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INTRODUCTION

1.1 WHAT IS FLUID POWER?

Utilization of fluid power is important because it is one of the three avail-able means of transmitting power. Other methods of transmitting powerare by utilizing mechanical means and by applying electrical energy. Todemonstrate this we will consider that we have a prime mover such as adiesel engine on one side of the room and a mechanical contrivance on theother. The objective is to see how, in a generic sense, power can be usedby the methods quoted above to perform the necessary mechanical work.

For mechanical power transmission, the prime mover is connected tothe device and, by use of gearboxes, pulleys, belts and clutches, the nec-essary work can be performed. With the electrical method, an electricalgenerator is used. The current developed can be carried through electricalcable to operate electrical motors, linear or rotary, modulation being pro-vided by variable resistance or solid state devices in the circuits. For fluidpower utilization, an oil pump is connected to the engine and instead ofelectrical cables, high pressure hose is used to convey pressurized fluid tomotors (again linear or rotary), pressure and flow modulation now beingprovided within the motors or by means of hydraulic valves. Any of thethree methods described may be used however, if an engineering systemrequires:

1. Minimum weight and volume

2. Large forces and low speeds

3. Instant reversibility1

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2 INTRODUCTION

4. Remote control

then the fluid power technique will often have significant competitive ad-vantages.

It is indeed unfortunate that design of fluid power systems is seldomtaught at four-year universities in the United States at the same time asformal teaching of power transmission systems involving mechanical andelectrical systems. Such comprehensive design teaching would demonstrateadequately the advantages of such systems. In some instances hydraulicspower transmission is the only technique that can be used. The most spec-tacular example is that of extending an aircraft’s control surface into a highvelocity airstream where the only technique available is that of using fluidpower actuators because of their high power to weight and volume to weightadvantages.

1.2 A BRIEF HISTORY OF FLUID POWER

The performance of mechanical work using pressurized and moving fluidsdates back for nearly six millennia. The Egyptians and Chinese used movingwater and wind to do work and records show that the advanced civilizationin China in 4000 B.C. constructed and utilized wooden valves to controlwater flow through pipes made of bamboo. In Egypt, the Nile River wasdammed so that irrigation could be performed. The Roman Empire alsoused aqueducts, reservoirs and valves to carry water to cities.

The above applications did of course use dynamic properties of fluidsand kinetic energy was employed to perform useful work. Fluid power is,however, customarily associated with the use of potential energy in pressur-ized fluids. The nearest example in antiquity which comes to mind is thequarrying of marble where holes were drilled in its surface, the holes werethen filled with water and the water compressed by hammering in woodenplugs. Pressures achieved as a result were sufficiently high to fracture themarble.

Little scientific progress was made in the Middle Ages in connectionwith fluid power and it was not until 1648 that a Frenchman, Blaise Pascal,formulated the law that states that pressure in a fluid is transmitted equallyin all directions. Practical use was made of this theory by the EnglishmanJoseph Bramah who built the first hydraulic press in the year 1795. Ap-proximately 50 years later, the Industrial Revolution in Great Britain ledto further development of the water press and other industrial machines.The growth was so rapid that by the late 1860s large cities had centralfluid power generating stations from which pressurized fluid was pumpedto factories. The development of internal combustion engines, manual and

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automatic controls, and electrical power during the latter part of the nine-teenth century, however, diminished the rate of growth of centralized fluidpower plants and the practice of such activity ceased.

Interest returned to fluid power at the century’s end due to its recog-nized unique advantages, and in 1906 the electric system for elevating andtraining guns in the battleship U.S.S. Virginia was replaced by a hydraulicsystem. In this installation a variable speed hydrostatic transmission sys-tem was used to maneuver the guns. Modern ships now make extensiveuse of fluid power for many services including winches, controllable pitchpropellers, rudder control, heavy freight elevators, and raising ammunitionfrom magazines to the guns.

The whole science of fluid power is concerned with the utilization ofeither a liquid or a gas as a fluid medium. Water and air were the mediafirst used. For some time, however, hydrocarbon based fluids (i.e. oils) havebeen the dominant liquids. Water based liquids are still used for specializedapplications where the flammability of hydrocarbon fluids is unacceptable.In this text, we will be dealing exclusively with hydrocarbon fluids andthus knowledge of their characteristics is required. It should be noted, how-ever, that there exists a fully developed comprehensive technology centeredaround pneumatic systems and there is significant industrial informationand manufacturing activity. The reader is referred to other sources for thisinformation.

1.3 FLUID POWER APPLICATIONS, PRESENTAND FUTURE

Current activity in fluid power technology includes its use to perform trans-mission and control functions. The growing field of robotics is giving theengineer the opportunity to perform sophisticated design studies for equip-ment used in many productive sectors such as aerospace, agriculture, auto-mated manufacture, construction, defense, energy and transportation. Theabove gives an indication of present and future career opportunities forthose with skills and experience in fluid power technology. With their in-creasing use, it is predicted that fluid power components will become lessexpensive, thereby further improving the competitive advantages of utiliz-ing fluid power as a power transmission medium.

With regard to fluid power components, considerable improvementshave been made in the design of seals, fluids, valves, conductors, pumpsand motors. The most significant advances in hydraulic system design,however, are seen in the area of controls. Electro-mechanical controls havediversified considerably and have led to many new hydraulic applications.

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4 INTRODUCTION

More recent developments have included the use of programmable con-trollers in conjunction with hydraulic systems. These controllers containdigitally operated electronic components and have programmable memorywith instructions to implement functions such as logic, sequencing, timing,and counting. Such modules may control many different types of machinesor processes. It is pleasing to note that fluid power applications are be-ing extended and should increasingly improve our quality of life by, amongother things, reducing the need for manual work to be performed.

Dependability has been improved by the development of easily servicedcartridge-type control valves with very long service life and minimum main-tenance. Due in part to greater demand, the above systems have been re-duced in cost, high pressure piping has been minimized, performance hasbeen improved, and there has been a simplification of maintenance proce-dures.

As will be demonstrated in this text, the improvements in physicalequipment have been accompanied by an enhanced ability to analyze theperformance of fluid power systems. Much of this development can be at-tributed to the dramatic improvement in computing power available to theengineer. More comprehensive analysis will provide a new level of perfor-mance for power transmission systems for machines of today and for thefuture.

1.4 ADVANTAGES OF USING FLUID POWERSYSTEMS

It was stated earlier that there are advantages to using hydraulic systemsrather than mechanical or electrical systems for specific applications andfor those applications using large powers. Some of these advantages aregiven below:

1. Force multiplication is possible by increasing actuator area or workingpressure. In addition, torques and forces generated by actuators arelimited only by pressure and as a result high power to weight ratioand high power to volume ratio are readily achievable.

2. It is possible to have a quick acting system with large (constant) forcesoperating at low speeds and with virtually instant reversibility. Inaddition, a wide speed range of operating conditions may be achieved.

3. A hydraulic system is relatively simple to construct with fewer movingparts than in comparable mechanical or electrical machines.

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4. Power transmission to remote locations is also possible provided thatconductors and actuators can be installed at these locations.

5. In most cases the hydraulic fluid circulated will act as a lubricant andwill also carry away the heat generated by the system.

6. A complex system may be constructed to perform a sequence of op-erations by means of mechanical devices such as cams, or electricaldevices such as solenoids, limit switches, or programmable electroniccontrols.

1.5 A PROBABLE FUTURE DEVELOPMENT

An example of a future development is the design, construction and mar-keting of a hybrid vehicle, where, instead of using electric generator/motorand power electronics, a hydraulic hybrid design could be advantageous.The U.S. Environmental Protection Agency has already built such a devicethat has achieved a fuel consumption saving of 55%. The conventional drivetrain from a stock 2003 four-wheel drive Ford Expedition was removed andreplaced by a hydraulic drive train [1]. In addition, the 5.4 L V8 gasolineengine was replaced by a 1.9 L Volkswagen 4 cylinder diesel engine. Thehydraulic system used two pumps and two accumulators. One of the pumpmotors switched between pumping and driving modes and pre-charged theaccumulators. During braking, the other pump motor helped to recoverbraking energy. The pump motor units worked together to pressurize oneaccumulator to 5000 lbf/in.2 and the other to 200 lbf/in.2. It is conjec-tured that hydraulic hybrid drive trains are particularly well suited to beused in frequently stopping vehicles such as school buses and urban deliverytrucks because the system captures large amounts of energy normally lostin braking in conventionally powered vehicles.

REFERENCES

1. ASME, 2004, Mechanical Engineering, 126(9), p. 13.

© 2006 by Taylor & Francis Group, LLC