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2 problem-solving skills2.1 OVERVIEWIn this chapter we begin discussing some of the steps that engineers follow when they solve problems and perform calculations in their daily work .Mechanical engineers are fluent with numbers,and they are organized when obtaining numerical answers to questions that involve a remarkable breadth of variables and physical properties.Some of the questions that you will encounter in your study of mechanical engineering are force,torque,thermal conductivity,shear stress,fluid viscosity[vi'skɔsəti] n. [物] 粘性，[物] 粘度,elastic modulus ,kinetic energy,Reynolds['renəldz] n. 雷诺兹（姓氏）number,specific heat,and so on.The list is very long indeed.The only way that you will make sense of so many quantities is to be very clear about them in calculations and when explaining results to others.Each quantity in mechanical engineering has two components :a numerical value and a unit.One is simply meaningless without others,and practicing engineers pay as close and careful attention to the units in a calculation as they do to the numbers.In the first portion of this chapter,we will discuss fundamental concepts for systems of units and conversions between them and a procedure for checking dimensional consistency that will serve you well in engineering calculations.Aside from the question of units,engineers must obtain numerical answers to questions- how strong ？how heavy？how much power?what temperature?—often in the face of uncertainty and incomplete information.At the start of a new design,for instance,the shape and dimensions of the final product are not known ;if they were ,then it wouldn,t be necessary to design in the first place .Only rough estimates of the force applied to a structure might be available.Likewise,exact values for material properties are rarely known,and there will always be some variation between samples of materials.Nevertheless,an engineer still has a job to do,and the design process must start somewhere.For that reason,engineer make approximations in order to assign numerical values to quantities that are otherwise unknown. Those approximations are understood to be imperfect,but they are better than random guesses and certainly better than nothing.Mechanical engineer use their common sense,experience,intuition,judgment,and physical laws to find answers through a process called order-of-magnitude approximation.These estimates are a first step toward reducing a concrete physical situation to its most essential and relevant pieces.Mechanical engineers are comfortable making reasoned approximations.In the following sections we begin a discussion of numerical values,unit systems,significant digits,and approximation.These principles and techniques will be applied further as we explore the"element"of mechanical engineering outlined in later chapter. After completing this chapter,you should be able to:● Report both a numerical value and its units in each calculationthat you perform.● List the base units in the United States Customary System andthe Systeme International d'unites,and state some of the derived units used in mechanical engineering.● Understand the need for proper bookkeeping of units whenmaking engineering calculations and the implications of not doing so.●Convert numerical quantities from the United States CustomarySystem to the Systeme International d'Unites,and vice versa.● Check your equations and calculations to verify that they aredimensionally consistent.● Understand how to perform order-of-magnitude approximations. 第二章解决问题的技能2.1概述这一章，我们开始探讨机械工程师在日常工作中，解决问题、进行计算时所采用的步骤。
Lesson1 the mechanical design process 1, the mechanical design processThe ultimate objective of mechanical design is to produce a useful product that satisfies the needs of a customer and that is safe ,efficient, reliable, economical, and practical to manufacture .think broadly when answering the question ,”who is the customer for the product or system I am about to design?”it is essential that you know the desires and expectations of all customers before product design. marketing professionals are often employed to manage the definition of customer expectations , but designers will likely work with them as a part of a product development team .many methods are used to determine what the customer wants. One popular method , called quality function deployment or QFD , seeks(1) to identify all of the features and performance factors that customers desire and (2) to assess the relative importance of these factors. the result of the QFD process is a detailed set of functions and design requirements for the product.It is also important to consider how the design process fits with all functions that must happen to deliver a satisfactory product to the customer and to service the product throughout its lift cycle. in fact, it is important to consider how the product will be disposed of after it hasserved its useful life . the total of all such function that affect the product is sometimes called the product realization process or PRP . some of the factors included in PRP are as follows:1 marketing functions to assess customer requirements2 research to determine the available technology that can reasonably be used in the product3 availability of materials and components that can be incorporated into the product4 product design and development5 performance testing6 documentation of the design7 vendor relationships and purchasing functions8 work-force skills9 physical plant and facilities available10 capability of manufacturing systems11production planning and control of production systems12production support systems and personnel13 quality systems requirements14 sales operations and time schedules15 cost targets and other competitive issues16 customer service requirements17 environmental concerns during manufacture , operation anddisposal of the product18 legal requirements19 availability of financial capitalCan you add to this list? You should be able to see that the design of a product is but one part of a comprehensive process. in this text, we will focus more carefully on the design process itself , but the producibility of designs must always be considered . this simultaneous consideration of product design and manufacturing process design is often called concurrent engineering.2 skills needed in mechanical designProduct engineers and mechanical designers use a wide range of skills and knowledge in their daily work . these skills and knowledge are included in the following :1sketching, technical drawing , and computer-aided design2 properties of materials, materials processing, and manufacturing processes3 applications of chemistry, such as corrosion protection, plating , and painting4 statics, dynamics, strength of materials , kinematics, and mechanisms5 fluid mechanics, thermodynamics, and heat transfer6 fluid power, the fundamentals of electrical phenomena, andindustrial controls7 experimental design and performance testing of materials and mechanical systems8 stress analysis9 specialized knowledge of the behavior of machine elements, such as gears, belt drives, chain drives, shafts, bearings, keys, splines, couplings, seals, spring, connections(bolted, riveted, welded, adhesive), electric motors, linear motion devices, clutches, and brakes10 creativity , problem solving ,and project management11 oral communication, listening, technical writing, and teamwork skills3 functions , design requirements and evaluation criteriaSection 1 emphasized the importance of carefully identifying the needs and expectations of the customer prior to beginning the design of a mechanical device. you can formulate these by producing clear , complete statements of functions, design requirements , and evaluation criteria:1 Functions tell us what the device must do , using general , nonquantitative statements that employ action phrases such as “to support a load”, “to lift a crate ”, “to transmit power”, or “to hold two structural members together”, etc2 Design requirements are detailed, usually quantitativestatements of expected performance levels, environmental conditions in which the device must operate, limitations on space or weight , or available materials and components that may be used3 Evaluation criteria are statements of desirable qualitative characteristics of a design that assist the designer in deciding which alternative design is optimum-that is , the design that maximizes benefits while minimizing disadvantagesTogether these elements can be called the specifications for the designMost designs progress through a cycle of activities are outlined in figure1.1. you should typically propose more than one possible alternative design concept. This is where creativity is exercised to produce truly novel designs .Each design concept must satisfy the functions and design requirements . a critical evaluation of the desirable features, advantages, and disadvantages of each design concept should be completed . then a rational decision analysis technique should use the evaluation criteria to decide which design concept is the optimum and , therefore , should be produced.Reading materialMechanical design is the process of designing and/or selecting mechanical components and putting them together to accomplish a desired function. Of course machine elements must be compatible ,must fit well together , and must perform safely and efficiently. The designer must consider not only the performance of the element being designed at a given time , but also the elements with which it must interface.To illustrate how the design of machine elements must be integrated with a larger mechanical design , let us consider the design of a speed reducer for the small tractor . suppose that,to accomplish the speed reduction ,you decide to design a double-reduction, spur gear speed reducer. You specify four gears , three shafts , six bearings, and a housing to hold the individual elements in proper relation to each other. The primary elements of the speed reducer are :1 the input shaft is to be connected to the power source, a gasoline engine whose output shaft rotates at 2000 rpm . a flexible coupling is to be employed to minimize difficulties with alignment .2 the first pair of gears, a and b ,causes a reduction in the speed of the intermediate shaft proportional to the ratio of the numbers of teeth in the gears. Gear b and c are both mounted to intermediate shaft and rotate at the same speed .3 a key is used at the interface between the hub of each gear and the shaft on which it is mounted to transmit torque between the gear and the shaft .4 the second pair of gears , c and d ,further reduces the speed of gear d and the output shaft to the range of 290 rpm to 295 rpm .5 the output shaft is to carry a chain sprocket.the chain drive ultimately is to be connected to the drive wheel of the tractor.6 each of the three shafts is supported by two ball bearings .making them statically determinate and allowing the analysis of forces and stresses using standard principles of mechanics.7 the bearings are held in a housing that is to be attached to the frame of the tractor . note the manner of holding each bearing so that the inner race rotates with the shaft while the outer race is held stationary.8 seals are on the input and output shafts to prohibit contaminants from entering the housing .9 details of how the active elements are to be installed , lubricated, and aligned are only suggested at this stage of the design process to demonstrate feasibility .one possible assembly process could be as follows.Start by placing the gearing the gears, keys, spacers, and bearings on their respective shaftsThen insert input shaft into its bearing seat on the left side of the housing.Insert the left end of intermediate shaft into its bearing seat whileengaging the teeth of gears a and b.Install the center bearing support to provide support for the bearing at the right side of input shaft.Install output shaft by placing its left bearing into the seat on the center bearing support while engaging gears c and d.Install the right side cover for the housing while placing the final two bearings in their seats.Ensure careful alignment of the shafts.Place gear lubricant in the lower part of housing .The arrangement of the gears, the placement of the bearings so that they straddle the gears , and the general configuration of the housing are also design decisions . the design process cannot rationally proceed until these kinds of decisions are made . when the overall design is conceptualized, the design of the individual machine elements in the speed reducer can proceed . you should recognize that you have already made many design decisions by rendering such a sketch . first, you choose spur gears rather than helical gears , a worm and worm gear , or bevel gears. in fact , other types of speed reduction devices –belt drives , chain drives, or many others—could be appreciate.1gearsfor the gear pairs ,you must specify the number of teeth in each gear ,the pitch (size) of the teeth , the pitch diameters, the face width ,and the material and its heat treatment . these specifications depend on considerations of strength and wear of the gear teeth and the motion requirements(kinematics ). You must also recognize that the gears must be mounted on shafts in a manner that ensures proper location of the gears, adequate torque transmitting capability from the gears to the shafts (as through keys ) and safe shaft design.2shaftshaving designed the gear pairs, next you will consider the shaft design . the shaft is loaded in bending and torsion because of the forces acting at the gear teeth . thus, its design must consider strength and rigidity , and it must permit the mounting of the gears and bearings . shafts of varying diameters may be used to provide shoulders against which to seat the gears and bearings. There may be keyseats cut into the shaft, the input and output shafts will extend beyond the housing to permit coupling with the engine and the drive axle . the type of coupling must be considered, as it can have a dramatic effect on the shaft stress analysis. Seals on the input and output shafts protect internal components.3bearingsdesign of the bearings is next. If rolling contact bearings are to be used , you will probably select commercially available bearings from a manufacturer’s catalog , rather than design a unique one . you mustfirst determine the magnitude of the loads on each bearing from the shaft analysis and the gear designs. The rotational speed and reasonable design life of the bearings and their compatibility with the shaft on which they are to be mounted must also be considered. For example , on the basis of the shaft analysis, you could specify the minimum allowable diameter at each bearing seat location to ensure safe stress levels . the bearing selected to support a particular part of the shaft ,then , must have a bore( inside diameter) no smaller than necessary. When a specific bearing is selected , the shaft at the bearing seat location and allowable tolerances must be specified ,according to the bearing manufacturer’s recommendations , to achieve proper operation and life expectancy of the bearing.。
第一章：机械工程及学科总论机械工程机械工程是以有关的自然科学和技术科学为理论基础，结合生产实践中的技术经验，研究和解决在开发、设计、制造、安装、运用和修理各种机械中的全部理论和实际问题的应用学科。
其基本构成是零件和机构，最终产品是机器。
机械就是机器和机构的总称。
2机械的组成（1）原动部分，是机械的动力源，机械依赖其驱动其他部分，如电动机、内燃机等。
（2）传动部分，是将原动部分的运动和动力传递给执行部分的中间装置，常由凸轮机构、齿轮机构等组成。
（3）控制部分，是控制机械的原动部分、执行部分和传动部分按一定的顺序和规律运动的装置，它包括各种控制机构、电气装置、计算机和液压系统等。
（4）执行部分，是直接完成机器预订功能的工作部分，如汽车的车轮、机床的主轴等。
机器是由若干不同零件组装而成：零件是组成机器的基本要素，即机器的最小制造单元。
各种机器经常用到的零件称为通用零件，如螺钉、螺母、轴、齿轮、弹簧等。
在特定的机器中用到的零件称为专用零件，如汽轮机中的叶片、起重机的吊钩、内燃机中的曲轴、连杆、活塞等。
构件是机器的运动单元，一般由若干个零件刚性联接而成，也可以是单一的零件。
机器的特征：1.都是由许多构件组合而成。
2.组成机器的各运动实体之间有确定的相对运动关系。
3.能实现能量的转换，代替或减轻人的劳动，完成有用的机械功。
凡具备上述三个特征的实体组合体称为机器。
机器组成完整的机器由原动机、工作机和传动装置三部分组成：原动机：机器的动力来源。
电动机、内燃机及液压机等。
工作机：处于整个机械传动路线终端，是完成工作任务的部分。
传动装置：主要作用是把动力部分的运动和动力传递给工作部分的中间环节。
6什么是机构：具有确定相对运动的各种实物的组合，即符合机器的前两个特征。
机构的特征：1.都是由许多构件组合而成。
2.组成机器的各运动实体之间有确定的相对运动关系。
机器与机构的区别：机构主要用来传递和变换运动，而机器主要用来传递和变换能量，从结构和运动学的角度分析，机器与机构之间并无区别。
一、概念1、零件：机械制造中的基本单元。
2、构件：机械运动的基本单元。
3、部件：由若干个零件组成的装配体。
4、机构：由两个以上构件通过活动连接以实现给定运动的组合体，其各组成部分之间具有一定的相对运动用来传递运动与动力，或实现某种特定的运动。
5、机器：由一个或一个以上的机构组成，具有确定的机械运动并完成一定有用工作过程的装置。
6、机械：机器和机构的总称。
7、机械工程：包括机械学和机械制造学两大学科。
二、简答1、零件和构件的区别：零件是机械制造中的基本单元，构件是机械运动的基本单元；构件可以是一个零件组成的，也可以是多个零部件组成的。
2、机器的特征：机器是由两个或两个以上构件组成的，各构件之间具有确定的机械运动，能够代替人类完成有用的机械功或转换机械能。
3、机械工程的涵义：机械工程以有关的自然科学和技术科学为其理论基础，结合生产实践中积累的实际经验，研究和解决在开发、设计、制造、安装、应用和维修各种机械中的全部理论和技术。
一、概念1、功能是产品在使用过程中表现出来的具体功用。
2、规格反映了产品适合的工作能力与范围。
3、性能是指产品工作能力与工作质量之间的差异。
4、功能分为必要功能和非必要功能。
5、一次动力机是把自然界的能源直接转变为机械能的装置。
6、二次动力机是把电能或由电能产生的液压能、气压能等转变为机械能的装置。
7、连续转动到间歇运动的变换与实现机构有槽轮机构和棘轮机构。
8、按其用途，螺旋传动可分为传力螺旋、传导螺旋和调整螺旋。
9、按化学成分、结合键的特点，工程材料可分为金属材料、非金属材料和复合材料三大类。
10、碳素结构钢Q235中数字表示屈服强度，45号钢中数字表示平均含碳量，灰铁HT200中数字表示抗拉强度，铸造碳钢ZG200-400中数字“400”表示抗拉强度。
11、金属材料、陶瓷、高分子材料构成三大固体材料。
12、为了改善钢的性能，在碳钢的基础上加入其它合金元素的钢称为合金钢。
13、碳素钢是指C的质量分数小于2.11%和含有少量Si、Mn、S、P等杂质元素的铁碳合金。
3.1 OVERVIEWAs introduced in Chapter 1,the primary objective of mechanical engineers is to make machines that work, that are useful, and that improve society in some way. Machines in general,and power transmission equipment in particular, are often constructed of standardized components or "building blocks."Just as an electrical engineer might select off-the-shelf resistors,capacitors,and transistors as the elements of a circuit。
mechanical engineers have good intuition for specifying the different available types ofbearing ,shafts ,gears,belts and other components in theirdesigns.Once the details of a machine have been worked out and the components for it have been selected ,the product then needs to be built.Mechanical engineers also devise manufacturing and assembly processes,and at the conclusion of this chapter we will introduce several common machine tools.A solid appreciation of machine components and tools is valuable at three levels.First,and from our primary standpoint in this textbook,a working knowledge of this hardware is important for you to develop a technical vocabulary!Mechanical engineering has its own precise language,and specialized terms are used to describe the construction and operation of machines.To communicate effectivelywith other engineers,you will need to learn,adopt,and share that language.Second,having such a background is necessary to select the poper component for an application and to specify the appropriate machine tool for a production process.Of course,it is not possible to list and describe every machine and component that embodies mechanical engineering principles,and that is not our intention in this chapter.However,by examining just a few machine components and tools,you will develop a growing appreciation for practical machinery issues.Finally,this chapter is a good place for you to begin the journey of understanding the inner workings of mechanical hardware.It's intellectually healthy for you to be curious about machines,wonder how they were made,dissect them,and think about they could have been made differently or better .3.1概述正如第1章介绍，机械工程师的首要目标是制造机器，制造能够工作的、有用的并以某种方式，改善社会的机器。
