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CHILDREN'S SCIENTIFIC LABORATORY
Muscles from the air. Children's Science Lab
Directory / Children's Science Lab With some stretch, we can assume that the pneumatic drive of machines is one of the oldest. After all, the wind has long served man both in the sails of ships and in the blades of mills. With a more rigorous approach, the pneumatic actuator is perhaps one of the youngest, and therefore very promising. It is used for power clamping of parts, movement of tools, stepping and intermittent feeds in a straight line and in a circle, used in cutting, pressing, assembling and many other technological operations. Suffice it to say that approximately half of industrial robots are pneumatically driven. Its basic idea is extremely simple. The compressor compresses the air. This gas "spring" stores stored potential energy until air is supplied to the air motor. When expanding, the potential energy will turn into the kinetic energy of the output link, for example, a piston with a rod, which, in turn, will set the working body of the machine in motion. In addition to the simplicity of design, the pneumatic actuator has many advantages. First of all, the working fluid is always at hand, it is literally taken "out of thin air". In addition, after use, it is thrown away there, and almost without any environmental troubles. And since air is hygienic compared to other working fluids, the pneumatic actuator is widely used in the food, electronic, pharmaceutical industries, as well as in precision instrumentation. Installations with a pneumatic drive, other things being equal, are cheaper, more reliable, work well in heat and cold, are not afraid of high humidity and dust, and guarantee complete fire, electrical and explosion safety. The service life of pneumatic actuators reaches 20000 hours, the working force reaches several tons, and the working speeds are 5 times higher than those of the hydraulic actuator, and both force and speed can be smoothly adjusted using very simple devices. In many cases, the pneumatic drive can be connected directly to the working body of the machine, which is thus driven without complex mechanical transmissions. Other important advantages compared to the electric drive are the ability to brake to a standstill under load for an unlimited time and the ability to do without overload protection devices. So, there are many advantages, and the pneumatic drive, of course, would be out of competition if it did not have equally numerous shortcomings. It is difficult to get rid of them, since these shortcomings are an organic continuation of the merits. They are due to the fact that the working fluid is air, a compressible gas. Due to this property, it is impossible to carry out smooth movement of the working bodies of the machine during load fluctuations, it is difficult to stop the tool at a strictly defined point, and the pneumatic command through the pipeline can only be transmitted at the speed of sound. Therefore, in some cases, hybrid systems are convenient: pneumohydraulic (if you need high smoothness or stopping accuracy) and electro-pneumatic (if you need to ensure speed). The advantage of the hydraulic drive is the ability to use high pressure of the working fluid (up to 500 atmospheres). It allows you to create forces of hundreds and thousands of tons with small cylinder sizes. Why is the same high pressure not used in the pneumatic actuator? Firstly, it is difficult to create it in an air compressor, and secondly, it is dangerous to use. When a pipeline breaks, the compressed air will blow the pieces apart like shrapnel. So, summing up, it can be argued that, except for those cases when great efforts and accuracy of fixing a part or a working tool are required, it is best to use a simple, cheap and reliable pneumatic actuator. A variety of mechanisms are used as pneumatic engines: membrane, piston, vane, turbine ... But it is not enough to have an engine that performs mechanical work, you also need to control its movement, and for this you need to solve three main tasks: change the direction of rectilinear and rotational motion, smoothly change its speed and smoothly regulate the generated working force. For this purpose, various pneumatic devices have been created.
Let's talk a little more about these mechanisms. The simplest air motor is a diaphragm actuator with a return spring that compresses during forward travel. Its main advantages are the simplicity of design, the tightness of the working cavity and only one command pneumatic line. And the main drawback is a relatively small working stroke. The membrane mechanism has found wide application in the petrochemical and gas industries, as well as in transport. It opens the doors of buses, activates the brakes of railway cars and trucks. The piston pneumatic cylinder is even more popular among machine builders. Single acting cylinders are similar to diaphragm motors and have the same advantages and disadvantages. Double-acting pneumatic cylinders provide significantly larger strokes, and therefore are more often used. Until recently, only the rod served as the output link in such engines. When compressed air is supplied to one of the cavities of the cylinder, the other cavity is connected to the atmosphere. Therefore, in a double-acting pneumatic cylinder, the piston with the rod can only be in two extreme stable positions - either the rod is fully retracted or fully extended. When the cylinder diameter is limited, a double or even triple pneumatic cylinder is used. It consists of two or three cylinders connected in series with each other, working on one common rod. In this case, the forces acting on the pistons add up. If the pneumatic cylinder is installed vertically, then when the compressed air supply is interrupted, its stem may fall under the action of gravity. To prevent this phenomenon, FESTO (Austria) has developed a pneumatic cylinder, in which the rod is securely fixed by a special mechanism, and is released again when compressed air is supplied. The transmission of motion using a rod has a number of disadvantages. First, the stem must be sealed. Secondly, when the rod is fully extended, the total length of the pneumatic cylinder almost doubles. Thirdly, the magnitude of the stroke is limited by the rigidity of the rod - with a long stroke, the rod will begin to bend. In recent years, a number of foreign firms have developed rodless pneumatic cylinders, devoid of these shortcomings. Thus, the FESTO company has developed a design in which strong permanent ring magnets are built into the piston and carriage. When the piston moves due to the action of magnetic forces, the outer movable carriage also moves along the axis of the cylinder. The working body of the machine is fastened with it. This provides the following benefits. Firstly, the total length of the cylinder does not change when the piston moves, and secondly, such a cylinder can provide a significantly larger stroke compared to a conventional one - up to 10 meters or more. In addition, a seal is only needed between the piston and the cylinder, and the cylinder itself, with two compressed air inlets, becomes a sealed structure. In a rodless pneumatic cylinder manufactured by ORIGA (Sweden), the piston is rigidly connected to a movable carriage placed on the outer surface of the cylinder through a sliding longitudinal slot. This gap is sealed with the help of two flexible steel tapes (internal and external) and permanent magnets. The rigid connection of the piston with the carriage ensures the dependence of the transmitted working force on the pressure of compressed air, which distinguishes this design from the previous one. In the pneumatic cylinder of the BOSCH (Germany) company there are rods on both sides of the piston, but they are a flexible steel tape. These belts are sealed against the cylinder and transmit the movement to the outer movable carriage through two rollers. When the piston moves to the right, the carriage moves to the left, and vice versa. The carriage is equipped with a pneumatic brake, which allows stopping it not only in extreme positions, but also in any intermediate position. However, the accuracy of such positioning is low. The hose air motor does not have a rod - a hollow rubber hose, along the axis of which a carriage with two rollers can move along its outer surface. In pneumatic cylinders at high speeds, the piston can create shocks at the ends of the stroke. To prevent them, pneumatic cylinders with braking have been created, which can be smoothly adjusted using throttles - holes of variable cross section. Rotating pneumatic cylinders are widely used to drive chucks, clamp workpieces and bar material on screw-cutting lathes. The supply of compressed air to them is carried out through a special coupling. The cylinder body can rotate around the longitudinal axis, while the clutch remains stationary. There are a number of shock technological operations, for example, stamping. Impact pneumatic cylinders have been developed for them, in which the potential energy of compressed air is converted into the kinetic energy of impact. Another type of pneumatic motors is chamber or balloon. They are used in clutches and brakes of presses, used as car jacks, "pneumomats" for lifting massive structures, for example, in aircraft construction, in the air suspension of car chassis. This suspension allows you to adjust the ground clearance (clearance) of the car. Often there is a need to rotate the working body of the machine. For this purpose, rotary air motors are used, most often piston and sliding (blade). In a piston piston, two pistons are connected by a common rod, on which there is a gear rack that meshes with the gear wheel. The shaft of the latter is the output link of the air motor. Under the action of compressed air, the pistons with the rod perform a reciprocating motion, which is converted into rotation of the output shaft. In a vane pneumatic motor, the body is made in the form of a ring with a fixed partition. Inside this housing, under the action of compressed air, a sealed blade (or gate), also associated with the output shaft, can rotate. In pneumatic motors, the potential energy of compressed air is converted into a multi-turn rotational motion of the output shaft. There are many types of pneumatic motors - gear, lamellar, turbine, screw. The most widespread are vane and turbine pneumatic motors, especially for driving pneumatic tools - drilling and grinding machines, screwdrivers, wrenches, scissors, files and many others. Their main advantage is complete electrical and explosion safety. It has already been said that the disadvantages of the pneumatic drive associated with the compressibility of air are deprived of the combined drive - pneumohydraulic. In this definition, the word "pneumo" is not in vain in the first place. The source of energy in it is compressed air. This drive consists of two cylinders - pneumatic and hydraulic, the pistons and rods of which are rigidly fastened to each other, which ensures high smoothness of movement. The travel speed is controlled by a throttle mounted on the bypass pipeline. If the communication between the cavities of the hydraulic cylinder is closed with a valve, due to the incompressibility of the liquid, it is possible to stop the piston with the rod in any intermediate position, that is, to carry out precise positioning. Such a combined drive has all the positive properties of its "parents", except for one: it does not create large working forces. This is understandable. After all, the source of energy is compressed air of low (compared to hydraulic drive) pressure. Great effort provides pneumohydraulic booster. In it, the energy source is compressed air, the pressure of which is transferred to the oil through the rod. In a hydraulic cylinder, the pressure is ten times greater than the pressure of compressed air - this depends on the ratio of the areas of the piston and rod. The use of pneumatic hydraulic boosters is especially convenient in the clamping devices of machine tools. In them, when moving the clamping jaws to contact with the product, low pressure is needed, and high pressure is needed to ensure the clamping of the product. Such amplifiers have also found application in the braking devices of various machines and in the drive of tools, for example, drills, where they provide increased torque. Such amplifiers from MEKMAN (Sweden) provide an oil pressure of 250 atmospheres with a compressed air pressure of only 10 atmospheres! Let us dwell in more detail on the use of a pneumatic actuator in industrial robotic manipulators. The development of robotics began with the creation of the simplest and lightest industrial robots, so the pneumatic drive turned out to be very useful. Usually the links of the manipulator are rigid structures. Each link is supplied with its own drive - just as the shoulder, forearm and hand of a person have their own muscles. The number of links (or their drives) determines the number of degrees of freedom of the robot. For most existing robots, this number does not exceed six or seven. But the number of degrees of mobility determines the maneuverability of the manipulator, including the ability to bypass or go around obstacles. The human hand has 22 degrees of mobility. Recently, a pneumatic actuator of variable stiffness has been developed in the USSR. Such a mechanism, resembling a snake, allows you to create a manipulator with an infinite number of degrees of freedom. It is a hollow flexible shell with several longitudinal chambers. When equal pressures are applied to all chambers, the manipulator occupies a vertical position, and when different pressures are applied, it bends towards the chambers with lower pressure. In France, a pneumatic robot "Cedrom-3" has been developed, which, like a worm, moves in a peristaltic way - due to successive stretching and contraction of its flexible "body". It consists of three sections. Each of them is an elastic corrugated tube similar to a gas mask hose. Such a robot "worm" can crawl along any channel, pipe, flat, convex or concave surface, in a horizontal or even vertical direction. It can make turns at an angle of up to 90 °, move in a loose environment - in sand, grain, snow, debris. At rest, this robot is 3 m long and 120 mm in diameter. Its weight is 10 kg, traction force is 80 kg, travel speed is more than 1 m/min. It can "creep" distances of more than 30 m and withstand temperatures up to 80 ° C. In Japan, tubular elastic chambers filled with compressed air are used to drive the robot. Such a drive is a piece of rubber tube enclosed in a braid of synthetic material. When compressed air is supplied, the tube begins to expand in diameter and contract in length in the axial direction - like a muscle. The rubber tube ends with metal plinths on both sides. To control each degree of mobility of the robot, two such rubber actuators are used. One of the metal bases of each drive is fixedly fixed, while the others are interconnected by a flexible cable thrown over a pulley. This pulley is connected to one of the links of the robot's arm. When the pressure in one of the actuators increases, it "shrinks", and when the pressure decreases by the same amount, the other actuator "relaxes" (that is, lengthens). As a result, the cable moves, rotates the pulley and the arm link of the robot. Such a robot is controlled by a microcomputer. Its low weight and flexibility make it harmless to humans. Due to its simplicity, this robot can be used to perform many simple operations, such as varnishing parts. Most importantly, through the use of rubber "muscles" filled with compressed air, it was possible to achieve an unprecedented ratio between the mass of the robot (6 kg) and the lifted load (2 kg) - 3:1. After all, this ratio is usually 10:1 or more. But such a robot is still far from a person. Recall that weightlifters lift weights that are 2-2,5 times their own. So it's too early for robot designers to calm down! What are the prospects for the development of pneumatic drive? According to the well-known company FESTO (Austria), the total volume of production of pneumatic actuators in Europe, the USA and Japan in 1986 amounted to 6,5 billion German marks. These funds are enough to produce 200.000 comfortable middle class cars! In the developed capitalist countries, many dozens of large and small firms produce pneumatic drive equipment of the widest range. The largest of these firms are FESTO, Wabco-Westinghouse (Germany), Martoier (Germany), Mekman (Sweden). The range of FESTO pneumatic actuator elements is several thousand units, including pneumatic cylinders of various types with diameters from 6 to 320 mm, a working stroke from several millimeters to several meters, and equipment controlling them of all sizes - with cross sections for air passage from 2,5 up to 20 mm. The CMEA countries also produce pneumatic drive equipment - in the People's Republic of Belarus, the GDR, and above all in Hungary (joint production with the Mekman firm) - of a wide assortment and high quality. Let's see what is the situation with the production and use of the pneumatic drive in the domestic industry. It cannot be called anything other than deplorable. The centralized production of pneumatic drive equipment and the supply of all branches of mechanical engineering with it is carried out by the Ministry of Machine Tool Industry. Only 4 enterprises are engaged in the production of pneumatic equipment in it, and pneumatic cylinders for mechanical engineering throughout the country are made by the Ordzhonikidze Experimental (!?) Pneumatic Equipment Plant. Its nomenclature consists of only 58 models of pneumatic cylinders. A small number of rotary pneumatic motors and long-stroke pneumatic cylinders for industrial robots are made by the Simferopol Production Association Pnevmatika. Miniature pneumatic cylinders and pneumatic cylinders with braking are not made by any enterprise of the Minstankoprom. In total, 150 models of pneumatic cylinders without braking are produced, and 1000 are required. Pneumatic cylinders with braking require 1200 models. There are only 4 models of rotary air motors, and 24 are required (all figures given are according to VNIIgidroprivod, Kharkov). Some branches of mechanical engineering have organized their own production of pneumatic actuators. So, the Mytishchi plant of electric train and metro cars produces pneumatic cylinders to drive the doors of these cars. The automotive industry produces diaphragm pneumatic actuators for automobile brake systems and bus door drives. However, these individual examples do not change the overall picture. The quality of rubbing surfaces and rubber seals of pneumatic equipment produced by the Ordzhonikidzevsky Experimental Plant and the Simferopol Production Association "Pnevmatika" is very poor. This leads to low reliability and insufficient equipment life. And this is while the pneumatic cylinders produced in VNR provide 50 million double strokes, which is enough for the entire service life of almost any machine! The situation with the production of distribution and control equipment, as well as compressed air preparation equipment, is no better. Its nomenclature is very narrow, and the quality and reliability (with the exception of the equipment of the Moscow Pnevmoapparat) are low. All this led to a very small use of the pneumatic drive in the domestic engineering industry. The miniature pneumatic drive equipment is especially lacking. When mastering the production of new machines under the licenses of foreign companies, it is necessary to transfer to domestic components, including those for the pneumatic drive. At the same time, each time the stumbling block is the lack of the necessary domestic pneumatic drive equipment, and it has to be bought abroad for foreign currency. The accelerated development of domestic mechanical engineering and the creation of high-level technology is impossible without the creation of a modern base for the production of all types of drives, including pneumatic ones. Without a quick and prompt solution to this most important task, our mechanical engineering will not be able to move forward and become competitive in the world market. Linear air motors Double acting pneumatic cylinder
Single acting pneumatic cylinder
Rodless pneumomagnetic cylinder
Rodless pneumatic cylinder
Rope pneumatic cylinder
Hose air motor
Pneumatic cylinder with braking
Rotating pneumatic cylinder
Impact pneumatic cylinder
Chamber air motor
Rotary air motors Piston with rack and pinion
Gate (vane)
Air motors Gear
Rotary vane
Turbine
Pneumohydraulic drive
Pneumohydraulic booster
Pneumatic engine "Snake"
Pneumatic "muscles"
Author: V.Levin
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