Page 226 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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Gear teeth can be obtained by forming methods and generating methods. When form
cutting, the tooth space takes the exact shape of cutter. When generating, a generating
tool is moved relative to the gear blank like two gears in mesh for the purpose of gear
tooth profile cutting [1]. A standardized tooth system has a specified pressure angle and
modulus to facilitate interchangeability and availability.
Hobbing is employed to make gears with a high degree of profile accuracy and excel-
lent surface furnishing at a high production rate. A hob takes a shape similar to a worm
with periodic relief grooves and is sharpened to provide cutting edges. While hobbing,
the hob is fed across the face width of gear blank as both the gear blank and the hob
rotate at a proper speed ratio. Shaping utilizes either a rack cutter or a pinion cutter to
cut gear teeth. The cutter is reciprocated back-and-forth across the face of gear blank
as the gear blank rotates. Milling uses a milling cutter to cut tooth spaces sequentially
by rotating the gear blank one circular pitch each time until a complete gear forms.
Hobbing, shaping and milling produce spur and helical gear teeth with good accuracy
for low speed, light load applications.
When gears operate at high speed or under heavy loads, gears may be subjected to
additional dynamic loads if errors exist in tooth profiles. Tooth surface quality must,
therefore, be enhanced by a finishing process following gear cutting operation. Gear
finishing, like shaving, grinding and honing, uses highly accurate tools to remove minute
amounts of material off teeth surfaces.
Shaving is a corrective process of scarping away tiny material to improve profile accu-
racy and surface finish. Grinding can be realized by form grinders or generating grinders,
similar to the cutting method just discussed. Both utilize abrasive wheels to grind both
sides of the space between two teeth. Honing tends to average surface irregularities,
removing nicks and burrs by a hone, a fine-grit abrasive gear-shaped tool [6]. Gear
finishing can improve dimensional accuracy, increase surface hardness, enhance load
sharing among gears and reduce vibration and noise. Gears in high-speed, high-load
and high-precision applications are often honed after grinding in order to produce a
very smooth surface finish.
Different manufacturing methods produce diverse accuracy grade gears. Gear accu-
racy grade defines the tolerances of various size gears manufactured to a specified accu-
racy. An experience-based guide to accuracy levels achieved by different manufacturing
methods for various applications are provided in Table 8.5. In ISO, GB and DIN stan-
dards, small number represents high accuracy, while in AGMA 2008 standard, large
number specifies high precision.
Gears are more expensive than belts and chains. Heavy loads, high speeds, long lives
and quiet operation require high quality gear manufacture; when loads and speeds are
less demanding, accuracy requirements may be relaxed. As expected, the manufacturing
cost increases sharply with increased precision.

8.6.4 Allowable Stresses

Tooth surface pitting and tooth breakage are two forms of failure in gears due to fatigue.
The strength and pitting resistance against failure can be evaluated by endurance
limits of gear materials. Endurance limits are important properties obtained from
fatigue tests, using a pair of accuracy grades 4∼6 ISO/GB standard spur gears, with
∘
module m = 3∼5 mm, centre distance a = 100 mm, pressure angle = 20 ,face