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Spur Inexpensive, simple to design, no thrust load is de-veloped
by the gearing, wide variety of manufac-turing options.
Can generate t noise, especially at highspeeds, and are
usually restricted to pitch-line ve-locities below 20 m/s (4000
ft/min).
Helical Useful for high speed and high power applications,quiet
at high speeds. Often used instead of spur
gears for high speed applications.
Generate a thrust load on a single face, more expen-sive than
spur gears.
Bevel High ef y (can be 98% or higher), can transferpower
across nonintersecting shafts. Spiral bevelgears transmit loads
evenly and are quieter thanstraight bevel.
Shaft alignment is critical, rolling element bearingsare
therefore often used with bevel gears. Thislimits power transfer
for high speed applications(where a journal bearing is preferable).
Can be ex-pensive.
Worm Compact and cost effective designs for large gearratios.
Ef y can be as high as 90% or as low as50%, and is lower than
for other gear sets.
Wear by abrasion is of higher concern than othergear types, can
be expensive. Generate very highthrust loads. Worm cannot be driven
by gear; wormmust drive gear.
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b e r o f t e e t h o n g e a r f o r w h i c h
g e o m e t r y f a c t o r i s d e s i r e d
0.16 0.20 0.24 0.28 0.32 0.36 0.40
Geometry factor, Yb
Number of teeth in mate
13 15 20 25 30 35 40 45 50
60
70
80
90
100
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100
90
80
70
60
50
40
30
20
10
N u m b e r o f t e e t h o n
g e a r f o r w h i c h
g e o m e t r y f a c t o r i s d e s i r e d
0.16 0.20 0.24 0.28 0.32 0.360.12
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60 80
100
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N u m b e r o f t e e t h i n g e
a r f o r w h i c h
g e o m e t r y f a c t o r i s
d e s i r e d
100
90
80
70
60
50
40
30
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Material Hardness Material Hardness Material factor
Case hardened steel 58 HRC Case hardened steel 60 HRC 0.85
Case hardened steel 55 HRC Case hardened steel 55 HRC 1.00
Flame hardened steel 50 HRC Case hardened steel 55 HRC 1.05
Flame hardened steel 50 HRC Flame hardened steel 50 HRC 1.05Oil
hardened steel 375-425 HB Oil hardened steel 375-425 HB 1.20
Heat treated steel 210-300 HB Case hardened steel 55 HRC
1.45
Cast iron – Case hardened steel 55 HRC 1.95
Cast iron – Flame hardened steel 55 HRC 2.00
Cast iron – Annealed steel 160-200 HB 2.10Cast iron – Cast iron
– 3.10
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dg = 2cd − dw
px =πdg
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Fundamentals of Machine Elements, 3rd
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