Mechanical Engineering

Summary

Mechanical engineering is the field of technology that deals with engines, machines, tools, and other mechanical devices and systems. This broad field of innovation, design, and production deals with machines that generate and use power, such as electric generators, motors, internal combustion engines, and turbines for power plants, as well as heating, ventilation, air-conditioning, and refrigeration systems. In many universities, mechanical engineering is integrated with nuclear, materials, aerospace, and biomedical engineering. The tools used by scientists, engineers, and technicians in other disciplines are usually designed by mechanical engineers. Robotics, microelectromechanical systems, and the development of nanotechnology and bioengineering technology constitute a major part of modern research in mechanical engineering.

Definition and Basic Principles

Mechanical engineering is the field dealing with the development and detailed design of systems to perform desired tasks. Developed from the discipline of designing the engines, power generators, tools, and mechanisms needed for mass manufacturing, it has grown into the broadest field of engineering, encompassing or touching most of the disciplines of science and engineering. Mechanical engineers take the laws of nature and apply them using rigorous mathematical principles to design mechanisms. The process of design implies innovation, implementation, and optimization to develop the most suitable solution to the specified problem, given its constraints and requirements. The field also includes studies of the various factors affecting the design and use of the mechanisms being considered.

The laws of physics and thermodynamics are at the root of mechanical engineering. Sir Isaac Newton's laws of motion and gravitation, the three laws of thermodynamics, and the laws of electromagnetism are fundamental to mechanical design.

Starting with the Industrial Revolution in the nineteenth century and going through the 1970s, mechanical engineering was generally focused on designing large machines and systems and automating production lines. Ever-stronger materials and larger structures were sought. In the 1990s and the first part of the twenty-first century, mechanical engineering saw rapid expansion into the world of ever-smaller machines, first in the field of micro and then nanomaterials, probes and machines, down to manipulating individual atoms. In this regime, short-range forces assume a completely different relationship to mass. This led to a new science integrating electromagnetics and quantum physics with the laws of motion and thermodynamics. Mechanical engineering also expanded to include the field of system design, developing tools to reduce the uncertainties in designing increasingly more complex systems composed of larger numbers of interacting elements.

Background and History

The engineering of tools and machines has been associated with systematic processes since humans first learned to select sticks or stones to swing and throw. The associations with mathematics, scientific prediction, and optimization are clear from the many contraptions that humans developed to help them get work done. In the third century BCE, for example, the mathematician Archimedes of Syracuse was associated with the construction of catapults to hurl projectiles at invading armies, who must themselves have had some engineering skills, as they eventually invaded his city and murdered him. Tools and weapons designed in the Middle Ages, from Asia to Europe and Africa, show amazing sophistication. In the thirteenth century, Mesopotamian engineer Al-Jazari invented the camshaft and the cam-slider mechanism and used them in water clocks and water-raising machines. In Italy, Leonardo da Vinci designed many devices, from portable bridges to water-powered engines.

The invention of the steam engine at the start of the Industrial Revolution is credited with the scientific development of the field that is now called mechanical engineering. In 1847, the Institution of Mechanical Engineers was founded in Birmingham, England. In North America, the American Society of Civil Engineers was founded in 1852, followed by the American Society of Mechanical Engineers in 1880. Most developments came through hard trial and error. However, the parallel efforts to develop retrospective and introspective summaries of these trials resulted in a growing body of scientific knowledge to guide further development.

Nevertheless, until the late nineteenth century, engineering was considered a second-rate profession and segregated from the “pure” sciences. Innovations were published through societies such as England's Royal Society only if the author was introduced and accepted by its prominent members, usually from wealthy landed nobility. Publications came from deep intellectual thinking by amateurs who supposedly did it for pleasure and amusement. Actual hands-on work and details were left to paid professionals, who were deemed lower class. Even in America, engineering schools were called trade schools and were separate from the universities that catered to those desiring liberal arts educations focused on the classics and languages from the Eurocentric point of view.

Rigorous logical thinking based on the experience of hands-on applications, which characterizes mechanical engineering, started gaining currency with the rise of a culture that elevated the dignity of labor in North America. It gained a major boost with the urgency brought about by several wars. From the time of the American Civil War to World War I, weapons such as firearms, tanks, and armored ships saw significant advancements and were joined by airplanes and motorized vehicles that functioned as ambulances. During these conflicts, the individual heroism that had marked earlier wars was eclipsed by the technological superiority and scientific organization delivered by mechanical engineers.

Concomitantly, principles of mass production were applied intensively and generated immense wealth in Europe and the United States. Great universities were established by people who rose from the working classes and made money through technological enterprises. The Great Depression collapsed the established manufacturing entities and forced a sharp rise in innovation as a means of survival. New engineering products developed rapidly, showing the value of mechanical engineering. World War II and the subsequent Cold War integrated science and engineering inseparably. The space race of the 1960s through the 1980s brought large government investments in both military and civilian aerospace engineering projects. These spun off commercial revolutions in computers, computer networks, materials science, and robotics. Engineering disciplines and knowledge exploded worldwide, and there is little superficial difference between engineering curricula in most countries of the world.

The advent of the Internet accelerated and completed this leveling of the knowledge field, setting up a sharper impetus for innovation based on science and engineering. Competition in manufacturing advanced the field of robotics so that cars made by robots in automated plants achieve superior quality more consistently than those built by skilled master craftsmen. Manufacturing based on robotics can respond more quickly to changing specifications and demand than human workers can.

Beginning in the 1990s, micromachines began to take on growing significance. Integrated microelectromechanical systems were developed using the techniques used in computer production. One by one, technology products once considered highly glamorous and hard to obtain—from calculators to smartphones—have been turned into mass-produced commodities available to most at an affordable cost. Other products—from personal computers and cameras to cars, rifles, music and television systems, and even jet airliners—are also heading for commoditization as a result of the integration of mechanical engineering with computers, robotics, and microelectromechanics.

How It Works

The most common idea of a mechanical engineer is one who designs machines that serve new and useful functions in an innovative manner. Often, these machines appear to be incredibly complex inside or extremely simple outside. Accurately accomplishing these miraculous designs is systematic, and good mechanical engineers make it look easy.

System Design. At the top level, system design starts with a rigorous analysis of the needs to be satisfied, the market for a product that satisfies those needs, the time available to do the design and manufacturing, and the resources that must be devoted. This step also includes an in-depth study of what has been done before. This leads to “requirements definition,” where the actual requirements of the design are carefully specified. Experienced designers believe that this step already determines more than 80 percent of the eventual cost of the product.

Next comes an initial estimate of the eventual system characteristics, performed using simple, commonsense logic, applying the laws of nature and observations of human behavior. This step uses results from benchmarking what has been achieved before and extrapolating some technologies to the time when they must be used in the manufacturing of the design. Once these rudimentary concept parameters and their relationships are established, various analyses of more detailed implications become possible. A performance estimation then identifies basic limits and determines whether the design “closes,” meeting all the needs and constraints specified at the beginning. Iterations on this process develop the best design. Innovations may be totally radical, which is relatively rare, or incremental in individual steps or aspects of the design based on new information, or on linking developments in different fields. In either case, extensive analysis is required before an innovation is built into a design. The design is then analyzed for ease and cost of manufacture. The “tooling,” or specific setups and machines required for mass manufacture, are considered.

A cost evaluation includes the costs of maintenance through the life cycle of the product. The entire process is then iterated on to minimize this cost. The design is then passed on to build prototypes, thereby gaining more experience on the manufacturing techniques needed. The prototypes are tested extensively to see if they meet the performance required and predicted by the design.

When these improvements are complete and the manufacturing line is set up, the product goes into mass manufacture. The engineers must stay engaged in the actual performance of the product through its delivery to the end user and learn from the customer's experience to design improvements to the product as quickly as possible. In modern concurrent engineering practice, designers attempt to achieve as much as possible of the manufacturing process design and economic optimization during the actual product design cycle to shorten the time to reach the market and the cost of the design cycle. The successful implementation of these processes requires both technical knowledge and experience from the mechanical engineers. These come from rigorous fields of knowledge, some of which are listed below.

Engineering Mechanics. The field of engineering mechanics integrates knowledge of statics, dynamics, elasticity, and strength of materials. These fields rigorously link mathematics, the laws of motion and gravitation, and material property relationships to derive general relations and analysis methods. Fundamental to all of engineering, these subfields are typically covered at the beginning of any course of study.

In statics, the concept of equilibrium from Newton's first law of motion is used to develop free-body diagrams showing various forces and reactions. These establish the conditions necessary for a structure to remain stable and describe relations between the loads in various elements.

In dynamics, Newton's second law of motion is used to obtain relations for the velocity and acceleration vectors for isolated bodies and systems of bodies and to develop the notions of angular momentum and moment of inertia.

The strength of materials is a general subject that derives relationships between material properties and loads using the concepts of elasticity and plasticity and the deflections of bodies under various types of loading. These analyses help the engineer predict the yield strength and the breaking strength of various structures if the material properties are known. Metals were the preferred choice of material for engineering for many decades, and methods to analyze structures made of them were highly refined, exploiting the isotropy of metal properties. Modern mechanical engineering requires materials the properties of which are much less uniform or exotic in other ways.

Graphics and Kinematics. Engineers and architects use graphics to communicate their designs precisely and unambiguously. Initially, learning to draw on paper was a major part of learning engineering skills. However, students now learn the principles of graphics using computer-aided design (CAD) software and computer graphics concepts. The drawing files can also be transferred quickly into machines that fabricate a part in computer-aided manufacturing (CAM). Rapid prototyping methods such as stereolithography construct an object from digital data generated by computer graphics.

The other use of graphics is to visualize and perfect a mechanism. Kinematics develops a systematic method to calculate the motions of elements, including their dependence on the motion of other elements. This field is crucial to developing, for instance, gears, cams, pistons, levers, manipulator arms, and robots. Machines that achieve very complex motions are designed using the field of kinematics.

Robotics and Control. The study of robotics starts with the complex equations that describe how the different parts satisfy the equations of motion with multiple degrees of freedom. Methods of solving large sets of algebraic equations quickly are critical in robotics. Robots are distinguished from mere manipulator arms by their ability to make decisions based on input rather than depend on a telepresence operator for commands. For instance, telepresence is adequate for operating a machine on the surface of the Moon, which is only a few seconds of round-trip signal travel time from Earth using electromagnetic signals. However, the round-trip time for a signal to Mars is several minutes, so a rover operating there cannot wait for commands from Earth regarding how to negotiate around an obstacle. A fully robotic rover is needed that can make decisions based on what its sensors tell it, like humans.

Entire manufacturing plants are operated using robotics and telepresence supervision. Complex maneuvers such as the rendezvous between two spacecraft, one of which may be spinning out of control, have been achieved in orbits in space, where the dynamics are difficult for a human to visualize. Flight control systems for aircraft have been implemented using robotics, including algorithms to land the aircraft safely and more precisely than human pilots can. These systems are developed using mathematical methods for solving differential equations rapidly, along with software to adjust parameters based on feedback.

Materials. The science of materials has advanced rapidly since the late twentieth century. Wood was once a material of choice for many engineering products, including bridges, aircraft wings, propellers, and train carriages. The fibrous nature of wood required considerable expertise from those choosing how to cut and lay sections of wood. As a natural product, its properties varied considerably from one specimen to another. Metals became much more convenient to use in design and fabrication because the energy to melt and shape metals cheaply became available. Various alloys were developed to tailor machinery for strength, flexibility, elasticity, corrosion resistance, and other desirable characteristics. Detailed tables of properties for these alloys were included in mechanical engineering handbooks.

Materials used to manufacture mass-produced items have migrated to molded plastics made of hydrocarbons derived from petroleum. The molds are shaped using such techniques as rapid prototyping and computer-generated data files from design software. Composite materials are tailored with fiber bundles arrayed along directions where high tensile strength is needed and much less strength along directions where high loads are not likely, thus achieving large savings in mass and weight.

Fluid Mechanics. The science of fluid mechanics is important to any machine or system that either contains or must move through water, air, or other gases or liquids (fluids). Fluid mechanics employs the laws of physics to derive conservation equations for specific packets of fluid (the Lagrangian approach) or for the flow through specified control volumes (Eulerian approach). These equations describe the physical laws of conservation of mass, momentum, and energy, relating forces and work to changes in flow properties. The properties of specific fluids are related through the thermal and caloric equations expressing their thermodynamic states. The speed of propagation of small disturbances, known as the speed of sound, is related to the dependence of pressure on density and hence on temperature. Various nondimensional groupings of flow and fluid properties—such as the Reynolds number, Mach number, and Froude number—are used to classify flow behavior. Increasingly, for many problems involving fluid flow through or around solid objects, calculations starting from the conservation equations are able to predict the loads and flow behavior reliably using the methods of computational fluid dynamics (CFD). However, the detailed prediction of turbulent flows remains beyond reach and is approximated through various turbulence models. Fluid-mechanic drag and the movements due to flow-induced pressure remain very difficult to calculate to the accuracy needed to improve vehicle designs.

Methods for measuring the properties of fluids and flows in their different states are important tools for mechanical engineers. Typically, measurements and experimental data are used at the design stage, well before the computational predictions become reliable for refined versions of the product.

Thermodynamics. Thermodynamics is the science behind converting heat to work and estimating the best theoretical performance that a system can achieve under given constraints. The three basic laws of temperature are the zeroth law, which defines temperature and thermal equilibrium; the first law, which describes the exchange between heat, work, and internal energy; and the second law, which defines the concept of entropy. Although these laws were empirically derived and have no closed-form proof, they give results identical to those that come from the law of conservation of energy and to notions of entropy derived from statistical mechanics of elementary particles traced to quantum theory. No one has yet been able to demonstrate a true perpetual motion machine, and it does not appear likely that anyone will. From the first law, various heat-engine cycles have been invented to obtain better performance suited to various constraints. Engineers working on power-generating engines, propulsion systems, heating systems, and air-conditioning and refrigeration systems try to select and optimize thermodynamic cycles and then use a figure of merit—a means of evaluating the performance of a device or system against the best theoretical performance that could be achieved—as a measure of the effectiveness of their design.

Heat Transfer. Heat can be transferred through conduction, convection, or radiation, and all three modes are used in heat exchangers and insulators. Cooling towers for nuclear plants, heat exchangers for nuclear reactors, automobile and home air-conditioners, and the radiators for the International Space Station are all designed from the basic principles of these modes of heat transfer. Some space vehicles are designed with heat shields that are ablative. The Thermos flask (which uses an evacuated space between two silvered glass walls) and windows with double and triple panes with coatings are examples of widely used products designed specifically to control heat transfer.

Machine Design. Machine design is at the core of mechanical engineering, bringing together the various disciplines of graphics, solid and fluid mechanics, heat transfer, kinematics, and system design in an organized approach to designing devices to perform specific functions. This field teaches engineers how to translate the requirements for a machine into a design. It includes procedures for choosing materials and processes, determining loads and deflections, failure theories, finite element analysis, and the basics of how to use various machine elements such as shafts, keys, couplings, bearings, fasteners, gears, clutches, and brakes.

Metrology. The science of metrology concerns measuring systems. Engineers deal with improving the accuracy, precision, linearity, sensitivity, signal-to-noise ratio, and frequency response of measuring systems. The precision with which dimensions are measured has a huge impact on the quality of engineering products. Large systems such as airliners are assembled from components built on different continents. For these to fit together at final assembly, each component must be manufactured to exacting tolerances, yet requiring too much accuracy sharply increases the cost of production. Metrology helps in specifying the tolerances required and ensuring that products are made to such tolerances.

Acoustics and Vibrations. These fields are similar in much of their terminology and analysis methods. They deal with wavelike motions in matter, their effects, and their control. Vibrations are rarely desirable, and their minimization is a goal of engineers in perfecting systems. Acoustics is important not only because minimizing noise is usually important, but also because engineers must be able to build machines to generate specific sounds, and because the audio signature is an important tool in diagnosing system status and behavior.

Production Engineering. Production engineering deals with improving the planning and implementation of the production process, designing efficient and precise tools to produce goods, laying out efficient assembly sequences and facilities, and setting up the flow of materials and supplies into the production line, and the control of quality and throughput rate. Production engineering is key to implementing the manufacturing step that translates engineering designs into competitive products.

Applications and Products

Conventional Applications. Mechanical engineering is applied to the design, manufacture, and testing of almost every product used by humans and to the machines that help humans build those products. The products most commonly associated with mechanical engineering include all vehicles, such as railway trains, buses, ships, cars, airplanes and spacecraft, cranes, engines, and electric or hydraulic motors of all kinds, heating, ventilation and air-conditioning systems, the machine tools used in mass manufacture, robots, agricultural tools, and the machinery in power plants. Several other fields of engineering such as aerospace, materials, nuclear, industrial, systems, naval architecture, computer, and biomedical developed and spun off at the interfaces of mechanical engineering with specialized applications. Although these fields have developed specialized theory and knowledge bases of their own, mechanical engineering continues to find application in the design and manufacture of their products.

Innovations in Materials. Carbon nanotubes have been heralded as a future super-material with strength hundreds of times that of steel for the same mass. Composite materials incorporating carbon already find wide use in various applications where high temperatures must be encountered. Metal matrix composites find use in primary structures even for commercial aircraft. Several “smart structures” have been developed, where sensors and actuators are incorporated into a material that has special properties to respond to stress and strain. These enable structures that will twist in a desired direction when bent or become stiffer or more flexible as desired, depending on electrical signals sent through the material. Materials capable of handling very low (cryogenic) temperatures are at the leading edge of research applications. Magnetic materials with highly organized structures have been developed, promising permanent magnets with many times the attraction of natural magnets.

Sustainable Systems. One very important growth area in mechanical engineering is in designing replacements for existing heating, ventilation, and air-conditioning systems, as well as power generators, that use environmentally benign materials yet achieve high thermodynamic efficiencies, minimizing heat emission into the atmosphere. This effort demands a great deal of innovation and is at the leading edge of research, both in new ways of generating power and reducing the need for power.

Medicine and Biology. Biomechanics is an application of mechanical science in biomedical and biological sciences that helps fabricate exoskeletons and prosthetics. Carbon nanotubes can be helpful in DNA recognition, immunology, and antiviral resistance. Nanorobotics, a combination of nanotechnology, mechanics, and biomaterials, can be used to treat medical conditions such as cancer, cerebral aneurysms, and kidney stones. Computational fluid dynamics helps understand blood flows, human organ dynamics, and surgical simulations.

Careers and Course Work

Mechanical engineers work in nearly every industry in innumerable functions. The curriculum in engineering school accordingly focuses on providing a firm foundation in the basic knowledge that enables problem-solving and continued learning through life. The core curriculum starts with basic mathematics, science, graphics, and an introduction to design. It continues to engineering mechanics, core subjects, and specialized electives. In twenty-first-century engineering schools, students can work on individual research and design projects that are invaluable in providing the students with perspective and integrating their problem-solving skills. Courses include mechanical and aerospace engineering, computational engineering, robotics engineering, and engineering mechanics.

After obtaining a bachelor's degree, some continue studying for a license or certificate. Though not required for many positions, these credentials can support future career goals, like management. Some positions require professional engineering licenses and bi-annual continuing education courses, but this varies by location. The American Society of Mechanical Engineers maintains a list of accredited education programs.

Mechanical engineers have a broad range of career choices. Traditional occupations include designing systems for energy, heating, ventilation, air-conditioning, pressure vessels and piping, automobiles, and railway equipment. Other options include designing bioengineering production systems, microelectromechanical systems, optical instrumentation, telecommunications equipment, and software. Many mechanical engineers also go on to management positions. Aspirants can work as auto research engineers, robotics engineers, or heating and cooling systems engineers.

Social Context and Future Prospects

Mechanical engineering attracts many students and offers a broad array of career opportunities. Students in mechanical engineering schools can range across numerous disciplines and create their own specialties. With nanomachines and biologically inspired self-assembling robots becoming realities, mechanical engineering has transformed from a field that generally focuses on big industries to one that emphasizes tiny and efficient machines. 

Mechanical engineers increasingly collaborate with professionals across industries to improve and create technology. For example, creating aerodynamic cars with the automobile industry or addressing global sanitation challenges with epidemiologists. The fields of robotics, automation, and artificial intelligence rely heavily on mechanical engineers' expertise. Additionally, energy-related studies and advancing green energy technology are critical topics of concern for modern mechanical engineers. The future of sustainable energy is intimately associated with advances in mechanical engineering.

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