Third Concept: Sector-Shaped Gear
2.6 C ASE S TUDY : C OMPUTER -A IDED D ESIGN : N ONINVASIVE M EDICAL I MAGING
Many times a CAD model is used not only to represent geometrical layouts but also to simulate the product’s performance.
2.6 C ASE S TUDY : C OMPUTER -A IDED D ESIGN : N ONINVASIVE
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computer control to perform the procedure. The particular system examined in this section consists of two syringes that deliver the contrast agent and a companion saline solution to the patient. The system includes pistons and cylinders as in traditional syringes, but an electronic motor automatically depresses the pistons to inject the chemicals precisely during the MRI session.
The syringes are used only once and then are discarded. Our case study in computer-aided engineering involves the connection or interface between the disposable syringe and the mechanism that automatically depresses the piston.
Mechanical engineers designed the connection between the syringe and the injection system so that a medical technician could quickly remove an empty syringe and install a fresh one. In addition, the engineers had to design the connection to be strong enough to securely lock the syringe into place and to neither leak nor break when subjected to high pressure during the injection. Engineers designed the system through a sequence of steps that leveraged computer-aided engineering software tools:
1. Design concept. After the system requirements were ascertained, potential solutions developed, and a fi nal solution selected, engineers then created a computer-based drawing of each component in the contrast agent’s injection system. The cross-sectional view of Figure 2.23 illustrates the concept for how the syringe interface, cylinder, and piston connect to one another and to the body of the injection machine.
2. Detailed design. As the concept was reviewed and discussed, engineers incorporated details that had, rightly so, not been addressed at the earlier concept stage. The drawing of Figure 2.23 was developed into the three- dimensional solid model shown in Figure 2.24(a) (see on page 68). Details that would be present in the fi nal manufactured component, such as the stiffening ribs shown in Figure 2.24(b), were then added to make the
2.6 Case Study: Computer-Aided Design: Noninvasive Medical Imaging
Syringe interface
Cylinder
Piston
Automated injection system
Connection for needle
Figure 2.23
A computer- generated drawing of the design concept for the syringe and its interface with the electronically controlled injection system.
Reprinted with permission by Medrad, Inc.
model as realistic and representative of the fi nal hardware as possible.
Engineers used such drawings to visualize the product and to describe its dimensions, shape, and function to others.
3. System simulation. As the syringe is inserted into the automated injection system, rotated, and snapped into place, the fl anges on the syringe interface are subjected to large locking forces that could cause it to crack and break. Using the CAD models, engineers analyzed the stresses at the syringe interface and modifi ed its design so that the fl anges would be strong enough for its intended use. The simulation predicted how the syringe interface would bend and distort as it is inserted into the injection system. If the stress is too large, the engineers modify the component’s shape or dimensions until the design had suffi cient mechanical strength.
4. Manufacturing process. The CAD models were then used to support the manufacturing of the product. On the basis of cost and required strength, the engineers decided that the syringe interface would be plastic and that molten material would be injected at high pressure into a mold.
Once the plastic cooled and solidifi ed, the mold would be opened and the fi nished part removed. The CAD models were then used to design the mold; Figure 2.25 depicts an exploded view of the mold’s fi nal design.
The digital models enabled the designers to simulate the molten plastic fl ow into the hollow portions of the mold and to verify that it would fi ll as expected.
Engineers could then quickly adjust in the CAD models the locations of the seams, injection points, and holes where air could bleed out of the mold. The results showed that air bubbles would not become trapped in the mold and the plastic would not cool and solidify before the mold
(a) (b)
Figure 2.24
(a) As the design evolved, the computer model was extended to represent the component in three dimensions. (b) The design was refi ned to refl ect all geometric features that would be present once the syringe interface was manufactured, such as the stiffening ribs.
Reprinted with permission by Medrad, Inc.
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became completely fi lled. But if a simulation did reveal such problems, as illustrated in Figure 2.26, the engineers could change the mold’s design until the performance was satisfactory.
5. Documentation. Finally, the mechanical engineers prepared detailed digital drawings of the syringe interface (Figure 2.27, see on page 70) and of the mold that would be used for its large-scale production. Technical reports, test data, and computer analysis were compiled and archived digitally to document the design. In the future, the syringe interface might be modifi ed for use in a new product, and the engineers working on that project would need to review the present design process before they build on it and develop a next-generation product.
Figure 2.25
Mechanical engineers designed each com- ponent of the mold that would be used to manufacture the syringe interface.
The components of the mold are shown here in an exploded view.
Reprinted with permission by Medrad, Inc.
Figure 2.26
A computer simula- tion of molten plastic
fl owing into and fi ll- ing the mold in order
to identify locations where air bubbles could potentially be trapped.
Reprinted with permission by Medrad, Inc.
Air pockets
2.6 Case Study: Computer-Aided Design: Noninvasive Medical Imaging
This case study highlights so-called seamless, or paperless, portions of a design process: A product can now be designed, analyzed, prototyped, and manufactured by the integration of digital simulation and computer analysis tools.
S UMMARY
The creative process behind mechanical design cannot be set forth fully in one chapter—or even in one textbook for that matter. Indeed, with this material as a starting point, you can continue to develop design skills and hands-on experience throughout your professional career. Even the most seasoned engineer grapples with all the decisions and trade-offs in a design process that transforms an idea into manufactured hardware that can be sold at a reasonable cost. The subject of mechanical design has many facets.
In this chapter we have introduced you to a basic design process and some of the issues that guide how a new product is designed, manufactured, and ultimately protected in the business environment through patents.
As we described in Chapter 1, engineers apply their skills in mathematics, science, and computer-aided engineering for the purpose of making things that work safely and transform lives. At the highest level, engineers apply the procedure of Section 2.2 to reduce an open-ended problem to a sequence of manageable steps: defining system requirements, conceptual design where concepts are generated and narrowed down, and detailed design where all the geometric, functional, and production details of a product are developed. Engineering is ultimately a business venture, and you should be aware of that broader context in which mechanical engineering is practiced. When developing a new product, an engineer, a team of engineers, or a company often wants to protect the new technology with a patent. Patents provide the inventor with a limited monopoly on the product in exchange for the invention being explained to others.
Figure 2.27
Final verifi ed and documented design of the syringe interface.
Reprinted with permission by Medrad, Inc.
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In the end, successful design is a function of creativity, elegance, usability, and cost. Throughout design, engineers use their judgment and make order- of-magnitude calculations to move ideas to concepts, and concepts to detailed designs. Engineers also specify the methods to be used to produce hardware, and those decisions are based on the quantity to be produced, the allowable cost, and the level of necessary precision. Although rapid prototyping is becoming an increasingly viable way to quickly manufacture custom products, the primary classes of mass production manufacturing processes still include casting, forming, machining, joining, and fi nishing. Each technique is well suited for a specifi c application based on the shape of the mechanical component, and the materials used. Machining operations are conducted using bandsaws, drill presses, milling machines, and lathes, and each of these machine tools uses a sharpened tool to remove material from a workpiece.
Machine tools can be numerically controlled to fabricate high-precision components based on designs that are developed through computer-aided engineering software packages.
Finally in this chapter, we explored two case studies to learn how design principles can be applied to problems at two different orders of magnitude.
These case studies presented the conceptual design of a small vehicle powered by renewable sources, and the design of a system to address the global issue of overstrained power grids in urban settings. A third case study was used to explore the “seamless” application of computer-aided design tools. Hopefully you recognize that design principles can be used to develop and produce a diverse set of products, systems, and services to meet the always complex technical, global, societal, and environmental challenges that our world faces.