Page 43 - Analysis and Design of Machine Elements

An Overview of Machine Design
must have corrosion resistance. Analysing potential failure mode is also helpful for 21
establishing required material properties for the element. For example, to resist
against excessive wear, hard materials are required; to resist against large deformation,
materials with large modulus of elasticity or stiffness are preferred.
Materials may be available in different forms. Proper manufacturing processes, that is,
machining, forming, joining, finishing and coating, are required to turn original shaped
materials into designed machine elements. Therefore, a material’s capability for form-
ing, joining or welding together with the initial material cost and processing costs are
important considerations in material selection. Besides, recycling, disposal, legal and
health issues are additional considerations [10].
After identifying, selecting, evaluating and ranking candidate materials, combined
with making compromises among constraints of properties, availability, machinability
and costs, a final selection decision can be made.

1.5 Unit Systems

A unit is a specified magnitude of a physical quantity [10]. Units chosen for any three
quantities of force, mass, length, and time are called base units, the fourth unit is the
derived unit. If force, length and time are base units, the system is a gravitational sys-
tem of units, such as the foot-pound-second (fps) system and inch-pound-second (ips)

Table 1.3 Commonly used design variables and their units.

Variables Symbol SI units ips units fps units

Force F N (newton) lb (pound) lb
Length l m (metre) in. (inch) ft (foot)
−1
2
2
Mass m kg (kilogram) lb s in. −1 slug (lb s ft )
Time t s (second) s s
Acceleration a ms −2 in. s −2 ft s −2
∘
∘
Angle rad or degree ( ) rad (radian) or radordegree( )
∘
degree ( )
Angular velocity rad s −1 rad s −1 rad s −1
Energy or work E, W J (joule) lb in. lb ft
Frequency f Hz (Hertz)
Moment or torque M,T Nm lb in. lbft
−1
Power P Nm s −1 = W lb in s −1 lb ft s ,hp
(watt)
Pressure p Pa (Pascal) psi, lb in. −2 psf
Stress , MPa psi, lb in. −2 –
Temperature T degrees Celsius degrees degrees
∘
∘
∘
( C) Fahrenheit ( F) Fahrenheit ( F)
Velocity ms −1 in s −1 ft s −1
Weight W N lb lb