167 DESIGN OF MACHINERY CHAPTER 1 ics is synonymous with human factors engineering.We often see reference to the good or bad ergonomics of an automobile interior or a household appliance.A machine de- signed with poor ergonomics will be uncomfortable and tiring to use and may even be dangerous.(Have you programmed your VCR lately,or set its clock?) There is a wealth of human factors data available in the literature.Some references are noted in the bibliography.The type of information which might be needed for a machine design problem ranges from dimensions of the human body and their distribu- Make the ma- tion among the population by age and gender,to the ability of the human body to with- chine fit the man stand accelerations in various directions,to typical strengths and force generating abili- ty in various positions.Obviously,if you are designing a device that will be controlled by a human (a grass shortener,perhaps),you need to know how much force the user can exert with hands held in various positions,what the user's reach is,and how much noise the ears can stand without damage.If your device will carry the user on it,you need data on the limits of acceleration which the body can tolerate.Data on all these topics exist. Much of it was developed by the government which regularly tests the ability of military personnel to withstand extreme environmental conditions.Part of the background re- search of any machine design problem should include some investigation of human factors. 1.9 THE ENGINEERING REPORT Communication of your ideas and results is a very important aspect of engineering. Many engineering students picture themselves in professional practice spending most of their time doing calculations of a nature similar to those they have done as students. Fortunately,this is seldom the case,as it would be very boring.Actually,engineers spend the largest percentage of their time communicating with others,either orally or in writ- ing.Engineers write proposals and technical reports,give presentations,and interact with support personnel and managers.When your design is done,it is usually necessary to present the results to your client,peers,or employer.The usual form of presentation is a formal engineering report.Thus,it is very important for the engineering student to develop his or her communication skills.You may be the cleverest person in the world. but no one will know that If you cannot communicate your ideas clearly and concisely. In fact,if you cannot explain what you have done,you probably don't understand it your- self.To give you some experience in this important skill,the design project assignments in later chapters are intended to be written up in formal engineering reports.Informa- tion on the writing of engineering reports can be found in the suggested readings in the bibliography at the end of this chapter. 1.10 UNITS There are several systems of units used in engineering.The most common in the United States are the U.S.foot-pound-second (fps)system,the U.S.inch-pound-second (ips) system,and the System International (SI).All systems are created from the choice of three of the quantities in the general expression of Newton's second law F= 2 (1.la)
ics is synonymous with human factors engineering. We often see reference to the good or bad ergonomics of an automobile interior or a household appliance. A machine designed with poor ergonomics will be uncomfortable and tiring to use and may even be dangerous. (Have you programmed your VCR lately, or set its clock?) There is a wealth of human factors data available in the literature. Some references are noted in the bibliography. The type of information which might be needed for a machine design problem ranges from dimensions of the human body and their distribution among the population by age and gender, to the ability of the human body to withstand accelerations in various directions, to typical strengths and force generating ability in various positions. Obviously, if you are designing a device that will be controlled by a human (a grass shortener, perhaps), you need to know how much force the user can exert with hands held in various positions, what the user's reach is, and how much noise the ears can stand without damage. If your device will carry the user on it, you need data on the limits of acceleration which the body can tolerate. Data on all these topics exist. Much of it was developed by the government which regularly tests the ability of military personnel to withstand extreme environmental conditions. Part of the background research of any machine design problem should include some investigation of human factors. 1.9 THE ENGINEERING REPORT Communication of your ideas and results is a very important aspect of engineering. Many engineering students picture themselves in professional practice spending most of their time doing calculations of a nature similar to those they have done as students. Fortunately, this is seldom the case, as it would be very boring. Actually, engineers spend the largest percentage of their time communicating with others, either orally or in writing. Engineers write proposals and technical reports, give presentations, and interact with support personnel and managers. When your design is done, it is usually necessary to present the results to your client, peers, or employer. The usual form of presentation is a formal engineering report. Thus, it is very important for the engineering student to develop his or her communication skills. You may be the cleverest person in the world, but no one will know that if you cannot communicate your ideas clearly and concisely. In fact, if you cannot explain what you have done, you probably don't understand it yourself. To give you some experience in this important skill, the design project assignments in later chapters are intended to be written up in formal engineering reports. Information on the writing of engineering reports can be found in the suggested readings in the bibliography at the end of this chapter. 1. 10 UNITS There are several systems of units used in engineering. The most common in the United States are the U.S. foot-pound-second (fps) system, the U.S. inch-pound-second (ips) system, and the System International (SI). All systems are created from the choice of three of the quantities in the general expression of Newton's second law
re F is force,m is mass,l is length,and t is time.The units for any three of these ibles can be chosen and the other is then derived in terms of the chosen units.The rce chosen units are called base units,and the remaining one is then a derived unit. Most of the confusion that surrounds the conversion of computations between ei- rone of the U.S.systems and the SI system is due to the fact that the SI system uses a fferent set of base units than the U.S.systems.Both U.S.systems choose force,length, ad rime as the base units.Mass is then a derived unit in the U.S.systems,and they are eferred to as gravitational systems because the value of mass is dependent on the local gravitational constant.The SI system chooses mass,length,and time as the base units and force is the derived unit.SI is then referred to as an absolute system since the mass is a base unit whose value is not dependent on local gravity. The U.S.foot-pound-second (fps)system requires that all lengths be measured in feet (ft),forces in pounds (Ib),and time in seconds (sec).Mass is then derived from Newton's law as mF2 (1.1b) and the units are: Pounds seconds squared per foot (Ib-sec2/ft)=slugs The U.S.inch-pound-second (ips)system requires that all lengths be measured in inches (in),forces in pounds (Ib),and time in seconds (sec).Mass is still derived from Newton's law,equation 1.1b,but the units are now: It is unfortunate that the mass unit in the ips system Pounds seconds squared per inch(Ib-sec2/in)=blobs has never officially been given a name such as the This mass unit is not slugs!It is worth twelve slugs or one blob." term slug used for mass in the fps system.The author Weight is defined as the force exerted on an object by gravity.Probably the most boldly suggests (with tongue common units error that students make is to mix up these two unit systems(fps and ips) only slightly in cheek)that when converting weight units (which are pounds force)to mass units.Note that the grav-this unit of mass in the ips itational acceleration constant(g)on earth at sea level is approximately 32.2 feet per system be called a blob (bl) second squared which is equivalent to 386 inches per second squared.The relationship to distinguish it more clearly from the shg (sl),and to between mass and weight is: help the student avoid some Mass weight gravitational acceleration of the common units errors listed above. m=形 (1.2) Twelve slugs =one blob. 8 It should be obvious that,if you measure all your lengths in inches and then use Blob does not sound amy g=32.2 feet/sec2 to compute mass,you will have an error of a factor of 12 in your re- sillier than slug,is easy to sults.This is a serious error,large enough to crash the airplane you designed.Even remember,implies mass, and has a convenient worse off is the student who neglects to convert weight to mass at all in his calculations. abbreviation (bl)which is an He will have an error of either 32.2 or 386 in his results.This is enough to sink the ship! anagram for the abbreviation for pound (Ib).Besides,if To even further add to the student's confusion about units is the common use of the you have ever seen a garden unit of pounds mass(Ibm).This unit is often used in fluid dynamics and thermodynam- slug,you know it looks just ics and comes about through the use of a slightly different form of Newton's equation: like a "little blob
* It is unfortunate that the mass unit in the ips system has never officially been given a name such as the term slug used for mass in the fps system. The author boldly suggests (with tongue only slightly in cheek) that this unit of mass in the ips system be called a blob (bl) to distinguish it more clearly from the slug (sl), and to help the student avoid some of the common units errors listed above. Twelve slugs = one blob. Blob does not sound any sillier than slug, is easy to remember, implies mass, and has a convenient abbreviation (bl) which is an anagram for the abbreviation for pound (Ib). Besides, if you have ever seen a garden slug, you know it looks just like a "little blob
18罗 DESIGN OF MACHINERY CHAPTER 1 F=ma (1.3) Be where m mass in Ibm'a acceleration and gc the gravitational constant. The value of the mass of an object measured in pounds mass (Ibm)is numerically egual to its weight in pounds force (Ib/).However the student must remember to divide the value of m in Ibm by gc when substituting into this form of Newton's equation.Thus the Ibm will be divided either by 32.2 or by 386 when calculating the dynamic force.The result will be the same as when the mass is expressed in either slugs or blobs in the F= ma form of the equation.Remember that in round numbers at sea level on earth: I Ibm=llbf I slug =32.2 Ibf I blob =386 Ibf The SI system requires that lengths be measured in meters (m),mass in kilograms (kg),and time in seconds (sec).This is sometimes also referred to as the mks system. Force is derived from Newton's law,equation 1.1b and the units are: kilogram-meters per second2(kg-m/sec2)=newtons Thus in the SI system there are distinct names for mass and force which helps alle- viate confusion.When converting between SI and U.S.systems,be alert to the fact that mass converts from kilograms (kg)to either slugs (sl)or blobs (bl),and force converts from newtons (N)to pounds (Ib).The gravitational constant (g)in the SI system is ap- proximately 9.81 m/sec2. The principal system of units used in this textbook will be the U.S.ips system.Most machine design in the United States is still done in this system.Table 1-4 shows some of the variables used in this text and their units.The inside front cover contains a table of conversion factors between the U.S,and SI systems. The student is cautioned to always check the units in any equation written for a prob- lem solution,whether in school or in professional practice after graduation.If properly written,an equation should cancel all units across the equal sign.If it does not,then you can be absolutely sure it is incorrect.Unfortunately,a unit balance in an equation does not guarantee that it is correct,as many other errors are possible.Always double-check your results.You might save a life. 1.11 WHAT'S TO COME In this text we will explore the topic of machine design in respect to the synthesis of mechanisms in order to accomplish desired motions or tasks,and also the analysis of these mechanisms in order to determine their rigid-body dynamic behavior.On the premise that we cannot analyze anything until it has been synthesized into existence,we will first explore the topic of synthesis of mechanisms.Then we will investigate the analysis of those and other mechanisms for their kinematic behavior.Finally,in Part II we will deal with the dynamic analysis of the forces and torques generated by these moving machines.These topics cover the essence of the early stages of a design project. Once the kinematics and kinetics of a design have been determined,most of the concep- tual design will have been accomplished.What then remains is detailed design-sizing the parts against failure.The topic of detailed design is discussed in other texts such as reference [8]
where m = mass in Ibm' a = acceleration and gc = the gravitational constant. The value of the mass of an object measured in pounds mass (Ibm) is numerically equal to its weight in pounds force (Ib/). However the student must remember to divide the value of m in Ibm by gc when substituting into this form of Newton's equation. Thus the Ibm will be divided either by 32.2 or by 386 when calculating the dynamic force. The result will be the same as when the mass is expressed in either slugs or blobs in the F = ma form of the equation. Remember that in round numbers at sea level on earth: I Ibm = llbf I slug = 32.2 Ibf I blob = 386 Ibf The SI system requires that lengths be measured in meters (m), mass in kilograms (kg), and time in seconds (sec). This is sometimes also referred to as the mks system. Force is derived from Newton's law, equation 1.1b and the units are: kilogram-meters per second2 (kg-m/sec2) = newtons Thus in the SI system there are distinct names for mass and force which helps alleviate confusion. When converting between SI and u.s. systems, be alert to the fact that mass converts from kilograms (kg) to either slugs (sl) or blobs (bl), and force converts from newtons (N) to pounds (Ib). The gravitational constant (g) in the SI system is approximately 9.81 m/sec2. The principal system of units used in this textbook will be the U.S. ips system. Most machine design in the United States is still done in this system. Table 1-4 shows some of the variables used in this text and their units. The inside front cover contains a table of conversion factors between the U.S, and SI systems. The student is cautioned to always check the units in any equation written for a problem solution, whether in school or in professional practice after graduation. If properly written, an equation should cancel all units across the equal sign. If it does not, then you can be absolutely sure it is incorrect. Unfortunately, a unit balance in an equation does not guarantee that it is correct, as many other errors are possible. Always double-check your results. You might save a life. 1.11 WHAT'S TO COME In this text we will explore the topic of machine design in respect to the synthesis of mechanisms in order to accomplish desired motions or tasks, and also the analysis of these mechanisms in order to determine their rigid-body dynamic behavior. On the premise that we cannot analyze anything until it has been synthesized into existence, we will first explore the topic of synthesis of mechanisms. Then we will investigate the analysis of those and other mechanisms for their kinematic behavior. Finally, in Part II we will deal with the dynamic analysis of the forces and torques generated by these moving machines. These topics cover the essence of the early stages of a design project. Once the kinematics and kinetics of a design have been determined, most of the conceptual design will have been accomplished. What then remains is detailed design-sizing the parts against failure. The topic of detailed design is discussed in other texts such as reference [8]
INTRODUCTION Table 1-4 Variables and Units Base Units in Boldface-Abbreviations in ( Variable Symbol ips unit fps unit Sl unit Force F pounds (Ib) pounds (b) newtons (N) Length inches (in) feet (ft) meters(m) Time t seconds(sec) seconds (sec) seconds (sec) Mass m Ib-sec2 in (bl) Ib-sec2/什(s kilograms (kg) Weight W pounds(Ib) pounds(Ib) newtons (N) Velocity in sec ft sec m/sec Acceleration in/sec2 什/sec2 m/sec2 Jerk i in/sec3 ft/sec3 m/sec3 Angle 0 degrees(deg) degrees (deg) degrees(deg) Angle 0 radians (rad) radians (rad) radians (rad) Angular velocity 0 rad/sec rad sec rad /sec Angular acceleration 令 rad/sec2 rad/sec2 rad/sec2 Angular jerk 0 rad/sec3 rad/sec3 rad/sec3 Torque lb-in lb-ft N-m Mass moment of inertia lb-in-sec2 lb-ft-sec2 N-m-sec2 Energy E in-lb ft-lb joules Power P in-b sec ft-lb sec watts Volume In3 3 m3 Weight density Y Ib/in3 Ib/什3 N/m3 Mass density P bl/in3 sl /ft3 kg /m3 1.12 REFERENCES 1 Rosenauer,N.,and A.H.Willis.(1967).Kinematics of Mechanisms.Dover Publications:New York,p.275 ff. 2 de Jonge,A.E.R.(1942)."What Is Wrong with 'Kinematics'and 'Mechanisms'?" Mechanical Engineering,64(April),pp.273-278. 3 Artobolevsky,I.I.(1975).Mechanisms in Modern Engineering Design.N.Wein- stein,translator.Vols.I-5.MIR Publishers:Moscow. 4 Erdman,A.E..ed.(1993).Modern Kinematics:Developments in the Last Forty Years.Wiley Series in Design Engineering.John Wiley Sons:New York. 5 Wallen,R.W.(1957)."Unlocking Human Creativity:"Proc.of Fourth Conference on Mechanisms,Purdue University,pp.2-8. 6 Dixon,J.R.(1995)."Knowledge Based Systems for Design."Journal of Mechanical Design,.117b(2),p.11
2家格炸 DESIGN OF MACHINERY CHAPTER 1 7 Suh,N.P.(1995)."Axiomatic Design of Mechanical Systems."Journal of Mechani- cal Design,117b(2),p.2. 8 Norton,R.L.(1996).Machine Design:An Integrated Approach.Prentice-Hall: Upper Saddle River,NJ. For additional information on the history of kinematics,the following are recommended: Artobolevsky,I.I.(1976)."Past Present and Future of the Theory of Machines and Mecha- nisms."Mechanism and Machine Theory.11,pp.353-361. Brown,H.T.(1869).Five Hundred and Seven Mechanical Movements.Brown,Coombs Co.:New York,republished by USM Corporation,Beverly,MA.,1970. de Jonge,A.E.R.(1942)."What Is Wrong with Kinematics'and 'Mechanisms"?"Mechanical Engineering.64(April),pp.273-278. de Jonge,A.E.R.(\943)."A Brief Account of Modem Kinematics."Transactions of the ASME,Pp.663-683. Erdman,A.E.,ed.(1993).Modern Kinematics:Developments in the Last Forty Years.Wiley Series in Design Engineering,John Wiley Sons:New York. Ferguson,E.S.(1962)."Kinematics of Mechanisms from the Time of Watt."United States National Museum Bulletin.228(27),pp.185-230. Freudenstein,F.(1959)."Trends in the Kinematics of Mechanisms."Applied Mechanics Reviews,12(9),September,pp.587-590. Hartenberg.R.S.,and J.Denavit.(1964).Kinematic Synthesis of Linkages.McGraw-Hill: New York,pp.1-27. Nolle,H.(1974)."Linkage Coupler Curve Synthesis:A Historical Review -II.Developments after 1875."Mechanism and Machine Theory.9,pp.325-348. Nolle,H.(1974)."Linkage Coupler Curve Synthesis:A Historical Review -I.Developments up to 1875."Mechanism and Machine Theory.9,pp.147-168. Nolle,H.(\975)."Linkage Coupler Curve Synthesis:A Historical Review -III.Spatial Synthesis and Optimization."Mechanism and Machine Theory.10,pp.41-55. Reuleaux,F.(1963).The Kinematics of Machinery.A.B.W.Kennedy,translator.Dover Publications:New York,pp.29-55. Strandh,S.(1979).A History of the Machine.A&W Publishers:New York. For additional information on creativity and the design process,the following are recommended: Alger,J.R.M.,and C.V.Hays.(1964).Creative Synthesis in Design.Prentice-Hall:Upper Saddle River,NJ. Allen,M.S.(1962).Morphological Creativity.Prentice-Hall:Upper Saddle River,NJ. Altschuller,G.(1984).Creativity as an Exact Science.Gordon and Breach:New York. Buhl,H.R.(960).Creative Engineering Design.lowa State University Press:Ames,IA
7 Suh, N. P. (1995). "Axiomatic Design of Mechanical Systems." Journal of Mechanical Design, 117b(2), p. 2. 8 Norton, R. L. (1996). Machine Design: An Integrated Approach. Prentice-Hall: Upper Saddle River, NJ. For additional information on the history of kinematics, the following are recommended: Artobolevsky, I. I. (1976). "Past Present and Future of the Theory of Machines and Mechanisms." Mechanism and Machine Theory, 11, pp. 353-361. Brown, H. T. (1869). Five Hundred and Seven Mechanical Movements. Brown, Coombs & Co.: New York, republished by USM Corporation, Beverly, MA., 1970. de Jonge, A. E. R. (1942). "What Is Wrong with 'Kinematics' and 'Mechanisms'?" Mechanical Engineering, 64(April), pp. 273-278. de Jonge, A. E. R. (\ 943). "A Brief Account of Modem Kinematics." Transactions of the ASME, pp. 663-683. Erdman, A. E., ed. (1993). Modern Kinematics: Developments in the Last Forty Years. Wiley Series in Design Engineering, John Wiley & Sons: New York. Ferguson, E. S. (1962). "Kinematics of Mechanisms from the Time of Watt." United States National Museum Bulletin, 228(27), pp. 185-230. Freudenstein, F. (1959). "Trends in the Kinematics of Mechanisms." Applied Mechanics Reviews, 12(9), September, pp. 587-590. Hartenberg, R. S., and J. Denavit. (1964). Kinematic Synthesis of Linkages. McGraw-Hill: New York, pp. 1-27. Nolle, H. (1974). "Linkage Coupler Curve Synthesis: A Historical Review - II. Developments after 1875." Mechanism and Machine Theory, 9, pp. 325 - 348. Nolle, H. (1974). "Linkage Coupler Curve Synthesis: A Historical Review -I. Developments up to 1875." Mechanism and Machine Theory, 9, pp. 147-168. Nolle, H. (\ 975). "Linkage Coupler Curve Synthesis: A Historical Review - III. Spatial Synthesis and Optimization." Mechanism and Machine Theory, 10, pp. 41-55. Reuleaux, F. (1963). The Kinematics of Machinery, A. B. W. Kennedy, translator. Dover Publications: New York, pp. 29-55. Strandh, S. (1979). A History of the Machine. A&W Publishers: New York. For additional information on creativity and the design process, the following are recommended: Alger, J. R. M., and C. V. Hays. (1964). Creative Synthesis in Design. Prentice-Hall: Upper Saddle River, NJ. Allen, M. S. (1962). Morphological Creativity. Prentice-Hall: Upper Saddle River, NJ. Altschuller, G. (1984). Creativity as an Exact Science. Gordon and Breach: New York. Buhl, H. R. (\960). Creative Engineering Design. Iowa State University Press: Ames, IA