Power Transmission Prashanth
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Transcript of Power Transmission Prashanth
8/3/2019 Power Transmission Prashanth
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POWER TRANSMISSION
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Transmission
• Power transmission is the movement of energy from its place of
generation to a location where it is applied to performing useful
work.
• Rotational motion transmitted from one mechanical element to
the other with the help of certain systems called Transmissionsystems or Drives.
• One that drives – Driving system.
• Other which is driven – Driven system.
• Usually round rods, called Shafts are employed to transmit
rotational motion.
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Types of Transmission systems
• Transmission systems are classified based on:
Distance between them
Speed
Power• Belt Drives
• Rope Drives
• Chain Drives
• Gear Drives
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Belt Drives
• Used to transmit power from one shaft to another when theshafts are some distance apart.
• Consists of an endless belt which is wrapped tightly over two
pulleys known as Driving and the Driven pulleys which in turn
connected to the rotating shafts.
• Motion from the Driving pulley is transmitted to the driven
pulley by frictional resistance between the belt and the surface
of the pulleys.
• Since there will be slip between the belt and surface of the
pulleys, the velocity ratio in the belt drives is never absolutely
constant.
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Belt Materials
• Belt must be strong, flexible & durable and must have a highcoefficient of friction.
• Commonly used materials are Leather, Fabric, Rubber, Balata,
Camel’s hair and Woven cotton.
• Belts are available with Flat, Round or V-Cross sections.• Usually Flat and V-Belts are used.
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Advantages of Flat belt drives
• Flexibility• Shock absorption
• Efficiency at high speeds.
• Protection against over load
• Resistance to abrasive and other harmful environments.
• Simplicity
• Smoothness of operation.
• Low cost• low maintenance and long life.
• Used to connect widely spaced shafts.
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Disadvantages Flat belt drives
• Less efficiency due to Slip and Creep• Not suitable for short distances
• Comparatively large size
• Belt joints reduce the life of the belt.
• Heat buildup occurs. Speed is limited to usually 7000feet per minute (35 meters per second). Powertransmission is limited to 370 kilowatts (500horsepower).
• Operating temperatures are usually restricted to – 31 to185°F ( – 35 to 85°C).
• Some adjustment of center distance or use of an idlerpulley is necessary for wear and stretch compensation.
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Types of Belt Drives
• Belt drives can be classified as:Open belt drive
Crossed belt drive
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Open Belt Drive
• Used to connect shafts, which are parallel and rotating in samedirection.
• Used when the distance between the shafts are quite large.
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Crossed Belt Drive
• Used to connect shafts, which are parallel and rotating inopposite directions.
• Used usually when the distance between the shafts are small.
• At a point where belt crosses, it rubs against itself and wears.
• To avoid wear, center distance between the shafts must begreater than 20b.
where, b is the width of the belt and operate at low velocity.
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Velocity Ratio of Belt Drive
• Velocity ratio of belt drive is defined as the ratio of angularvelocity of Driver pulley to the angular velocity of Driven
pulley,
where,NL and NS are the rotational speeds of thelarge and the small pulley respectively,
dS and dL are the diameters of the largeand the small pulley respectively,
t is the belt thickness.
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Slip in Belt Drives
•When a belt is transmitting power, there is always slip in asmall amount between belt and the pulleys and is due to
insufficient frictional grip between belt and the rim of pulley.
• The difference between the linear speeds of the pulley rim and
the belt on it is the measure of slip & expressed in percentage.
Where,
Total Percentage, s = s1 + s2
s is the total percentage of the belt slip,
s1 & s2 are the percentage slip between driving
pulley & the belt and Driven pulley & the belt
respectively.
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Nomenclature of Open Belt Drive
dL - Diameter of the larger pulleydS
– Diameter of the smaller pulley
αL- Angle of wrap of the larger pulley
αS – Angle of wrap of the smaller pulley
C- Center distance between the two pulleys
αL= 180ο + 2β
αS= 180ο - 2β
Where angle β is,
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Nomenclature of Crossed Belt Drive
dL - Diameter of the larger pulley
dS – Diameter of the smaller pulley
αL- Angle of wrap of the larger pulley
αS – Angle of wrap of the smaller pulleyC- Center distance between the two pulleys
αL= αS
= 180ο + 2β
Where angle β is,
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Belt Tensions
• Due to the presence of friction between the pulley and the belt
surfaces, tensions on both the sides of the belt are not equal.• So it is important that one has to identify the higher tension side
and the lower tension side.
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Belt Tensions
• When the driving pulley rotates (in this case, anti-clock wise),
from the fundamental concept of friction, we know that the belt
will oppose the motion of the pulley.
• Thereby, the friction, f on the belt will be opposite to the motion
of the pulley.
• Friction in the belt acts in the direction, as shown in the Figure,and will impart a motion on the belt in the same direction.
• The friction f acts in the same direction as T2.
• Equilibrium of the belt segment suggests that T1 is higher than T2.
• Here, we will refer T1
as the tight side and T2
as the slack side,
i.e., T1 is higher tension side and T2
is lower tension side.
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Ratio of Belt Tensions
where,
α is the Angle of Lap,
T1 & T2 are the Tight and Slack side Tensions in the belt,dφ is the angle subtending at the center of the pulley,
T & (T+dT) are the Tensions at the two extremities of the belt,
dN is the Normal reaction, and
μdN is the force of friction opposing the slip & actingperpendicular to dN.
In case of Flat Belts,
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Power Transmitted by the Belt
• Depends on the arc of contact, difference in the belt tensions,coefficient of friction and center distance.
• Pulley having lower value of μα (smaller pulley) governs the
power transmission.
Where,
P is the power transmission in Watt and
v is the belt velocity in m/s
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Creep in Belt Drives
• Presence of friction between pulley and belt causes differentialtension in the belt.
• This differential tension causes the belt to elongate or contract
and create a relative motion between the belt and the pulley
surface.• This relative motion between the belt and the pulley surface is
created due to the phenomena known as Creep.
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Pulleys• Stepped Pulley
Used for changing the speed of the Driven shaft while theDriving shaft runs at constant speed.
• Tight & Loose Pulley
Used when the Driven shaft is to be started or stopped
whenever desired without stopping the belt.Tight pulley (Fast pulley) keyed to the shaft.
Loose pulley turns loose upon the driven shaft and is keptin place by the hub of the tight pulley and a collar.
• Jockey or Idler Pulley
Power transmitting capacity of the drive is seriouslyreduced by Idler pulley.
Idler increases the angle of wrap and reduce the belttensions required for a given power.
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V-Belts• Used in high power transmission.
• Transmit power between the shafts whose center distance isshort.
• Advantages
Transmit power with no slip.
Used for short center distance drives.
High velocity ratio and high efficiency.
Drive is smooth and maintenance is low.
• Disadvantages
Not economical
Cannot be used with larger center distances.
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Rope Drives
• Rope drives used to transmit power over long distances.
• Rope runs over pulleys, called Sheaves having groovedsurfaces.
• Ropes are made of Cotton, Hemp, Manila or Steel wires.
• Main advantage is that a number of separate drives may betaken from one driving pulley.
• In case of multiple rope system, even if one rope breaks, thecontinuity of power is maintained.
• Advantages
High efficiency
Low cost
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Types of Rope Drives
• Rope Drives are classified as
Fibre ropes
Wire ropes
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Fibre Ropes
• Fibre ropes are made of Leather, Cotton and Rubberizedfabric.
• Fibre ropes upto 60m length can be used.
• Fiber ropes used in lighter applications like printing machines.
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Wire Ropes
• Wire ropes are made of Cast steel and Alloy steel.• Wire rope upto 150m length can be used.
• Wire ropes used in heavier applications like hoisting,
elevators, mines, suspension bridges.
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Gear Drives
• A gear is a component within a transmission device thattransmits rotational force to another gear or device.
• Gears are toothed wheels used to transmit power from one shaft
to another when a constant speed ratio is desired and the
distance between the shaft is relatively small.
• Gears are mounted on the axles or shafts and keyed to them.
• Two gears mounted one on each of the Driving and Driven
shafts are arranged so that the teeth of one will mesh with the
teeth of other.
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Types of Gears
• Gears are classified according to the position of axes of theshafts.
Parallel
Spur Gear
Helical Gear
Double Helical Gear
Herringbone Gear
Rack and Pinion
Intersecting
Bevel Gear
Non-intersecting and Non-parallel
Worm gears
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Spur Gear • Teeth is parallel to axis of rotation.
• Transmit power from one shaft toanother parallel shaft.
• Used in Electric screwdriver,
oscillating sprinkler, windup alarmclock, washing machine and clothesdryer.
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Spur Gears
Spur gear
Gear
Pinion
Transmission between parallel shafts
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External and Internal Spur Gear…
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Helical Gear • The teeth on helical gears are cut at an angle to the face of the
gear.
• This gradual engagement makes helical gears operate much
more smoothly and quietly than spur gears.
• One interesting thing about helical gears is that if the angles of
the gear teeth are correct, they can be mounted on
perpendicular shafts, adjusting the rotation angle by 90
degrees.
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Helical Gear…
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Helical Gears
Parallel or non parallel shafts
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Herringbone Gears
• To avoid axial thrust, two helical
gears of opposite hand can be
mounted side by side, to cancel
resulting thrust forces.
• Herringbone gears are mostly
used on heavy machinery.
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Rack and Pinion
• Rack and pinion gears are used to
convert rotation (From the pinion)
into linear motion (of the rack)
• A perfect example of this is the
steering system on many cars
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Rack and Pinion
Conversion of rotary to linear motion or vice versa
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Bevel Gears
• Bevel gears are useful when the direction of a shaft's rotation
needs to be changed.
• They are usually mounted on shafts that are 90 degrees apart,but can be designed to work at other angles as well.
• The teeth on bevel gears can be straight, helical or spiral.
• locomotives, marine applications, automobiles, printingpresses, cooling towers, power plants, steel plants, railwaytrack inspection machines, etc.
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Straight and Spiral Bevel Gears
Shafts with intersecting axes
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Worm and Worm Gear
• Worm gears are used when large gear reductions are needed. Itis common for worm gears to have reductions of 20:1, and
even up to 300:1 or greater.
• Many worm gears have an interesting property that no other
gear set has: the worm can easily turn the gear, but the gear
cannot turn the worm.
• Worm gears are used widely in material handling and
transportation machinery, machine tools, automobiles etc..
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Worm Gear
Transmitting motion between non parallel and non intersecting
shafts,
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Advantages & Disadvantages
• No slip• High transmission efficiency
• Compact and effective for transmission over shorter distances
• Complex manufacturing procedure
• Large center distance transmission is not possible
• Applications
Automobile gear boxes
Differential
MachinesWatches
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Nomenclature
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Internal Spur Gear System
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Gear Trains
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Gear Box
Semi Automatic Transmission
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Semi Automatic Transmission