Page 40 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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bronze alloys include phosphor bronze, leaded phosphor bronze, aluminium bronze and
silicon bronze; while cast bronze alloys have tin bronze, leaded tin bronze, nickel tin
bronze and aluminium bronze [9]. Bronzes are softer than ferrous alloys but have good
strength, machinability and wear resistance. They run well against steel or cast iron when
lubricated and can support high loads and operate at high temperatures. These qualities
are utilized in sliding bearings and worm gearing where the combination of heavy loads
and high sliding velocities places them in analogous situations [10].
1.4.1.4 Polymers
Polymers, usually referred to as plastics, fall into two main groups; that is, thermoplas-
tics and thermosets. Thermoplastics are softened with heat and include acetal, acrylic,
acrylonitrile-butadiene-styrene (ABS), nylon, polycarbonate, polyimide and polyvinyl
chloride (PVC). They are generally impact resistant. Thermosets are generally heat resis-
tant, including epoxy, phenolic and polyester [9, 10]. Both types are inexpensive, light
in weight, resistant to corrosion and wear. Also, they have a low coefficient of friction,
low strength and provide quiet and smooth operation.
1.4.1.5 Composite Materials
Composite materials are comprised of matrix materials and reinforcement materials.
Each remains distinct and separate from each other. Matrix materials are various plastic
resins like nylon, epoxy or polyester and metals, while reinforcements include glass, car-
bon and SiC (silicon carbide) in the form of continuous fibres, either straight or woven,
short chopped fibres and particulates [8–10]. The reinforcement provides stiffness and
strength, and the matrix holds the materials together and transfers load to the reinforce-
ment materials.
Unlike isotropic, homogeneous engineering materials that have identical material
properties in every direction, the material properties of composites vary with both
location and direction. The orientation of multiple laminates can be optimized to reach
a desirable high strength-to-weight ratio or high stiffness-to-weight ratio. Therefore,
composite materials are becoming increasingly popular in automotive, marine, aircraft
and spacecraft applications to realize lightweight design with high structural stiffness
and excellent strength performance.

1.4.2 Material Properties
Material properties, including mechanical, physical, chemical, dimensional and
processing properties, are important considerations in material selection. Among
them, mechanical properties are especially crucial in determining the size and shape
of machine elements. They also decide the performance of machine element under
operating conditions.
Mechanical properties refer to strength, stiffness, hardness, toughness, ductility,
impact, and creep and so on. Material strength considered for design calculation
depends largely on the selected materials and loading conditions. For example, yield
strengths are mainly applied to ductile materials; ultimate tensile strength for brittle
materials; ultimate compressive strength for ceramics and glasses and fatigue strength
for cyclic loading. Material stiffness is represented by the elastic modulus or Young’s
modulus and Poisson’s ratio, which is almost the same for all steels [10]. The service