power transmission prashanth

8/3/2019 Power Transmission Prashanth

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POWER TRANSMISSION



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.



Types of Transmission systems

• Transmission systems are classified based on:

Distance between them

Speed

Power• Belt Drives

• Rope Drives

• Chain Drives

• Gear Drives



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.



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.



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.



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.



Types of Belt Drives

• Belt drives can be classified as:Open belt drive

Crossed belt drive



Open Belt Drive

• Used to connect shafts, which are parallel and rotating in samedirection.

• Used when the distance between the shafts are quite large.



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.



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.



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.



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,



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,



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.



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.



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,



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



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.



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.



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.



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



Types of Rope Drives

• Rope Drives are classified as

Fibre ropes

Wire ropes



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.



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.



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.



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



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.



Spur Gears

Spur gear

Gear

Pinion

Transmission between parallel shafts



External and Internal Spur Gear…



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.



Helical Gear…



Helical Gears

Parallel or non parallel shafts



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.



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



Rack and Pinion

Conversion of rotary to linear motion or vice versa



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.



Straight and Spiral Bevel Gears

Shafts with intersecting axes



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..



Worm Gear

Transmitting motion between non parallel and non intersecting

shafts,



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



Nomenclature



Internal Spur Gear System



Gear Trains



Gear Box

Semi Automatic Transmission



Semi Automatic Transmission

power transmission prashanth

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