B.tech i eme u 5 transmission of motion and power

1
Course : B.Tech Mechanical
Subject : Elements of Mechanical Engineering
Unit-5

Contents:
 Introduction
 Methods of drive,
 Power transmission elements
 shaft Spindle and axle
 Belt-drive
 Pulleys
 Power transmitted by a belt, Chain
drive, Friction drive, Gear drive

Introduction
 Need:
◦ Every actuation or system need power or
motion to drive itself from source.
◦ Thus transmission of power from a
source(such as an engine or motor)
through a machine to an output actuation
is needed to do all machine tasks.
◦ This task can be achieved by different
methods of drives.

Different Terms of TPM
 Driving member
 Driven member
 Idler member
 Pulley
 Belt
 Chain
 Rope
 Gear

Methods of drive
Belts
Chains
Gears
Used when the distance between
the shaft centers is LARGE
the shaft centers is LARGE and no
slip is permitted
the shaft centers is adequately
less

Belts and Belt Drive
What is Belt?
A belt is continuous bond of flexible
material passing over pulleys to
transmit motion from one shaft to
another.
1

Belt and Belt Drive…
 Flat Belt:
◦ It has narrow rectangular cross-section
◦ Used for their simplicity due to minimum
bending stress on pulleys
◦ Load capacity depends on its width
◦ Only one belt is used in whole drive.
◦ Materials used for Belts are Leather,
Rubber, Textile, Balata, steel.

 V Belt:
◦ It has trapezoidal section running on
pulleys with grooves cut to match belt.
◦ Normal angle between the sides of the
groove is 40 deg.
◦ Used when distance between two shafts
are too small for flat belt drive.
◦ More efficient than flat belt due to wedge
action in groove.
◦ Made up of fabric coated with rubber.

 Round Belt:
◦ It has round section.
◦ Diameter range is 3 to12 mm but usually
from 4 to 8 mm.
◦ Used to transmit low power, mainly in
instruments, table-type machine tools, in
clothing industry and household
appliances.
◦ Made up of leather. Canvas and rubber.

 Belt Drive:
◦ It consist of of driver and driven pulleys
and the belt which is mounted on pulleys
with certain amount of tension and
transmits peripheral force by friction.
◦ Two types of drives:
 Open belt drive
 Cross belt drive

◦ Open belt drive
 The drive in which Parallel shafts rotate in
same direction is called open belt drive.
 Belt is subject to tension and bending.

◦ Cross belt drive
 The drive in which parallel shafts rotate in
opposite direction.
 More wear and tear of belt in this drive, but it
transmit more power than open belt drive.
2

Open belt drive Vs. Close belt drive
Open Belt Drive Closed Belt Drive
Both driver and the driven rotates in
the same direction
Driver and driven rotates in opposite
direction
When the shafts are horizontal,
inclined it is effective to transmit the
power
Even if the shafts are vertical it is
effective to transmit the power
As there is no rubbing point, the life of
the belt is more
Due to the rubbing point, the life of
the belt reduces.
Require less length of the belt
compared to crossed belt drive for
same centre distance, pulley
diameters.
Require more length of belt compared
to open belt drive for the same centre
distance, pulley diameters.

◦ Application of belt drives:
 Transmit power from low or medium capacity
electric motors to operative machines
 To transmit power from small prime movers

◦ Transmission of power in belt drive:
 Driving pulley L and driven pulley M is
connected my belt. The driving pulley pulls the
belt from one side and delivers the same to the
other.
 Hence tension is in tight side will be more than
slack side.
T1 = tension in the tight side, N
T2 = tension in the slack side, N
v = velocity of the belt, m/s
Now effective turning force = T1 - T2
Hence, work = Force x Distance = (T1 - T2 ) v
Nm/s
Thus. Power = (T1 - T2 ) v

Pulleys and Pulley Drive…
Types of Pulley Drive:
◦ Idler pulley
◦ Stepped pulley
◦ Fast-Loose pulley

Chains and Chain Drive
◦ What is chain?
A chain consists of links connected by
joints which provide for flexibility for chain.

Chains and Chain Drive…
◦ Chain drive:
 A chain drive consists of two sprockets
and chain
 Most often, the power is conveyed by a
roller chain, known as the drive chain,
passing over a sprocket gear, with the
teeth of the gear meshing with the holes
in the links of the chain.
 The gear is turned, and this pulls the
chain putting mechanical force into the
system

 Advantages of Chain Drives
◦ Do not slip
◦ Maintain constant and precise speed.
◦ Good service life
◦ Easy to install and repair
 Disadvantages of Chain Drives
◦ Noisy
◦ Need lubrication
◦ Weight of the chain

 Roller Chain Drive:
◦ The hollow rollers are held inside two flat link plates
which are joined together by sleeves or bushing
passing inside the rollers.

 Roller Chain Drive:
◦ Consecutive sets of such assemblies are connected
together with another pair of plates called pin link
plates, which in turn are held by central pins passing
through the sleeves. Rivets are used to join the link
plates. Sometimes the roller link plates are joined
together by rollers themselves and no sleeves are
used.

 Silent Chain Drive:
It consist of a number of
flat links, tooth shaped at
ends and joined together
by long cross pins. The
sprockets are usually
wider than those of the
roller chain and have a
central groove which
holds retained plates
provided in the central
links for keeping the
chain on the sprocket
securely.

 Use of chain drive:
◦ Motorcycles
◦ Bicycles
◦ Automobiles
◦ Conveyers
◦ Agricultural machinery
◦ Oil-well drilling machines
◦ Machine tools

 What is Gear?
◦ A gear is wheel provided with teeth which mesh
with the teethe on another wheel, or on to a
rack, so as to give a positive transmission of
motion from one component to another.
Gears and Gear Drive…

Types of Gears:
According to the position of axes of the shafts.
a. Parallel
1.Spur Gear
2.Helical Gear
3.Rack and Pinion
b. Intersecting
Bevel Gear
c. Non-intersecting and Non-parallel
worm and worm gears

TYPES OF
GEARS
4

 Spur gears
Teeth is parallel to axis of
rotation
Transmit power from one shaft
to another parallel shaft
 USES
Generally it is used in Electric
screwdriver, oscillating
sprinkler, windup alarm clock,
washing machine and clothes

 Spur gear types:
◦ External type
◦ Internal type

 Helical gears
The teeth of the gear are helix around
the gear. The helical gear run more
smoothly and more quietly at high
speed and curvilinear contact of gear
teeth giving gradual engagement.
 USES
Generally it is used in automobile
power transmission.

 Bevel gears
Bevel gear teeth are varying in
cross section along the tooth
width. The axis of two moving
gears are inclined in the bevel
gear.
USES
When the axis of the two shafts
are inclined and intersect each
other

 Worm and worm wheel
A worm has one or more number of helical
threads of trapezoidal shape cut on it.
USES
To transmit power
between two shafts
having their axis at
right angles and non-
coplaner. Like in
drilling machine.

 Rack and pinion
Rack is a spur gear of infinite diameter. The
rack is mesh with another small gear known as
pinion
USES
It is used to convert
rotary motion into
linear motion.
Such as lathe , drilling ,
planning machines.

 Pitch circle. It is an imaginary circle which by pure rolling
action would give the same motion as the actual gear.
 Pitch circle diameter. It is the diameter of the pitch circle.
The size of the gear is usually specified by the pitch circle
diameter. It is also known as pitch diameter.
 Pitch point. It is a common point of contact between two
pitch circles.
 Pitch surface. It is the surface of the rolling discs which the
meshing gears have replaced at the pitch circle.
 Pressure angle or angle of obliquity. It is the angle
between the common normal to two gear teeth at the point of
contact and the common tangent at the pitch point. It is
usually denoted by φ. The standard pressure angles are 14
1/2 ° and 20°.

 Addendum. It is the radial distance of a tooth from the pitch
circle to the top of the tooth.
 Dedendum. It is the radial distance of a tooth from the pitch
circle to the bottom of the tooth.
 Addendum circle. It is the circle drawn through the top of the
teeth and is concentric with the pitch circle.
 Dedendum circle. It is the circle drawn through the bottom of
the teeth. It is also called root circle.
Note : Root circle diameter =
Pitch circle diameter × cos φ ,
where φ is the pressure angle.

Circular pitch. It is the distance measured on the circumference
of the pitch circle from a point of one tooth to the corresponding
point on the next tooth. It is usually denoted by Pc
,Mathematically,
 A little consideration will show that the two gears will mesh
together correctly, if the two wheels have the same circular
pitch.
Note : If D1 and D2 are the diameters of the two meshing gears
having the teeth T1 and T2 respectively, then for them to mesh
correctly,

Diametral pitch. It is the ratio of number of teeth to the pitch
circle diameter in millimetres. It is denoted by pd.
Mathematically,
Module. It is the ratio of the pitch circle diameter in millimeters to
the number of teeth. It is usually denoted by m. Mathematically,
Clearance. It is the radial distance from the top of the tooth to the
bottom of the tooth, in a meshing gear. A circle passing through
the top of the meshing gear is known as clearance circle.
Total depth. It is the radial distance between the addendum and
the dedendum circles of a gear. It is equal to the sum of the
addendum and dedendum.

Working depth. It is the radial distance from the addendum circle to
the clearance circle. It is equal to the sum of the addendum of the
two meshing gears.
Tooth thickness. It is the width of the tooth measured along the pitch
circle.
Tooth space . It is the width of space between the two adjacent teeth
measured along the pitch circle.
Backlash. It is the difference between the tooth space and the tooth
thickness, as measured along the pitch circle. Theoretically, the
backlash should be zero, but in actual practice some backlash must
be allowed to prevent jamming of the teeth due to tooth errors and
thermal expansion.

Face of tooth. It is the surface of the gear tooth above the pitch
surface.
Flank of tooth. It is the surface of the gear tooth below the pitch
surface.
Top land. It is the surface of the top of the tooth.
Face width. It is the width of the gear tooth measured parallel to its
axis.
Profile. It is the curve formed by the face and flank of the tooth.
Fillet radius. It is the radius that connects the root circle to the profile
of the tooth.

Path of contact. It is the path traced by the
point of contact of two teeth from the
beginning to the end of engagement.
Length of the path of contact. It is the
length of the common normal cut-off by the
addendum circles of the wheel and pinion.
Arc of contact. It is the path traced by a
point on the pitch circle from the beginning
to the end of engagement of a given pair of
teeth. The arc of contact consists of two
parts, i.e.
(a) Arc of approach. It is the portion of the
path of contact from the beginning of the
engagement to the pitch point.
(b) Arc of recess. It is the portion of the
path of contact from the pitch point to the
end of the engagement of a pair of teeth.

Gear Trains
 When two or more gears are made to mesh
with each other to transmit power from one
shaft to other. Such an arrangement is called
gear train.
Types:
 Simple gear train
 Compound gear train
 Epicyclic gear train

Simple gear train
 When there is only one gear on each
shaft, then it is known as simple gear
train.
6

Simple gear train…
 When the distance between the two gears is large
and we need Constant velocity ratio:
 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
2. By providing one or more intermediate gears.
The first method (i.e. providing large sized gears) is very
inconvenient and uneconomical method
whereas the latter method (i.e. providing one or more
intermediate gear) is very convenient and economical.

 Intermediate gears:
Intermediate gears are called
idle gears, as they do not
effect the speed ratio or train
value of the system.
1. To connect gears where a
large centre distance is
required
2. To obtain the desired
direction of motion of the
driven gear(i.e. clockwise or
anticlockwise)

 Intermediate gears:
when the number of intermediate gears are odd,
the motion of both the gears (i.e. driver and driven
or follower) is like.
If the number of intermediate gears are even, the
motion of the driven or follower will be in the
opposite direction of the driver.

ADVANTAGES of Simple Gear Train
 to connect gears where a large center distance is
required
 to obtain desired direction of motion of the driven
gear ( CW or CCW)
 to obtain high speed ratio

Compound gear train
 When there are more than one gear on a
shaft , then the gear train is called a
compound train of gear.

ADVANTAGES of Compound Gear Train
 A much larger speed reduction from the first
shaft to the last shaft can be obtained with
small gear.
 If a simple gear trains used to give a large
speed reduction, the last gear has to be very
large.

Epicyclic Gear Train
 When there is relative
motion between two or
more of the axes of
wheels, such
arrangement is called
epicyclic gear train.
 A small gear at the
center called the sun,
several medium sized
gears called the planets
and a large external
gear called the ring gear.

ADVANTAGES of Epicyclic Gear
Train
•They have higher gear ratios.
•They are popular for automatic transmissions in
automobiles.
•They are also used in bicycles for controlling power
of pedaling automatically or manually.
•They are also used for power transmission
between internal combustion engine and an electric
motor.

Review Questions
Short Answer Questions
1. Differentiate between open belt and crossed belt
drive.
2. What are the commonly used materials for flat belts?
3. List out the applications of belt drives.
4. What do you mean by slip in a belt drive?
5. Differentiate between belt drive and chain drive.
Long Answer Questions
1. Explain with neat sketch, the types of various drives.
2. List out the advantages and disadvantages of all
drives.
3. Compare flat belt and V belts.
4. Briefly explain chain drives.
5. Briefly explain pulley drives.

Sources/Links
1. Image references
2. http://mechteacher.com/mt/wp-content/uploads/2013/05/Flat-belt-
drive.png
3. http://www.globalspec.com/ImageRepository/LearnMore/20135/crossed
%20belt%20drive0cb067c5f9ea41978ae3d270411d8715.png
4. http://www.thecartech.com/subjects/auto_eng/Gear_Types_files/image00
1.jpg
5. http://file1.cucdc.com/cwfiles/11/66011/picture/F5.4%20Nomenclature
%20of%20the%20spur%20gear%20teeth.jpg
6. http://nptel.ac.in/courses/Webcourse-contents/IIT-
Delhi/Kinematics%20of%20Machine/site/gear/pics/image012b.png
 Content References
 – Elements of Mechanical Engineering by H.G. Katariya, J.P
Hadiya, S.M.Bhatt , Books India Publication.
 -Elements of Mechanical Engineering by V.K.Manglik, PHI
 -Elements of Mechanical Engineering by R.K Rajput.
 -Elements of Mechanical Engineering by P.S.Desai & S.B.Soni

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