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FIGURE 1-1
A Freehand Sketch of a Trailer Hitch Assembly for a Tractor
While often incomplete in terms of detail needed for manufacture, the engineering sketch should contain enough information to allow the development of an engineering model for design and analysis. This may include critical, if approximate, dimensional information, some material assumptions, and any other data germane to its function that is needed for further analysis. The engineering sketch captures some of the givens and assumptions made, even implicitly, at the outset of the design process.
1.5
COMPUTER-AIDED DESIGN AND ENGINEERING
The computer has created a true revolution in engineering design and analysis. Problems whose solution methods have literally been known for centuries but that only a generation ago were practically unsolvable due to their high computational demands can now be solved in minutes on inexpensive microcomputers. Tedious graphical solution methods were developed in the past to circumvent the lack of computational power available from slide rules. Some of these graphical solution methods still have value in that they can show the results in an understandable form. But one can no longer “do engineering” without using its latest and most powerful tool, the computer.
Computer-Aided Design (CAD)
As the design progresses, the crude freehand sketches made at the earliest stages will be supplanted by formal drawings made either with conventional drafting equipment or, as is increasingly common, with computer-aided design or computer-aided drafting software. If the distinction between these two terms (both of which share the acronym CAD) was ever clear (a subject for debate which will be avoided here), then that distinction is fading as more sophisticated CAD software becomes available. The original CAD systems of a generation ago were essentially drafting tools that allowed the creation of computer-generated multiview drawings similar to those done for centuries before by hand on a drafting board. The data stored in these early CAD systems were
12
MACHINE DESIGN -
An Integrated Approach
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strictly two-dimensional representations of the orthographic projections of the part’s true 3-D geometry. Only the edges of the part were defined in the database. This is called a wireframe model. Some 3-D CAD packages use wireframe representation as well.
Present versions of most CAD software packages allow (and sometimes require) that the geometry of the parts be encoded in a 3-D data base as solid models. In a solid model the edges and the faces of the part are defined. From this 3-D information, the conventional 2-D orthographic views can be automatically generated if desired. The major advantage of creating a 3-D solid-model geometric data base for any design is that its mass-property information can be rapidly calculated. (This is not possible in a 2-D or 3-D wireframe model.) For example, in designing a machine part, we need to determine the location of its center of gravity (CG), its mass, its mass moment of inertia, and its cross-sectional geometries at various locations. Determining this information from a 2-D model must be done outside the CAD package. That is tedious to do and can only be approximate when the geometry is complex. But, if the part is designed in a solid modeling CAD system such as ProEngineer, [7] Unigraphics, [4] or one of many others, the mass properties can be calculated for the most complicated part geometries.
Solid modeling systems usually provide an interface to one or more Finite Element Analysis (FEA) programs and allow direct transfer of the model’s geometry to the FEA package for stress, vibration, and heat transfer analysis. Some CAD systems include a mesh-generation feature which creates the FEA mesh automatically before sending the data to the FEA software. This combination of tools provides an extremely powerful means to obtain superior designs whose stresses are more accurately known than would be possible by conventional analysis techniques when the geometry is complex.
While it is highly likely that the students reading this textbook will be using CAD
tools including finite element or boundary element analysis (BEA) methods in their professional practice, it is still necessary that the fundamentals of applied stress analysis be thoroughly understood. That is the purpose of this text. FEA techniques will be discussed in Chapters 4 and 8 but will not be emphasized in this text. Rather we will concentrate on the classical stress-analysis techniques in order to lay the foundation for a thorough understanding of the fundamentals and their application to machine design.
FEA and BEA methods are rapidly becoming the methods of choice for the solution of complicated stress-analysis problems. However, there is danger in using those techniques without a solid understanding of the theory behind them. These methods will always give some results. Unfortunately, those results can be incorrect if the problem was not well formulated and well meshed with proper boundary conditions applied. Being able to recognize incorrect results from a computer-aided solution is extremely important to the success of any design. Chapter 8 provides a brief introduction to FEA.
The student should take courses in FEA and BEA to become familiar with these tools.
Figure 1-2 shows a solid model of the ball bracket from Figure 1-1 that was created in a CAD software package. The shaded, isometric view in the upper right corner shows that the solid volume of the part is defined. The other three views show orthographic projections of the part. Figure 1-3 shows the mass-properties data which are calculated by the software. Figure 1-4 shows a wireframe rendering of the same part generated from the solid geometry data base. A wireframe version is used principally to speed up the screen-drawing time when working on the model. There is much less wireframe display information to calculate than for the solid rendering of Figure 1-2.
Chapter 1