Application
•Safety is of paramount importance in all designs. Understand and obey all applicable laws, codes, and standards.
•Apply appropriate safety factors when conducting design and analysis.
•The design specification should account for all design requirements and foreseeable needs.
•Research and test. This will improve your results and help prevent mistakes.
•Avoid overly descriptive terms early in the design specification phase. They can limit creativity.
•State and question assumptions made during the specification and design process.
•Build your list of known mechanical devices to maximize your synthesis capabilities. Work to improve your understanding and ‘feel for’ forces, distances, and machining methods.
•Allow enough time for research and conceptualization. Ensure that the work environment allows long periods of concentration. This is often a challenge in the modern office.
•Come up with multiple solutions for every problem and select the best one. Use a decision matrix to aid the selection process.
•When visualization stalls, some designers find it helpful to begin drawing or modeling the known surfaces, parts, and points in an assembly. A second round of visualization after accurately modeling what is known is often more fruitful.
Proper units are critical when performing measurement and analysis. Most engineering books contain some discussion of units and unit conversions. In addition, unit conversion tools abound on the Internet. This section provides an overview of the most commonly used engineering units and conversions.
•M. Lindeburg, Mechanical Engineering Reference Manual for the PE Exam, 11th Ed., Professional Publications, Inc., Belmont, CA, 2001
•Oberg, Jones, Horton, Ryffel, Machinery’s Handbook, 28th Ed., Industrial Press, New York, NY, 2008
•Unit Converter Express Online: www.unitconverters.net
•Unit Conversion Tools Online: www.unit-conversion.info
•Online Conversion Website: www.onlineconversion.com
•Unit Conversion Website: www.unitconversion.org
ENGINEERING UNITS
Consistency of units is essential in all engineering calculations. Checking units will often turn up problems with a calculation. In the United States, both the Imperial (inch or foot) and SI (metric) systems are used. Some commonly used engineering units in both systems can be found in Table 1-3.
Table 1-3: Selected Engineering Units
There are two common ways to represent Imperial units in the United States: the inch-pound-second system (ips) and the foot-pound-second system (fps). The ips system is more commonly used in machinery design, though certain calculations are often done in the fps system. It is important to note that the unit of pounds mass (lbm) is numerically equal to pounds force (lbf) and must be divided by the gravitational constant before it is used in equations calling for mass. Imperial units of mass are blobs (ips basis) or slugs (fps basis). The values of gravitational acceleration in the Imperial system are as follows:
ips: gc = 386.4 lbm·in/lbf·s2 (or blobs)
fps: gc = 32.174 lbm·ft/lbf·s2 (or slugs)
The metric system, or SI system, is a base-10 system of units that uses prefixes to designate orders of magnitude. The basis of length in the metric system is the meter (m). Common metric prefixes can be found in Table 1-4. Most metric machinery design is done using millimeters (mm) as the standard measure of length.
Table 1-4: Common Metric Prefixes
UNIT CONVERSIONS
Conversion between Imperial and metric units is a fact of life for many designers. Some common engineering unit conversions are listed in Table 1-5.
Table 1-5: Selected Unit Conversions
Calculation is an essential part of engineering design. It is critical that units be consistent, the correct equations used for the application, and appropriate factors of safety applied. Consult trusted references for calculation guidance. Some common machinery
