Page 225 - Analysis and Design of Machine Elements

8.6.2.1 Commonly Used Gear Materials Gear Drives 203
Common gear materials include ferrous, nonferrous metals and nonmetallic materials.
Steels combine superior characteristics of high strength and moderate cost. Their prop-
erties, as well as surface hardness, can be improved via proper heat treatments. Most
of moderate to heavy-duty speed reducers use medium carbon steel and alloy steels for
gear materials. Wrought steels and cast steels can be used for gear blanks. For large sized
gears, cast steels are preferred.
Both grey and nodular cast iron are used for large gears in low speed open gearings.
The inexpensive cast iron gears have good wear resistance and high vibration damping
capacity. They are usually paired with steel pinions to obtain reasonable strength with
quiet operation.
Other commonly used materials are nonferrous metals, like bronze. The lower mod-
ulus of elasticity of copper alloys provides greater tooth deflection and improves load
sharing among teeth. Since bronze and steel run well together, the combination of a
steel pinion and a bronze gear is often used. Brass is an inexpensive material usually for
clock gears.
Nonmetallic gears, such as plastic gears, can operate quietly under light load without
lubrication. They have low weight, good wear and corrosion resistance, Plastic gears
can be moulded into final form without subsequent machining, which greatly reduce
production cost. Nonmetallic gearings are increasingly used in the area where low cost
and quiet operation take precedence over strength requirements.

8.6.2.2 Heat Treatments
Gear blanks made of medium carbon steels or alloy steels are usually first normalized
and tempered before being machined. Such treatment produces soft tooth surfaces with
hardness less than 350HBW. These gears are limited to general applications. If both
meshing gears have soft tooth surfaces, the pinion tooth surfaces are treated 30–50HBW
higher than those of the gear. This is because pinions have to experience more stress
cycles with relatively thin root thicknesses compared with mating gears.
The gears with hard tooth surfaces (hardness ≥38–65HRC) are firstly produced by
milling, shaping or hobbing, followed by case hardening processes, such as carburizing
or nitriding, to improve surface hardness. Carburizing produces surface hardness within
a range of 55–65 HRC, resulting in almost the highest strength for gears. Nitriding pro-
duces a hard but thin case and should be avoided if overload or shock will be expe-
rienced, because this case is not sufficiently strong to resist such loads [3]. Local sur-
face heating by induction hardening or oxyacetylene flame hardening followed by rapid
quenching are used to produce a high-hardness, wear-resistant surface layer of gear
teeth. Finally, gear teeth are finished by grinding, shaving or honing to achieve high
precision.

8.6.3 Gear Manufacturing and Quality

Gears are made by a variety of manufacturing methods to achieve various qualities. The
selection of the proper method involves a balanced consideration of design require-
ments, the capability and limitation of manufacturing methods and costs. Gears may be
initially cut by hobbing, shaping, milling and so on, followed by a finishing process such
as shaving, grinding, lapping, honing or burnishing to obtain high precision gear teeth.