Machine Designers Reference. J. Marrs
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 design calculations include geometric relationships, moments of inertia, stress, strain (deformation), and fatigue analysis.
•Beer, Johnston, Mechanics of Materials, McGraw-Hill Inc., New York, NY, 1981
•M. Lindeburg, Mechanical Engineering Reference Manual for the PE Exam, 11th Ed., Professional Publications, Inc., Belmont, CA, 2001
•R. L. Norton, Machine Design: An Integrated Approach, 4th Ed., Prentice Hall, Upper Saddle River, NJ, 2011
•Oberg, Jones, Horton, Ryffel, Machinery’s Handbook, 28th Ed., Industrial Press, New York, NY, 2008
The relationships in Tables 1-6 through 1-20 are particularly helpful for machinery design. Many of these equations are used in the context of later chapters, but are reproduced here as a quick reference.
Table 1-6: Useful Equations and Values
Table 1-7: Right Triangle Relationships
Units: angles are in degrees
Table 1-8: Areas and Perimeters
Units: angles are in degrees unless otherwise noted
Table 1-9: Volumes and Surface Areas
Table 1-10: Area Moments of Inertia
Table 1-11: Mass Moments of Inertia
Table 1-12: Beam Deflections and Forces
Units: always check all units for consistency
Table 1-13: Column Buckling
Units: always check all units for consistency
Table 1-14: Critical Speed of Rotating Shafts
Units: length in inches, force in pounds (lbf), speed in RPM
Table 1-15: Stress and Strain
Units: always check all units for consistency
Be sure to consult the recommended resources if combined stresses are present.
Table 1-16: Fatigue Equations
Table