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TRANSCRIPT
Chapter 13
Gear Trains
11/23/2013
Dr. Mohammad Suliman Abuhiba, PE 1
13.2. Types of Gear Trains
1. Simple gear train
2. Compound gear train
3. Reverted gear train
4. Epicyclic gear train: axes of shafts on which the
gears are mounted may move relative to a fixed
axis.
In the first three types of gear trains, axes of
shafts over which the gears are mounted are
fixed relative to each other
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13.3. Simple Gear Train
Simple gear train: only one gear on each shaft
Gears are represented by their pitch circles.
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13.3. Simple Gear Train
N1 = Speed of gear 1(driver) in rpm
N2 = Speed of gear 2 (driven or follower) in rpm
T1 = Number of teeth on gear 1
T2 = Number of teeth on gear 2
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13.3. Simple Gear Train
When the number of intermediate gears are odd,
the driver & driven rotate in the same direction
If number of intermediate gears are even, the
motion of the driven will be in the opposite
direction of the driver
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13.3. Simple Gear Train
Consider a simple train of gears with one
intermediate gear
N1 = Speed of driver in rpm
N2 = Speed of intermediate gear in rpm
N3 = Speed of driven or follower in rpm
T1 = Number of teeth on driver,
T2 = Number of teeth on intermediate gear
T3 = Number of teeth on driven or follower
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13.3. Simple Gear Train
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13.3. Simple Gear Train
Speed ratio & train value, in a simple train of
gears, is independent of size & number of
intermediate gears.
Intermediate gears are called idle gears
The idle gears are used to:
1. connect gears where a large center distance is
required
2. obtain desired direction of motion of driven gear
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13.4. Compound Gear Train
Compound train of gears: more than one gear on a shaft,
Fig. 13.2.
Used Whenever distance between driver & driven has to
be bridged over by intermediate gears and at the same
time a great speed ratio is required
Each intermediate shaft has two gears rigidly fixed to it
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13.4. Compound Gear Train
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13.4. Compound Gear Train
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13.4. Compound Gear Train
Usually for a speed reduction in excess of 7 to 1,
a simple train is not used and a compound train
or worm gearing is employed.
Gears which mesh must have the same circular
pitch or module
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Example 13.1
The gearing of a machine tool is shown in Fig. 13.3.
The motor shaft is connected to gear A & rotates at
975 rpm. Gear wheels B, C, D and E are fixed to
parallel shafts rotating together. The final gear F is
fixed on output shaft. What is the speed of gear F ?
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Gear A B C D E F
No. of teeth 20 25 50 75 26 65
13.5. Design of Spur Gears x = Distance between centers of two shafts
N1 = Speed of the driver
T1 = Number of teeth on the driver
d1 = Pitch circle diameter of the driver
N2 , T2 & d2 = Corresponding values for driven
pc = Circular pitch
Distance between centers of two shafts
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Example 13.2
Two parallel shafts, about 600 mm apart are to be
connected by spur gears. One shaft is to run at 360
rpm and the other at 120 rpm. Design the gears, if
the circular pitch is to be 25 mm.
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13.6. Reverted Gear Train
Reverted gear train: axes of
1st driver & last driven are
co-axial (Fig. 13.4)
motion of 1st gear & last
gear is like
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13.6. Reverted Gear Train
T1 = # of teeth on gear 1
r1 = Pitch circle radius of
gear 1
N1 = Speed of gear 1 in
rpm
T2, T3, T4 = Number of
teeth on respective gears
r2, r3, r4 = Pitch circle radii
of respective gears
N2, N3, N4 = Speed of
respective gears in rpm
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13.6. Reverted Gear Train
Circular pitch or module of all gears is assumed
to be same
number of teeth on each gear is directly
proportional to its circumference or radius.
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Example 13.3
The speed ratio of the
reverted gear train, Fig.
13.5, is to be 12. The
module pitch of gears A
& B is 3.125 mm and
of gears C & D is 2.5
mm. Calculate the
suitable numbers of
teeth for the gears. No
gear is to have less
than 24 teeth.
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13.7 Epicyclic Gear
Train
Axes of the shafts, over
which the gears are
mounted, may move
relative to a fixed axis.
Epicyclic gear trains are
useful for transmitting
high velocity ratios with
gears of moderate size
in a comparatively
lesser space.
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13.8. Velocity Ratios of Epicyclic
Gear Train
1. Tabular method
2. Algebraic method
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13.8. Velocity Ratios of Epicyclic
Gear Train - Tabular method
TA = Number of
teeth on gear A
TB = Number of
teeth on gear B
CCW = +ve
CW = -ve
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13.8. Velocity Ratios of Epicyclic
Gear Train - Tabular method
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Step
No.
Conditions of motion Revolutions of elements
Arm C Gear A Gear B
1 Arm fixed-gear A rotates through +1 rev 0 +1 -TA / TB
2 Arm fixed-gear A rotates through +x rev 0 +x -x TA / TB
3 Add + y revolutions to all elements +y +y +y
4 Total motion +y x+y y -x TA / TB
2. Algebraic method
Example 13.4
In an epicyclic gear train, an arm carries two gears A and B
having 36 and 45 teeth respectively. If the arm rotates at
150 rpm in the ccw direction about the center of gear A
which is fixed, determine the speed of gear B.
If gear A instead of being fixed, makes 300 rpm in the cw
direction, what will be the speed of gear B ?
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Example 13.5
In a reverted epicyclic gear train, arm A carries two gears B
and C and a compound gear D - E. Gear B meshes with
gear E and gear C meshes with gear D. The number of
teeth on gears B, C and D are 75, 30 and 90 respectively.
Find the speed and direction of gear C when gear B is fixed
and arm A makes 100 rpm clockwise.
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Example 13.6
An epicyclic gear consists of
three gears A, B and C. Gear
A has 72 internal teeth and
gear C has 32 external teeth.
Gear B meshes with both A
and C and is carried on an
arm EF which rotates about
the center of A at 18 rpm. If
gear A is fixed, determine the
speed of gears B & C.
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Example 13.7
How many revolutions does the
arm, to which the pinions B and C
are attached, make:
1. when A makes one revolution
cw and D makes half a
revolution ccw
2. when A makes one revolution
cw and D is stationary
The number of teeth on gears A &
D are 40 and 90 respectively.
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Example 13.8 In an epicyclic gear train, the internal
wheels A and B and compound wheels
C and D rotate independently about
axis O. The wheels E and F rotate on
pins fixed to the arm G. E gears with A
& C and F gears with B & D. All the
wheels have the same module and the
number of teeth are: TC= 28; TD= 26;
TE= TF= 18.
1. Find the number of teeth on A & B
2. If arm G makes 100 rpm cw and A
is fixed, find the speed of B
3. If arm G makes 100 rpm. Cw and
wheel A makes 10 rpm ccw; find the
speed of wheel B.
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Example 13.9
In an epicyclic gear of the ‘sun and
planet’ type shown, the pitch circle
diameter of the internally toothed ring
is to be 224 mm and the module 4
mm. When the ring D is stationary, the
spider A, which carries three planet
wheels C of equal size, is to make one
revolution in the same sense as the
sun wheel B for every five revolutions
of the driving spindle carrying the sun
wheel B. Determine suitable numbers
of teeth for all the wheels.
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Example 13.10
Two shafts A and B are co-axial. A gear C (50 teeth)
is rigidly mounted on shaft A. A compound gear D-E
gears with C and an internal gear G. D has 20 teeth
and gears with C. E has 35 teeth and gears with an
internal gear G. Gear G is fixed and is concentric
with the shaft axis. The compound gear D-E is
mounted on a pin which projects from an arm keyed
to the shaft B. Find the number of teeth on internal
gear G assuming that all gears have the same
module. If shaft A rotates at 110 rpm, find the speed
of shaft B.
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Example 13.10 11/23/2013
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Evaluation Quiz
Quiz will be held on
Wednesday 20/11/2013
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