机械工程导论外文

机械工程导论外文内容摘要：

brightened in the final image. With such information． a physician or surgeon can develop an improved diagnosis of the patient’s medical condition. Contrast agents must be injected in a precise and safe manner. For that reason, automated mechanical syringes under puter control are often used to perform the procedure. The particular system examined in this section prises two syringes that deliver the contrast agent and a panion saline solution to the patient. The system includes pistons and cylinders as in traditional syringes, but an electronic motor automatically depresses the pistons to precisely inject the chemicals during the MRI session. The syringes are used only once, and then they are disposed. A constraint on the product39。 s design is the method by which the syringes are inserted, held in the automated injection system, and then removed. Our case study in puteraided engineering involves the connection or interface between the disposable syringe itself and the mechanism that automatically depresses the piston. Mechanical engineers design the connection between the syringe and the injection system so that a medical technician can quickly remove an empty syringe and install a fresh one. In addition, the connection must be strong enough to securely lock the syringe into place and to neither leak nor break when it is subjected to high pressure during the injection. Engineers designed the syringe and its connection to the injection system through a sequence of steps that draw extensively on puteraided design tools: 1. Concept. Engineers first created a puterbased drawing of each ponent in the injection system. The crosssectional view of Figure (a) illustrates how the syringe interface, cylinder, and piston connect to one another and to the body of the automated injection machine. At the design concept stage, engineers fixed their ideas with an approximate representation of the product。 recognizing that many details remained to be resolved. 2. Detailed design evolution. As the concept was reviewed and discussed, engineers began incorporating realistic features and tending to details that had, rightly so, not been addressed at the earlier concept stage. The final shape of the syringe interface was established by including all of the geometric features that would be present once the ponent was ultimately manufactured. The drawing of Figure (a) was first developed into the threedimensional solid model shown in Figure (a). Each detail that would be present in the final ponent, even the stiffening ribs shown in Figure (b), was then built into the puter model so as to make it as realistic and representative of finished hardware as possible. Engineers used such drawings to visualize the product and describe its dimensions, shape, and function to others. In addition, the drawings were developed in such a format that other puteraided engineering tools could directly import the threedimensional representation, simplifying subsequent stress and manufacturing analyses. 3. Strength and deformation analyses. As the syringe is inserted into the automated injection system by the MRI technician, rotated, and snapped into place, the flanges on the syringe interface are subjected to large locking forces that could cause it to crack and break. Because the assembly is used in a precision medical environment, it is critical that engineers design each ponent to be as reliable as possible. In the next step along the product development process, engineers analyzed the syringe interface and modified its design so that the flanges would be strong enough for its intended use. Many puteraided mechanical engineering tools are patible with one another, enough so that data files for the dimensions and shape of a ponent can be transferred between software packages. In that manner, the threedimensional puter model of Figure (b) was directly transferred from a drafting software package into one that is used to analyze stress. In a virtual environment, before any parts were actually produced, mechanical engineers simulated how the syringe interface would bend and distort as it is inserted into the injection system (Figure ). If the stress or deformation was predicted to be too large, the engineers would iterate back to the previous step and modify the shape or dimensions until the design had sufficient mechanical strength. As is almost always the case, the process involved several iterations in which the design was repeatedly analyzed, modified, and reanalyzed until the performance requirements were met and the syringe interface would not be expected to break during use. 4. Manufacturing process. In the next step, mechanical engineers needed to determine which tools and processes would be used to manufacture the product. Engineering involves designing not only the product but also the techniques that will be used to manufacture it. The engineers in this case decided that the syringe interface would be produced from plastic and that molten material would be injected at high pressure into a mold. Once the plastic cooled and solidified, the mold would be opened and the finished part could be removed. The mechanical engineers therefore needed to design the mold and verify that it would fill with molten plastic as expected. Figure depicts an exploded view of the mold39。 s final design. However, before the mold itself was machined, engineers first used puteraided engineering tools to analyze and refine it. As shown in Figure , the injection molding process was simulated as molten plastic flowed, into and filled the hollow portions of the mold. In virtual puter simulations, engineers were able to adjust the locations of the injection points, air bleed points, and seams in the mold until the results showed that air bubbles would not bee＿ trapped in the mold and that the pla。