Design & Fabrication Of Automatic Pneumatic Lift - Online Article
Abstract
In order to implement the ideas one has to make the model of that. Human beings are restricted to some extent in bearing loads but they are not restricted for innovations. The technology is so much advanced that every problem can be solved, resources are easily available.
As a human being is able to take away a limited amount of load (50Kg or more than that), it vary from person to person. But there are many things which a human being can not be. As every one knows that human mind can do anything, any problem can be solved in this world.
To sort out this problem "LIFT" is introduced. This is a machine which has the capability of taking various loads.
Through this article one is able to make the lift. This article covers the following topics:
- Construction of an Automatic Pneumatic Lift.
- To give it electronic control.
- Study of circuits & different types of components used in lift.
- Design for lifting the weight.
Introduction
The pneumatic lift is a device used for carrying passengers or goods from one floor to another in multistoried building. The hydraulic lifts are of two types, namely:
- Direct acting pneumatic lift
- Suspended pneumatic lift
Direct acting Pneumatic lift
It consists of a ram, sliding in a fixed cylinder. At the top of the sliding ram, a cage (on which the persons may stand or goods may be placed) is fitted. The liquid under pressure flows into the fixed cylinder. This liquid exerts force on the sliding ram, which moves vertically up and thus raises the cage to the required height.
The cage is moved in the downward direction, by removing the liquid from the fixed cylinder.
Suspended Pneumatic Lift
It is a modified form of the direct acting pneumatic lift. It consists of a cage (on which persons may stand or goods may be placed) which is suspended from a wire rope. A jigger, consisting of a fixed cylinder, a sliding ram and a set of two pulley blocks, is provided at the foot of the hole of the cage. One of the pulley blocks is movable and the other is a fixed one. The end of the sliding ram is connected to the movable pulley block. A wire rope, one end of which is fixed and other end is taken round all the pulleys of the movable and fixed blocks and finally over the guide pulley. The cage is suspended from the other end of the rope. The raising or lowering of the cage of the lift is done by the jigger.
When water under high pressure is admitted into the fixed cylinder of the jigger, the sliding ram is forced to move towards left. As one end of the sliding ram is connected to the movable pulley block and hence the movable pulley block moves towards the left, thus increasing the distance between two pulley blocks. The wire rope connected to the cage is pulled and the cage is lifted. For lowering the cage, water from the fixed cylinder is taken out. The sliding ram moves towards right and hence movable pulley block also moves towards right. This decrease the distance between two pulley blocks and the cage is lowered due to increased length of the rope.
Some Basics
Hydraulics is the application of fluid mechanics to engineering devices involving liquids, usually water or oil. Hydraulics deals with such problems as the flow of fluids through pipes or in open channels and the design of storage dams, pumps, and water turbines. With other devices it deals with the control or use of liquids, such as nozzles, valves, jets, and flow meters.
Two of the most important applications of hydraulics are in the design of hydraulic actuators and hydraulic presses; these are based on Pascal's law, which states that the pressure exerted on a liquid is the same in all directions. Because force equals pressure multiplied by area, forces can be greatly amplified by having liquid enclosed between two movable pistons of different area. If, for instance, one piston has a unit area of 1 and the other a unit area of 10, a unit force of 1 applied to the smaller piston, corresponding to a pressure of 1 per unit area will result in a force of 10 per unit area on the larger piston. This mechanical advantage can be used in such hydraulic actuators as the brake on a motorcar, where the relatively small force applied at the pedal is greatly multiplied to produce a large force at the brake shoe. The control flaps of aircraft are actuated by similar hydraulic systems. Hydraulic jacks and lifts are used for raising vehicles in service stations and for lifting heavy loads in the construction industry. Hydraulic presses, which were invented by the British engineer Joseph Bramah in 1796, are employed to shape, extrude, or stamp metals and to test materials under high pressures. Units developing a force of more than 4 million kg (9 million lb) have been developed for shaping whole aircraft sections.
Pneumatic Lift
The hydraulic lift works on the principle that the effort required to move something is the product of the force and the distance the object is moved. By using an incompressible fluid to transmit the force, the hydraulic lift allows a small force applied over a large distance to have the same effect as a large force applied over a small distance. In this way, a small hand pump may be used to lift a car. In order to fill the large cylinder under the car with fluid, however, the small pump must be operated many times.
Force
Force, in physics, is any action or influence that accelerates, or changes the velocity of, an object. Force is a vector, which means that it has both direction and magnitude. When several forces act on an object, the forces can be combined to give a resultant, or net force. The resultant force acting on an object, the object's mass, and the acceleration of the object are all related to each other by Newton's second law of motion, named after the 17th-century physicist and mathematician Isaac Newton. The second law of motion states that the acceleration an object experiences multiplied by the mass of the object is equal to the resultant force acting on an object. Thus, if a given force acts on two objects of different mass, the object with a larger mass will have a lower acceleration.
An object experiences a force when it is pushed or pulled by another object. For example, shoving a stationary shopping trolley applies a force that causes the shopping trolley to accelerate. An object can also experience a force because of the influence of a field. For example, a dropped ball accelerates towards the ground because of the presence of the gravitational field; electrical charges attract or repel each other because of the presence of an electric field.
Usually, several forces act on an object at once. If multiple forces combine to give a resultant force that is zero, then the object will not accelerate; the object will either remain motionless or continue moving at a constant velocity. For example, if a person pushes a shopping trolley with a force equal in magnitude to the force of friction that opposes the trolley's motion; the forces will cancel, giving a resultant force of zero. As a result, the trolley will move down the aisle with a constant velocity. If the person suddenly stops pushing, the only force acting on the trolley is the frictional force. Since the resultant force is no longer zero, the trolley accelerates: its velocity drops to zero. (Acceleration such that, in which speed is reduced, is also known as deceleration.)
In SI units the unit of force is the Newton, which is the force that imparts to an object with a mass of 1 kg an acceleration of 1 m/sec^{2}.
Pressure
Pressure, force per unit area exerted at right angles to a surface. A standing person exerts a pressure on the ground equal to his or her weight divided by the area of the feet in contact with the ground. The pressure is greater if the person stands on tiptoe; and it can be reduced by, for example, people walking on snow by donning snowshoes, which are made large in order to spread the body weight over a larger area.
The effects of pressure can be seen in many everyday situations. The gas in a balloon exerts an outward pressure on the material of the balloon. At the same time the air outside the balloon exerts an inward pressure on it (as it does on all other objects on the Earth). When the balloon is inflated, the gas inside it exerts a greater pressure outward than the inward pressure of the atmosphere. The excess of the pressure inside the balloon keeps it stretched taut. If excess gas is let out of the balloon, the internal and external pressures become equal and cancel each other out; the material of the balloon becomes limp.
The pressure of the atmosphere on the human body would crush it if the equal internal pressure of the fluids in the body not counterbalances it. A deep-sea diver experiences enormous additional pressures from the weight of the water above: but the equally high pressure of the air inside the diving suits and hence inside the diver's body provides protection against these pressures.
In the SI, or international system of units, used in scientific work, pressure is expressed in terms of Newton per square meter (Pascal). In non-scientific contexts it is often expressed in kilograms weight per square centimeter, or pounds weight per square inch. Another frequently used unit of pressure is the atmosphere (atm), defined as equal to the pressure exerted by a column of the liquid metal mercury exactly 760 mm (29.92 inch) high. This unit is very close to the average pressure of the atmosphere at sea level. It corresponds to 101.325 kilopascals (kPa) or 14.696 lb-wt/sq in.
Pressure Gauges
Most gauges record the difference between a fluid's pressure and local atmospheric pressure. For small pressure differences, a U-tube manometer is used. It consists of a U-shaped tube with one end connected to the container and the other open to the atmosphere. Filled with a liquid, such as water, oil, or mercury, the difference in the liquid surface levels in the two manometer legs indicates the difference between the pressure and local atmospheric pressure. For higher-pressure differences, a Bourdon gauge, named after the French inventor Eugene Bourdon, is used. This consists of a hollow metal tube with an oval cross-section, bent in the shape of a hook. One end of the tube is closed, the other open and connected to the measurement region. If pressure (above local atmospheric pressure) is applied, the oval cross-section will become more nearly circular, and at the same time the tube will straighten out slightly. The resulting motion of the closed end, proportional to the pressure, can then be measured via a pointer or needle connected to the end through a suitable linkage. Gauges used for recording rapidly fluctuating pressures commonly employ piezoelectric or electrostatic sensing elements that can provide an instantaneous response.
When a pressure gauge measures the difference between the fluid pressure and the local atmospheric pressure, the latter must be added to the gauge's indication to arrive at the true absolute pressure. A negative reading corresponds to a partial vacuum.
Low gas pressure (down to about 10^{-6} mm mercury absolute) can be measured by the so-called McLeod gauge: a measured volume of gas at the unknown low pressure is compressed at constant temperature to a much smaller volume, and then the pressure is measured directly with a manometer. The unknown pressure is then calculated from Boyle's law. For still lower pressures, various gauges depending on radiation, ionization, or molecular effects are used.
Range
Pressures may range from 10^{-8} to 10^{-2} mm of mercury (absolute) for high-vacuum work to thousands of atmospheres for hydraulic presses and controls. Pressures in the range of millions of atmospheres have been obtained for experimental purposes; for the manufacture of artificial diamonds, pressures of about 70,000 atmospheres, together with temperatures in excess of 2770° C (5000° F), are required.
In the atmosphere the decreasing weight of the overlying air column with altitude leads to a reduction in local atmospheric pressure. Thus the pressure decreases from its sea-level value of 101.325 kPa (14.696 lb-wt/sq in) to about 89 per cent of this value at 1 km (0.62 mi), and to about 26 per cent at 10 km (6.2 mi).
Partial pressure is the term applied to the effective pressure that a single constituent exerts in a mixture of gases. Total atmospheric pressure is equal to the sum of the partial pressures of the atmosphere's constituents (oxygen, nitrogen, carbon dioxide, and rare gases).
Fluid
Fluid Mechanics is a physical science that deals with the action of fluids at rest or in motion and with engineering applications and devices using fluids. Fluid mechanics is basic to such diverse fields as aeronautics, chemical, civil, and mechanical engineering, meteorology, naval architecture and oceanography.
Fluid mechanics can be subdivided into two major areas: fluid static or hydrostatics, which deals with fluids at rest, and fluid dynamics, concerned with fluids in motion. The term hydrodynamics is applied to the flow of liquids or to low-velocity gas flows in which the gas can be considered as being essentially incompressible. Aerodynamics or gas dynamics is concerned with the behavior of gases when velocity and pressure changes are sufficiently large to require inclusion of the compressibility effects.
Applications of fluid mechanics include jet propulsion, turbine, compressors, and pumps. The utilization of water and oil pressure in engineering is the field of hydraulics.
Fluid Statics or Hydrostatics
A fundamental characteristic of any fluid at rest is that the force exerted on any particle within the fluid is the same in all directions. If the forces were unequal, the particle would move in the direction of the resultant force. It follows that the force per unit area, the pressure, exerted by the fluid against the walls of an arbitrarily shaped containing vessel is perpendicular to the walls at every point. If the pressure were not perpendicular an unbalanced tangential force component would exist and the fluid would move along the wall.
The French mathematician and philosopher Blaze Pascal first formulated this concept in a slightly extended form in 1647, Known as Pascal's law. It states that the pressure applied to an enclosed fluid is transmitted equally in all directions and to all parts of the enclosing vessel, if pressure differences due to the weight of the fluid can be neglected. This law has extremely important applications in hydraulics.
The top surface of a liquid at rest in an open vessel will always be perpendicular to the resultant forces acting on it. If gravity is the only force, the surface will be horizontal. If other forces in addition to gravity act, then the "free" surface will adjust itself. For instance, if a glass of water is spun rapidly about its vertical axis, both gravity and centrifugal forces will act on the water and the surface will form a parabola that is perpendicular to the resultant force.
If gravity is the only force acting on a liquid contained in an open vessel, the pressure at any point within the liquid is directly proportional to the weight of a vertical column of that liquid above that point. This, in turn, is proportional to the depth of the point below the surface and is independent of the size or shape of the container. Thus the pressure at the bottom of a vertical pipe 2.54 cm (1 inch) in diameter and 15 m (about 50 ft) high that is filled with water is the same as the pressure at the bottom of a lake about 15 m (about 50 ft) deep. Similarly, if a pipe 30 m (100 ft) long is filled with water, and slanted so that the top is only 15 m (50 ft) above the bottom vertically, the water will exert the same pressure at the bottom of the pipe, even though the distance along the pipe is much greater than the height of the vertical pipe. The weight of a column of fresh water 30 cm (12 inch) high and with a cross section of 6.5 sq cm (1 sq inch) is 195 g (0.435 lb) and this will be the force exerted at the bottom. A column of the same height but 12 times the diameter will have 144 times the volume and will weigh 144 times as much, but the pressure, which is force per unit area, will remain identical. The pressure at the bottom of a mercury column of the same height will be 13.6 times as great, as mercury is 13.6 times as dense as water.
The second important principle of fluid static's was discovered by the Greek mathematician and philosopher Archimedes. Archimedes' principle states that a submerged body is subject to a buoyancy force that is equal to the weight of the fluid displaced by that body. This explains why a heavily laden ship floats; its total weight equals exactly the weight of the water that it displaces, and this weight exerts the buoyant force supporting the ship.
The point at which all forces producing the buoyant effect, or up thrust, may be considered to act is called the center of buoyancy and is the center of gravity of the displaced fluid. When the ship is upright, this point is on the vertical centerline of the ship; similarly the center of gravity of the ship structure is also on this line. When the ship rolls, the center of buoyancy moves sideways and the line of action of the buoyancy force, which is always vertical, intersects the centerline of the ship at a point called the metacenter. For stability, the metacenter must be above the center of gravity of the ship, irrespective of the position of the center of buoyancy. In the vast majority of ships the center of buoyancy is actually below the center of gravity.
Archimedes' principle also makes possible the determination of the density of an object that is so irregular in shape that its volume cannot be measured directly. If the object is weighed first in air and then in water, the difference in weights will equal the weight of the volume of the water displaced, which is the same as the volume of the object. Thus the density of the object (weight divided by volume) can readily be determined. In very high-precision weighing, the weight of the displaced air also has to be accounted for in arriving at the correct volume and density.
Fluid Dynamics or Hydrodynamics
This branch of fluid mechanics deals with the laws of fluids in motion; these laws are considerably more complex and, in spite of the greater practical importance of fluid dynamics, only a few basic ideas can be discussed here.
Interest in fluid dynamics dates from the earliest engineering application of fluids in machines. Archimedes made an early contribution by his invention of the screw pump, if the tradition ascribing it to him is true. The pushing action of the Archimedes screw is similar to that of the corkscrew like device in a meat grinder. Other hydraulic machines and devices were developed by the Romans, who not only used Archimedes' screw for irrigation and mine pumping but also built extensive aqueduct systems, some of which are still in use. The Roman architect and engineer Vitruvius invented the horizontal waterwheel during the 1st century bc, which revolutionized corn milling.
Despite the early practical applications of fluid dynamics, little or no understanding of the basic theory existed, and development lagged accordingly. After Archimedes, more than 1,800 years elapsed before the next significant scientific advance was made by the Italian mathematician and physicist Evangelista Torricelli who invented the barometer in 1643, and formulated Torricelli's law, which related the efflux velocity of a liquid through an orifice in a vessel to the liquid height above it. The next great advance in the development of fluid mechanics had to await the formulation of the laws of motion by the English mathematician and physicist Isaac Newton. These laws were first applied to fluids by the Swiss mathematician Leonhard Euler, who derived the basic equations for a frictionless, or inviscid, fluid.
Euler first recognized that dynamical laws for fluids can only be expressed in a relatively simple form if the fluid is assumed incompressible and ideal, that is, if the effects of friction or viscosity can be neglected. Because, however, this is never the case for real fluids in motion, the results of such an analysis can only serve as an estimate for those flows in which viscous effects are small.
Incompressible & Frictionless Flows
These flows follow Bernoulli`s Principle, named after the Swiss mathematician and scientist Daniel Bernoulli. The principle states that the total mechanical energy of an incompressible and in viscid (frictionless) flow is constant along a streamline. Streamlines are imaginary flow lines that are always parallel to the local direction of the flow, and that for steady flow are also the lines followed by individual fluid particles. Bernoulli's principle leads to an interrelationship between pressure effects, velocity effects, and gravity effects, and indicates that the velocity increases as the pressure decreases. This principle is important in nozzle design and in flow measurements, and it can also be used to predict the lift of a wing in flight.
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