Chapter 1 Introduction to fluid power

Chapter 1: Introduction to fluid power

2011


1.1.What is fluid power? Fluid power is the technology that deals with the generation, control, and transmission of power, using pressurized fluids. It can be said that fluid power is the muscle that moves industry. This because fluid power is used to push, pulls, regulates or drives virtually all the machines of modern industry. For example, fluid power steers and brake automobiles, launches spacecraft, moves earth, harvest crops, mines coal, drives machine tools, control air planes, 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.

Fluid power is called hydraulics when the fluid is a liquid and is called pneumatic when the fluid is gas. Thus fluid power is the general term used for both hydraulics and pneumatics. Hydraulic systems use liquids such as petroleum oils, synthetic oils and water. 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 widel y 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.

There are actually two different types of fluid systems: fluid transport and fluid power. Fluid transport systems have as their sole objective of the delivery of 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 resulting in a linear motion, whereas a fluid motor produces a torque resulting in rotary motion. Thus in a fluid power systems, cylinders and motors (which are also called actuators), provide the muscle to do the desired work. Of course, control components such as valves are needed to ensure that the work is done smoothly, accurately, efficiently and safely. Hydraulic and pneumatic control lecture notes by Siraj K.

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Liquids provide a very rigid medium for transmitting power and thus can operate under high pressures to provide huge forces and torques to drive loads with utmost accuracy and precession. Figure 1.1 shows a hydraulic chain saw that is ideal for large tree trimming applications from an aerial bucket as well as for cut up removal jobs. These chain saws are commonly used by electric power line crews because they are light weight, dependable, quite, and safer than gasolinepowered saws. The chain saw, which uses a hydraulic gear motor, has a total weight of 6.7 lb. it operates with a flow rate range of 4 to 8 gpm and a pressure range of 1000 to 2000 psi.

Figure 1.1 Hydraulic chain saws

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 addition, provisions can be made to control the operation of the pneumatic actuators that drive the loads. Thus pneumatic systems can be used effectively in applications where low pressures can be used because the loads to be driven do not require large forces. Figure 1.2 shows a pneumatically controlled dexterous hand designed to study machine dexterity and human manipulations such as robotics and tactile sensing. 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 opposing thumb. Each joint is positioned by pneumatic actuators (located in actuator pack) driving a high-strength tendon.

Hydraulic and pneumatic control lecture notes by Siraj K.

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Liquids provide a very rigid medium for transmitting power and thus can operate under high pressures to provide huge forces and torques to drive loads with utmost accuracy and precession. Figure 1.1 shows a hydraulic chain saw that is ideal for large tree trimming applications from an aerial bucket as well as for cut up removal jobs. These chain saws are commonly used by electric power line crews because they are light weight, dependable, quite, and safer than gasolinepowered saws. The chain saw, which uses a hydraulic gear motor, has a total weight of 6.7 lb. it operates with a flow rate range of 4 to 8 gpm and a pressure range of 1000 to 2000 psi.

Figure 1.1 Hydraulic chain saws

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 addition, provisions can be made to control the operation of the pneumatic actuators that drive the loads. Thus pneumatic systems can be used effectively in applications where low pressures can be used because the loads to be driven do not require large forces. Figure 1.2 shows a pneumatically controlled dexterous hand designed to study machine dexterity and human manipulations such as robotics and tactile sensing. 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 opposing thumb. Each joint is positioned by pneumatic actuators (located in actuator pack) driving a high-strength tendon.


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Figure 1.2 pneumatically controlled dexterous hand

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 1650 with discovery of Pascal’s law: pressure is transmitted undiminished in a confined body of fluid. Pascal found that when he rammed a cork down in to jug completely completely full of wine, wine, the bottom of of the jug broke and fell out. Pascal Pascal law indicated indicated that the pressures were equal at the top and bottom of jug. However, the jug has small opening area at the top and the large area at the bottom. Thus, the bottom absorbs greater force due to its larger area.

In 1750, Bernoulli developed his law of conservation of energy for a fluid flowing in the 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 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, 1870, was being used to drive hydraulic equipment such as a cranes, presses, Hydraulic and pneumatic control lecture notes by Siraj K.

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winches, extruding machines, hydraulic jacks, shearing machines, and riveting machines. In these systems, steam engines drove hydraulic water pumps, which delivered water at moderate pressures through pipes to industrial plants for powering 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 technology. Thus 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 years of the nineteenth century. The modern era of fluid power is considered to have in 1906 when a hydraulic system was developed to replace electrical system for elevating and controlling guns on the battleship USS Virginia. For this application, the hydraulic system developed used oil instead of water. This change in hydraulic fluid and subsequent solution of sealing problems were significant milestones in the rebirth of fluid power.

In 1926 the United State developed the first utilized, packaged hydraulic system consisting of a pump, controls and actuator. The military requirements leading 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 system for radar and sonar. During World War II the 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.

The expanding economy that followed World War II led to the present situation 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, robots, and machine tools. In the automobile alone, fluid power is used in hydraulic brakes, automotive transmissions, power steering, power brakes, air conditioning, lubrication, water coolant and gasoline pumping systems. The innovative use of modern Hydraulic and pneumatic control lecture notes by Siraj K.

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technology such as electrohydraulic closed-loop systems, microprocessors and improved materials for component construction will continue to advance the performance fluid power systems.

Figure 1.3 shows the space shuttle Columbia, powered by fluid thrust forces, soaring from its launch pad. The space shuttle takes off like a rocket and winged orbiter then maneuvers around Earth like a spaceship. After completing its mission it lands on a runway like an airplane. Unlike earlier manned spacecraft, which were good for only one flight, the shuttle orbiter and rocket boosters can be used again and again. Only the external tank is expanded on each launch. Figure 1.4 provides a cutaway view of the shuttle vehicle, identifying its main components, many of which are hydraulically actuated.

Figure 1.3 the space shuttle Colombia soaring its launch pad (Courtesy of NASA, Washington, D.C.)


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Figure 1.4 Cutaway view of space shuttle vehicle identifying its main components (Courtesy of NASA, Washington, D.C.)

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 method to use, it is important to know the salient of the features of each type. For example, fluid systems can transmit more power economically over greater distances than mechanical types. However, fluid systems are restricted to shorter distances than 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 the 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 electrical systems.

For example, the

performance of an electromagnet is limited by the saturation limit of steel. On the other hand, the power capacity of fluid system 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 increase productivity. This includes remote and direct control of production operations, manufacturing processes, and Hydraulic and pneumatic control lecture notes by Siraj K.

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materials handling. Fluid power is well suited for these automation applications because of advantages in the following four major categories

1. Ease and accuracy of control : by the use of simple levers and push buttons, 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-thousandth of an inch. Figure 1.5 shows a fluid power system that allows an air craft pilot to raise and lower his landing gear. When the pilot moves the lever of 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.

Figure 1.5 hydraulic operation of aircraft landing gear. (Courtesy of National Fluid Power Association, Milwaukee, Wisconsin) 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 1.6 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 10-ft radius ( a torque of 20,000 ft.lb) under rough operating conditions.


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Figure 1.6. Hydraulically driven turntable for handling huge logs. (Courtesy of Eaton Corp, Fluid Power Division, Eden Prairie, Minnesota) 3.

Constant force or torque: a fluid power system is capable of providing constant force or

torque regardless of speed changes. This is accomplished whether the work out put moves a few inches per hour, several hundred inches per minute, a few revolutions per hour, or thousands of revolutions per minute. Figure 1.7 depicts an application in oceanography that involves the exploration and development of the ocean’s resources of the benefit of humankind. In the instance, it is important for the operator to apply a desired constant grabbing force through the use of the grapping hooks.

Figure 1.7. Fluid power applications in oceanography. (Courtesy of National Fluid Power Association, Milwaukee, Wisconsin.) Hydraulic and pneumatic control lecture notes by Siraj K.

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Chapter 1: Introduction to fluid power 4.

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Simplicity, safety, economy: in general, fluid power systems use fewer moving parts than

comparable mechanical or electrical systems. Thus, they are simpler to maintain and operate. This, in turn, maximizes safety, compactness, and reliability. Figure 1.8 shows a fluid power steering control system designed for transportation vehicles. The steering unit (shown attached to the steering wheel column in figure 1.8) consists of a manually operated directional control valve and meter in a single body. Because the steering unit is fully fluid-linked, mechanical linkages, universal joints, bearings, reduction gears, and so forth, are eliminated. This provides a simple, compact system. In addition, very little input torque is required to produce the steering control needed. Additional benefits of fluid power systems include instantly reversible motion, automatic protection against overloads, and infinitely variable speed control. Fluid power systems also have the highest power-per- weight ratio of any known power source.

Figure 1.8. Fluid power steering control system for transportation vehicles. (Courtesy of Eaton Corp, Fluid Power Division, Eden Prairie, Minnesota.)

Fluid power is used in a diverse range of applications from mobile construction and aerospace equipment to powering industrial machinery, and offers several advantages over other types of motive force. With fluid power systems, a single source of fluid pressure (compressor or pump) can power many axes or fluid power devices. The power source can be located where space is not critical. Because much of the size and weight of the fluid power system is off-loaded onto the power unit, the individual actuators can be small compared to the power they produce. In Hydraulic and pneumatic control lecture notes by Siraj K.

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addition, they are often quieter, and generate less heat than electric actuators. Fluid power actuators can also be used in hazardous environments where electric sparks must be avoided

By using accumulators to store energy, the hydraulic power unit only needs to provide slightly more than the average demand, increasing efficiencies for machines with varying load cycles. In applications such as presses where a constant holding pressure or torque must be applied, hydraulic actuators have a big advantage because no energy is used while they are not moving, whereas a motor draws a large amount of current to maintain torque even while stopped. Most motors will overheat and fail under these conditions.

Hydraulic cylinders are very smooth and efficient for linear movement. There are no poles that cause cogging and no need for backlash compensation. Drawbacks of fluid power In the spite of all the previously mentioned advantages of fluid power, it is not a panacea for all power transmission applications. Fluid power systems also have some drawbacks. For example, hydraulic oils are messy (dirty or disordered) and leakage is impossible to eliminate completely. Hydraulic lines can burst, possibly resulting 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 is an oil leak occurs in an area of hot equipment. In the pneumatic systems, components such as compressed air tank and accumulators potentially explosive if the pressure is allowed to increase beyond safe design limits. Therefore each application must be studied thoroughly to determine the best overall system to employ. 1.4.Applications of fluid power

Fluid power is extensively used in manufacturing, construction, transportation, agriculture, mining, military operations, health, and even recreation. The list is almost endless. System sizes range from miniature to massive, but fluid power principles provide the needed power, force, and control. Fluid power has been a key contributing factor in the development of current agricultural equipment. Modern farm equipment uses hydraulics extensively.


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These uses range from simple hydraulic cylinders that raise and lower implements to complex devices that maintain clearances, adjust torque, and provide easy control of speed and direction on tractors and a variety of specialized planting, harvesting, and processing equipment. Fluid power is used in some form in all modern transportation systems designed to move people and products.

These uses range from automobiles to complex, wide-body aircraft found on international flights. Specific examples of the application of fluid power principles include hydraulic and pneumatic braking systems, power-assisted steering found on most forms of wheeled vehicles, hydrostatic transmissions that provide almost unlimited speed and torque control, and

suspension systems that use hydraulic and/or pneumatic dampening Applications vary and components have different appearances in the various applications.

The construction industry is a very diverse industry. Construction activities include the building of residences and all types of commercial structures, roads and highways, irrigation systems, harbor facilities, and a wide variety of other construction-related activities. The industry makes use of many types of earth-moving equipment, material-handling equipment, and specialized fastening and finishing devices.

Mining companies use fluid power both in open-pit and underground operations. Spectacular examples of an application in this industry are the huge shovels used in coal strip mining operations. These shovels remove the overburden from veins of coal that are near the surface. Some of these shovels are several stories high and they can remove multiple cubic yards of material during each pass of the scoop. The shovels use large numbers of fluid power systems and circuits for movement and control.

Although a number of fluid power applications have already been presented, the following additional examples show more fully the widespread use of fluid power in today’s world.

i. Fluid power drives high-wire overhead tram: most overhead trams require a tow cable to travel

up or down steep inclines. However, the 22-passenger, hydraulically powered and


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controlled sky-tram shown in figure 1.9 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 design of the sky-tram drive is a map (driven by a standard eight-cylinder gasoline engine), which supplies pressurized fluid to four hydraulic motors. Each motor drives two friction drive wheels. Eight drives 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.

Figure 1.9. Hydraulically powered Sky-tram. (Courtesy of Sky-tram systems, Inc., Scottsdale, Arizona)

ii. 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 1.10 shows hydraulically driven elevator 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 delivers full-torque rotary power from start up to full rpm.


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Figure 1.10. Hydraulically driven elevator conveyor for use in harvesting of corn. (Courtesy of Eaton Corp., Fluid Power Division, iii.

Hydraulic power brush drives: figure 1.11 shows a fluid power brush drive used for

cleaning roads, floors and so forth in various industrial locations. Mounted directly at the hub of the front and side sweep scrub brushes, compact hydraulic motors power right where it’s needed. They eliminate bulky mechanical linkages for efficient, lightweight machine design. The result is continuous, rugged industrial cleaning action at the flip of a simple valve.

Figure 1.12. Hydraulic power brush drive. (Courtesy of Eaton Corp., Fluid Power Division, Eden Prairie, Minnesota.)


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iv. Fluid power is the muscle in industrial lift trucks: figure 1.12 shows an industrial hydraulic lift truck with a 5000-lb capacity. The hydraulic system includes dual action tilt cylinders and hoist cylinder. Tilting action is smooth and sure for better load stability and easier load placement. A lowering valve in the hoist cylinder controls the speed of descent. Fluid power steering is available as an optional feature.

Figure 1.12. Industrial hydraulic lift truck (Courtesy of Eaton Corp., Industrial Truck Division, Philadelphia, Pennsylvania.) v. Fluid power drives excavators: figure 1.13 shown an excavator whose hydraulically actuated bucket digs soil from the ground and drops the soil into a dump truck at a construction site. A total of four hydraulic cylinders are used to drive the three pin-connected members called the boom, stick and bucket. The boom is the member that is pinned at one end to the cab frame. The stick is the actual member that is pinned at one end to the boom and pinned at its other end to the bucket. Two of the cylinders connect the cab frame to the boom. A third cylinder connects the boom to the stick and the fourth cylinder connects the stick to the bucket. For the excavator shown in the figure 1.13, the volume capacity of the large size bucket is 4.2 cu yd and the maximum lifting capacity at ground level is 41,000 lb.


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Figure 1.13. Hydraulic powered excavator. (Courtesy of John Deere Co., Moline, Illinois.) vi. Hydraulic power robotic dexterous arm: figure 1.14 shows a hydraulically powered robotic arm that has the strength and dexterity to torque down bolts with its fingers and yet can gingerly pick up an egg shell. This robotic arm is adept at using human tools such as hammers, electric drills, and tweezers and can even bat a baseball. The arm has a hand with a thumb and two fingers, as well as wrist, elbow and shoulder. The control system is capable of accepting computer or human operator control inputs. The system can be designed for carrying out hazardous applications in the subsea, utilities or nuclear environments and it is also available in a range of sizes from human proportions to 6 ft long.

Figure 1.14. Hydraulically Powered Dexterous Arm. (Courtesy of Sarcos, Inc., Salt Lake City, Utah)


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1.5.Components of a fluid power system Hydraulic system There are six basic components required in a hydraulic system (refer to figure 1.15): 1. A tank (reservoir) to hold the hydraulic oil 2. A pump to force the oil through the system 3. An electric motor or other power source to drive the pump 4. Valves to control oil direction, pressure and flow rate 5. An actuator to convert the pressure of the oil in to mechanical force or torque to do useful work. Actuators can either be cylinders to provide linear motion, as shown in figure 1.15, or motors (hydraulic) to provide rotary motion, as shown in figure 1.16 6. Piping, which carries the oil from one location to another

Figure 1.15. Basic hydraulic system with linear hydraulic actuator (cylinder) (Courtesy of Sperry Vickers, Sperry Rand Corp., Troy Michigan)

Figure 1.16. Basic hydraulic system with rotary hydraulic actuator (motor) (Courtesy of Sperry Vickers, Sperry Rand Corp., Troy Michigan)


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Of course, the sophistication and complexity of hydraulic systems will vary depending on the specific applications. This is also true of the individual components that comprise the hydraulic system. As an example, refer to figure 1.17, which shows two different sized, complete hydraulic power units designed for two uniquely different applications. Each unit is a complete, packaged power system containing its own electric motor, pump, shaft coupling, reservoir and miscellaneous piping, pressure gages, valves and other components as required for proper operation.

Figure 1.17. Two different-sized, complete hydraulic power units. (Courtesy of Continental Hydraulics, Division of Continental Machines Inc. , Savage, Minnesota)

The advantages of hydraulic systems over other methods of power transmission are•

Simpler design. In most cases, a few pre-engineered components will replace complicated mechanical linkages.

•

Flexibility. Hydraulic components can be located with considerable flexibility. Pipes and hoses instead of mechanical elements virtually eliminate location problems.

•

Smoothness. Hydraulic systems are smooth and quiet in operation. Vibration is kept to a minimum.

•

Control. Control of a wide range of speed and forces is easily possible.

•

Cost. High efficiency with minimum friction loss keeps the cost of a power transmission at a minimum.

•

Overload protection. Automatic valves guard the system against a breakdown from overloading.


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The main disadvantage of a hydraulic system are maintaining the precision parts when they are exposed to bad climates and dirty atmospheres. Protection against rust, corrosion, dirt, oil deterioration, and other adverse environmental conditions is very important. The following paragraphs discuss several basic hydraulic systems.

a.

Hydraulic Jack. In this system (Figure 1-18), a reservoir and a system of valves has

been added to Pascal's hydraulic lever to stroke a small cylinder or pump continuously and raise a large piston or an actuator a notch with each stroke. Diagram A shows an intake stroke. An outlet check valve closes by pressure under a load, and an inlet check valve opens so that liquid from the reservoir fills the pumping chamber. Diagram B shows the pump stroking downward. An inlet check valve closes by pressure and an outlet valve opens. More liquid is pumped under a large piston to raise it. To lower a load, a third valve (needle valve) opens, which opens an area under a large piston to the reservoir. The load then pushes the piston down and forces the liquid into the reservoir.

Figure 1.18. Hydraulic Jack


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Motor-Reversing System. Figure 1-19 shows a power-driven pump operating a

reversible rotary motor. A reversing valve directs fluid to either side of the motor and back to the reservoir. A relief valve protects the system against excess pressure and can bypass pump output to the reservoir, if pressure rises too high.

Figure 1.19. Motor reversing system Open-Center System.

In this system, a control-valve spool must be open in the center to allow pump flow to pass through the valve and return to the reservoir. Figure 1-20 shows this system in the neutral position. To operate several functions simultaneously, an open-center system must have the correct connections, which are discussed below. An open-center system is efficient on single functions but is limited with multiple functions.


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Figure 1.20. Open-center system

(1) Series Connection. Figure 1-21 shows an open-center system with a series connection.

Oil from a pump is routed to the three control valves in series. The return from the first valve is routed to the inlet of the second, and so on. In neutral, the oil passes through the valves in series and returns to the reservoir, as the arrows indicate. When a control valve is operated, the incoming oil is diverted to the cylinder that the valve serves. Return liquid from the cylinder is directed through the return line and on to the next valve.

Figure 1.21. Open-center system with a series connection This system is satisfactory as long as only one valve is operating at a time. When this happens, the full output of the pump at full system pressure is available to that function. However, if more Hydraulic and pneumatic control lecture notes by Siraj K.

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than one valve is operating, the total of the pressures required for each function cannot exceed the system's relief setting.

(2) Series/Parallel Connection. Figure 1-22 shows a variation on the series connection. Oil from

the pump is routed through the control valves in series, as well as in parallel. The valves are sometimes stacked to allow for extra passages. In neutral, a liquid passes through the valves in series, as the arrows indicate. However, when any valve is operating, the return is closed and the oil is available to all the valves through the parallel connection.

Figure 1.22. Open-center system with a series and parallel connection

When two or more valves are operated at once, the cylinder that needs the least pressure will operate first, then the cylinder with the next least, and so on. This ability to operate two or more valves simultaneously is an advantage over the series connection. (2)

Flow Divider . Figure 1-23 shows an open-center system with a flow divider. A flow

divider takes the volume of oil from a pump and divides it between two functions. For example, a flow divider might be designed to open the left side first in case both control valves were actuated simultaneously. Or, it might divide the oil to both sides, equally or by percentage. With this system, a pump must be large enough to operate all the functions simultaneously. It must also supply all the liquid at the maximum pressure of the highest function, meaning large amounts of hp are wasted when operating only one control valve.


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Figure 1.23. Open-center system with a flow divider

Closed-Center System.

In this system, a pump can rest when the oil is not required to operate a function. This means that a control valve is closed in the center, stopping the flow of the oil from the pump. Figure 1-24 shows a closed-center system. To operate several functions simultaneously, a closed-center system have the following connections:

Figure 1.24. Closed-center system (1) Fixed-Displacement Pump and Accumulator . Figure 1-25 shows a closed-center system. In

this system, a pump of small but constant volume charges an accumulator. When an accumulator


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is charged to full pressure, an unloading valve diverts the pump flow back to a reservoir. A check valve traps the pressured oil in the circuit.

Figure 1.25. Fixed-displacement pump and accumulator When a control valve is operated, an accumulator discharges its oil and actuates a cylinder. As pressure begins to drop, an unloading valve directs the pump flow to an accumulator to recharge the flow. This system, using a small capacity pump, is effective when operating oil is needed only for a short time. However, when the functions need a lot of oil for longer periods, an accumulator system cannot handle it unless the accumulator is very large.

(2) Variable-Displacement Pump. Figure 1-26 shows a closed-center system with a variable-

displacement pump in the neutral mode. When in neutral, oil is pumped until the pressure rises to a predetermined level. A pressure-regulating valve allows the pump to shut off by itself and maintain this pressure to the valve. When the control valve is operating, oil is diverted from the pump to the bottom of a cylinder. The drop in pressure caused by connecting the pump's pressure line to the bottom of the cylinder causes the pump to go back to work, pumping oil to the bottom of the piston and raising the load.


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Figure 1.26. Variable-displacement pump When the valve moves, the top of the piston connects to a return line, which allows the return oil that was forced from the piston to return to the reservoir or pump. When the valve returns to neutral, oil is trapped on both sides of the cylinder, and the pressure passage from the pump is dead-ended. After this sequence, the pump rests. Moving the spool in the downward position directs oil to the top of the piston, moving the load downward. The oil from the bottom of the piston is sent into the return line.

Figure 2-27 shows this closed-center system with a charging pump, which pumps oil from the reservoir to the variable-displacement pump. The charging pump supplies only the makeup oil required in a system and provides some inlet pressure to make a variable-displacement pump more efficient. The return oil from a system's functions is sent directly to the inlet of a variabledisplacement pump.


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Figure 1.27. Closed-center system with a charging pump Because today's machines need more hydraulic power, a closed-center system is more advantageous. For example, on a tractor, oil may be required for power steering, power brakes, remote cylinders, three-point hitches, loaders, and other mounted equipment. In most cases, each function requires a different quantity of oil. With a closed-center system, the quantity of oil to each function can be controlled by line or valve size or by orificing with less heat build up when compared to the flow dividers necessary in a comparable open-center system. Other advantages of a closed-center system are that•

It does not require relief valves because the pump simply shuts off by itself when standby

pressure is reached. This prevents heat buildup in systems where relief pressure is frequently reached. •

It has lines, valves, and cylinders that can be tailored to the flow requirements of each

function. •

It has an available reserve flow to ensure full hydraulic speed at low engine revolutions per

minute (rpm). More functions can be served. •

It is more efficient on functions such as brakes, which require force but very little piston

movement. By holding the valve open, standby pressure is constantly applied to the brake piston with no efficiency loss because the pump has returned to standby


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Pneumatic system

The reason for using pneumatics, or any other type of energy transmission on a machine, is to perform work. The accomplishment of work requires the application of kinetic energy to a resisting object resulting in the object moving through a distance. In a pneumatic system, energy is stored in a potential state under the form of compressed air. Working energy (kinetic energy and pressure) results in a pneumatic system when the compressed air is allowed to expand. For example, a tank is charged to 100 PSIA with compressed air. When the valve at the tank outlet is opened, the air inside the tank expands until the pressure inside the tank equals the atmospheric pressure. Air expansion takes the form of airflow. To perform any applicable amount of work then, a device is needed which can supply an air tank with a sufficient amount of air at a desired pressure. This device is positive displacement compressor.

What a Positive Displacement Compressor Consists of

A positive displacement compressor basically consists of a movable member inside housing. The compressor has a piston for a movable member. The piston is connected to a crankshaft, which is in turn connected to a prime mover (electric motor, internal combustion engine). At inlet and outlet ports, valves allow air to enter and exit the chamber.

How a Positive Displacement Compressor Works

As the crankshaft pulls the piston down, an increasing volume is formed within the housing. This action causes the trapped air in the piston bore to expand, reducing its pressure. When pressure differential becomes high enough, the inlet valve opens, allowing atmospheric air to flow in. With the piston at the bottom of its stroke, inlet valve closes. The piston starts its upward movement to reduce the air volume which consequently increases its pressure and temperature. When pressure differential between the compressor chamber and discharge line is high enough, the discharge valve opens, allowing air to pass into an air receiver tank for storage.


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Fig. 1-28: Basic pneumatic power arrangement.


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Properties of Compressed Air

•

Components have long working life resulting in longer system reliability

•

Environmentally friendly

•

Safety issues are minimized e.g. Fire hazards; unaffected by overloads (actuators stall or slip)

•

pneumatic actuators in a system do not produce heat (except for friction)

Properties of Gases

Three important variables –

1. Temperature, T

–

2. Pressure, P

–

3. Volume, V

Gas laws describe relationships between these variables Bernoulli’s Law •

When there is fluid flow through a tube of varying diameters –

the total energy in the system is constant

–

the velocity is inversely proportional to the pressure

–

V 1 is less than V 2

–

P1 is higher than P2


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Boyle’s Law

The pressure of a given mass of gas is inversely proportional to its volume (providing the gas remains at constant temperature)

Isothermal (equal temperature) Properties of Gases (Boyle’s Law) P1 ∗ V 2

=

P2 ∗ V 2

Or P2

=

P1 ∗ V 1 V 2

Where: P1

=

Initial pressure (Psia)

P2

=

Final pressure (Psia)

V 1

=

V 2

=

Initial volume (cf) Final volume (cf)


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Charles’s Law

When the pressure of a confined gas remains constant, the volume of the gas is directly proportional to the absolute temperature. A given mass of gas increases in volume by 1/273 of its volume for every degree Celsius rise in temperature or 1/459.7 for every degree Fahrenheit rise in temperature. e.g. Hot air balloon Isobaric - equal pressure V 1 V 2

=

T 1 T 2

Where: V1 = initial volume V2 = resulting volume T1 = initial absolute temperature T2 = resulting absolute temperature A volume of air in an accumulator is submerged in a bucket of ice water (32 degrees F). If you remove the accumulator from the ice water and place it in a bucket of boiling water what would the resulting volume be. V 2

=

V 1 ∗ T 2 T 1

Pascal’s Law

Pressure at any point in a closed body of fluid is the same in all directions, exerting equal force on equal areas. F = P ∗ A

Where: F = force P = pressure A = area


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Fluid power symbols

Power Generation Component Symbols

Power Distribution Component Symbols


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Power Deployment Component Symbols

Power Regulation Component Symbols

Fluid Conditioning Component Symbols

Pneumatic symbols


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Chapter 1 Introduction to fluid power

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