

1 MIT International Journal of Mechanical Engineering, Vol. 5, No. 2, August 2015, pp Design and Fabrication of Hydraulic Scissor Lift Doli Rani* Faculty of Mechanical Engineering Department, Sunderdeep Group of Institutions, Ghaziabad, U.P., India Nitin Agarwal Faculty of Mechanical Engineering Department, MIT, Moradabad, U.P., India Vineet Tirth Faculty of Mechanical Engineering Department, MIT, Moradabad, U.P., India ABSTRACT The following paper describes the design as well as analysis of a hydraulic scissor lift having two levels. 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. A Scissor lift is the type platform that can usually move vertically. This mechanism is achieved by the use of link, folding support in crisscross pattern known as a Pantograph. 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 vertically. 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 along with their sketches with the help of design software CATIA V5 followed by stress analysis on COMSOL. Further fabrication of all the parts and assembly is carried out. Keywords: Hydraulic Scissor, COSMOL, Scissor lift, Hydraulic circuit, software CATIA V5. І. INTRODUCTION 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 [1]. 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 [2]. 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).

2 MIT International Journal of Mechanical Engineering, Vol. 5, No. 2, August 2015, pp 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 [1-3]. Fig. 2 Fig. 1 ІІ. Working principle 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 [3]. 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. Each scissors lift is designed and manufactured to meet the industry safety requirements set forth in ANSI MH2 9.1, and is by far the most popular and efficient of all styles of scissors tables used in material handling applications. Fig. 3 COMPONENTS OF SCISSOR LIFT SCISSOR ARMS PLATFORM BASE FRAME PINNED JOINTS SPACING SHAFT HYDRAULIC CIRCUIT 1. 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.

3 MIT International Journal of Mechanical Engineering, Vol. 5, No. 2, August 2015, pp 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 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. 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 often the culprit when a scissors lift over-compresses under load, or otherwise bounces (like a spring) during operation. 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 [4]. 7. 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 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. Off-centered 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 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 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). ІІІ. DESIGN ANALYSIS Design Considerations 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 Fig. 4: Standards for single acting cylinders.

4 MIT International Journal of Mechanical Engineering, Vol. 5, No. 2, August 2015, pp Pressure = 315 bar; Material structure steel st-42 hollow tube; Tensile strength = 42kgf/mm 2 = N/mm 2 ; FOS = 4 [5]. 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) 2. Design of Piston Rod For piston rod material of mild steel EN 8, σt = N / mm 2. 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) 3. Design of End Cover Material used Mild steel; Based on strength basis F = d tc σt (5) The thickness is found by industrial formula tc = d (3 σw / 16 P) (6) Where σw = working stress 4. Piston Head Piston head diameter is 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; σy= yield point; L = length of rod; I = radius of gyration; K = Minimum radius of gyration and is given by K = I / A (7) Critical load using Euler s Formula F cr = C π 2 E / (L / K) 2 (8) F cr = π 2 E I / 4 L 2 (9) 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 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 and load kept on it is taken by the two cylinders and four supports which are made up of C Channels. ІV. RESULTS AND DISCUSSION MATLAB environment was used for the design calculations while Maple15 was used for the simulation of the model. The simulation enables the engineer to calculate the pressures and forces of the hydraulic cylinders. Results from the Simulated Model Maple software was used to simulate Figures 5 and 6 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. Fig. 5: Model of Hydraulic scissors Lift

5 MIT International Journal of Mechanical Engineering, Vol. 5, No. 2, August 2015, pp The operations of the lift is controlled by controlling the rate of descent by altering the open area of an orifice that chokes the flow of hydraulic fluid back into the sump, while the rate of ascent is controlled by the open area of a spool valve, along with the system pressure and cylinder dimensions. The effect of increasing the load on the gantry can also be investigated. Four plots were generated from the simulation, which explains the operations of the behaviour of the circuit. Fig. 6 2: D Schematic of the Hydraulic Circuit of the Scissor Lift Fig. 8: Volumetric flow rate against time (probe is located at the same place with the pressure probe) Fig. 7: Pressure against time (between port A and the port to the check valve) Fig. 9 (a): force against time

6 MIT International Journal of Mechanical Engineering, Vol. 5, No. 2, August 2015, pp 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. Table 2 S.No. Quantity Value 1. Stress at the cylinder, stc Load at piston rod,l Diameter of piston rod, Dp Minimum thickness of End cover, Tc 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. Fig. 9 (b): Stroke (length) against time Figure 9: 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 4.1 using the equations. Table 1: Input variables and values associated with the design S.No. Component Variables/ Value Unit symbols 1. Outer diameter, D o Mm Cylinder Inner diameter, Di Mm Pressure 315 Bar 2. End cover stress EST MPA Factor of safety FOS 4 Tensile stress Tensile stress on piston rod Sts 103 N/mm 2 Stp N/mm 2 5. Working stress W st 800 N/mm 2 6. Piston head diameter D ph Mm 7. Young Modulus E 207 GPA 8. Yield point stress Ypst 250 MPA V. CONCLUSIONS 1. The design and fabrication of a portable work platform elevated 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 design for high load also if a suitable high capacity 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 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. 6. The device affords plenty of scope for modifications for 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.

7 MIT International Journal of Mechanical Engineering, Vol. 5, No. 2, August 2015, pp REFERENCES [1]. Barsel, R.K., Fluid Mechanics, 2nd Edition, John Wiley & Sons, [2]. Gupta, R.K., Machine Design, 4 th Edition, Eurasia Publishing House, Ltd., [3]. Franklin Mill, Aerial Lift Safety: Operating Requirements retrieved online: 21/04/2011. [4]. Hedge, R.K., Mechanical Engineering Science, 3 rd Edition, John Wiley & Sons, 1995 [5]. Khurmi, R.S. and Gupta, R. K., Theory of Machines, 2nd Edition, Chaurasia Publishing House, Ltd., [6]. Understanding Scissor Lift Deflection, retrieved online at www. Autoquip. com, 21/04/2011. [7]. WCB Standards: A324 Forklift Mounted Work Platforms Retrieved Online 21/04/2011. [8]. Elevating Work Platform, Retrieved online at www. Wikipedia, 21/04/2011.