For Problems 15–28, perform the indicated conversion of units. (Refer to Appendix 16 for conversion factors.) Express the results with the appropriate prefix as illustrated in Tables 1–3 and 1–4.
15. Convert a shaft diameter of 1.75 in to mm.
16. Convert the length of a conveyor from 46 ft to meters.
17. Convert the torque developed by a motor of 12 550 lb
#
in to N#
m.18. A wide-flange steel-beam shape, W12*14, has a cross-sectional area of 4.12 in2. Convert the area to mm2.
19. The W12*14 beam shape has a section modulus of 14.8 in3. Convert it to mm3.
20. The W12*14 beam shape has a moment of inertia of 88.0 in4. Convert it to mm4.
21. What standard steel equal leg angle would have a cross-sectional area closest to (but greater than) 750 mm2? See Appendix 15.
22. An electric motor is rated at 7.5 hp. What is its rating in watts (W)?
23. A vendor lists the ultimate tensile strength of a steel to be 127 000 psi. Compute the strength in MPa.
24. Compute the weight of a steel shaft, 35.0 mm in di- ameter and 675 mm long. (See Appendix 3 for the density of steel.)
25. A torsional spring requires a torque of 180 lb
#
in torotate it 35°. Convert the torque to N
#
m and the ro- tation to radians. If the scale of the spring is defined as the applied torque per unit of angular rotation, compute the spring scale in both unit systems.26. To compute the energy used by a motor, multiply the power that it draws by the time of operation. Con- sider a motor that draws 12.5 hp for 16 h/day, five days per week. Compute the energy used by the mo- tor for one year. Express the result in ft
#
lb and W#
h.27. One unit used for fluid viscosity in Chapter 16 of this book is the reyn, defined as 1.0 lb
#
s/in2. If alubricating oil has a viscosity of 3.75 reyn, convert the viscosity to the standard units in the U.S. Customary System lb
#
s/ft2 and in the SI (N#
s/m2).28. The life of a bearing supporting a rotating shaft is expressed in number of revolutions. Compute the life of a bearing that rotates 1750 rpm continuously for 24 h/day for five years.
25 The Big Picture
You Are the Designer
2–1 Objectives of This Chapter 2–2 Properties of Materials
2–3 Classification of Metals and Alloys 2–4 Variability of Material Properties Data 2–5 Carbon and Alloy Steel
2–6 Conditions for Steels and Heat Treatment 2–7 Stainless Steels
2–8 Structural Steel 2–9 Tool Steels 2–10 Cast Iron
2–11 Powdered Metals 2–12 Aluminum
2–13 Zinc Alloys and Magnesium 2–14 Nickel-based Alloys and Titanium 2–15 Copper, Brass, and Bronze 2–16 Plastics
2–17 Composite Materials 2–18 Materials Selection
Materials in
Mechanical Design
t W O
the Big PictUre
Discussion Map
■
■ You must understand the behavior of materials to make good design decisions and to communicate with suppliers and man- ufacturing staff.
Discover
Examine consumer products, industrial machinery, automo- biles, and construction machinery.
What materials are used for the various parts?
Why do you think those materials were specified?
How were they processed?
What material properties were important to the decisions to use particular materials?
Examine the appendices tables, and refer to them later as you read about specific materials.
Materials in Mechanical Design
This chapter summarizes the design properties of a variety of materials. The appendices include data for many examples of these materials in many conditions.
It is the designer’s responsibility to specify suitable materials for each component of a mechanical device.
Your initial efforts in specifying a material for a par- ticular component of a mechanical design should be
directed to the basic kind of material to be used. Keep an open mind until you have specified the functions of the component, the kinds and magnitudes of loads it will carry, and the environment in which it must
operate. Your selection of a material must consider its physical and mechanical properties and match them to the expectations placed on it. First consider the following classes of materials:
Metals and their
alloys Plastics Composites
Elastomers Woods Ceramics and glasses Each of these classes contains a large number of specific materials covering a wide range of actual properties. However, you probably know from your experience the general behavior of each kind and have some feel for the applications in which each is typi- cally used. Most of the applications considered in the study of design of machine elements in this book use metal alloys, plastics, and composites.
Satisfactory performance of machine components and systems depends greatly on the materials that the designer specifies. As a designer, you must understand how materials behave, what properties of the mate- rial affect the performance of the parts, and how you should interpret the large amounts of data available on material properties. Your ability to effectively com- municate your specifications for materials with suppli- ers, purchasing agents, metallurgists, manufacturing process personnel, heat treatment personnel, plastics molders, machinists, and quality assurance specialists often has a strong influence on the success of a design.
Explore what kinds of materials are used in con- sumer products, industrial machinery, automobiles, construction machinery, and other devices and sys- tems that you come into contact with each day. Make judgments about why each material was specified for a particular application. Where do you see steel being used? Contrast that usage with where aluminum or other nonferrous materials are used. How are the products produced? Can you find different parts that are machined, cast, forged, roll-formed, and welded?
Why do you think those processes were specified for those particular products?
Document several applications for plastics and describe the different forms that are available and that have been made by different manufacturing pro- cesses. Which are made by plastic molding processes, vacuum forming, blow molding, and others? Can you identify parts made from composite materials that have a significant amount of high-strength fibers embedded in a plastic matrix? Check out sporting goods and parts of cars, trucks, and airplanes.
From the products that you found from the exploration outlined previously, identify the basic properties of the materials that were important to the designers: strength, rigidity (stiffness), weight (den- sity), corrosion resistance, appearance, machinability, weldability, ease of forming, cost, and others.
This chapter focuses on material selection and the use of material property data in design decisions, rather than on the metallurgy or chemistry of the materials.
One of the uses of the information in this chapter is as a glossary of terms that you can use throughout the book; important terms are given in italic type. Also, there are numerous references to Appendix 3 through 13, where tables of data for material properties are given. Go there now and see what kinds of data are provided. Then you can study the tables in more depth as you read the text. Note that many of the problems that you will solve in this book and the design projects that you complete will use data from these tables.
The subject of this chapter is very broad and it is not possible to include in this book all the detail you may need for each design situation that arises as you study this book or in your career. Huge amounts of additional information are available on the Internet and the numerous sites listed at the end of this chap- ter give suggestions that may lead to what you need.
Many of the listed sites are referred to within this chapter, but some are not and they provide alternate ways for you to search for what you need.
Now apply some of what you have gained from The Big Picture exploration to a specific design situa- tion as outlined in You Are the Designer, which follows.
ARE THE DESIGnER
You are part of a team responsible for the design of an electric lawn mower for the household market. One of your tasks is to specify suitable materials for the various components. Consider your own experience with such lawn mowers and think what materials would be used for these key components: wheels, axles, housing, and blade. What are their functions? What conditions of service will each encounter? What is one reasonable type of material for each component and what general properties should it have? How could they be manufactured? Possible answers to these questions follow.
Wheels
Function: Support the weight of the mower. Permit easy, rolling movement. Provide for mounting on an axle. Ensure safe opera- tion on flat or sloped lawn surfaces.
Conditions of service: Must operate on grass, hard surfaces, and soft earth. Exposed to water, lawn fertilizers, and general outdoor conditions. Will carry moderate loads. Requires an attractive appearance.
YOU
7. Describe cast irons and several kinds of gray iron, ductile iron, and malleable iron.
8. Describe powdered metals and their properties and uses.
9. Describe several types of tool steels and carbides and their typical uses.
10. Describe aluminum alloys and their conditions, such as strain hardening and heat treatment.
11. Describe the nature and typical properties of zinc, titanium, copper, brass, bronze, and nickel-based alloys.
12. Describe several types of plastics, both thermoset- ting and thermoplastic, and their typical properties and uses.
13. Describe several kinds of composite materials and their typical properties and uses.
14. Implement a rational material selection process.
2–2 PrOPerties OF Materials
Machine elements are very often made from one of the metals or metal alloys such as steel, aluminum, cast iron, zinc, titanium, or bronze. This section describes the important properties of materials as they affect mechani- cal design.
Strength, elastic, and ductility properties for met- als, plastics, and other types of materials are usually determined from a tensile test in which a sample of the material, typically in the form of a round or flat bar, is clamped between jaws and pulled slowly until it breaks in tension. The magnitude of the force on the bar and the corresponding change in length (strain) are monitored and recorded continuously during the test. Because the This simplified example of the material selection process
should help you to understand the importance of the information provided in this chapter about the behavior of materials commonly used in the design of machine elements. A more comprehensive discussion of material selection occurs at the end of this chapter.
2–1 OBJectiVes OF this chaPter
After completing this chapter, you will be able to:
1. State the types of material properties that are impor- tant to the design of mechanical devices and systems.
2. Define the following terms: tensile strength, yield strength, proportional limit, elastic limit, modulus of elasticity in tension, ductility and percent elon- gation, shear strength, Poisson’s ratio, modulus of elasticity in shear, hardness, machinability, impact strength, creep density, coefficient of thermal expan- sion, thermal conductivity, and electrical resistivity.
3. Describe the nature of carbon and alloy steels, the number-designation system for steels, and the effect of several kinds of alloying elements on the proper- ties of steels.
4. Describe the manner of designating the condition and heat treatment of steels, including hot rolling, cold drawing, annealing, normalizing, through- hardening, tempering, and case hardening by flame hardening, induction hardening, and carburizing.
5. Describe stainless steels and recognize many of the types that are commercially available.
6. Describe structural steels and recognize many of their designations and uses.
One reasonable material: One-piece plastic wheel incorporat- ing the tire, rim, and hub. Must have good strength, stiffness, toughness, and wear resistance.
Manufacturing method: Plastic injection molding Axles
Function: Transfer the weight of mower from the housing to the wheels. Allow rotation of the wheels. Maintain location of the wheels relative to the housing.
Conditions of service: Exposure to general outdoor conditions.
Moderate loads.
One possible material: Steel rod with provisions for mounting wheels and attaching to housing. Requires moderate strength, stiffness, and corrosion resistance.
Manufacturing method: Commercially available cylindrical rod.
Parts of the rod may need machining.
Housing
Function: Support, safely enclose, and protect operating components, including the blade and motor. Accommodate the attachment of two axles and a handle. Permit cut grass to exit the cutting area.
Conditions of service: Moderate loads and vibration due to motor. Possible shock loads from wheels. Multiple attachment points for axles, handle, and motor. Exposed to wet grass and general outdoor conditions. Requires attractive appearance.
One possible material: Heavy-duty plastic with good strength, stiffness, impact resistance, toughness, and weather resistance.
Manufacturing method: Plastic injection molding. May require machining for holes and mounting points for the motor.
Blade
Function: Cut blades of grass and weeds while rotating at high speed. Facilitate connection to motor shaft. Operate safely when foreign objects are encountered, such as stones, sticks, or metal pieces.
Conditions of service: Normally moderate loads. Occasional shock and impact loads. Must be capable of sharpening a portion of the blade to ensure clean cutting of grass. Maintain sharpness for reasonable time during use.
One possible material: Steel with high strength, stiffness, impact resistance, toughness, and corrosion resistance.
Manufacturing method: Stamping from flat steel strip. Machin- ing and/or grinding for cutting edge.
the load acts over a smaller area, and the actual stress continues to increase until failure. It is very difficult to follow the reduction in diameter during the necking- down process, so it has become customary to use the peak of the curve as the tensile strength, although it is a more conservative value.
Yield Strength, s
yThat portion of the stress–strain diagram where there is a large increase in strain with little or no increase in stress is called the yield strength (sy). This property indicates that the material has, in fact, yielded or elongated plastically, permanently, and to a large degree. If the point of yielding is quite noticeable, as it is in Figure 2–1, the property is called the yield point rather than the yield strength. This is typical of a plain carbon hot-rolled steel.
Figure 2–2 shows the stress–strain diagram that is typical of a nonferrous metal such as aluminum or titanium or of certain high-strength steels. Notice that there is no pronounced yield point, but the material has actually yielded at or near the stress level indicated as sy. That point is determined by the offset method, in which a line is drawn parallel to the straight-line portion of the curve and is offset to the right by a set amount, usually 0.20% strain (0.002 in/in). The inter- section of this line and the stress–strain curve defines the material’s yield strength. In this book, the term yield strength will be used for sy, regardless of whether the material exhibits a true yield point or whether the offset method is used.