This document discusses different types of gear trains: 1. Simple gear train which uses one gear on each shaft to transmit motion. 2. Compound gear train which uses more than one gear on a shaft. 3. Reverted gear train where the first and last gears are co-axial and rotate in the same direction. 4. Epicyclic gear train where the gear axes can move relative to a fixed axis, allowing one gear to drive another in circular motion. Formulas for speed ratio and train value are provided for each gear train type. Examples of applications like differentials are also mentioned.

1. Sub:KOM GEAR TRAINS

2. Introduction to the gear train
Definition:
When two or more gears are made to mesh with each
other to transmit power from one shaft to another, such
a combination is called ‘gear train or train of toothed
wheels’.
Types of gear trains:
1.Simple gear train,
2.Compound gear train,
3.Reverted gear train, and
4.Epicyclic gear train.

3. Simple gear train
Definition:
When there is only one gear on
each shaft as shown in fig., it
is known as simple gear train.
When the distance between
two shafts is small, the two
gears 1 and 2 are made to
mesh with each other to
transmit motion from one
shaft to other as shown in fig.
Since the gear 1 drives 2,
therefore gear 1 is called the
driver and the gear 2 is called
the driven or follower.

4. Simple gear train
Since the speed ratio of gear train is the ratio of the
speed of the driver to the speed of the driven or follower
and the ratio of speeds of any pair of gears in mesh is
the inverse of their number of teeth, therefore
It may be noted that ratio of the speed of the driven to
the speed of the driver is known as train value of the
gear train. Mathematically,
Speed ratio =
Train value =

5. Sometimes, the distance between the two gears is large.
The motion from one gear to another, in such a case, may
be transmitted by either of the following two methods :
1.By providing the large sized gear, or
2.By providing one or more intermediate gears.
A little consideration will show that the former method
is very inconvenient and uneconomical ; whereas the
latter method is very economical.
It may be noted that when the number of intermediate
gears odd, the motion of both gears is like but if the
number of intermediate gears even, the motion of both
gears is unlike.

6. It is seen that the speed ratio and the train value of
gears, is independent of the size and the number of
intermediate gears.
These intermediate gears are called idle gears, as they
do not effect the speed ratio or train value of the system.
The idle gears are used for the following two purposes :
1.To connect gears where a large distance is required,
and
2.To obtain the desired direction of motion of the driven
gear(i.e. clockwise or anticlockwise).

7. Compound gear train
Definition:
When there are more than one
gear a shaft as shown in fig., it
is known as compound gear
train.
Compound gear trains are
useful in bridging over the
space between the driver and
the driven.

8. Compound gear train
Whenever the distance between the driver and the
driven has to be bridged over by intermediate gears and
at the same time a great speed ratio is required, then
the advantage of intermediate gears is intensified by
providing compound gears on intermediate shafts.
In this case, each intermediate shaft has two gears
rigidly fixed to it so they may have the same speed.
One of these two gears mashes with the driver and the
other with the driven attached to the next shaft as
shown in fig.

9. Speed ratio of compound gear train is given by,
Train value of compound gear train is given by,
The advantage of compound train over a simple train is
that a much longer speed reduction from one shaft to the
last shaft can be obtained with small gears.
Speed ratio
Train value

10. Design of spur gears
Sometimes the spur gears are to
be designed for the given velocity
ratio and the distance between
the centers of their shafts.
x
Where,
x=distance between the centers
of two shafts,
d1=pitch circle diameter of the
driver,
d2=corresponding value for the
driven

11. Reverted gear train
Definition:
When the axes of the first gear and
the last gear are co-axial, then the
gear train is known as reverted
gear train as shown in figure.
Here the gear 1 drives the gear 2 in
the opposite direction.
Since the gear 2 and 3 form a
compound gear and the gear 3 will
rotate in the same direction as of gear
2.
the gear 3 will drive the gear 4 in the
same direction as of gear 1.
hence the motion of the first gear and
the last gear is like.

12. Let T1=number teeth on gear 1,
r1=pitch circle radius of gear 1, and
N1=speed of gear 1 in r.p.m.
Similarly,
T2, T3, T4=number of teeth on respective gears,
r2, r3, r4=pitch circle radius of respective gears, and
N2, N3, N4=speed of respective gears in r.p.m.
Since the distance between the centers of the shafts of gears 1
and 2 as well as gears 3 and 4 is same, therefore
r1+r2=r3+r4
Also the pitch of the gears assumed to be the same, therefore
number of teeth on each gear is directly proportional to its
circumference or radius.
T1+T2=T3+T4
And
Speed ratio

13. Epicyclic gear train
In an epicyclic gear train, the axes of
the shafts, over which the gears are
mounted, may move relative to a
fixed axis.
A simple epicyclic gear train is
shown in figure.
When a gear 1 and arm 3 and gear
2 and arm 3 have a common axix.
If the arm is fixed, the gear train is
simple and gear 1 can drive gear 2
or vice-versa, but is gear 1 is fixed
and the arm is rotated about the
axis then the gear 2 is forced to
rotate upon and around the gear 1.
Such a motion is called epicyclic.

14. Velocity ratio of epicyclic gear train
The following two methods may be used for finding out the velocity ratio of
an epicyclic gear train.
1.Tabular method, and
2.Algebraic method.
Since the gears 1 and 2 are meshing directly, therefore they will revolve in
opposite directions.
Since the arm is fixed, therefore its speed Nc=0.
If the gear 1 is fixed, then Na=0.
=
=
=

15. Compound epicylic gear train-sun and planet gear

16. Epicylic gear train with bevel gears :
Humpage’s speed reduction gear. Differential gear of
an automobile.

17. 2014
By,
Dhaval Maru
Parth Panchal
Shardul Bhumbhani
Thank you.