Wheel and chain must clear frame for all deflections. Wheel should move straight up and down.

274 CHAPTER 9 Product Generation 9.6.3 Develop Connections: Create and Refine Interfaces for Functions for the Mount Vision Bicycle Rear Suspension This section focuses on the connections between the components. On the Marin Mount Vision Pro, the connections are those between the links in the four-bar linkage, those connecting the shock to the bike and those that connect the fixed parts together. We will consider these in order. For the four-bar linkage, the con- nections are the four pivots. These must have one degree of freedom and thus can be either bearings or flexures. For most mountain bikes, either rolling ele- ment bearings or bushings are used, but some have used flexures. Considering Fig. 9.27, the shock can be mounted in many different ways. It can be mounted between any two elements that move closer together as the system deflects; for example, element C and the frame, elements A and B, and so on. The addition of the shock adds two more pivots to the assembly making a total of six pivoting connections. The Marin engineers reduced the number of pivots by mounting the shock between linkage pivots 2 and 4. As the suspension system deflects, pivot 2 moves toward pivot 4. In fact, the engineers, when determining the lengths of all the seven members, took the needed change in length of the shock as an additional constraint. The decision to mount the shock in this manner made the design of linkage more challenging and connections more complex, but the trade-off for fewer pivots made this worthwhile. Pivots 2 and 4 need to have the link and shock free to rotate about the axel shown as a centerline in Fig. 9.29. Note in Fig. 9.28, the amount of rotation of these elements is small, only a few degrees in some cases. Bearings that operate primarily in one position and only move a small amount from that position present Frame Shock Link Link Figure 9.29 The components in pivots 2 and 4. 9.6 Generating a Suspension Design for the Marin 2008 Mount Vision Pro Bicycle 275 Figure 9.30 Final design of pivot 2. Reprinted with permission of Fox Racing Shox. their own design problems as small deflections do not force the lubricant to flow to all the areas. The final connection at pivot 4 is shown in Fig. 9.30. Connections between components that are moving relative to each other need to be addressed. They are refined in Section 9.6.4. 9.6.4 Develop Components for the Mount Vision Bicycle Rear Suspension Finally, the actual components need to be developed. For the Marin engineers these parts needed to be light in weight, manufacturable in volumes that matched the sales projections, and had a look that would attract sales. Thus, these parts were a combination of structure and eye candy. We will discuss three of the components here, link A, the ball bearing in link C, and the lower part of the rear stay. Link A is a very simple component that needs to connect pivots 1 and 2. The final component, like many on the bike is forged aluminum with the bearing mounting surfaces machined. It is shown in two views in Fig. 9.31. The bearing between the axel and the link, shown pressed into the link in Fig. 9.31, is a rolling element ball bearing. As mentioned earlier, this bearing does not rotate very much and thus requires special consideration. The final bearing chosen was one that was specially designed for aircraft control systems, another application with small, repetitive motions. The rear stay components could have been made out of round aluminum tubes welded together as with most aluminum bikes. However, to get a better “look,” 276 CHAPTER 9 Product Generation Figure 9.31 Link A. Reprinted with permission of Marin Bicycles. the designers wanted tubes that curved, and to save weight, the engineers wanted tubes that tapered. As shown in Figs. 2.10 and 9.28 these two requirements were met. The manufacturing method used is called hydroforming. To hydroform, a round tube is put in a die and then the tube is filled with high-pressure liquid causing it to deform and be shaped by the die. 9.7 SUMMARY ■ A Bill of Materials is a parts list—an index to the product. ■ Products must be developed from concepts through concurrent development of form, material, and production methods. This process is driven by the functional decomposition discussed in Chap. 7. ■ Form is bound by the geometric constraints and defined by the configuration of connected components. ■ The development of most components and assemblies starts at their inter- faces, or connections, since for the most part function occurs at the interfaces between components. ■ Product development is an iterative loop that requires the development of new concepts, the decomposition of the product into subassemblies and compo- nents, the refinement of the product toward a final configuration, and the patching of features to help find a good product design. ■ Vendor selection is an important part of the design process. 9.8 SOURCES Ashby, M. F.: Materials Selection in Mechanical Design, Pergamon Press, Oxford, U.K., 1992. An excellent text on materials selection. There is a computer program available imple- menting the approach in this text. Blanding, D.: Exact Constraint: Machine Design Using Kinematic Principles, ASME Press, 1999. The best reference on the design or connections between components. Written by a design engineer from Kodak. Budinski, K.: Engineering Materials: Properties and Selection, Reston, Va., 1979. A text on materials written with the engineer in mind.

9.9 Exercises

277 Snead, C. S.: Group Technology: Foundations for Competitive Manufacturing, Van Nostrand Reinhold, New York, 1989. An overview of group technology for classifying components. Tjalve, E.: A Short Course in Industrial Design, Newnes-Butterworths, London-Boston, 1979. An excellent book on the development of form. Ullman, D. G., S. Wood, and D. Craig: “The Importance of Drawing in the Mechanical Design Process,” Computers and Graphics, Vol. 14, No. 2 1990, pp. 263–274. A paper that itemizes the different uses of graphical representations in mechanical design. Information on modular systems and architecture are from Alizon, F., Shooter, S. B. and Simpson, T. W.: “Improving an Existing Product Family based on CommonalityDiversity, Modularity, and Cost,” Design Studies, 2007 Vol. 28, No. 4, pp. 387–409. Qureshi, A., J. T. Murphy, B. Kuchinsky, C. C. Seepersad, K. L. Wood and D. D. Jensen: “Principles of Product Flexibility,” ASME IDETCCIE Advances in Design Automation Conference , Philadelphia, Pa., 2006. Paper Number: DETC2006-99583. Tripathy, Anshuman, and Steven D. Eppinger: “A System Architecture Approach to Global Product Development,” MIT, Sloan School of Management, Working Paper Number 4645-07, March 2007. 9.9 EXERCISES

9.1 Develop a bill of materials for

a. A stapler

b. A bicycle brake caliper

c. A hole punch

9.2 For the original design problem Exercise 4.1, develop a product layout drawing or solid

model by doing these:

a. Develop the spatial constraints.

b. Develop a refined house of quality and function diagrams for the most critical interface. c. Develop connections and components for the product. d. Show the force flow through the product for its most critical loading. 9.3 For the redesign problem Exercise 4.2: a. Identify the spatial constraints for all important operating sequences. b. At critical interfaces, identify the energy, information, and material flows. c. Develop a refined house of quality and function diagrams for the most critical interface.

d. Develop new connections and components for the product.

e. Show the force flow through the product for its most critical loading.

9.4 Determine the force flow in

a. A bicycle chain.

b. A car door being opened.

c. A paper hole punch.

d. Your body while holding a 5-kg weight straight out in front of you with your

left hand. 278 CHAPTER 9 Product Generation

9.5 For a part you designed, decide whether to make it or buy it from a vendor. The cost-

estimating templates available on the website for plastic part and machined part cost estimation might be of help. See Sections 11.2.3 and 11.2.4 for discussion about these cost estimators. 9.10 ON THE WEB A template for the following document is available on the book’s website: www.mhhe.comUllman4e ■ Bill Of Materials ■ Make or Buy 10 C H A P T E R Product Evaluation for Performance and the Effects of Variation KEY QUESTIONS ■ Which is best to evaluate the product performance, analytical models or physical testing? ■ What is a P-diagram and how does it help identify noise? ■ How are trade-offs made? ■ What are the three types of noises and how do they affect product quality? ■ Why is tolerance stacking important during assembly? ■ How is robust design used to ensure quality? 10.1 INTRODUCTION The primary goal in this chapter is to compare the performance of the product to the engineering specifications developed earlier in the design project. Performance is the measure of behavior, and the behavior of the product results from the design effort to meet the intended function. Thus, part of the goal is to track and ensure understanding of the functional development of the product. If the functional development is not understood, the product may exhibit unintended behaviors. Another subgoal is to design in quality. Although this chapter is about “evaluation for performance,” it gives another opportunity to be sure that a quality product is developed—that it will always work as it was designed to. Best practices for product evaluation are listed in Table 10.1, an extension of Table 4.1. The first eight best practices are covered in this chapter. The remainder of the best practices listed in the table are aimed at other, nonperformance product evaluation techniques and are covered in Chap. 11. Although all of these best 279 280 CHAPTER 10 Product Evaluation for Performance and the Effects of Variation Table 10.1 Best practices for product evaluation ■ Monitoring functional change Sec. 10.2 ■ Goals of performance evaluation Sec. 10.3 ■ Trade-off management Sec. 10.4 ■ Accuracy, variation, and noise Sec. 10.5 ■ Modeling for performance evaluation Sec. 10.6 ■ Tolerance analysis Sec. 10.7 ■ Sensitivity analysis Sec. 10.8 ■ Robust design Secs. 10.9 and 10.10 ■ Design for cost DFC Sec. 11.2 ■ Value engineering Sec. 11.3 ■ Design for manufacture DFM Sec. 11.4 ■ Design for assembly DFA Sec. 11.5 ■ Design for reliability DFR Sec. 11.6 ■ Design for test and maintenance Sec. 11.7 ■ Design for the environment Sec. 11.8 practices are discussed as techniques for product evaluation, they all contribute to the generation of the product as part of the iterative generateevaluate cycle. 10.2 MONITORING FUNCTIONAL CHANGE Although the main goal of evaluation is comparing product performance with engineering targets, it is equally important to track changes made in the function of the product. Conceptual designs were developed first by functionally modeling the problem and then, on the basis of that model, developing potential concepts to fulfill these functions. This transformation from function to concept does not end the usefulness of the functional modeling tool. As the form is refined from concept to product, new functions are added. An obvious question about this process arises: What benefit is there in refin- ing the function model as the form is evolving? The answer is that by updating the functional breakdown, the functions that the product must accomplish can be kept very clear. Nearly every decision about the form of an object adds something, either desirable or undesirable to the function of the object. It is important not to add functions that are counter to those desired. For example, in the design of the Marin Mount Vision suspension, the decision to use the air shock necessitated an interface between the user and shock to add air and to adjust the dampening. The final shock chosen, the Fox Float RP23 Fig 10.1, shows the air valve and adjust- ment handle near the top of the unit. The exact steps a user must go through to add air to the shock were made clear by refining the function occurring at the interface between the user and the air valve on the shock. Besides tracking the functional Every feature added brings with it new intended functionality. It is unintended functionality that can hurt you. 10.3 The Goals of Performance Evaluation 281 Figure 10.1 Fox Float RP23 used on the Marin Mount Vision. Reprinted with permission of Fox Racing Shox. evolution of the product, the refinement of the functional decomposition also aids in the evaluation of potential failure modes covered in Chap. 11. Finally, tracking the evolution of function means continuously updating the flow models of energy, information, and materials. It is these flows that determine the performance of the product. As the product matures, the intended function and actual behavior merge and so what was, in conceptual design, concern for “the desired” now turns to measuring “the reality.” 10.3 THE GOALS OF PERFORMANCE EVALUATION In Chap. 6, we developed a set of engineering requirements based on the needs of the customer. For each of these requirements, a specific target was set. The goal now is to evaluate the product design relative to these targets. Since the targets are represented as numerical values, the evaluation can only occur after the product is refined to the point that numerical engineering measures can be made. In Chap. 8, the concepts developed were not yet refined enough to compare with the targets and were thus compared with measures that are more abstract. Evaluation can now be based on comparison with the engineering requirements. Beyond comparison to the requirements, effective evaluation procedures should