INTRODUCTION
The rotation motion is the ideal and
the simplest means of transmission of
mechanical power with negligible
losses. Rotational motion can be
transmitted from one mechanical
element to the other with the help of
certain systems known as
transmission systems or drives.
These systems are driven by a prime
mover or transmit the rotational
motion to the various parts of a
machine within itself. Usually, shafts
are employed to transmit rotational
motion.
The one that drives is called the
driving system.
The other which is driven is called
driven system.
BELT DRIVE:
Belt Drives are a type of frictional
drives used for transmitting powers
from one shaft to another by means of
pulleys which rotate at the same
speed or at the different speed.
A belt drive is shown in the figure. Consists of two
pulleys over which an endless belt it passed over them.
The mechanical power or rotary motion is transmitted
from the driving pulley to the driven pulley because of the
frictional grip that exists between the belt and the pulley
surface.
The portion of the belt which is having less tension is
called slack side and the one which has higher
tension is called tight side. The effective pulling power
of the belt that causes the rotation of the driven pulley is
the difference in tension on the slack and tight side.
The tensions in the tight and slack sides of the belt
depend on the angle of contact, the belt drives have to be
arranged such that the slack side comes above and the
tight side comes below the pulleys.
This arrangement increases the angled contact of
the belt on the driven side. Sometimes in a belt-
drive, there is always a possibility of some slipping
taking place between the belt and the pulleys which
cause the driven pulley to rotate at a lesser speed,
consequently reduces the power transmission.
Hence belt drives are said to be not a Positive type
of power transmission system.
Types of Belts:
There are four commonly used types of belts are:
Flat Belt
V- Belt
Circular Belt
Flat belt: This belt has a rectangular cross-section.
These belts are capable of transmitting power over long
distances between pulley centers. The efficiency of this
drive is around 98% and produces little noise.
V-belts: V-belts also used with grooved pulleys, V-
belts are trapezoidal in cross-section. These belts
permit large speed ration and can transmit higher
power. Multiple drives are possible.
Circular: This type of belt has a circular cross-
section and is used with the grooved pulleys.
Types of Belt Drives:
The following are the 5 main types of Belt Drives:
Open belt drive.
Cross belt drive.
Stepped cone pulley or speed cone drive.
Fast and loose pulleys.
Jockey pulley drive.
1. Open Belt Drive
In these types of belt drive, the belt is
employing when the two parallel shafts have to
rotate in the same direction.
When the shafts are far apart, the lower side of
the belt should be the tight side and the upper
side must be the slack side.
This is because, when the upper side becomes
the slack side, it will sag due to its own weight
and thus increase the arc of contact.
2. Cross Belt Drive
These types of belt drives, the belt is employing
when two parallel shafts have to rotate in the
opposite direction. At the junction where the belts
cross, it rubs against itself and wears off.
To avoid excessive wear, the shafts must be
placed at a maximum distance from each other and
operated at very low speeds.
3. Stepped Cone Pulley or
Speed Cone Drive
A stepped cone pulley also known for a speed cone
is showing in the fig.
These types of belt drives are used when the
speed of the driven shaft is to be changed very
frequently as in the case of machine tools such
as lathe, drilling machine, etc.
A stepped cone pulley is an integral casting
having three or number of pulleys of different sizes
one adjacent to the other as shown in fig.
One set of stepped cone pulley is mounted in
reverse on the driven shaft. An endless belt will be
wrapped around one pair of pulleys.
By shifting the belt from one pair of pulleys to
the other, the speed of the driven shaft can be
varied.
The diameter of the driving and driven pulleys is
such that the same belt will operate when shifted on
different pairs of pulleys.
4. Fast and Loose Pulley
Drive
A fast and loose pulley drive is showing in fig.
These types of belt drives are used when the
driven or machine shaft is to be started or stopped
whenever desired without interfering with the driving
shaft.
A pulley which is keyed to the machine shaft is
called a fast pulley and run at the same speed as
that of the machine shaft.
A loose pulley runs freely over the machine shaft
and is incapable of transmitting any power.
When the driven shaft is required to be stopped,
the belt is pushed on to the loose pulley by means of
a sliding bar having belt forks.
5. Jockey Pulley Drive
In an open belt drive arrangement, if the centre
distance is small, or if the driven pulleys are very
small.
Then the arc of contact of the belt with the
driven pulley will be very small, which reduces the
tensions in the belt, or if the required tension of the
belt cannot be obtained by other means, an idler
pulley, called jockey pulley is placed on the slack
side of the belt as shown in fig.
Which increase the arc of contact and thus the
tension which results in increased power
transmission.
Slip and Creep In Belt Drive:
Slip in Belts:
Consider an open belt drive rotating in a clockwise
direction, this rotating of the belt over the pulleys is
assumed to be due to firm frictional grip between the belt
and the pulleys.
When this frictional grip becomes insufficient, there is a
possibility of forwarding motion of driver without pulley with
it, this is known as the Slip in a belt.
Therefore slip may be defined as the relative motion
between the pulley and the belt in it. This reduces the
velocity ratio and usually expressed in % and it is denoted
by S.
Creep in Belts:
Consider as an open belt drive rotating in the clockwise direction.
The portion of the belt leaving the driven and entering the driver is
known as the tight side and a portion of the belt leaving the driver
and entering, this known as the slack side.
During rotation, there is an expansion of a belt on a tight side and
contraction of the belt on the slack side.
Due to this uneven expansion and contraction of the belt over the
pulleys, there will be a relative movement (motion) of the belt over
the pulleys this phenomenon is known as Creep in a belt.
Advantages and Disadvantages of
Flat Belts
The following are the advantages and disadvantage of
belts drive.
Advantages:
1. Flexible, simple in construction, smooth operations.
2. Efficient at high speeds and protects against
overload.
3. Running and maintenance cost is low.
4. Relatively long life and easy to work with.
Disadvantages:
1. Loss of power sues to slip and creep in turn results in
low efficiency.
2. Not preferred for short-centre distances.
3. Because of the endlessness of the belt, joints reduce
the life of the belt.
4. Not a positive drive.
Materials Used For Belts
The materials used for belts must be strong, flexible and
durable. It should have a high coefficient of friction. The
various materials used are:
1. Leather
2. Fabric
3. Rubber
4. Balata
Leather: The leather may be oak-tanned or mineral salt
tanned ex: Chrome tanned. When the thickness of the belt
required is more than, two or more strips are cemented
together. Leather belts require periodic cleaning.
Fabric: Fabric belts are made by folding canvas or cotton
ducks is a layer (depending on the required thickness) and
stitching together.
Rubber: The belts are made of Fabric with a rubber layer.
These are used in sawmills, paper mills, etc.
Balata: The belts are made out of these materials are
similar to rubber belts expect that balata gym is used
instead of rubber. The belts of these materials are acid
and waterproof but cannot be used where the
temperature is above 45°.
V-Belts:
The V-belts or “V” types of belt drives are widely used in
high power transmission. V-belts are trapezoidal in section
as shown in the figure. They are molded as endless loops
from rubber reinforced with fibrous material. They run in
the v-grooves made in the pulleys.
The wedging action of the belts in the V-grooves enables
them to transmit higher torques. The power transmitted by
a v-belt drive and be increased by operating with several
belts placed side.
This form of drive is used extensively in all classes
of machinery transmitting power from 0.5 kW up to
150kW.
Advantages of the V-belt Drive over a
Flat Belt Drive
Following are the advantages of the v-belt drive over
the flat belt drive.
I. The V-belt drive gives compactness due to the small
distance between the centers of pulleys.
II. The drive is positive because the slip between the belt
and the pulley groove is negligible.
III. Since the V-belts are made endless and there is no
trouble, therefore the drive is smooth.
IV. It provides longer life, 3 to 5 years.
V. The operation of the belt and pulley is quiet.
VI. It can be easily installed and removed.
VII. The belt has the ability to cushion the shock when
machines are started.
VIII. The high-velocity ratio (maximum 10) may be
obtained.
IX. V-belt may be operated in either direction with the
tight side of the belt at the top or bottom.
X. Therefore the power transmitted by v-belts is more
than flat belts.
XI. The centre line may be horizontal, vertical or inclined.
Selection of Belt Drive
The following factors are considered in the
selection of belt drives.
The speed of the driver and driven pulleys
Speed reduction ratio
Power to be transmitted
Center distance between the shaft
Shaft layout
Positive drive requirements
