1
FIGURE 1-6
An FEA Mesh Applied to the Solid Model of the Ball Bracket in the CAD System is included, with or without the solid geometry aspects, the process is called CAE. Many commercial software packages do one or more aspects of CAE. The FEA and BEA software packages mentioned above are in this category. See Chapter 8 for more information on FEA. Dynamic force simulations of mechanisms can be done with such packages as ADAMS[5] and Working Model. [6] Some software packages such as ProEngineer, [7] Solidworks,[12] Unigraphics, [4] and others combine aspects of a CAD
system with general analysis capabilities. These constraint-based programs allow constraints to be applied to the design which can control the part geometry as the design parameters are varied.
Other classes of tools for CAE are equation solvers such as MATLAB[11] , Mathcad, [9] TK Solver, [8] and spreadsheets such as Excel. [10] These are general-purpose tools that will allow any combination of equations to be encoded in a convenient form and then will manipulate the equation set (i.e., the engineering model) for different trial data and conveniently display tabular and graphic output. Equation solvers are invaluable for the solution of force, stress, and deflection equations in machine-design problems because they allow rapid “what-if” calculations to be done. The effects of dimensional or material changes on the stresses and deflections in the part can be seen instantly. In the absence of a true solid modeling system, an equation solver also can be used to approximate the part’s mass properties while iterating the geometry and material properties of trial part designs. Rapid iteration to an acceptable solution is thus enhanced.
The CD-ROM included with the text contains a large number of models for various equation solvers that support the examples and case studies presented in the text.
Introductions to the use of TK Solver and Mathcad along with examples of their use are provided as PDF files on the CD-ROM. In addition, some custom-written computer programs, MOHR, CONTACT, ASDEQ, FOURBAR, FIVEBAR, SIXBAR, SLIDER, DYNACAM, and MATRIX are provided on the CD-ROM to aid in the calculation of dynamic loads and stresses when solving the open-ended design problems assigned.
16
MACHINE DESIGN -
An Integrated Approach
1
However, one must be aware that these computer tools are just tools and are not a substitute for the human brain. Without a thorough understanding of the engineering fundamentals on the part of the user, the computer will not give good results. Garbage in, garbage out. Caveat Lector.
Computational Accuracy
Computers and calculators make it very easy to obtain numerical answers having many significant figures. Before writing down all those digits, you are advised to recall the accuracy of your initial assumptions and given data. If, for example, your applied loads were known to only two significant figures, it is incorrect and misleading to express the calculated stresses to more significant figures than your input data possessed. However, it is valid and appropriate to make all intermediate calculations to the greatest accuracy available in your computational tools. This will minimize computational round-off errors. But, when done, round off the results to a level consistent with your known or assumed data.
1.6
THE ENGINEERING REPORT*
Communication of your ideas and results is a very important aspect of engineering.
Many engineering students picture themselves in professional practice spending most of their time doing calculations of a nature similar to those they have done as students.
Fortunately, this is seldom the case, as it would be very boring. Actually, engineers spend a large percentage of their time communicating with others, either orally or in writing. Engineers write proposals and technical reports, give presentations, and interact with support personnel. When your design is done, it is usually necessary to present the results to your client, peers, or employer. The usual form of presentation is a formal engineering report. In addition to a written description of the design, these reports will usually contain engineering drawings or sketches as described earlier, as well as tables and graphs of data calculated from the engineering model.
It is very important for engineering students to develop their communication skills.
You may be the cleverest person in the world, but no one will know that if you cannot communicate your ideas clearly and concisely. In fact, if you cannot explain what you have done, you probably don’t understand it yourself. The design-project assignments in Chapter 9 are intended to be written up in formal engineering reports to give you some experience in this important skill of technical communication. Information on writing engineering reports can be found in the suggested readings listed in the bibliography.
1.7
FACTORS OF SAFETY AND DESIGN CODES
* Excerpted from Norton,
The quality of a design can be measured by many criteria. It is always necessary to Design of Machinery, 3ed
calculate one or more factors of safety to estimate the likelihood of failure. There may McGraw-Hill, New York, 2004,
with the publisher’s permission.
be legislated, or generally accepted, design codes that must be adhered to as well.
Chapter 1