chapter 4 ppt 1.ppt

Power transmission
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Introduction
 Rotating elements which possess mechanical energy has to
be utilized at required place by transmitting.
 From prime mover to machine
 From one shaft to another
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Transmission system
 The system that is used to transmit power from one
mechanical element to another mechanical element.
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Types of transmitting system
 Belt drives
 Rope drives
 Chain drives
 Gear drives
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Factors to select transmission
system
 Distance between driver and driven pulley shaft.
 Operational speed.
 Power to be transmitted.
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Belt drive
Power is to be
transmitted between
the parallel shaft.
Consists of two pulleys
over which a endless
belt is passed
encircling the both.
Rotary motion is
transmitted from
driving pulley to driven
pulley.
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Friction is a helpful
agent.
Tension in one side of
the belt
Portion of the belt
having less tension is
called slack side.
Portion of the belt
having high tension is
called tight side.
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Terminology of a belt drive
 Driver : in a transmission system the one which drives or
supplies power to other mechanical element.
 Driven : in a transmission system the one which follows the
driver or receives power from driver.
 Tight side : the portion of the belt in maximum tension.
Denoted by T1 Newton.
 Slack side : the portion of the belt in minimum tension.
Denoted by T2 Newton.
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 Arc / angle of contact : it is the portion of the belt which is in
contact with pulley surface. Denoted by
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Belt materials
 Rubber
 Leather
 Canvas
 Cotton
 Steel
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Classification
 Open belt drive
 Closed belt drive
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Open belt drive
Both driver and driven
pulley rotate in both
direction.
Belt is passed over
driver and driven.
Driver pulley pulls the
belt from one side and
delivers to other side.
Tension is more in
lower side then upper
side.
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Cross belt drive
Driven rotates in
opposite direction to
that of driver.
At the point were the
belt crosses it rubs
against each other and
there will be wear.
To avoid this speed of
belt should be less
than
15 m/s.
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Comparison between Open belt drive and 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.
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n1/n2 = d2/d1
i.e., speed of driving Diameter of driven
speed of driven Diameter of driver
The ratio n1 d2 is called as
n2 d1 or “transmission ratio”
of the belt drives
n1 d2 + t
n2 d1 + t
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The relative motion between the belt and the pulley surface due to
contraction and expansion of belt is defined as “creep”.
Creep increases with load as it is caused by the elasticity of the
belt.
It reduces the speed of the driven pulley which results in loss of
power transmission.
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Relative motion between pulley and the belt
passing over it is defined as “slip”.
Velocity ratio n1 d2 100
( when slip is considered) = =
n2 d1 100-S
Where S= % slip
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Idler pulley www.bookspar.com | Website for
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Stepped Pulley www.bookspar.com | Website for
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Fast and loose pulley www.bookspar.com | Website for
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 Running and maintenance cost is low.
 Possibility to transmit power over a moderately long distance.
 Efficient at high speeds.
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 Not preferred for short centre distance.
 Belt joints reduces the life of the belt.
 Loss of power due to slip and creep.
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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.
Simple gear train (SGT)
Compound gear train (CGT)
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Simple gear train
 Arrangement of gears in series is known as simple gear train.
 Intermediate gears are provided between the driver and driven.
The function of the idler gears is
1. To cover the space between the driver and driven gears and to
2. Obtain the desired direction of driven
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Compound gear train
 When two or more gears are compounded, then the gear train is known as
compound gear train.
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