Page 83 - Shigley's Mechanical Engineering Design

bud29281_ch02_031-070.qxd 11/11/09 09:34 PM Page 58 Debd Hard Disk1:Desktop Folder:Temp Work:Satya 10/11/09:

58 Mechanical Engineering Design
Admiralty metal (28 percent Zn) contains 1 percent tin, which imparts excellent cor-
rosion resistance, especially to saltwater. It has good strength and ductility but only fair
machining and working characteristics. Because of its corrosion resistance it is used in
power-plant and chemical equipment. Aluminum brass (22 percent Zn) contains 2 percent
aluminum and is used for the same purposes as admiralty metal, because it has nearly the
same properties and characteristics. In the form of tubing or piping, it is favored over
admiralty metal, because it has better resistance to erosion caused by high-velocity water.
Brass with 36 to 40 Percent Zinc
Brasses with more than 38 percent zinc are less ductile than cartridge brass and cannot
be cold-worked as severely. They are frequently hot-worked and extruded. Muntz metal
(40 percent Zn) is low in cost and mildly corrosion-resistant. Naval brass has the same
composition as Muntz metal except for the addition of 0.75 percent tin, which con-
tributes to the corrosion resistance.
Bronze
Silicon bronze, containing 3 percent silicon and 1 percent manganese in addition to the
copper, has mechanical properties equal to those of mild steel, as well as good corro-
sion resistance. It can be hot- or cold-worked, machined, or welded. It is useful wher-
ever corrosion resistance combined with strength is required.
Phosphor bronze, made with up to 11 percent tin and containing small amounts of
phosphorus, is especially resistant to fatigue and corrosion. It has a high tensile strength
and a high capacity to absorb energy, and it is also resistant to wear. These properties
make it very useful as a spring material.
Aluminum bronze is a heat-treatable alloy containing up to 12 percent aluminum. This
alloy has strength and corrosion-resistance properties that are better than those of brass, and
in addition, its properties may be varied over a wide range by cold working, heat treating,
or changing the composition. When iron is added in amounts up to 4 percent, the alloy has
a high endurance limit, a high shock resistance, and excellent wear resistance.
Beryllium bronze is another heat-treatable alloy, containing about 2 percent beryl-
lium. This alloy is very corrosion resistant and has high strength, hardness, and resis-
tance to wear. Although it is expensive, it is used for springs and other parts subjected
to fatigue loading where corrosion resistance is required.
With slight modification most copper-based alloys are available in cast form.

2–19 Plastics
The term thermoplastics is used to mean any plastic that flows or is moldable when heat
is applied to it; the term is sometimes applied to plastics moldable under pressure. Such
plastics can be remolded when heated.
A thermoset is a plastic for which the polymerization process is finished in a hot
molding press where the plastic is liquefied under pressure. Thermoset plastics cannot
be remolded.
Table 2–2 lists some of the most widely used thermoplastics, together with some
of their characteristics and the range of their properties. Table 2–3, listing some of the
thermosets, is similar. These tables are presented for information only and should not
be used to make a final design decision. The range of properties and characteristics that
can be obtained with plastics is very great. The influence of many factors, such as cost,
moldability, coefficient of friction, weathering, impact strength, and the effect of fillers
and reinforcements, must be considered. Manufacturers’ catalogs will be found quite
helpful in making possible selections.