Budynas-Nisbett:Shigley's I.Basics 1.Introduction to T©The McGraw-Hill 21 Mechanical Engineering Mechanical Engineering Companies,2008 Design,Eighth Edition Design Introduction to Mechanical Engineering Design Cost Estimates There are many ways of obtaining relative cost figures so that two or more designs can be roughly compared.A certain amount of judgment may be required in some instances.For example,we can compare the relative value of two automobiles by comparing the dollar cost per pound of weight.Another way to compare the cost of one design with another is simply to count the number of parts.The design having the smaller number of parts is likely to cost less.Many other cost estimators can be used,depending upon the application,such as area,volume,horsepower,torque, capacity,speed,and various performance ratios. 1-8 Safety and Product Liability The strict liability concept of product liability generally prevails in the United States. This concept states that the manufacturer of an article is liable for any damage or harm that results because of a defect.And it doesn't matter whether the manufacturer knew about the defect,or even could have known about it.For example,suppose an article was manufactured,say,10 years ago.And suppose at that time the article could not have been considered defective on the basis of all technological knowledge then available. Ten years later,according to the concept of strict liability,the manufacturer is still liable.Thus,under this concept,the plaintiff needs only to prove that the article was defective and that the defect caused some damage or harm.Negligence of the manu- facturer need not be proved. The best approaches to the prevention of product liability are good engineering in analysis and design,quality control,and comprehensive testing procedures.Advertising managers often make glowing promises in the warranties and sales literature for a prod- uct.These statements should be reviewed carefully by the engineering staff to eliminate excessive promises and to insert adequate warnings and instructions for use. 1-9 Stress and Strength The survival of many products depends on how the designer adjusts the maximum stresses in a component to be less than the component's strength at specific locations of interest.The designer must allow the maximum stress to be less than the strength by a sufficient margin so that despite the uncertainties,failure is rare. In focusing on the stress-strength comparison at a critical(controlling)location, we often look for "strength in the geometry and condition of use."Strengths are the magnitudes of stresses at which something of interest occurs,such as the proportional limit,0.2 percent-offset yielding,or fracture.In many cases,such events represent the stress level at which loss of function occurs. Strength is a property of a material or of a mechanical element.The strength of an element depends on the choice,the treatment,and the processing of the material. Consider,for example,a shipment of springs.We can associate a strength with a spe- cific spring.When this spring is incorporated into a machine,external forces are applied that result in load-induced stresses in the spring,the magnitudes of which depend on its geometry and are independent of the material and its processing.If the spring is removed from the machine unharmed,the stress due to the external forces will return For an overview of estimating manufacturing costs.see Chap.11.KarlT.Ulrich and Steven D.Eppinger. Product Design and Development.3rd ed.,McGraw-Hill,New York,2004.Budynas−Nisbett: Shigley’s Mechanical Engineering Design, Eighth Edition I. Basics 1. Introduction to Mechanical Engineering Design © The McGraw−Hill 21 Companies, 2008 Introduction to Mechanical Engineering Design 15 Cost Estimates There are many ways of obtaining relative cost figures so that two or more designs can be roughly compared. A certain amount of judgment may be required in some instances. For example, we can compare the relative value of two automobiles by comparing the dollar cost per pound of weight. Another way to compare the cost of one design with another is simply to count the number of parts. The design having the smaller number of parts is likely to cost less. Many other cost estimators can be used, depending upon the application, such as area, volume, horsepower, torque, capacity, speed, and various performance ratios.9 1–8 Safety and Product Liability The strict liability concept of product liability generally prevails in the United States. This concept states that the manufacturer of an article is liable for any damage or harm that results because of a defect. And it doesn’t matter whether the manufacturer knew about the defect, or even could have known about it. For example, suppose an article was manufactured, say, 10 years ago. And suppose at that time the article could not have been considered defective on the basis of all technological knowledge then available. Ten years later, according to the concept of strict liability, the manufacturer is still liable. Thus, under this concept, the plaintiff needs only to prove that the article was defective and that the defect caused some damage or harm. Negligence of the manu￾facturer need not be proved. The best approaches to the prevention of product liability are good engineering in analysis and design, quality control, and comprehensive testing procedures. Advertising managers often make glowing promises in the warranties and sales literature for a prod￾uct. These statements should be reviewed carefully by the engineering staff to eliminate excessive promises and to insert adequate warnings and instructions for use. 1–9 Stress and Strength The survival of many products depends on how the designer adjusts the maximum stresses in a component to be less than the component’s strength at specific locations of interest. The designer must allow the maximum stress to be less than the strength by a sufficient margin so that despite the uncertainties, failure is rare. In focusing on the stress-strength comparison at a critical (controlling) location, we often look for “strength in the geometry and condition of use.” Strengths are the magnitudes of stresses at which something of interest occurs, such as the proportional limit, 0.2 percent-offset yielding, or fracture. In many cases, such events represent the stress level at which loss of function occurs. Strength is a property of a material or of a mechanical element. The strength of an element depends on the choice, the treatment, and the processing of the material. Consider, for example, a shipment of springs. We can associate a strength with a spe￾cific spring. When this spring is incorporated into a machine, external forces are applied that result in load-induced stresses in the spring, the magnitudes of which depend on its geometry and are independent of the material and its processing. If the spring is removed from the machine unharmed, the stress due to the external forces will return 9 For an overview of estimating manufacturing costs, see Chap. 11, Karl T. Ulrich and Steven D. Eppinger, Product Design and Development, 3rd ed., McGraw-Hill, New York, 2004