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1. Gears
Mechanical Engineering Dept. 1

2. Applications of Gears
• Toys and Small Mechanisms – small, low load, low cost
• Appliance gears – long life, low noise & cost, low to
moderate load
• Power transmission – long life, high load and speed
• Aerospace gears – light weight, moderate to high load
• Control gears – long life, low noise, precision gears
Mechanical Engineering Dept. 2

3. Mechanical Engineering Dept. 3

4. Types of Gears
Gear (large gear)
Spur gears – tooth profile is parallel to
the axis of rotation, transmits motion
between parallel shafts.
Internal gears
Pinion (small gear)
Helical gears – teeth are inclined to
the axis of rotation, the angle provides
more gradual engagement of the teeth
during meshing, transmits motion
between parallel shafts.
Mechanical Engineering Dept. 4

5. Types of Gears
Bevel gears – teeth are formed on a Straight
conical surface, used to transfer bevel gear
motion between non-parallel and
intersecting shafts.
Spiral
bevel gear
Mechanical Engineering Dept. 5

6. Types of Gears
Worm gear sets – consists of a
helical gear and a power screw (worm),
used to transfer motion between non-
parallel and non-intersecting shafts.
Rack and Pinion sets – a special
Pinion
case of spur gears with the gear
having an infinitely large diameter,
the teeth are laid flat.
Rack
Mechanical Engineering Dept. 6

7. Types of gears
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8. Nomenclature of Spur Gear Teeth
Clearance
Fillet radius
Pitch circle
gear diam.
Base Circle
Backlash = (tooth spacing)driven gear – (tooth thickness)driver , measured
on the pitch circle.
Mechanical Engineering Dept. 8

9. Useful Relations
P=N/d
P = diametral pitch, teeth per inch
N = number of teeth
d = pitch diameter (gear diameter)
p (circular pitch) = πd / N
Pp = π
Metric system
m (module, mm) = d / N
Mechanical Engineering Dept. 9

10. Standard Tooth Specifications
Pressure angle
Base tion
of ac
circle Line
Pitch Pressure angle φ
circle
Pitch
Pitch circle
line
Base
circle
Line of centers
Standard pressure angles, 14.5o (old), 20o, and 25o
Two mating gears must have the same diametral pitch, P,
and pressure angle, φ.
Mechanical Engineering Dept. 10

11. Template for teeth of standard gears
11

12. Standard Gear Specifications
Parameter Coarse pitch Fine pitch
(pd=N/d<20) (pd=N/d>20)
Pressure angle, φ 200 or 250 (not common) 200
Addendum, a 1/pd 1/pd
Dedendum, b 1.25/pd 1.25/pd
Working depth 2.00/pd 2.00/pd
Whole depth 2.25/pd 2.2/pd+0.002
Circular tooth thickness 1.571/pd (≅circular pitch/2) 1.571/pd
Fillet radius 0.30/pd Not standardized
Clearance 0.25/pd 0.25/pd+0.002
Minimum width at top 0.25/pd Not standardized
land
Circular pitch π/pd π/pd 12

13. Kinematics of Gear Trains
Conventional gear trains
ω3 N2 ω5 N4
, ω3 = ω4 ,
ω2 = N3 ω4 = N5
Speed ratio
ω5 output
= = input
ω2
mV = e = train value
Reverted gear train – output shaft is concentric
with the input shaft. Center distances of the
stages must be equal.
Mechanical Engineering Dept. 13

14. Planetary (or Epicyclic) Gears
• Gears whose centers can move
• Used to achieve large speed reductions in
compact space
• Can achieve different reduction ratios by
holding different combinations of gears
fixed
• Used in automatic transmissions of cars
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15. Planetary gear
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16. Components of a planetary gear
Planet
Carrier
Input shaft
Sun gear
Ring gear
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17. A variant of a planetary gear
Carrier
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18. Connecting the main engine to the propeller
through a reduction gear
Propeller, Output flange
operates at about 100
RPM Engine
Gea
r
Engine operates at
about 400 RPM
18

19. Gear box
Stick shift
Synchronizers
The gear box is in first gear, second gear
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20. Planetary gears in automotive
transmission
Planetary gears
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