What is Fluid Power?

1.1 WHAT IS FLUID POWER?

Utilization of fluid power is important because it is one of the three available

means of transmitting power. Other methods of transmitting power

are by utilizing mechanical means and by applying electrical energy. To

demonstrate this we will consider that we have a prime mover such as a

diesel engine on one side of the room and a mechanical contrivance on the

other. The objective is to see how, in a generic sense, power can be used

by the methods quoted above to perform the necessary mechanical work.

For mechanical power transmission, the prime mover is connected to

the device and, by use of gearboxes, pulleys, belts and clutches, the necessary

work can be performed. With the electrical method, an electrical

generator is used. The current developed can be carried through electrical

cable to operate electrical motors, linear or rotary, modulation being provided

by variable resistance or solid state devices in the circuits. For fluid

power utilization, an oil pump is connected to the engine and instead of

electrical cables, high pressure hose is used to convey pressurized fluid to

motors (again linear or rotary), pressure and flow modulation now being

provided within the motors or by means of hydraulic valves.

Any of the three methods described may be used however, if an engineering system

requires:

1. Minimum weight and volume

2. Large forces and low speeds

3. Instant reversibility

4. Remote control

then the fluid power technique will often have significant competitive advantages.

It is indeed unfortunate that design of fluid power systems is seldom taught at four-year universities in the United States at the same time as formal teaching of power transmission systems involving mechanical and electrical systems. Such comprehensive design teaching would demonstrate adequately the advantages of such systems. In some instances hydraulics

power transmission is the only technique that can be used. The most spectacular example is that of extending an aircraft’s control surface into a high velocity airstream where the only technique available is that of using fluid power actuators because of their high power to weight and volume to weight advantages.

1.2 A BRIEF HISTORY OF FLUID POWER

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

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

Little scientific progress was made in the Middle Ages in connection with 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 equally in all directions. Practical use was made of this theory by the Englishman Joseph Bramah who built the first hydraulic press in the year 1795. Approximately

50 years later, the Industrial Revolution in Great Britain led to further development of the water press and other industrial machines.

The growth was so rapid that by the late 1860s large cities had central fluid power generating stations from which pressurized fluid was pumped to factories. The development of internal combustion engines, manual andautomatic controls, and electrical power during the latter part of the nineteenth century, however, diminished the rate of growth of centralized fluid

power plants and the practice of such activity ceased.

Interest returned to fluid power at the century’s end due to its recognized unique advantages, and in 1906 the electric system for elevating and training guns in the battleship U.S.S. Virginia was replaced by a hydraulic system. In this installation a variable speed hydrostatic transmission system was used to maneuver the guns. Modern ships now make extensive

use of fluid power for many services including winches, controllable pitch propellers, rudder control, heavy freight elevators, and raising ammunition from magazines to the guns.

The whole science of fluid power is concerned with the utilization of either a liquid or a gas as a fluid medium. Water and air were the media first used. For some time, however, hydrocarbon based fluids (i.e. oils) have been the dominant liquids. Water based liquids are still used for specialized applications where the flammability of hydrocarbon fluids is unacceptable.

In this text, we will be dealing exclusively with hydrocarbon fluids and thus knowledge of their characteristics is required. It should be noted, however, that there exists a fully developed comprehensive technology centered around pneumatic systems and there is significant industrial information and manufacturing activity. The reader is referred to other sources for this

information.

1.3 FLUID POWER APPLICATIONS, PRESENT AND FUTURE

Current activity in fluid power technology includes its use to perform transmission and control functions. The growing field of robotics is giving the engineer the opportunity to perform sophisticated design studies for equipment used in many productive sectors such as aerospace, agriculture, automated manufacture, construction, defense, energy and transportation. The

above gives an indication of present and future career opportunities for those with skills and experience in fluid power technology. With their increasing use, it is predicted that fluid power components will become less expensive, thereby further improving the competitive advantages of utilizing fluid power as a power transmission medium.

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

More recent developments have included the use of programmable controllers in conjunction with hydraulic systems. These controllers contain digitally operated electronic components and have programmable memory with instructions to implement functions such as logic, sequencing, timing, and counting. Such modules may control many different types of machines

or processes. It is pleasing to note that fluid power applications are being extended and should increasingly improve our quality of life by, among other things, reducing the need for manual work to be performed.

Dependability has been improved by the development of easily serviced cartridge-type control valves with very long service life and minimum maintenance.

Due in part to greater demand, the above systems have been reduced in cost, high pressure piping has been minimized, performance has been improved, and there has been a simplification of maintenance procedures. As will be demonstrated in this text, the improvements in physical

equipment have been accompanied by an enhanced ability to analyze the performance of fluid power systems. Much of this development can be attributed to the dramatic improvement in computing power available to the engineer. More comprehensive analysis will provide a new level of performance for power transmission systems for machines of today and for the

future.

1.4 ADVANTAGES OF USING FLUID POWER SYSTEMS

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

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

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

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

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

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

6. A complex system may be constructed to perform a sequence of operations by means of mechanical devices such as cams, or electrical devices such as solenoids, limit switches, or programmable electronic controls.

1.5 A PROBABLE FUTURE DEVELOPMENT

An example of a future development is the design, construction and marketing of a hybrid vehicle, where, instead of using electric generator/motor and power electronics, a hydraulic hybrid design could be advantageous.

The U.S. Environmental Protection Agency has already built such a device that has achieved a fuel consumption saving of 55%. The conventional drive train from a stock 2003 four-wheel drive Ford Expedition was removed and replaced by a hydraulic drive train [1]. In addition, the 5.4 L V8 gasoline engine was replaced by a 1.9 L Volkswagen 4 cylinder diesel engine. The hydraulic system used two pumps and two accumulators. One of the pump

motors switched between pumping and driving modes and pre-charged the accumulators. During braking, the other pump motor helped to recover braking energy. The pump motor units worked together to pressurize one accumulator to 5000 lbf/in.2 and the other to 200 lbf/in.2. It is conjectured that hydraulic hybrid drive trains are particularly well suited to be used in frequently stopping vehicles such as school buses and urban delivery trucks because the system captures large amounts of energy normally lost in braking in conventionally powered vehicles.

REFERENCES

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