Gear Trains
Gears
A gear is a wheel with teeth on its outer edge.
The teeth of one gear mesh (or engage) with the teeth of another.
Above
Gears meshing or engaged
Gears
Driver and Driven
Two meshed gears always rotate in opposite directions.
Spur Gears
Driven gear
Driver gear
Gears
Idler gear
Driver
Idler gear
Driven
Gear train
•
It is a combination of gears that transmit motion from one shaft to
another.
•
It becomes necessary when it is required to obtain a large speed
reduction within a small space.
•
Types of gear trains:




Simple gear train
Compound gear train
Reverted gear train
Planetary and epicyclic gear train
Simple Gear Train
• Multiple gears can be connected together to form a gear train.
Each shaft carries only one
gear wheel.
Intermediate gears are known as
Idler Gears.
Compound Gear Train
If two gear wheels are mounted on a common shaft
then it’s a Compound Gear train.
Driver
Compound
Gear
Driven
Reverted Gear Train
• The driving and driven gears are coaxial or coincident.
Planetary or epicyclic Gear train
• These gears are useful to transmit very
high-velocity ratios with gears of smaller
sizes in a lesser space.
Simple Gear Train
•
Two external gears of a pair always move in opposite directions.
•
All odd-numbered gears move in one direction and all even-numbered
gears in the opposite direction.
Simple Gear Train
The speed ratio (or velocity ratio) of gear train is the ratio of the speed of the
driver to the speed of the driven or follower and ratio of speeds of any pair of
gears in mesh is the inverse of their number of teeth, therefore
Inverse of the speed ratio is known as the train
value of the gear train
Simple Gear Train
N1 = Speed of driver in r.p.m.,
N2 = Speed of intermediate gear in r.p.m.,
N3 = Speed of driven or follower in r.p.m.,
T1 = Number of teeth on driver,
T2 = Number of teeth on intermediate gear, and
T3 = Number of teeth on driven or follower.
Since the driving gear 1 is in mesh with the intermediate gear 2, therefore
speed ratio for these two gears is
Similarly, as the intermediate gear 2 is in mesh with the driven gear 3,
therefore speed ratio for these two gears is
Compound Gear Train
The speed ratio of compound gear train is
Example
The gearing of a machine tool is shown in Fig. 13.3. The motor shaft is
connected to gear A and rotates at 975 r.p.m. The gear wheels B, C, D
and E are fixed to parallel shafts rotating together. The final gear F is
fixed on the output shaft. What is the speed of gear F ? The number of
teeth on each gear are as given below :
Gear
A
B
C
D
E
F
No. Of
Teeth
20
50
25
75
26
65
Solution
Reverted Gear Train
When the axes of the first gear (i.e. first driver) and the
last gear (i.e. last driven or follower) are co-axial, then
the gear train is known as reverted gear train.
T1, T2, T3, T4 = Number of teeth on respective gears,
r1,r2, r3, r4 = Pitch circle radii of respective gears,
N1,N2, N3, N4 = Speed of respective gears in r.p.m.
Epicyclic Gear Train
 A gear train having a relative motion of
axes is called a planetary or an epicyclic
gear train.
 In epicylic train, the axis of at least one
of the gears also moves relative to the
frame.
Analysis of Epicyclic Gear Train
Tabular Method
 Lock the arm and assume the other wheels free to rotate.
 Turn any convenient gear through one revolution in the clockwise direction and
record the number revolution made by each of the other wheels.
 Multiply all the above recordings by x and write the same in the second row. This
is equivalent to the statement that the chosen wheel is given x revolutions in the
clockwise direction keeping the arm fixed.
 Add y to all the quantities in the second row and make the recordings in the third
row. This amounts to the fact that by locking the whole system, it is turned
through y revolutions in the clockwise direction. Thus the arm makes y
revolutions, the chosen wheel (y + x) revolutions, and so on.
 Apply the given conditions and find the values of x and y.
Example
Example