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附錄
附錄1
英文原文
Introduction to Fluid Power
1.1 WHAT IS FLUID POWER?
Fluid power is the technology that deals with the generation, control, and trans mission of power-using pressurized fluids. It can be said that fluid power is the muscle that moves industry. This is because fluid power is used to push, pull, reg ulate, or drive virtually all the machines of modern industry. For example, fluid power steers and brakes automobiles, launches spacecraft, moves earth, harvests crops, mines coal, drives machine tools, controls airplanes, processes food, and even drills teeth. In fact, it is almost impossible to find a manufactured product that hasn’t been “fluid-powered” in some way at some stage of its production or distribution.
Since a fluid can be either a liquid or a gas, fluid power is actually the gen eral term used for hydraulics and pneumatics. Hydraulic systems use liquids such as petroleum oils, water, synthetic oils, and even molten metals. The first hydraulic fluid to be used was water because it is readily available. However, water has many deficiencies. It freezes readily, is a relatively poor lubricant, and tends to rust metal components. Hydraulic oils are far superior and hence are widely used in lieu of water. Pneumatic systems use air as the gas medium because air is very abundant and can be readily exhausted into the atmosphere after completing its assigned task.
It should be realized that there are actually two different types of fluid sys tems: fluid transport and fluid power.
Fluid transport systems have as their sole objective the delivery of a fluid from one location to another to accomplish some useful purpose. Examples include pumping stations for pumping water to homes, Cross-country gas lines, and systems where chemical processing takes place as various fluids are brought together.
Fluid power systems are designed specifically to perform work. The work is accomplished by a pressurized fluid bearing directly on an operating fluid cylinder or fluid motor. A fluid cylinder produces a force, whereas a fluid motor produces a torque. Fluid cylinders and motors thus provide the muscle to do the desired work. Of course, control components are also needed to ensure that the work is done smoothly, accurately, efficiently, and safely.
Liquids provide a very rigid medium for transmitting power and thus can pro vide huge forces to move loads with utmost accuracy and precision. On the other hand, pneumatic systems exhibit spongy characteristics due to the compressibility of air. However, pneumatic systems are less expensive to build and operate. In ad dition, provisions can be made to control the operation of the pneumatic actuators that drive the loads.
Fluid power equipment ranges in size from huge hydraulic presses to minia ture fluid logic components used to build reliable control systems.
How versatile is fluid power? In terms of brute power, a feather touch by an operator can control hundreds of horsepower and transmit it to any location where a hose or pipe can go. In terms of precision such as applications in the machine tool industry, tolerances of one ten-thousandth of an inch can be achieved and repeated over and over again. Fluid power is not merely a powerful muscle; it is a controlled, flexible muscle that provides power smoothly, efficiently, safely, and precisely to accomplish useful work.
Figure 1-1 shows a pneumatically controlled dextrous hand designed to study machine dexterity and human manipulation in applications such as robotics and tactile sensing. Servo-controlled pneumatic actuators give the hand human-like grasping and manipulating capability. Key operating characteristics include high speed in performing manipulation tasks, strength to easily grasp hand-sized objects that have varying densities, and force grasping control. The hand possesses three fingers and an opposing thumb, each with four degrees of freedom. Each joint is positioned by two pneumatic actuators (located in an actuator pack with the con trolling servo valve) driving a high-strength tendon. Performance and configuration constraints concerning the weight, size, geometry, cleanliness, and availability of individual actuators led to the choice of pneumatic actuation.
1.2 HISTORY OF FLUID POWER
Fluid power is probably as old as civilization itself. Ancient historical accounts show that water was used for centuries to produce power by means of water wheels, and air was used to turn windmills and propel ships. However, these early uses of fluid power required the movement of huge quantities of fluid because of the relatively low pressures provided by nature.
Fluid power technology actually began in 1650 with the discovery of Pascal’s law: Pressure is transmitted undiminished in a confined body of fluid.
Pascal found that when he rammed a cork down into a jug completely full of wine, the bottom of the jug broke and fell out. Pascal’s law indicated that the pres sures were equal at the top and bottom of the jug. However, the jug has a small opening area at the top and a large area at the bottom. Thus, the bottom absorbs a greater force due to its larger area.
In 1750, Bernoulli developed his law of conservation of energy for a fluid flowing in a pipeline. Pascal’s law and Bernoulli’s law operate at the very heart of all fluid power applications and are used for analysis purposes. However, it was not until the Industrial Revolution of 1850 in Great Britain that these laws would actually be applied to industry. Up to this time, electrical energy had not been developed to power the machines of industry. Instead, it was fluid power that, by 1870, was being used to drive hydraulic equipment such as cranes, presses, winches, extruding machines, hydraulic jacks, shearing machines, and riveting ma chines. In these systems, steam engines drove hydraulic water pumps, which de livered water at moderate pressures through pipes to industrial plants for pow ering the various machines. These early hydraulic systems had a number of deficiencies such as sealing problems because the designs had evolved more as an art than a science.
Then, late in the nineteenth century, electricity emerged as a dominant tech nology. This resulted in a shift of development effort away from fluid power. Electrical power was soon found to be superior to hydraulics for transmitting power over great distances. There was very little development in fluid power technology during the last 10 yr of the nineteenth century.
The modern era of fluid power is considered to have begun in 1906 when a hydraulic system was developed to replace electrical systems for elevating and con trolling guns on the battleship USS Virginia. For this application, the hydraulic sys tem developed used oil instead of water. This change in hydraulic fluid and the sub sequent solution of sealing problems were significant milestones in the rebirth of fluid power.
In 1926 the United States developed the first unitized, packaged hydraulic sys tem consisting of a pump, controls, and actuator. The military requirements lead ing up to World War II kept fluid power applications and developments going at a good pace. The naval industry had used fluid power for cargo handling, winches, propeller pitch control, submarine control systems, operation of shipboard aircraft elevators, and drive systems for radar and sonar.
During and after World War lithe aviation and aerospace industry provided the impetus for many advances in fluid power technology. Examples include Hydraulic-actuated landing gears, cargo doors, gun drives, and flight control devices such as rudders, ailerons, and elevons for aircraft. Figure 1-2 shows the space shut tle Columbia, powered by fluid thrust forces, soaring from its launch pad. The space shuttle takes off like a rocket and the winged orbiter then maneuvers around Earth like a spaceship. After completing its mission it lands on a runway like an airplane. Unlike earlier manned space craft, which were good for only one flight, the shuttle orbiter and rocket boosters can be used again and again. Only the external tank is expended on each launch. Figure 1-3 provides a cutaway view of the shuttle ve hicle, identifying its main components, many of which are hydraulically actuated.
The expanding economy that followed World War II led to the present situa tion where there are virtually a limitless number of fluid power applications. Today fluid power is used extensively in practically every branch of industry. Some typical applications are in automobiles, tractors, airplanes, missiles, boats, and machine tools. In the automobile alone, fluid power is utilized in hydraulic brakes, automotive trans missions, power steering, power brakes, air conditioning, lubrication, water coolant, and gasoline pumping systems. The innovative use of modern technology such as electro-hydraulic closed-loop systems, microprocessors, and improved materials for com ponent construction will continue to advance the performance of fluid power systems.
Relative to automotive applications, Fig. 1-4 is a diagram showing the Bendix Hydro-Boost Power Brake System. The basic system consists of an open center spool valve and hydraulic cylinder assembled in a single unit (see Fig. 1-5). Operating pressure is supplied by the power steering pump. Hydro-Boost provides a power assist to operate a dual master-cylinder braking system. Normally mounted on the engine compartment fire wall, it is designed to provide specific “brake-feel” characteristics throughout a wide range of pedal forces and travel. A spring accu mulator stores energy for reverse stops. From one to three stops are available de pending on the magnitude and duration of the brake application. This system was developed by Bendix Corporation as an answer to crowded engine compartments and replaces the large vacuum units.
1.3 ADVANTAGES OF FLUID POWER
There are three basic methods of transmitting power: electrical, mechanical, and fluid power. Most applications actually use a combination of the three methods to obtain the most efficient overall system. To properly determine which principle method to use, it is important to know the salient features of each type. For ex ample, fluid systems can transmit power more economically over greater distances than can mechanical types. However, fluid systems are restricted to shorter dis tances than are electrical systems.
The secret of fluid power’s success and widespread use is its versatility and manageability. Fluid power is not hindered by the geometry of the machine, as is the case in mechanical systems. Also, power can be transmitted in almost limitless quantities because fluid systems are not so limited by the physical limitations of materials as are the electrical systems. For example, the performance of an elec tromagnet is limited by the saturation limit of steel. On the other hand, the power capacity of fluid systems is limited only by the physical strength of the material (such as steel) used for each component.
Industry is going to depend more and more on automation in order to in crease productivity. This includes remote and direct control of production opera tions, manufacturing processes, and materials handling. Fluid power is the muscle of automation because of advantages in the following four major categories.
1. Ease and accuracy of control. By the use of simple levers and push but tons, the operator of a fluid power system can readily start, stop, speed up or slow down, and position forces that provide any desired horsepower with tolerances as precise as one ten-thousandth of an inch. Figure 1-6 shows a fluid power system that allows an aircraft pilot to raise and lower his landing gear. When the pilot moves a small control valve in one direction, oil under pressure flows to one end of the cylinder to lower the landing gear. To retract the landing gear, the pilot moves the valve lever in the opposite direction, allowing oil to flow into the other end of the cylinder.
2. Multiplication of force. A fluid power system (without using cumbersome gears, pulleys, and levers) can multiply forces simply and efficiently from a fraction of an ounce to several hundred tons of output. Figure 1-7 shows an application where a rugged, powerful drive is required for handling huge logs. In this case, a turntable, which is driven by a hydraulic motor, can carry a 20,000-lb load at a lo ft radius (a torque of 200,000 ft ib) under rough operating conditions.
3. Constant force or torque. Only fluid power systems are capable of pro viding constant force or torque regardless of speed changes. This is accomplished whether the work output moves a few inches per hour, several hundred inches per minute, a few revolutions per hour, or thousands of revolutions per minute. Figure 1-8 depicts an application in oceanography that involves the exploration and de velopment of the ocean’s resources for the benefit of humankind. In this instance, it is important for the operator to apply a desired constant grabbing force through the use of the grappling hooks.
4. Simplicity, safety, economy. In general, fluid power systems use fewer mov ing parts than comparable mechanical or electrical systems. Thus, they are simpler to maintain and operate. This, in turn, maximizes safety, compactness, and relia bility. Figure 1-9 shows a power steering control designed for off-highway vehicles. The steering unit (shown attached to the steering wheel column in Fig. 1-9) con sists of a manually operated directional control valve and meter in a single body. See Fig. 1-10 for a cutaway of this steering unit. Because the steering unit is fully fluid-linked, mechanical linkages, universal joints, bearings, reduction gears, etc., are eliminated. This provides a simple, compact system. In addition, very little in put torque is required to produce the control needed for the toughest applications. This is important where limitations of control space require a small steering wheel and it becomes necessary to reduce operator fatigue. The compact design and ver satility of the control system allow the unit to control many large and high-pow ered systems with a high degree of reliability. The steering unit shown in Fig. 1-10 contains a check valve that converts the unit to a hand-operated pump for emer gency power-off steering.
Additional benefits of fluid power systems include instantly reversible mo tion, automatic protection against overloads, and infinitely variable speed control. Fluid power systems also have the highest horsepower-per-weight ratio of any known power source.
Drawbacks of Fluid Power
In spite of all these highly desirable features of fluid power, it is not a panacea for all power transmission problems. Hydraulic systems also have some drawbacks. Hydraulic oils are messy, and leakage is impossible to eliminate completely. Hydraulic lines can burst, possibly resulting in injuries to people due to high-speed oil jets and flying pieces of metal, if proper design is not implemented. Prolonged exposure to loud noise such as that emanating from pumps, can result in loss of hearing. Also, most hydraulic oils can cause fires if an oil leak occurs in an area of hot equipment. Therefore, each ap plication must be studied thoroughly to determine the best overall design. It is hoped that this book will not only assist the reader in developing the ability to make these types of system selection decisions but also present in a straightforward way the tech niques for designing, analyzing, and troubleshooting basic fluid power systems.
1.4 APPLICATIONS OF FLUID POWER
Although a number of cases of fluid power have already been presented in this chapter, the following additional applications should give the reader a broader view of the widespread use of fluid power in today’s world.
1. fluid power drives high-wire overhead tram. Most overhead trams require a haulage or tow cable to travel up or down steep inclines. However, the 22-passenger, 12,000-lb hydraulically powered and controlled Sky-tram shown in Fig. 1-11 is unique. It is self-propelled and travels on a stationary cable. Because the tram moves instead of the cable, the operator can stop, start, and reverse any one car completely independently of any other car in the tram system. Integral to the design of the Sky-tram drive is a pump (driven by a standard eight-cylinder gaso line engine), which supplies pressurized fluid to four hydraulic motors. Each mo tor drives two friction drive wheels.
Eight drive wheels on top of the cables support and propel the tram car. On steep inclines, high driving torque is required for ascent and high braking torque for descent. Dual compensation of the four hydraulic motors provides efficient pro portioning of available horsepower to meet the variable torque demands.
2. fluid power is applied to harvesting corn. The world’s dependence on the United States for food has resulted in a great demand for agricultural equipment development. Fluid power is being applied to solve many of the problems dealing with the harvesting of food crops. Figure 1-12 shows a hydraulically driven eleva tor conveyor system, which is used to send harvested, husked ears of corn to a wagon trailer. Mounted directly to the chain-drive conveyor, a hydraulic motor de livers full-torque rotary power from start-up to full rpm.
3．Hydraulics power brush drives. Figure 1-13 shows a fluid power—driven brush drive used for cleaning roads, floors, etc., in various industrial locations. Mounted directly at the hub of the front and side sweep-scrub brushes, compact hydraulic motors place power right where it’s needed. They eliminate bulky me chanical linkages for efficient, lightweight machine design. The result is continuous, rugged industrial cleaning action at the flip of a simple valve.
4. fluid power positions and holds parts for welding. In Fig. 1-14, we see an example of a welding operation in which a farm equipment manufacturer applied hydraulics for positioning and holding parts while welding is done. It is a typical example of how fluid power can be used in manufacturing and production opera tions to reduce costs and increase production. This particular application required a sequencing system for fast, positive holding. This was accomplished by placing a restrictor (sequence valve) on the flow of oil in the line leading to the second of the two cylinders (rams), as illustrated in Fig. 1-15. The first cylinder extends to the end of its stroke. Oil pressure then builds up, overcoming the restrictor setting, and the second cylinder extends to complete the “hold” cycle. This unique welding ap plication of hydraulics was initiated to increase productivity by making more parts per hour. In addition, the use of hydraulics reduced scrap rates and operator fa tigue as well as increasing productivity from 5 pieces per shift to more than 20—a 400 % increase.
5. Fluid power performs bridge maintenance. A municipality had used fluid power for years as a means for removing stress from structural members of bridges, making repairs, and replacing beams. As many as four or five bulky, low-pressure hand pumps and jacking ram setups were used to remove stress from beams need ing replacement. Labor costs were high, and no accurate methods existed for record ing pressures. An excessive downtime problem dictated that a new system be de signed for the job. A modern fluid power system was designed that located several 100-ton rams on the bridge structure, as illustrated in Fig. 1-16. One portable pump was used to actuate all of the rams by the use of a special manifold. This made it easy to remove stress from members needing repair or replacement. This new fluid power system cut the setup time and labor costs for each repair job to one-third that required with the hand pump and jacking ram setups previously used
6. Fluid power is the muscle in industrial lift trucks. Figure 1-17