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An Overview of Machine Design

1.1 Introduction

1.1.1 Machines and Machine Elements

A machine is a device that employs power to accomplish a desired function to benefit humankind. It is generally composed of a power source to provide power and movement, and an executive device to fulfil intended function. In between is a transmission system and controller. Power sources can be prime movers, that is, a machine utilizes a natural source of energy to produce power, like an internal combustion engine; or a secondary mover, like an electric motor, which receives energy directly or indirectly from a generator driven by a prime mover [1]. Take a car as an example; power sources, either engines in motor vehicles or batteries in electrical vehicles, provide power to executive devices; for example, wheels, through a transmission system (including couplings, clutches, shafts, powertrain etc.). The control system, that is, steering systems and brakes, control car movement. Lights, meters and windscreen wipers are accessories that facilitate a car running properly.

Machines involve a vast variety of mechanical products in various fields of manufacturing, transportation, aerospace, construction, agricultural, energy and many others. 'Products' covers industrial robots, machine tools and automated assembly systems in manufacturing systems; automobiles, trains, ships and aircraft in transportation vehicles; mobile cranes, dump trucks and concrete mixers in construction equipment. Household appliances, like vacuum cleaners, washing machines and air-conditioning systems are also machines.

A similar concept, that is, mechanism, is a combination of elements formed and connected to transmit motion in a predetermined fashion. Typical mechanisms include linkages, cams and follower systems, gears and gear trains and so on. There is no clear division between mechanisms and machines. If the transmitted forces or power are significant, it is considered a machine; otherwise, it is considered a mechanism [2]. Machinery is a derived term and refers to a grouping of mechanisms and machines [1].

A machine composes individual machine elements properly designed and arranged to work together. Machine elements are the fundamental components of a machine, and are broadly classified as universal elements, such as bolts, keys, splines, pins, belts, chains, gears, bearings and springs that are widely used in different kinds of machines, and special elements such as turbine blades, crankshafts and aircraft propellers, which perform specific functions [3]. This book focuses on the analysis and design of universal machine elements.

1.1.2 The Scope of Machine Design

Design is widely considered to be the central or distinguishing activity of engineering [4]. It aims to create and execute a purposeful plan to meet commercial, industrial and social needs. When design is discussed in mechanical engineering domain, especially about mechanical products or machines, it is termed mechanical or machine design.

Machine design is the art of envisioning, creating and developing a brand new or improving on an existing mechanical device for the fulfilment of human needs, with due regard for resource conservation and environmental impact [ 1,5]. It is an innovative, iterative, decision making and problem-solving process involving comprehensive utilization of scientific knowledge and creative capability. Designers are required to generate concepts and decide deterministic dimensions for devices or products from limited information, ambiguous and sometimes even partially contradictory requirements to achieve users' objectives while satisfying a set of specified constraints. Therefore, the initial design is usually tentative. With more variables gradually determined, material, geometry, as well as manufacturing and tolerance details are fine-tuned during iteration until final optimum design is achieved [5].

Machine design generally has more than one solution. Since design is a rational process of choosing among design alternatives [6], it greatly depends on the designer's knowledge, previous experience, design method and design philosophies to solve a specific problem. Consider, for example, the design of an automobile. A large number of models are available on the market, and all of them can fulfil the function of transportation. The differences among them are the operation convenience, comfort, aesthetic appearance, cost and so on. These features decide the competitiveness and sales of products.

Analysis and design are indispensable aspects in the process of machine design. Analysis is concerned with predicting the response of an existing or a tentatively designed machine under specified inputs, which is especially important in creative design process; while design attempts to decide the dimensions and shapes of machines to meet performance requirements. Integrating analysis and design skills during the design process distinguishes an outstanding design engineer from a good one [7]. Since design is an evolutionary process, a tentative design is firstly proposed and then analysed to see if it satisfies the given specifications. If not, which is the usual case; the tentative design is revised with changes involving geometry, material and loads, and is analysed again until it satisfies the specified design requirements.

Extensive multidisciplinary skills and knowledge are employed in the design process to convert inadequate, vague requirements into a product that is functional, safe, reliable, manufacturable, competitive and marketable. Engineering science includes mechanics of solids and fluids, materials science, manufacturing processes and so on. Designers are required to use computers and graphics tools to visualize design plans. Since designers often work collaboratively on teams, communication skills, teamwork ability and presentation skills are equally important.

To design a machine or machine element successfully, design engineers not only need to develop competence in understanding and applying scientific knowledge, empirical information, professional judgement and ingenuity in solving practical problems, but also cultivate a strong sense of responsibility and professional work ethic. Mechanical design involves almost all the disciplines of mechanical engineering [8-10]. The extensive knowledge and skills required for a mechanical designer are briefly summarized as:

• Competence in mathematics, statics, dynamics, mechanics of materials, kinematics and mechanisms to facilitate load, stress and strength analyses; an advanced CAD/CAE (computer-aided design or engineering) or FEM (finite element method) technique is preferred;
• Familiar with engineering materials and their properties, materials processing, heat treatments and manufacturing processes;
• Knowledge of tribology, fluid mechanics, heat transfer, electrical and information technology and controls;
• Creativity, complex problem-solving capability and project management skills;
• Competence in graphical representation by sketches, engineering drawing, CAD tools and 3D visualization to convert mental design concepts into technical drawings;
• Both verbal and written communication skills and presentation skills to articulate design projects, describe constraints and limitations and present proposals and technical reports;
• Teamwork and collaboration capability, and a sense of social and ethical responsibilities.

Students are expected to appreciate that machine design is an integration of physical and engineering considerations with social concerns, with an aim to design machines with satisfactory lives and high reliabilities. With the globalization of business world, future design engineers are expected to feel comfortable to work in a vibrant and multicultural environment, learn to satisfy the needs of customers in a competent, responsible, ethical and professional manner and able to communicate complex aspects of design verbally and graphically to other members in both national and international concurrent design teams.

1.2 Machine Design

1.2.1 Machine Design Considerations

An important assessment of design quality is machine's safe and reliable performance of its intended function for the prescribed design life without serious breakdown. Besides, machine design involves a multitude of considerations, as summarized in Table 1.1. It is unrealistic to satisfy all these considerations, as some are seemingly incompatible. Designers are challenged to recognize incongruities and find compromises between these discrepancies. During the design process, the total life cycle of a product, from initial ideation, design, manufacturing, assembly to service and final disposal, should be reviewed, and situations that may practically occur during manufacturing, transporting, storing, installing, servicing and disposal be evaluated. The main concerns selected from Table 1.1 are discussed next and will be addressed throughout the book.

Table 1.1 Machine design considerations.